- Email: [email protected]
Design of a large-format high-resolution streak camera with a planar photocathode
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Design of a large-format high-resolution streak camera with a planar photocathode Zhang Jing-Jin a,b , Liu Ai-lin b , Yang Qin-lao a ,∗, Zong Fang-ke a,b ,∗∗, Guo Bao-ping a,b a b
College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China College of Electronic and Information Engineering, Shenzhen University, Shenzhen 51806, China
ARTICLE
INFO
Keywords: Streak camera Ultraviolet X-ray Large-format detection area High spatial resolution High temporal resolution
ABSTRACT Streak cameras are widely used as ultrafast diagnostic tools in several research fields. With an increase in the complexity of the experiments being conducted, large-format streak cameras with high spatiotemporal resolution are required for wide-band, high-precision spectroscopic recording. This paper presents the design of a high-resolution large-format streak camera with a planar photocathode. To alleviate the effect of field curvature for a large-format photocathode and to improve its spatial resolution, a weak–strong double-focusing lens coupled with a spherical fluorescent screen is used. A high voltage is also used to reduce transit time dispersion in order to improve temporal resolution. Experimental results show that the minimum effective diameter of the planar photocathode is 40 mm, compare to the existing 30 mm photocathode, which can achieve a minimum spatial resolution of 1000 line pairs and a temporal resolution of approximately 4 ps. The dynamic spatial resolution achieved is 20 lp/mm and the deflection sensitivity is approximately 64 kV/mm.
1. Introduction Streak cameras convert temporal information of an ultrafast signal into spatial domain and can achieve picoseconds or even femtosecond measurement with high spatial resolution. Therefore, they are widely used as ultrafast diagnostic tools when researching high energy density physics [1–3], laser induced plasmas [4–6], and particularly in physical experiments of inertial confinement fusion (ICF) [7–9]. With an increase in the complexity of the experiments being conducted, large-format streak tubes with a high resolution over the entire spatial– temporal imaging plane are required for wide-band, high-precision spectroscopic recording [10,11]. Similarly, target imaging experiments used to investigate radiation flow requires a large field of view with high resolution across the photocathode. Divided field-of-view imaging, such as multichannel recording and pinhole array imaging, also requires a large number of resolution elements across the photocathode. To meet the demands described above, a large-format streak camera with a high spatiotemporal resolution should be developed. To implement a large format, the effect of field curvature needs to be alleviated; this effect causes electrons from the edges of the photocathode to arrive later and at a different focus than electrons from the center. By employing a curved photocathode–acceleration system, Lawrence Livermore National Laboratory developed a second-generation X-ray
streak camera [12] (P2XSC), with a concentric sphere-type electrostatic focusing streak tube, the RCA C73435, as the major component. This could minimize aberrations and considerably improve off-center focusing. However, the working voltage up to 25 kV of the RCA C73435 [12] can easily produce arc and damage the photocathode and even the metal acceleration grid unit. In addition, such high anode voltage is not conducive to improve the deflection sensitivity of streak camera. Therefore, it needs to increase the slope of scanning voltage to improve the temporal resolution. And this may easily lead to the breakdown of electronic components, while also easily lead to jitter of scanning starting signal, which is not conducive to synchronization. Moreover, for X-ray, when developing a photocathode streak tube, the Parylene film [13] must be bent to prepare a spherical substrate and ensuring a smooth surface of the substrate is a very difficult task. In this paper, the idea of using a weak–strong bifocal lens was proposed for the first time, which can not only alleviate the influence of the field curvature, but also make the electron beam maintain a high speed to reduce the influence of space charge effect and then improve the temporal resolution. Based on this, we designed a large-format streak tube with a planar photocathode at 12 kV anode voltages, compared to the 25 kV of the RCA 73 435. The electronic optical system was designed using the finite difference method in the MATLAB software,
∗ Corresponding author. ∗∗ Corresponding author at: College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. E-mail addresses: [email protected] (Z. Jing-Jin), [email protected] (L. Ai-lin), [email protected] (Y. Qin-lao), [email protected] (Z. Fang-ke), [email protected] (G. Bao-ping).
https://doi.org/10.1016/j.nima.2019.163076 Received 17 July 2019; Received in revised form 29 September 2019; Accepted 31 October 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
photocathode exit surface have different flight trajectories due to different emission angles, which eventually result in the secondary electrons arriving at the image plane of the streak camera not as a point but as a diffuse spot, and the flying time of the secondary electrons varies. However, the angular distribution of secondary electrons in photocathode is difficult to obtain through experiments. Therefore, some researchers verify that the angular distribution of secondary electrons obeys Lambert cosine distribution [19] by Monte Carlo simulation. Therefore, we considered the original angle distribution to be a Lambert distribution, and the sampling results obtained using the MC methods are shown in Fig. 2. Then, the electron trajectories were calculated using the classical fourth-order Runge–Kutta method. 3. Design concept Fig. 1. Sample of initial energy distribution.
When designing the streak tube structure, a magnification ratio larger than one may lead to an increased focusing scope, which is detrimental to the improvement of temporal resolution. Therefore, an electronic optical system with inverted images is preferable. However, a magnification less than zero means that the odd number of cross section between electron trajectories and axis. The intersection axis point in the streak tube will mostly be located near the anode aperture because of the drastic change in the field gradient. From the Scherzer series [18] expansion, the radial force of the electric field on electrons can be given as follows.
and the electron trajectories were calculated by the classical fourthorder Runge–Kutta method. In addition, the dynamic spatiotemporal resolution was also simulated. An experiment was conducted to test the parameters of the streak tube, including its dynamic spatiotemporal resolution and dynamic deflection sensitivity. The results showed that the minimum effective diameter of the planar photocathode is 40 mm and the temporal resolution is approximately 4 ps; in addition, the dynamic spatial resolution is 20 lp/mm and the deflection sensitivity is approximately 64 kV/mm.
𝑒 𝐹𝑟 = − 𝑉 ′′ (𝑧) 𝑟 2
(1)
Eq. (1) shows that the convergence force of the electron beam is proportional to the off-axis height. In other words, a large off-axis distance of electron beams near the anode aperture may lead to early focusing, which will aggravate the effect of field curvature. Therefore, in order to alleviate the field curvature effect, it is necessary to decrease the off-axis height of the electron beam near the anode aperture. As we know, the refractive index of an electron beam cannot exhibit sudden variation; therefore, the action of height reduction can only be achieved gradually before the electron beam reaches the anode aperture. In 1969, D.J. Bradley introduced an acceleration grid near the photocathode to enhance the field between them, thereby improving the temporal resolution of the streak tube from nanoseconds to picoseconds. Therefore, in our design, the acceleration grid is used to improve the temporal resolution of the streak tube. Nevertheless, the application of an acceleration grid increases the energy of the electron beam; thus, it may need to be decelerated while converging. However, according to research conducted by Niu Han-ben, the space charge effect of the streak tube mainly occurs in the low-energy region. In order to reduce the influence of the space charge effect on temporal resolution and dynamic range caused by electron deceleration, the energy of an electron beam must be maintained at a constant level after deceleration, which will weaken the focusing effect. In addition, the application of multiple focusing scopes will increase the focusing scope of the tube, which is not conducive to improvement of the temporal resolution. Based on the above presented arguments, the streak tube designed in this study has the following structural characteristics: the electron beams accelerated by the photocathode–grid system are focused by the first weak convergent lens; then, a divergent lens is used to maintain the electron beam off-axis height when moving to the anode aperture. A strong convergent lens near the anode aperture is used to achieve the final focusing of the electrons. As a result, the off-axis height of the electron beam before moving to the anode aperture decreases with the first weak focusing and the following divergent lens; the high-energy motion state is still maintained, which is conducive to an improvement of the temporal resolution.
2. Design method When designing the streak tube structure, the boundary conditions (including the location and structural parameters of each pole as well as their voltage) were first set; then, the potential distribution of the entire tube was calculated by applying the finite difference method, in which an algorithm, successive over-relaxation, was iterated thousands of times with a truncation error of 10−12 . Compare to the quantum efficiency of the cathode, the spectral response sensitivity is easier to measure, therefore is usually used to describe the quantum efficiency of the cathode. Some studies [14] have shown that for the gold cathode, when X-ray is incident vertically, the optimum thickness of the transmission gold cathode should be 10 nm, and then the sensitivity of the energy spectrum response decreases linearly with the increase of the thickness. Thus, if the surface of the gold cathode is rough and uneven, it is easy to cause the overall uniformity of its quantum efficiency. Because the surface finish of the photocathode is related to the preparation process of the cathode, the photocathode is simplified in the process of theoretical design, and the surface quantum efficiency is considered to be uniform. The initial energy distribution of electrons was generated based on the Monte Carlo (MC) [15] sampling of the Maxwell distribution. Because of the unavailability of a suitable ultrafast X-ray source, the experiments below are carried out using a third harmonic of a Quantronix Integra-C Ti–sapphire laser system with a pulse width of 130 fs to excite the Au photocathode. The work function of bulk Au is 5.1 eV [16], which is larger than the photon energy of the 266 nm light used in the testing 4.66 eV. However, the oxidized polycrystalline Au thin film of the photocathode surface reduces the work function to 4.2 eV, which yields a 0.5 eV wide secondary electron energy distribution with 266 nm light illumination [17,18]. Therefore, the most probable energy generated by Monte Carlo Method was set to be 0.5 eV owing to the use of a Au cathode. The final distribution of 50 000 electrons is shown in Fig. 1. The angular distribution of secondary electrons is an important factor that directly affects the temporal and spatial resolution of streak camera. Secondary electrons emitted from the same position of the 2
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. Sample of initial angle distribution: (a) initial angle distribution; (b) cosine of initial angle. Table 1 Parameters of the streak tube. Tube length (mm)
Length of drift zone (mm)
Diameter (mm)
Length of photocathode (mm)
Magnification ratio
Total voltage (kV)
600
283
84
50
1.32
12
Fig. 3. The schematic of streak tube.
3.1. Structure of streak tube Fig. 3 shows the schematic of the streak tube, and Fig. 4 is the electron-optical structure and axial potential distribution of the streak tube; Table 1 lists the parameters of the tube. In Fig. 3(a), ‘‘Cathode’’ refers to the photocathode and ‘‘Grid’’ refers to the acceleration grid component; the distance between the photocathode and the grid is 6 mm. The field strength is 1.75 kV/mm shown in Table 2. ‘‘F1’’ is the first focusing pole; it combines with Grid to form a weak focusing lens used for the first focusing step. It can also be used to fine-tune the magnification of the streak tube. ‘‘A1’’ is the first anode and ‘‘F2’’ is the second focusing pole. ‘‘F1’’, ‘‘A1’’, and ‘‘F2’’ are combined to form a divergent lens to enhance the energy of the electron beam after the first focusing step. Finally, ‘‘Anode’’ is combined with F2 to form a strong convergent lens to achieve a second focusing scope of the electron beam. The electron beam that passes the Anode maintains a uniform motion. Fig. 5 shows the trajectory of the electrons in the streak tube; the trajectory is calculated by the classical fourth-order Runge–Kutta method. Table 2 lists the voltage potential of each pole.
Fig. 5. Trajectory of electron beams.
Table 2 Voltage of each pole. Pole
PC
Grid
F1
A1
F2
Anode
Screen
Voltage (kV)
−12
−1.5
−6
−1.5
−10.35
0
0
Fig. 4. Structure of streak tube: (a) electron-optical system; (b) axial potential distribution.
3
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Table 3 Spot diameters at a 598 mm image plane (μm). Height (mm)
deflection system is not uniform whereas its axial component is very small and can be neglected. The deflection displacement of the electron beam on the screen is as follows. [( ) ] 𝑎𝑉𝐷𝑒𝑓 𝑙𝑒𝑐𝑡 𝑎𝑑2 𝑑 𝐷= + 𝑙 𝑙𝑛 2 − 𝑎 (2) ) ( 𝑑2 − 𝑑1 𝑑1 2𝑈𝑎 𝑑2 − 𝑑1
Electron number
0.1 5 10 15 20 25
500
1500
3000
25 21 24 39 98 203
36 30 31 51 118 282
50 51 52 73 115 256
Therefore, the sensitivity of the inclined plate deflection system can be given as follows. [( ) ] 𝑎𝑑2 𝑑 𝐷 𝑎 𝑃𝑖𝑛𝑐𝑙𝑖𝑛𝑒𝑑 = = + 𝑙 𝑙𝑛 2 − 𝑎 (3) ( ) 𝑉𝐷𝑒𝑓 𝑙𝑒𝑐𝑡 𝑑2 − 𝑑1 𝑑1 2𝑈𝑎 𝑑2 − 𝑑1
Table 4 Location of optimal image plane and scattered spot. Height (mm)
Location of optima image plane (mm)
FWHM of scattered spot on optimal image plane (μm)
0.1 5 10 15 20 25
598 598 597 595 590 582
36 32 33 35 38 72
When designing a deflection system, the plate spacing d (including 𝑑1 and 𝑑2 ) and width w can be determined by tracing the electron trajectory of the on-axis and off-axis 25 mm emission electron beams and then observing the beam spot distribution at the exit of the anode aperture. When determining the length a, the influence on spatial and temporal resolution must be considered. Eq. (2) shows that the deflection displacement is limited by the maximum diameter of the fluorescent screen 2D. In other words, this also limits the deflection sensitivity of the deflection plate. However, to obtain a high temporal resolution, it is better to maintain a high deflection sensitivity. Therefore, the length 𝒂 cannot be too large. Meanwhile, for a shorter length a, a high deflection sensitivity increases the applied voltage on the deflection plate; thus, the edge lens effect of the plate exit may be too obvious to obtain a high dynamic spatial resolution. The structural parameters of the deflection plate selected are as follows.
3.2. Calculation of spatial resolution To investigate the spatial resolution characteristics, electron beams are emitted at different off-axis heights. By calculating the electron trajectories, the electron beams emitted at different off-axis heights are imaged at 598 mm. The initial spatial distribution follows a Gauss distribution with a full width at half maximum (FWHM) of 5 μm. The initial temporal distribution is a Gauss pulse with a FWHM of 10 ps. To investigate the influence of different current densities on spatial resolution, the corresponding electron numbers are calculated. The off-axis heights and spot diameters on a 598 mm image plane under different conditions (500, 1500, and 3000 with current densities of 1.273 A/cm2 , 3.820 A/cm2 , and 7.639 A/cm2 , respectively) are shown in Table 3.
d1 = 6 mm, d2 = 7.5 mm, a = 40 mm, w = 40 mm
(4)
Here, 𝑑1 and 𝑑2 correspond to the spacing between the inlet and outlet plates in Fig. 5, respectively. It can be seen from Table 1 that the length of the drift scope of the tube is 283 mm and the distance between the inclined deflection plate outlet and the fluorescent screen in Fig. 3 is 𝑙 = 243 mm; moreover, 𝑈𝑎 = 12 kV. Using Eq. (4), the deflection sensitivity can be calculated to be 64.9 mm∕kV. The aim of the tube design is to develop a streak camera with a high temporal resolution of 5 ps (2% of the full screen scanning speed), limits the full screen scanning speed to be approximately 2.5 ns. Because the size of the experimental fluorescent screen is 52 mm in the follow-up experimental test, but the blank edge of the optical fiber panel is reserved in practice, the actual effective size is selected as 50 mm, therefore, the scanning speed can be obtained as 𝑉𝑠𝑐𝑎𝑛 = 2 × 107 m∕s./ Then, the slope of the scan voltage is approximately 𝑈𝑠𝑐𝑎𝑛 = 𝑣𝑠𝑐𝑎𝑛 𝑃𝑖𝑛𝑐𝑙𝑖𝑛𝑑𝑒 ≈ 0.308 kV∕ns.
3.3. Measures to improve image quality In wide-beam imaging devices, the effect of field curvature on image quality is proportional to the square of the object height. It can be seen from Table 3 that the farther away from the axis the electron beam is, the wider is the distribution of the scattered spot on the 598 mm image plane. At the same time, we calculated the distribution of spots of each off-axis electron beam on its optimal image plane as shown in Table 4. From Table 4, it is obvious that the imaging performance will be greatly improved through the application of a spherical fluorescent screen. Therefore, in order to further reduce the effect of field curvature, a spherical fluorescent screen can be used to improve the imaging performance of a streak tube.
3.4.2. Calculation of dynamic spatial resolution Dynamic spatial resolution refers to the minimum distance between two points that can be resolved by streak camera in the process of mapping temporal information to spatial information by electrostatic deflection system. The dynamic spatial resolution is calculated by determining the diameter of the scattered spot on the fluorescent screen after deflection of the electron beam. To demonstrate this more intuitively, the spatial mask plate on the photocathode is simulated. Fig. 7 shows the electron trajectory including falling points on a 598 mm image plane after dynamically calculating the emitted electron beams near the photocathode center, with the launch settings are as follows.
3.4. Calculation of dynamic characteristics The dynamic characteristics of the streak tube include its dynamic spatial resolution and its temporal resolution. The function of a streak tube is to map temporal information into spatial information by scanning, which is realized by a deflection system. Therefore, the design of a deflection system should be considered when calculating the dynamic characteristics of the streak tube. 3.4.1. Design of deflection system Compared with a flat plate deflection system, an inclined plate deflection system has higher deflection sensitivity and is more conducive to improve the temporal resolution, as shown in Fig. 6. Unlike the electric field in the former, the electric field in the inclined plate
(1) The initial spatial distribution of each streak simulated in the slit direction in space is the normal distribution of the 10 μm FWHM with the distance between the streaks being 40 μm. The spatial distribution of fringes in the scanning direction is the normal distribution of the 10 μm FWHM. 4
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 6. Inclined plate deflection system.
Fig. 7. Simulated result of dynamic spatial resolution. Table 5 Parameters of fringes in pulses. Streak pair no.
1 2
Streak no.
1.1 1.2 2.1 2.2
Two-sided coordinates of FWHM (cm) Left side
Right side
−0.00456 0.000767 −0.00419 0.00107
−0.000685 0.00464 −0.00147 0.00415
It can be seen from Table 5 that the diameter of the scattered spot of streak pair no. 1 can be approximated to 40 μm while that of no. 2 is approximately 30 μm (we conclude that the difference between them is because the dynamic deflection voltage may change the image position with the fixed focusing voltage, that is, the effect of deflection aberration). In other words, the streak tube designed in this study has a minimum theoretical dynamic spatial resolution of 25 lp/mm.
Diameter of scattered spot (μm)
38.5 39 29 30.8
(2) The normal distribution of the initial time of each pulse is 1 ps FWHM, two pulses were simulated continually and the pulse interval is 50 ps (peak interval). (3) Each streak emits 500 electrons, and the space charge effect is taken into account throughout the calculation.
3.4.3. Calculation of temporal resolution When calculating the temporal resolution of the entire tube, the Fabry–Perot etalon is simulated by setting the temporal interval between pulses. Because of the space charge effect, when the photoelectron current density is high, the pulse will be widened and the calculation accuracy will be affected. To reduce the influence of the space charge effect on the calculation accuracy of temporal resolution, the initial emission is set as follows.
Fig. 8 shows a histogram distribution of the electron trajectory in the slit direction corresponding to the two pulses. Table 5 lists the coordinates of the half-height and half-width of the electron beam of each streak image in each pulse.
(1) The uniform distribution of the photocathode slit direction in space is within 100 μm, whereas the scanning direction is still taken as the normal distribution of FWHM 10 μm. (2) Three continuous pulses were simulated, each with FWHMs of each 1 ps, peak-to-peak intervals of 50 ps, and 500 electrons per 5
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 8. Falling points of dynamic electron trajectories. Table 6 Pulse correlation coordinates. Pulse No.
Coordinates of pulse peak (cm)
1 2 3
0.7538 0.6583 0.5597
Two-side coordinates of FWHM (cm) Left side
Right side
0.7511 0.6484 0.5522
0.7587 0.6648 0.5675
The width of spline (μm)
11
the width value in Table 6, the temporal resolution corresponding to each spline width can be deduced as follows. 𝛥𝜏𝑠𝑝𝑙𝑖𝑛𝑒 =
50 ps (7538−6583)∕11
= 0.576 ps
(5)
Subsequently, the spline number of the first pulse can be obtained from the coordinates of the two sides of the FWHM of the first pulse 𝑁𝑠𝑝𝑙𝑖𝑛𝑒 = 7587−7511 ≈ 7; therefore, the temporal resolution of the entire 11 streak tube could be T = 𝑁𝑠𝑝𝑙𝑖𝑛𝑒 ∗ 𝛥𝜏𝑠𝑝𝑙𝑖𝑛𝑒 ≈ 4 ps. 4. Experiments
Fig. 9. Dynamic temporal resolution calculation results: (a) the electron trajectories’ falling points of three pulse sequences on the fluorescent screen; (b) the histogram distribution of each pulse in the scanning direction.
4.1. Design of spatial resolution reticle Based on the simulation results, we have developed a streak tube and tested it both statically and dynamically. The test data show that it fits perfectly to the design results. The structural sketch of a reticle on the photocathode used in the test is shown in Fig. 10. The center circle is a marker point, which simplifies locating the center of the image when imaging. The diameter of the photocathode is 50 mm, on which the reticles are separated by a spatial resolved reticle and a temporal resolved light through a hole with an interval of 1 mm. Among them, the width of the light through holes is 20 μm, whereas the spatial resolved reticle width is 100 μm.
pulse, which corresponds to a current density of 0.637 A/cm2 per pulse. Fig. 9 shows the simulated results in the scanning direction of the fluorescent screen. Table 6 shows the coordinates of the pulse peaks in Fig. 9(b) and the coordinates of the two sides of the space corresponding to the pulse width. In Fig. 9(b), the second and third pulses are wider than the first. According to the results of the simulation calculation, we conclude that the phenomenon is caused by the large calculation time-step (1 ps), which implies that the first pulse should be preferable for calculating the temporal resolution. At the beginning of the transmitting pulse sequence, the pulse interval is set to 50 ps. Therefore, from the relationship between the coordinates of the peaks of the first and second pulses, combined with
4.2. Fabrication of photocathode As mentioned above, Au or CsI are commonly used as photocathode materials for ultraviolet and soft X-ray streak cameras. Generally, the soft X-ray streak camera typically uses an open vacuum system to replace the photocathode at any time. Although CsI has higher 6
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 10. Structural sketch of reticle on photocathode.
quantum efficiency, owing to the inevitable need to be exposed to the atmosphere (water vapor) during cathode replacement, CsI is liable to combine with water molecules in the air, which results in deliquescence and affects the emission characteristics. In contrast, Au cathode has better stability and conductivity, and it can be prepared by an ordinary coating machine. Therefore, Au is used as the cathode material of the streak tube. Li Yu-kun et al. discussed the relationship between the spectral response sensitivity and the thickness of Au photocathode material for incident X-ray energy of 1 keV. It was proved that the thickness of the Au cathode should be approximately 10 nm for optimum spectral response sensitivity, whereas the corresponding thickness of the Au cathode should be approximately 40 nm for ultraviolet band testing. In this case, it is difficult to form a self-supporting structure with the Au cathode material, so it requires an organic thin film substrate of a certain thickness to support the cathode, and it is hoped that the substrate has no effect on the quantum efficiency of the photocathode material. Compared with 240 nm thick Formvar film and 800 nm thick Polypropylene films, 100 nm thick Parylene [13] film has better transmittance in the energy range of 0.1 keV to 10 keV. Furthermore, its melting point is high, it does not react with acid and alkali, and its mechanical strength is high. Therefore, 100 nm thick Parylene film is chosen as the substrate material. Parylene thin films were prepared by the vapor deposition method. The film thickness was cut along the glass edge and opened to float on the water surface. Then, the cathode sheet was lifted from the water, dried, and then evaporated with an Au cathode material having a thickness of 40 nm. The finished cathode is shown in Fig. 11.
Fig. 11. Photocathode fabricated with Au.
4.3. Fabrication of spherical fluorescent screen According to Table 4, the curvature radius of the spherical screen can be calculated from the right triangle by using (6).
Fig. 12. Fabricated spherical fluorescent screen: (a) photo of spherical phosphor screen; (b) luminescence of screen illuminated by an ultraviolet lamp.
𝑑 2 + ℎ2 (6) 2𝑑 Among them, 𝜌𝑠 is the curvature radius of the spherical fluorescent screen; 𝒅 is the height of the spherical gap (i.e., the distance between the ideal image plane and the screen, the maximum height of the spherical gap is 16 mm from Table 4); and 𝒉 is the viewing height. According to the actual size of the screen, the curvature of the screen can be obtained as 26 mm (which is the maximum size we could fabricate). Thus, the curvature radius should be 44 mm. However, the fluorescent powder could not be deposited if the curvature radius was less than 64 mm [20].
𝜌𝑠 =
Fig. 12 shows the fabricated physical spherical fluorescent screen. The fluorescent screen is fabricated by depositing P20 phosphor powder on the spherical surface (input surface is spherical — output surface is flat) of the optical fiber panel by the centrifugal deposition method, and then attaching it to the metal electrode by oxygen arc welding so that the fluorescent screen can be removed or replaced at any time in the experiment. 7
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
geometrical optical imaging system, a Pettzval surface also exists in the streak tube. Ideally, this surface should be symmetrical with respect to the photocathode center, and the resolution on both sides should be symmetrical if the screen is located in the corresponding axial position at the cathode center. However, the image plane rotation (the existence of assembly errors and the influence of magnetic field in the surrounding environment) makes the side of the image plane that is close to the screen has better spatial resolution whereas the other side deteriorates, as shown in Fig. 11. Fig. 10 shows that the image center of any two adjacent reticle plates is 4 mm apart; therefore, we can conclude that the effective resolution range of each group of reticle plates (including reticle plates with resolution line pairs, rectangular through holes without resolution line pairs, and the spacing between them) is 4 mm. Thus, it can be seen from Fig. 14 that the total imaging length is at least 32 mm. Within this range, according to Fig. 15(b), it can be observed that the total number of resolution elements 𝑁 is calculated as follow.
Fig. 13. The experiment set up of static parameters test.
4.4. Static parameters test When testing the static spatial resolution, the entire photocathode is irradiated by an ultraviolet lamp (without an image intensifier for the large light intensity of the source), and the data are recorded by PI company’s scientific air-cooled CCD (27 mm ∗ 27 mm, 2048 Pixel ∗ 2048 Pixel). The test set up is shown in Fig. 13, and test results are shown in Fig. 14 (owing to the limited size of CCD, the relative position between CCD and the screen can only be adjusted twice to collect images). The diameter of the fluorescent screen used in the experiment is 52 mm. From Table 1, we can see that the magnification of the streak tube designed in this paper is 1.35, so only the image range with the photocathode of diameter 38 mm could be obtained. However, to retain the central marker point during the test, the position of the CCD cannot be adjusted to the periphery. Therefore, we can only detect the imaging range in Fig. 10 from 17 lp/mm on the left side of center to 18 lp/mm on the right side (to make it easier to identify, we performed an inverse phase operation on the image color). Taking the circle point in Fig. 14 as the origin, the distance between the corresponding position and the center of the pattern with different resolution can be determined according to the structure size given in Fig. 10, which is the transverse axis. Fig. 15(a) is the corresponding contrast calculated from Figs. 14, and 15(b) is the limit spatial resolution calculated from this contrast. Table 4 indicates that the ideal image plane of the streak tube designed in this paper is not planar. In other words, similar to the
𝑁 = (30 + 31 + 31 + 32 + 24 + 27 + 28 + 25) × 4 = 912 lp
(7)
As mentioned above, to improve the imaging quality, spherical fluorescent screens can be used to mitigate the effects of field curvature. The curvature radius of the spherical screen is 64 mm and the corresponding height of the spherical gap is 5.5 mm. Table 4 shows that the optimum shape of the electron beam is almost identical to that of the spherical screen in the cathode range of at least 30 mm around the center, which was proved directly from the experimental results in Fig. 14. Additionally, it can be seen from Table 4 that the spherical screen is still effective in improving the image contrast of electron beams emitted in the range of 15 mm to 20 mm off-axis height. Therefore, according to the streak image contrast on both sides of Fig. 15(b) and the effective range of the spherical screen, we infer that the left-most reticle plate of the cathode center (corresponding to the 15 lp/mm and 16 lp/mm dividing board on the left side of Fig. 10) has a limit spatial resolution at least 15 lp/mm, that is, at least 9 mm near the edge of cathode is still distinguishable and can provide at least 135 resolution elements of information. In other words, in the static mode, the effective diameter of the photocathode designed in this paper can reach at least 40 mm, which can provide 1000 resolution elements of information.
Fig. 14. Test result of static spatial resolution: (a) left side image of the photocathode center; (b) right side image of the photocathode center.
8
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 15. Information of reticle image: (a) contrast ratio of the different height streak image; (b) limit spatial resolution of the streak tube.
obtained by using the X-ray source. The incident X-ray or UV pulse is converted into photoelectrons. The UV light does not produce exactly the same electron energy distribution as the X-ray [16]. However, the initial photoelectron energy distribution is much lower than the energy distribution achieved by the accelerating field of the photocathode and grid, as shown in Fig. 3(a). In the test, Quantronix Integra-C triple frequency Ti: Sapphire ultraviolet femtosecond laser (130 fs@266 nm with repetition frequency is 1 kHz) is used as the light source, the P.I.N. probe is illuminated by 800 nm red light to produce the trigger signal, and the trigger signal passes through the delay circuit to trigger the scanning circuit for the deflect system. Simultaneously, every single 266 nm ultraviolet light output from the laser is input into the Fabry–Perot etalon comprising all-mirror M4 and ultraviolet semi-reflective lens Lens to generate the temporal pulse sequence and then illuminate the photocathode of streak tube to produce the photoelectrons. After the enhancement of MCP, the final images are recorded by a CCD system (Princeton Instruments) and stored in the local computer, as shown in Fig. 18.
Fig. 16. Slope of scan voltage.
After the static spatial resolution test is completed, the scanning voltage slope is measured by an oscilloscope before the dynamic test, and the result is 300 V/ns, as shown in Fig. 16. Then, the deflection sensitivity test is conducted. When the streak tube works in static mode, the total voltage of the streak tube is adjusted to 12 kV (working voltage) whereas the voltage of the focus pole is adjusted to focus normally; then, the cathode is directly irradiated by the ultraviolet lamp. At this time, both deflection plates are grounded, and the coordinate of the scanning direction of the streak image is recorded as S1. Next, the total tube voltage is maintained to keep the lower deflection plate electrode grounded; meanwhile, the upper deflection plate electrode applies DC of 100 V. Subsequently, the position of the streak image is recorded again, as S2; this is subtracted from S1 and divided by the corresponding potential difference to yield the deflection sensitivity. The results are shown in Fig. 17. In Fig. 17, the cross marking is located at the cathode center, the locations shown in the Y coordinate in the lower left corner are 1386 and 908, and the sizes of the CCD pixels used are 13.5 μm; thus, the deflection sensitivity of the deflection plate can be obtained as follows:
𝑃inclined_test =
(1386 − 908) 𝑝𝑖𝑥𝑒𝑙 ∗ 13.5 μm = 64.53 mm∕kV 100 V
An ultraviolet lamp is used in the static test, so the streak image of almost the entire photocathode can be recorded, whereas the femtosecond laser used in the dynamic test can only be adjusted to the center of the irradiated cathode because of the limited laser spot size. Fig. 19 shows the test result of dynamic spatial resolution. The streak image is on the left side and the intensity distribution along the transverse axis is on the right side (the streak image broadens significantly when the first pulse laser input is too strong, so the streak image corresponding to the second pulse is selected). From the number of peaks on the curve, it can be seen that the dynamic spatial resolution in the center of the photocathode is 20 lp/mm (@contrast 10%) at least. When measuring the temporal resolution, the laser spot is adjusted to the corresponding position of the temporal-resolved light through hole on the right side of the cathode center, yielding a reading of 15 lp/mm, as shown in Fig. 10. The temporal-resolved element size corresponding to each CCD pixel at a fixed time interval (corresponding time length of Fabry–Perot etalon) is calculated from the interval between the streak images produced by the temporal pulse sequence, and then the single streak image is measured. The time width of a single pulse can be measured by the number of pixels occupied by the FWHM of the intensity distribution, as shown in Fig. 20.
(8)
The length of Fabry–Perot etalons comprising M4 and Lens in Fig. 18 is 21.4 mm, and the corresponding temporal interval is 141.2 ps. The temporal resolution of the entire streak tube can be obtained as in Section 3.4.3 above, which is calculated as
4.5. Dynamic parameters test The dynamic measurement includes temporal resolution and dynamic spatial resolution. Because of the lack of X-ray source, the experiments are conducted using the UV light pulse, as used by other researchers [21]. The temporal resolution is almost identical to that
T=6
pixel∗141.2 ps∕218 pixel
≈ 4 ps
(9)
9
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 17. Image of deflection sensitivity tests: The left part is when both plates are kept grounded and the right part is the image when DC of 100 V is applied onto the upper deflect plates while the lower one is kept at zero.
Fig. 18. Dynamic test schematic.
Fig. 19. Image of dynamic spatial resolution.
10
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 20. Image of temporal resolution.
5. Conclusions
[2] A.S. Moore, J. Benstead, M.F. Ahmed, J. Morton, T.M. Guymer, R. Soufli, T. Pardini, R.L. Hibbard, C.G. Bailey, P.M. Bell, S. Hau-Riege, M. Bedzyk, M.J. Shoup, S.P. Regan, T. Agliata, R. Jungquist, D.W. Schmidt, L.B. Kot, W.J. Garbett, M.S. Rubery, J.W. Skidmore, E. Gullikson, F. Salmassi, Two-color spatial and temporal temperature measurements using a streaked soft x-ray imager, Rev. Sci. Instrum. 87 (11) (2016) 055110–055128. [3] Kenta Mukojima, Shinji Kanzaki, Kota Kawanishi, Katsuyoshi Sato, Tadashi Abukawa, Streak-camera reflection high-energy electron diffraction for dynamics of surface crystallography, Surf. Sci. 636 (2015) 25–30. [4] N.S. Vorobiev, P.B. Gornostaev, V.I. Lozovoi, A.V. Smirnov, E.V. Shashkov, M. Ya. Schelev, A PS-1/S1 picosecond streak camera in experimental physics, Instrum. Exp. Tech. 59 (4) (2016) 551–556. [5] J. Siegel, G. Epurescu, A. Perea, F.J. Gordillo-Vázquez, J. Gonzalo, C.N. Afonso, Temporally and spectrally resolved imaging of laser-induced plasmas, Opt. Lett. 29 (19) (2004) 2228–2230. [6] N. Kawahara, J.L. Beduneau, T. Nakayama, E. Tomita, Y. Ikeda, Spatially, temporally, and spectrally resolved measurement of laser-induced plasma in air, Appl. Phys. B 86 (4) (2007) 605–614. [7] J. Benstead, A.S. Moore, M.F. Ahmed, J. Morton, T.M. Guymer, R. Soufli, T. Pardini, R.L. Hibbard, C.G. Bailey, P.M. Bell, S. Hau-Riege, M. Bedzyk, I.I.I. M.J. Shoup, S. Reagan, T. Agliata, R. Jungquist, D.W. Schmidt, L.B. Kot, W.J. Garbett, M.S. Rubery, J.W. Skidmore, E. Gullikson, F. Salmassi, A new streaked soft x-ray imager for the National Ignition Facility, Rev. Sci. Instrum. 87 (5) (2016) 055110. [8] K.W. Hill, M. Bitter, L. Delgado-Aparicio, P.C. Efthimion, R. Ellis, L. Gao, J. Maddox, Pablant N.A.1, M.B. Schneider, H. Chen, S. Ayers, R.L. Kauffman, A.G. MacPhee, P. Beiersdorfer, R. Bettencourt, T. Ma, R.C. Nora, H.A. Scott, D.B. Thorn, J.D. Kilkenny, D. Nelson, M. Shoup, Y. Maron, Development of a high resolution x-ray spectrometer for the National Ignition Facility (NIF), Rev. Sci. Instrum. 87 (11) (2016) 11E344. [9] J.R. Howorth, J.S. Milnes, Y. Fisher, A. Jadwin, R. Boni, P.A. Jaanimagi, The development of an improved streak tube for fusion diagnostics, Rev. Sci. Instrum. 87 (11) (2016) 287–294. [10] J.A. Oertel, et al., A large format gated X-ray framing camera, SPIE Proc. 5194 (2003) 214–222. [11] P.A. Jaanimagi, Breaking the 100-fs barrier with a streak camera, SPIE Proc. 5194 (2003) 171–182. [12] Ke-Xun Sun, William Nishimura, A Second-Generation X-ray Streak Camera With True Large Format, High Dynamic Range, and High Reliability. [C] Proc. SPIE 5920. [13] Yu Jiajin, Xiang Huizhu, Gao Quanlan, Lei Zhiyuan, C8H8 film for experiments of high temperature plasma, Nucl. Fusion Plasma Phys. 12 (3) (1992) 189–192. [14] Li Yu-kun, Chen Tao, Deng Bo, Yuan Zheng, Cao Zhu-rong, Energy spectral response of photocathode of soft X-ray streak camera, High Power Laser Part. Beams 26 (2) (2014) 022002. [15] Xu Shu-yan, Application of Monte Carlo Method in Experimental Nuclear Physics, second ed., Atomic Energy Press, 2006, pp. 48–52. [16] M.M. Shakya, Z.H. Chang, Achieving 280fs resolution with a streak camera by reducing the deflection dispersion, Appl. Phys. Lett. 87 (2005) 041103. [17] J. Feng, H.J. Shin, J.R. Nasiatka, W. Wan, A.T. Young, G. Huang, A. Comin, J. Byrd, H.A. Padmore, An x-ray streak camera with high spatio-temporal resolution, Appl. Phys. Lett. 91 (2007) 134102. [18] H.Z. Cai, W.Y. Fu, D. Wang, Y.F. Lei, J.Y. Liu, Three-strip microchannel plate gated x-ray framing camera, Sensors Actuators A 285 (2019) 355–361. [19] Zhou Li Wei, Electron Optics with Wide Beam Focusing, Beijing Institute of Technology Press, Beijing, China, 1993, p. 133.
A large detection area and high spatiotemporal resolution X-ray streak camera with a planar photocathode was developed to meet the requirements of ICF experiments and high density physics research. First, the electron-optical system of the streak tube was designed via numerical simulation. Further, we developed the streak tube and experimentally tested the parameters of the streak tube, including the dynamic spatiotemporal resolution and the dynamic deflect sensitivity. The test results indicate that the optimal diameter of the planar photocathode is at least 40 mm, and the temporal resolution is approximately 4 ps. In addition, the dynamic spatial resolution is 20 lp/mm, whereas the deflect sensitivity is approximately 64 kV/mm. Further work will develop a vastly superior streak camera that can achieve a spatial resolution of 1500 line pairs in a planar photocathode of 50 mm in diameter. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Zhang Jing-Jin: Funding acquisition, Software, Methodology, Writing — original draft. Liu Ai-lin: Resources, Data curation, Formal analysis. Yang Qin-lao: Project administration, Investigation, Validation. Zong Fang-ke: Writing — review & editing. Guo Bao-ping: Conceptualization, Supervision. Acknowledgment We would like to thank Editage (www.editage.cn) for English language editing. Funding This paper was granted by National Natural Science Foundations of China (NSFC) (No. 11805137); Science and Technology Program of Shenzhen (JCYJ 20170818102618203, JCYJ 20170818141616714). References [1] Y.P. Opachich, P.M. Bell, D.K. Bradley, N. Chen, J. Feng, A. Gopal, B. Hatch, T.J. Hilsabeck, E. Huffman, J.A. Koch1, O.L. Landen, A.G. MacPhee, S.R. Nagel, S. Udin, Structured photocathodes for improved high-energy x-ray efficiency in streak cameras, Rev. Sci. Instrum. 87 (11) (2016) S228. 11
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
[20] Zhang Jing-jin, Lei Bao-guo, Yang Qin-lao, Improvement of imaging performance for X-ray image tube, J. Shenzhen Univ. (Sci. Eng.) 34 (1) (2017) 14–19.
[21] T.J. Hilsabeck, J.D. Hares, J.D. Kilkenny, P.M. Bell, A.K.L. Dymoke-Bradshaw, J.A. Koch, P.M. Celliers, D.K. Bradley, T. McCarville, M. Pivovaroff, R. Soufli, R. Bionta, Pulse-dilation enhanced gated optical imager with 5 ps resolution, Rev. Sci. Instrum. 81 (10) (2010) E317.
(a) P.M. Bell, J.D. Kilkenny, R.L. Hanks, O.L. Landen, Measurements with a 35 psec gate timemicrochannel plate camera, Proc. SPIE 1346 (1991) 456–464.
12
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Design of a large-format high-resolution streak camera with a planar photocathode Zhang Jing-Jin a,b , Liu Ai-lin b , Yang Qin-lao a ,∗, Zong Fang-ke a,b ,∗∗, Guo Bao-ping a,b a b
College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China College of Electronic and Information Engineering, Shenzhen University, Shenzhen 51806, China
ARTICLE
INFO
Keywords: Streak camera Ultraviolet X-ray Large-format detection area High spatial resolution High temporal resolution
ABSTRACT Streak cameras are widely used as ultrafast diagnostic tools in several research fields. With an increase in the complexity of the experiments being conducted, large-format streak cameras with high spatiotemporal resolution are required for wide-band, high-precision spectroscopic recording. This paper presents the design of a high-resolution large-format streak camera with a planar photocathode. To alleviate the effect of field curvature for a large-format photocathode and to improve its spatial resolution, a weak–strong double-focusing lens coupled with a spherical fluorescent screen is used. A high voltage is also used to reduce transit time dispersion in order to improve temporal resolution. Experimental results show that the minimum effective diameter of the planar photocathode is 40 mm, compare to the existing 30 mm photocathode, which can achieve a minimum spatial resolution of 1000 line pairs and a temporal resolution of approximately 4 ps. The dynamic spatial resolution achieved is 20 lp/mm and the deflection sensitivity is approximately 64 kV/mm.
1. Introduction Streak cameras convert temporal information of an ultrafast signal into spatial domain and can achieve picoseconds or even femtosecond measurement with high spatial resolution. Therefore, they are widely used as ultrafast diagnostic tools when researching high energy density physics [1–3], laser induced plasmas [4–6], and particularly in physical experiments of inertial confinement fusion (ICF) [7–9]. With an increase in the complexity of the experiments being conducted, large-format streak tubes with a high resolution over the entire spatial– temporal imaging plane are required for wide-band, high-precision spectroscopic recording [10,11]. Similarly, target imaging experiments used to investigate radiation flow requires a large field of view with high resolution across the photocathode. Divided field-of-view imaging, such as multichannel recording and pinhole array imaging, also requires a large number of resolution elements across the photocathode. To meet the demands described above, a large-format streak camera with a high spatiotemporal resolution should be developed. To implement a large format, the effect of field curvature needs to be alleviated; this effect causes electrons from the edges of the photocathode to arrive later and at a different focus than electrons from the center. By employing a curved photocathode–acceleration system, Lawrence Livermore National Laboratory developed a second-generation X-ray
streak camera [12] (P2XSC), with a concentric sphere-type electrostatic focusing streak tube, the RCA C73435, as the major component. This could minimize aberrations and considerably improve off-center focusing. However, the working voltage up to 25 kV of the RCA C73435 [12] can easily produce arc and damage the photocathode and even the metal acceleration grid unit. In addition, such high anode voltage is not conducive to improve the deflection sensitivity of streak camera. Therefore, it needs to increase the slope of scanning voltage to improve the temporal resolution. And this may easily lead to the breakdown of electronic components, while also easily lead to jitter of scanning starting signal, which is not conducive to synchronization. Moreover, for X-ray, when developing a photocathode streak tube, the Parylene film [13] must be bent to prepare a spherical substrate and ensuring a smooth surface of the substrate is a very difficult task. In this paper, the idea of using a weak–strong bifocal lens was proposed for the first time, which can not only alleviate the influence of the field curvature, but also make the electron beam maintain a high speed to reduce the influence of space charge effect and then improve the temporal resolution. Based on this, we designed a large-format streak tube with a planar photocathode at 12 kV anode voltages, compared to the 25 kV of the RCA 73 435. The electronic optical system was designed using the finite difference method in the MATLAB software,
∗ Corresponding author. ∗∗ Corresponding author at: College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. E-mail addresses: [email protected] (Z. Jing-Jin), [email protected] (L. Ai-lin), [email protected] (Y. Qin-lao), [email protected] (Z. Fang-ke), [email protected] (G. Bao-ping).
https://doi.org/10.1016/j.nima.2019.163076 Received 17 July 2019; Received in revised form 29 September 2019; Accepted 31 October 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
photocathode exit surface have different flight trajectories due to different emission angles, which eventually result in the secondary electrons arriving at the image plane of the streak camera not as a point but as a diffuse spot, and the flying time of the secondary electrons varies. However, the angular distribution of secondary electrons in photocathode is difficult to obtain through experiments. Therefore, some researchers verify that the angular distribution of secondary electrons obeys Lambert cosine distribution [19] by Monte Carlo simulation. Therefore, we considered the original angle distribution to be a Lambert distribution, and the sampling results obtained using the MC methods are shown in Fig. 2. Then, the electron trajectories were calculated using the classical fourth-order Runge–Kutta method. 3. Design concept Fig. 1. Sample of initial energy distribution.
When designing the streak tube structure, a magnification ratio larger than one may lead to an increased focusing scope, which is detrimental to the improvement of temporal resolution. Therefore, an electronic optical system with inverted images is preferable. However, a magnification less than zero means that the odd number of cross section between electron trajectories and axis. The intersection axis point in the streak tube will mostly be located near the anode aperture because of the drastic change in the field gradient. From the Scherzer series [18] expansion, the radial force of the electric field on electrons can be given as follows.
and the electron trajectories were calculated by the classical fourthorder Runge–Kutta method. In addition, the dynamic spatiotemporal resolution was also simulated. An experiment was conducted to test the parameters of the streak tube, including its dynamic spatiotemporal resolution and dynamic deflection sensitivity. The results showed that the minimum effective diameter of the planar photocathode is 40 mm and the temporal resolution is approximately 4 ps; in addition, the dynamic spatial resolution is 20 lp/mm and the deflection sensitivity is approximately 64 kV/mm.
𝑒 𝐹𝑟 = − 𝑉 ′′ (𝑧) 𝑟 2
(1)
Eq. (1) shows that the convergence force of the electron beam is proportional to the off-axis height. In other words, a large off-axis distance of electron beams near the anode aperture may lead to early focusing, which will aggravate the effect of field curvature. Therefore, in order to alleviate the field curvature effect, it is necessary to decrease the off-axis height of the electron beam near the anode aperture. As we know, the refractive index of an electron beam cannot exhibit sudden variation; therefore, the action of height reduction can only be achieved gradually before the electron beam reaches the anode aperture. In 1969, D.J. Bradley introduced an acceleration grid near the photocathode to enhance the field between them, thereby improving the temporal resolution of the streak tube from nanoseconds to picoseconds. Therefore, in our design, the acceleration grid is used to improve the temporal resolution of the streak tube. Nevertheless, the application of an acceleration grid increases the energy of the electron beam; thus, it may need to be decelerated while converging. However, according to research conducted by Niu Han-ben, the space charge effect of the streak tube mainly occurs in the low-energy region. In order to reduce the influence of the space charge effect on temporal resolution and dynamic range caused by electron deceleration, the energy of an electron beam must be maintained at a constant level after deceleration, which will weaken the focusing effect. In addition, the application of multiple focusing scopes will increase the focusing scope of the tube, which is not conducive to improvement of the temporal resolution. Based on the above presented arguments, the streak tube designed in this study has the following structural characteristics: the electron beams accelerated by the photocathode–grid system are focused by the first weak convergent lens; then, a divergent lens is used to maintain the electron beam off-axis height when moving to the anode aperture. A strong convergent lens near the anode aperture is used to achieve the final focusing of the electrons. As a result, the off-axis height of the electron beam before moving to the anode aperture decreases with the first weak focusing and the following divergent lens; the high-energy motion state is still maintained, which is conducive to an improvement of the temporal resolution.
2. Design method When designing the streak tube structure, the boundary conditions (including the location and structural parameters of each pole as well as their voltage) were first set; then, the potential distribution of the entire tube was calculated by applying the finite difference method, in which an algorithm, successive over-relaxation, was iterated thousands of times with a truncation error of 10−12 . Compare to the quantum efficiency of the cathode, the spectral response sensitivity is easier to measure, therefore is usually used to describe the quantum efficiency of the cathode. Some studies [14] have shown that for the gold cathode, when X-ray is incident vertically, the optimum thickness of the transmission gold cathode should be 10 nm, and then the sensitivity of the energy spectrum response decreases linearly with the increase of the thickness. Thus, if the surface of the gold cathode is rough and uneven, it is easy to cause the overall uniformity of its quantum efficiency. Because the surface finish of the photocathode is related to the preparation process of the cathode, the photocathode is simplified in the process of theoretical design, and the surface quantum efficiency is considered to be uniform. The initial energy distribution of electrons was generated based on the Monte Carlo (MC) [15] sampling of the Maxwell distribution. Because of the unavailability of a suitable ultrafast X-ray source, the experiments below are carried out using a third harmonic of a Quantronix Integra-C Ti–sapphire laser system with a pulse width of 130 fs to excite the Au photocathode. The work function of bulk Au is 5.1 eV [16], which is larger than the photon energy of the 266 nm light used in the testing 4.66 eV. However, the oxidized polycrystalline Au thin film of the photocathode surface reduces the work function to 4.2 eV, which yields a 0.5 eV wide secondary electron energy distribution with 266 nm light illumination [17,18]. Therefore, the most probable energy generated by Monte Carlo Method was set to be 0.5 eV owing to the use of a Au cathode. The final distribution of 50 000 electrons is shown in Fig. 1. The angular distribution of secondary electrons is an important factor that directly affects the temporal and spatial resolution of streak camera. Secondary electrons emitted from the same position of the 2
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. Sample of initial angle distribution: (a) initial angle distribution; (b) cosine of initial angle. Table 1 Parameters of the streak tube. Tube length (mm)
Length of drift zone (mm)
Diameter (mm)
Length of photocathode (mm)
Magnification ratio
Total voltage (kV)
600
283
84
50
1.32
12
Fig. 3. The schematic of streak tube.
3.1. Structure of streak tube Fig. 3 shows the schematic of the streak tube, and Fig. 4 is the electron-optical structure and axial potential distribution of the streak tube; Table 1 lists the parameters of the tube. In Fig. 3(a), ‘‘Cathode’’ refers to the photocathode and ‘‘Grid’’ refers to the acceleration grid component; the distance between the photocathode and the grid is 6 mm. The field strength is 1.75 kV/mm shown in Table 2. ‘‘F1’’ is the first focusing pole; it combines with Grid to form a weak focusing lens used for the first focusing step. It can also be used to fine-tune the magnification of the streak tube. ‘‘A1’’ is the first anode and ‘‘F2’’ is the second focusing pole. ‘‘F1’’, ‘‘A1’’, and ‘‘F2’’ are combined to form a divergent lens to enhance the energy of the electron beam after the first focusing step. Finally, ‘‘Anode’’ is combined with F2 to form a strong convergent lens to achieve a second focusing scope of the electron beam. The electron beam that passes the Anode maintains a uniform motion. Fig. 5 shows the trajectory of the electrons in the streak tube; the trajectory is calculated by the classical fourth-order Runge–Kutta method. Table 2 lists the voltage potential of each pole.
Fig. 5. Trajectory of electron beams.
Table 2 Voltage of each pole. Pole
PC
Grid
F1
A1
F2
Anode
Screen
Voltage (kV)
−12
−1.5
−6
−1.5
−10.35
0
0
Fig. 4. Structure of streak tube: (a) electron-optical system; (b) axial potential distribution.
3
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Table 3 Spot diameters at a 598 mm image plane (μm). Height (mm)
deflection system is not uniform whereas its axial component is very small and can be neglected. The deflection displacement of the electron beam on the screen is as follows. [( ) ] 𝑎𝑉𝐷𝑒𝑓 𝑙𝑒𝑐𝑡 𝑎𝑑2 𝑑 𝐷= + 𝑙 𝑙𝑛 2 − 𝑎 (2) ) ( 𝑑2 − 𝑑1 𝑑1 2𝑈𝑎 𝑑2 − 𝑑1
Electron number
0.1 5 10 15 20 25
500
1500
3000
25 21 24 39 98 203
36 30 31 51 118 282
50 51 52 73 115 256
Therefore, the sensitivity of the inclined plate deflection system can be given as follows. [( ) ] 𝑎𝑑2 𝑑 𝐷 𝑎 𝑃𝑖𝑛𝑐𝑙𝑖𝑛𝑒𝑑 = = + 𝑙 𝑙𝑛 2 − 𝑎 (3) ( ) 𝑉𝐷𝑒𝑓 𝑙𝑒𝑐𝑡 𝑑2 − 𝑑1 𝑑1 2𝑈𝑎 𝑑2 − 𝑑1
Table 4 Location of optimal image plane and scattered spot. Height (mm)
Location of optima image plane (mm)
FWHM of scattered spot on optimal image plane (μm)
0.1 5 10 15 20 25
598 598 597 595 590 582
36 32 33 35 38 72
When designing a deflection system, the plate spacing d (including 𝑑1 and 𝑑2 ) and width w can be determined by tracing the electron trajectory of the on-axis and off-axis 25 mm emission electron beams and then observing the beam spot distribution at the exit of the anode aperture. When determining the length a, the influence on spatial and temporal resolution must be considered. Eq. (2) shows that the deflection displacement is limited by the maximum diameter of the fluorescent screen 2D. In other words, this also limits the deflection sensitivity of the deflection plate. However, to obtain a high temporal resolution, it is better to maintain a high deflection sensitivity. Therefore, the length 𝒂 cannot be too large. Meanwhile, for a shorter length a, a high deflection sensitivity increases the applied voltage on the deflection plate; thus, the edge lens effect of the plate exit may be too obvious to obtain a high dynamic spatial resolution. The structural parameters of the deflection plate selected are as follows.
3.2. Calculation of spatial resolution To investigate the spatial resolution characteristics, electron beams are emitted at different off-axis heights. By calculating the electron trajectories, the electron beams emitted at different off-axis heights are imaged at 598 mm. The initial spatial distribution follows a Gauss distribution with a full width at half maximum (FWHM) of 5 μm. The initial temporal distribution is a Gauss pulse with a FWHM of 10 ps. To investigate the influence of different current densities on spatial resolution, the corresponding electron numbers are calculated. The off-axis heights and spot diameters on a 598 mm image plane under different conditions (500, 1500, and 3000 with current densities of 1.273 A/cm2 , 3.820 A/cm2 , and 7.639 A/cm2 , respectively) are shown in Table 3.
d1 = 6 mm, d2 = 7.5 mm, a = 40 mm, w = 40 mm
(4)
Here, 𝑑1 and 𝑑2 correspond to the spacing between the inlet and outlet plates in Fig. 5, respectively. It can be seen from Table 1 that the length of the drift scope of the tube is 283 mm and the distance between the inclined deflection plate outlet and the fluorescent screen in Fig. 3 is 𝑙 = 243 mm; moreover, 𝑈𝑎 = 12 kV. Using Eq. (4), the deflection sensitivity can be calculated to be 64.9 mm∕kV. The aim of the tube design is to develop a streak camera with a high temporal resolution of 5 ps (2% of the full screen scanning speed), limits the full screen scanning speed to be approximately 2.5 ns. Because the size of the experimental fluorescent screen is 52 mm in the follow-up experimental test, but the blank edge of the optical fiber panel is reserved in practice, the actual effective size is selected as 50 mm, therefore, the scanning speed can be obtained as 𝑉𝑠𝑐𝑎𝑛 = 2 × 107 m∕s./ Then, the slope of the scan voltage is approximately 𝑈𝑠𝑐𝑎𝑛 = 𝑣𝑠𝑐𝑎𝑛 𝑃𝑖𝑛𝑐𝑙𝑖𝑛𝑑𝑒 ≈ 0.308 kV∕ns.
3.3. Measures to improve image quality In wide-beam imaging devices, the effect of field curvature on image quality is proportional to the square of the object height. It can be seen from Table 3 that the farther away from the axis the electron beam is, the wider is the distribution of the scattered spot on the 598 mm image plane. At the same time, we calculated the distribution of spots of each off-axis electron beam on its optimal image plane as shown in Table 4. From Table 4, it is obvious that the imaging performance will be greatly improved through the application of a spherical fluorescent screen. Therefore, in order to further reduce the effect of field curvature, a spherical fluorescent screen can be used to improve the imaging performance of a streak tube.
3.4.2. Calculation of dynamic spatial resolution Dynamic spatial resolution refers to the minimum distance between two points that can be resolved by streak camera in the process of mapping temporal information to spatial information by electrostatic deflection system. The dynamic spatial resolution is calculated by determining the diameter of the scattered spot on the fluorescent screen after deflection of the electron beam. To demonstrate this more intuitively, the spatial mask plate on the photocathode is simulated. Fig. 7 shows the electron trajectory including falling points on a 598 mm image plane after dynamically calculating the emitted electron beams near the photocathode center, with the launch settings are as follows.
3.4. Calculation of dynamic characteristics The dynamic characteristics of the streak tube include its dynamic spatial resolution and its temporal resolution. The function of a streak tube is to map temporal information into spatial information by scanning, which is realized by a deflection system. Therefore, the design of a deflection system should be considered when calculating the dynamic characteristics of the streak tube. 3.4.1. Design of deflection system Compared with a flat plate deflection system, an inclined plate deflection system has higher deflection sensitivity and is more conducive to improve the temporal resolution, as shown in Fig. 6. Unlike the electric field in the former, the electric field in the inclined plate
(1) The initial spatial distribution of each streak simulated in the slit direction in space is the normal distribution of the 10 μm FWHM with the distance between the streaks being 40 μm. The spatial distribution of fringes in the scanning direction is the normal distribution of the 10 μm FWHM. 4
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 6. Inclined plate deflection system.
Fig. 7. Simulated result of dynamic spatial resolution. Table 5 Parameters of fringes in pulses. Streak pair no.
1 2
Streak no.
1.1 1.2 2.1 2.2
Two-sided coordinates of FWHM (cm) Left side
Right side
−0.00456 0.000767 −0.00419 0.00107
−0.000685 0.00464 −0.00147 0.00415
It can be seen from Table 5 that the diameter of the scattered spot of streak pair no. 1 can be approximated to 40 μm while that of no. 2 is approximately 30 μm (we conclude that the difference between them is because the dynamic deflection voltage may change the image position with the fixed focusing voltage, that is, the effect of deflection aberration). In other words, the streak tube designed in this study has a minimum theoretical dynamic spatial resolution of 25 lp/mm.
Diameter of scattered spot (μm)
38.5 39 29 30.8
(2) The normal distribution of the initial time of each pulse is 1 ps FWHM, two pulses were simulated continually and the pulse interval is 50 ps (peak interval). (3) Each streak emits 500 electrons, and the space charge effect is taken into account throughout the calculation.
3.4.3. Calculation of temporal resolution When calculating the temporal resolution of the entire tube, the Fabry–Perot etalon is simulated by setting the temporal interval between pulses. Because of the space charge effect, when the photoelectron current density is high, the pulse will be widened and the calculation accuracy will be affected. To reduce the influence of the space charge effect on the calculation accuracy of temporal resolution, the initial emission is set as follows.
Fig. 8 shows a histogram distribution of the electron trajectory in the slit direction corresponding to the two pulses. Table 5 lists the coordinates of the half-height and half-width of the electron beam of each streak image in each pulse.
(1) The uniform distribution of the photocathode slit direction in space is within 100 μm, whereas the scanning direction is still taken as the normal distribution of FWHM 10 μm. (2) Three continuous pulses were simulated, each with FWHMs of each 1 ps, peak-to-peak intervals of 50 ps, and 500 electrons per 5
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 8. Falling points of dynamic electron trajectories. Table 6 Pulse correlation coordinates. Pulse No.
Coordinates of pulse peak (cm)
1 2 3
0.7538 0.6583 0.5597
Two-side coordinates of FWHM (cm) Left side
Right side
0.7511 0.6484 0.5522
0.7587 0.6648 0.5675
The width of spline (μm)
11
the width value in Table 6, the temporal resolution corresponding to each spline width can be deduced as follows. 𝛥𝜏𝑠𝑝𝑙𝑖𝑛𝑒 =
50 ps (7538−6583)∕11
= 0.576 ps
(5)
Subsequently, the spline number of the first pulse can be obtained from the coordinates of the two sides of the FWHM of the first pulse 𝑁𝑠𝑝𝑙𝑖𝑛𝑒 = 7587−7511 ≈ 7; therefore, the temporal resolution of the entire 11 streak tube could be T = 𝑁𝑠𝑝𝑙𝑖𝑛𝑒 ∗ 𝛥𝜏𝑠𝑝𝑙𝑖𝑛𝑒 ≈ 4 ps. 4. Experiments
Fig. 9. Dynamic temporal resolution calculation results: (a) the electron trajectories’ falling points of three pulse sequences on the fluorescent screen; (b) the histogram distribution of each pulse in the scanning direction.
4.1. Design of spatial resolution reticle Based on the simulation results, we have developed a streak tube and tested it both statically and dynamically. The test data show that it fits perfectly to the design results. The structural sketch of a reticle on the photocathode used in the test is shown in Fig. 10. The center circle is a marker point, which simplifies locating the center of the image when imaging. The diameter of the photocathode is 50 mm, on which the reticles are separated by a spatial resolved reticle and a temporal resolved light through a hole with an interval of 1 mm. Among them, the width of the light through holes is 20 μm, whereas the spatial resolved reticle width is 100 μm.
pulse, which corresponds to a current density of 0.637 A/cm2 per pulse. Fig. 9 shows the simulated results in the scanning direction of the fluorescent screen. Table 6 shows the coordinates of the pulse peaks in Fig. 9(b) and the coordinates of the two sides of the space corresponding to the pulse width. In Fig. 9(b), the second and third pulses are wider than the first. According to the results of the simulation calculation, we conclude that the phenomenon is caused by the large calculation time-step (1 ps), which implies that the first pulse should be preferable for calculating the temporal resolution. At the beginning of the transmitting pulse sequence, the pulse interval is set to 50 ps. Therefore, from the relationship between the coordinates of the peaks of the first and second pulses, combined with
4.2. Fabrication of photocathode As mentioned above, Au or CsI are commonly used as photocathode materials for ultraviolet and soft X-ray streak cameras. Generally, the soft X-ray streak camera typically uses an open vacuum system to replace the photocathode at any time. Although CsI has higher 6
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 10. Structural sketch of reticle on photocathode.
quantum efficiency, owing to the inevitable need to be exposed to the atmosphere (water vapor) during cathode replacement, CsI is liable to combine with water molecules in the air, which results in deliquescence and affects the emission characteristics. In contrast, Au cathode has better stability and conductivity, and it can be prepared by an ordinary coating machine. Therefore, Au is used as the cathode material of the streak tube. Li Yu-kun et al. discussed the relationship between the spectral response sensitivity and the thickness of Au photocathode material for incident X-ray energy of 1 keV. It was proved that the thickness of the Au cathode should be approximately 10 nm for optimum spectral response sensitivity, whereas the corresponding thickness of the Au cathode should be approximately 40 nm for ultraviolet band testing. In this case, it is difficult to form a self-supporting structure with the Au cathode material, so it requires an organic thin film substrate of a certain thickness to support the cathode, and it is hoped that the substrate has no effect on the quantum efficiency of the photocathode material. Compared with 240 nm thick Formvar film and 800 nm thick Polypropylene films, 100 nm thick Parylene [13] film has better transmittance in the energy range of 0.1 keV to 10 keV. Furthermore, its melting point is high, it does not react with acid and alkali, and its mechanical strength is high. Therefore, 100 nm thick Parylene film is chosen as the substrate material. Parylene thin films were prepared by the vapor deposition method. The film thickness was cut along the glass edge and opened to float on the water surface. Then, the cathode sheet was lifted from the water, dried, and then evaporated with an Au cathode material having a thickness of 40 nm. The finished cathode is shown in Fig. 11.
Fig. 11. Photocathode fabricated with Au.
4.3. Fabrication of spherical fluorescent screen According to Table 4, the curvature radius of the spherical screen can be calculated from the right triangle by using (6).
Fig. 12. Fabricated spherical fluorescent screen: (a) photo of spherical phosphor screen; (b) luminescence of screen illuminated by an ultraviolet lamp.
𝑑 2 + ℎ2 (6) 2𝑑 Among them, 𝜌𝑠 is the curvature radius of the spherical fluorescent screen; 𝒅 is the height of the spherical gap (i.e., the distance between the ideal image plane and the screen, the maximum height of the spherical gap is 16 mm from Table 4); and 𝒉 is the viewing height. According to the actual size of the screen, the curvature of the screen can be obtained as 26 mm (which is the maximum size we could fabricate). Thus, the curvature radius should be 44 mm. However, the fluorescent powder could not be deposited if the curvature radius was less than 64 mm [20].
𝜌𝑠 =
Fig. 12 shows the fabricated physical spherical fluorescent screen. The fluorescent screen is fabricated by depositing P20 phosphor powder on the spherical surface (input surface is spherical — output surface is flat) of the optical fiber panel by the centrifugal deposition method, and then attaching it to the metal electrode by oxygen arc welding so that the fluorescent screen can be removed or replaced at any time in the experiment. 7
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
geometrical optical imaging system, a Pettzval surface also exists in the streak tube. Ideally, this surface should be symmetrical with respect to the photocathode center, and the resolution on both sides should be symmetrical if the screen is located in the corresponding axial position at the cathode center. However, the image plane rotation (the existence of assembly errors and the influence of magnetic field in the surrounding environment) makes the side of the image plane that is close to the screen has better spatial resolution whereas the other side deteriorates, as shown in Fig. 11. Fig. 10 shows that the image center of any two adjacent reticle plates is 4 mm apart; therefore, we can conclude that the effective resolution range of each group of reticle plates (including reticle plates with resolution line pairs, rectangular through holes without resolution line pairs, and the spacing between them) is 4 mm. Thus, it can be seen from Fig. 14 that the total imaging length is at least 32 mm. Within this range, according to Fig. 15(b), it can be observed that the total number of resolution elements 𝑁 is calculated as follow.
Fig. 13. The experiment set up of static parameters test.
4.4. Static parameters test When testing the static spatial resolution, the entire photocathode is irradiated by an ultraviolet lamp (without an image intensifier for the large light intensity of the source), and the data are recorded by PI company’s scientific air-cooled CCD (27 mm ∗ 27 mm, 2048 Pixel ∗ 2048 Pixel). The test set up is shown in Fig. 13, and test results are shown in Fig. 14 (owing to the limited size of CCD, the relative position between CCD and the screen can only be adjusted twice to collect images). The diameter of the fluorescent screen used in the experiment is 52 mm. From Table 1, we can see that the magnification of the streak tube designed in this paper is 1.35, so only the image range with the photocathode of diameter 38 mm could be obtained. However, to retain the central marker point during the test, the position of the CCD cannot be adjusted to the periphery. Therefore, we can only detect the imaging range in Fig. 10 from 17 lp/mm on the left side of center to 18 lp/mm on the right side (to make it easier to identify, we performed an inverse phase operation on the image color). Taking the circle point in Fig. 14 as the origin, the distance between the corresponding position and the center of the pattern with different resolution can be determined according to the structure size given in Fig. 10, which is the transverse axis. Fig. 15(a) is the corresponding contrast calculated from Figs. 14, and 15(b) is the limit spatial resolution calculated from this contrast. Table 4 indicates that the ideal image plane of the streak tube designed in this paper is not planar. In other words, similar to the
𝑁 = (30 + 31 + 31 + 32 + 24 + 27 + 28 + 25) × 4 = 912 lp
(7)
As mentioned above, to improve the imaging quality, spherical fluorescent screens can be used to mitigate the effects of field curvature. The curvature radius of the spherical screen is 64 mm and the corresponding height of the spherical gap is 5.5 mm. Table 4 shows that the optimum shape of the electron beam is almost identical to that of the spherical screen in the cathode range of at least 30 mm around the center, which was proved directly from the experimental results in Fig. 14. Additionally, it can be seen from Table 4 that the spherical screen is still effective in improving the image contrast of electron beams emitted in the range of 15 mm to 20 mm off-axis height. Therefore, according to the streak image contrast on both sides of Fig. 15(b) and the effective range of the spherical screen, we infer that the left-most reticle plate of the cathode center (corresponding to the 15 lp/mm and 16 lp/mm dividing board on the left side of Fig. 10) has a limit spatial resolution at least 15 lp/mm, that is, at least 9 mm near the edge of cathode is still distinguishable and can provide at least 135 resolution elements of information. In other words, in the static mode, the effective diameter of the photocathode designed in this paper can reach at least 40 mm, which can provide 1000 resolution elements of information.
Fig. 14. Test result of static spatial resolution: (a) left side image of the photocathode center; (b) right side image of the photocathode center.
8
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 15. Information of reticle image: (a) contrast ratio of the different height streak image; (b) limit spatial resolution of the streak tube.
obtained by using the X-ray source. The incident X-ray or UV pulse is converted into photoelectrons. The UV light does not produce exactly the same electron energy distribution as the X-ray [16]. However, the initial photoelectron energy distribution is much lower than the energy distribution achieved by the accelerating field of the photocathode and grid, as shown in Fig. 3(a). In the test, Quantronix Integra-C triple frequency Ti: Sapphire ultraviolet femtosecond laser (130 fs@266 nm with repetition frequency is 1 kHz) is used as the light source, the P.I.N. probe is illuminated by 800 nm red light to produce the trigger signal, and the trigger signal passes through the delay circuit to trigger the scanning circuit for the deflect system. Simultaneously, every single 266 nm ultraviolet light output from the laser is input into the Fabry–Perot etalon comprising all-mirror M4 and ultraviolet semi-reflective lens Lens to generate the temporal pulse sequence and then illuminate the photocathode of streak tube to produce the photoelectrons. After the enhancement of MCP, the final images are recorded by a CCD system (Princeton Instruments) and stored in the local computer, as shown in Fig. 18.
Fig. 16. Slope of scan voltage.
After the static spatial resolution test is completed, the scanning voltage slope is measured by an oscilloscope before the dynamic test, and the result is 300 V/ns, as shown in Fig. 16. Then, the deflection sensitivity test is conducted. When the streak tube works in static mode, the total voltage of the streak tube is adjusted to 12 kV (working voltage) whereas the voltage of the focus pole is adjusted to focus normally; then, the cathode is directly irradiated by the ultraviolet lamp. At this time, both deflection plates are grounded, and the coordinate of the scanning direction of the streak image is recorded as S1. Next, the total tube voltage is maintained to keep the lower deflection plate electrode grounded; meanwhile, the upper deflection plate electrode applies DC of 100 V. Subsequently, the position of the streak image is recorded again, as S2; this is subtracted from S1 and divided by the corresponding potential difference to yield the deflection sensitivity. The results are shown in Fig. 17. In Fig. 17, the cross marking is located at the cathode center, the locations shown in the Y coordinate in the lower left corner are 1386 and 908, and the sizes of the CCD pixels used are 13.5 μm; thus, the deflection sensitivity of the deflection plate can be obtained as follows:
𝑃inclined_test =
(1386 − 908) 𝑝𝑖𝑥𝑒𝑙 ∗ 13.5 μm = 64.53 mm∕kV 100 V
An ultraviolet lamp is used in the static test, so the streak image of almost the entire photocathode can be recorded, whereas the femtosecond laser used in the dynamic test can only be adjusted to the center of the irradiated cathode because of the limited laser spot size. Fig. 19 shows the test result of dynamic spatial resolution. The streak image is on the left side and the intensity distribution along the transverse axis is on the right side (the streak image broadens significantly when the first pulse laser input is too strong, so the streak image corresponding to the second pulse is selected). From the number of peaks on the curve, it can be seen that the dynamic spatial resolution in the center of the photocathode is 20 lp/mm (@contrast 10%) at least. When measuring the temporal resolution, the laser spot is adjusted to the corresponding position of the temporal-resolved light through hole on the right side of the cathode center, yielding a reading of 15 lp/mm, as shown in Fig. 10. The temporal-resolved element size corresponding to each CCD pixel at a fixed time interval (corresponding time length of Fabry–Perot etalon) is calculated from the interval between the streak images produced by the temporal pulse sequence, and then the single streak image is measured. The time width of a single pulse can be measured by the number of pixels occupied by the FWHM of the intensity distribution, as shown in Fig. 20.
(8)
The length of Fabry–Perot etalons comprising M4 and Lens in Fig. 18 is 21.4 mm, and the corresponding temporal interval is 141.2 ps. The temporal resolution of the entire streak tube can be obtained as in Section 3.4.3 above, which is calculated as
4.5. Dynamic parameters test The dynamic measurement includes temporal resolution and dynamic spatial resolution. Because of the lack of X-ray source, the experiments are conducted using the UV light pulse, as used by other researchers [21]. The temporal resolution is almost identical to that
T=6
pixel∗141.2 ps∕218 pixel
≈ 4 ps
(9)
9
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 17. Image of deflection sensitivity tests: The left part is when both plates are kept grounded and the right part is the image when DC of 100 V is applied onto the upper deflect plates while the lower one is kept at zero.
Fig. 18. Dynamic test schematic.
Fig. 19. Image of dynamic spatial resolution.
10
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 20. Image of temporal resolution.
5. Conclusions
[2] A.S. Moore, J. Benstead, M.F. Ahmed, J. Morton, T.M. Guymer, R. Soufli, T. Pardini, R.L. Hibbard, C.G. Bailey, P.M. Bell, S. Hau-Riege, M. Bedzyk, M.J. Shoup, S.P. Regan, T. Agliata, R. Jungquist, D.W. Schmidt, L.B. Kot, W.J. Garbett, M.S. Rubery, J.W. Skidmore, E. Gullikson, F. Salmassi, Two-color spatial and temporal temperature measurements using a streaked soft x-ray imager, Rev. Sci. Instrum. 87 (11) (2016) 055110–055128. [3] Kenta Mukojima, Shinji Kanzaki, Kota Kawanishi, Katsuyoshi Sato, Tadashi Abukawa, Streak-camera reflection high-energy electron diffraction for dynamics of surface crystallography, Surf. Sci. 636 (2015) 25–30. [4] N.S. Vorobiev, P.B. Gornostaev, V.I. Lozovoi, A.V. Smirnov, E.V. Shashkov, M. Ya. Schelev, A PS-1/S1 picosecond streak camera in experimental physics, Instrum. Exp. Tech. 59 (4) (2016) 551–556. [5] J. Siegel, G. Epurescu, A. Perea, F.J. Gordillo-Vázquez, J. Gonzalo, C.N. Afonso, Temporally and spectrally resolved imaging of laser-induced plasmas, Opt. Lett. 29 (19) (2004) 2228–2230. [6] N. Kawahara, J.L. Beduneau, T. Nakayama, E. Tomita, Y. Ikeda, Spatially, temporally, and spectrally resolved measurement of laser-induced plasma in air, Appl. Phys. B 86 (4) (2007) 605–614. [7] J. Benstead, A.S. Moore, M.F. Ahmed, J. Morton, T.M. Guymer, R. Soufli, T. Pardini, R.L. Hibbard, C.G. Bailey, P.M. Bell, S. Hau-Riege, M. Bedzyk, I.I.I. M.J. Shoup, S. Reagan, T. Agliata, R. Jungquist, D.W. Schmidt, L.B. Kot, W.J. Garbett, M.S. Rubery, J.W. Skidmore, E. Gullikson, F. Salmassi, A new streaked soft x-ray imager for the National Ignition Facility, Rev. Sci. Instrum. 87 (5) (2016) 055110. [8] K.W. Hill, M. Bitter, L. Delgado-Aparicio, P.C. Efthimion, R. Ellis, L. Gao, J. Maddox, Pablant N.A.1, M.B. Schneider, H. Chen, S. Ayers, R.L. Kauffman, A.G. MacPhee, P. Beiersdorfer, R. Bettencourt, T. Ma, R.C. Nora, H.A. Scott, D.B. Thorn, J.D. Kilkenny, D. Nelson, M. Shoup, Y. Maron, Development of a high resolution x-ray spectrometer for the National Ignition Facility (NIF), Rev. Sci. Instrum. 87 (11) (2016) 11E344. [9] J.R. Howorth, J.S. Milnes, Y. Fisher, A. Jadwin, R. Boni, P.A. Jaanimagi, The development of an improved streak tube for fusion diagnostics, Rev. Sci. Instrum. 87 (11) (2016) 287–294. [10] J.A. Oertel, et al., A large format gated X-ray framing camera, SPIE Proc. 5194 (2003) 214–222. [11] P.A. Jaanimagi, Breaking the 100-fs barrier with a streak camera, SPIE Proc. 5194 (2003) 171–182. [12] Ke-Xun Sun, William Nishimura, A Second-Generation X-ray Streak Camera With True Large Format, High Dynamic Range, and High Reliability. [C] Proc. SPIE 5920. [13] Yu Jiajin, Xiang Huizhu, Gao Quanlan, Lei Zhiyuan, C8H8 film for experiments of high temperature plasma, Nucl. Fusion Plasma Phys. 12 (3) (1992) 189–192. [14] Li Yu-kun, Chen Tao, Deng Bo, Yuan Zheng, Cao Zhu-rong, Energy spectral response of photocathode of soft X-ray streak camera, High Power Laser Part. Beams 26 (2) (2014) 022002. [15] Xu Shu-yan, Application of Monte Carlo Method in Experimental Nuclear Physics, second ed., Atomic Energy Press, 2006, pp. 48–52. [16] M.M. Shakya, Z.H. Chang, Achieving 280fs resolution with a streak camera by reducing the deflection dispersion, Appl. Phys. Lett. 87 (2005) 041103. [17] J. Feng, H.J. Shin, J.R. Nasiatka, W. Wan, A.T. Young, G. Huang, A. Comin, J. Byrd, H.A. Padmore, An x-ray streak camera with high spatio-temporal resolution, Appl. Phys. Lett. 91 (2007) 134102. [18] H.Z. Cai, W.Y. Fu, D. Wang, Y.F. Lei, J.Y. Liu, Three-strip microchannel plate gated x-ray framing camera, Sensors Actuators A 285 (2019) 355–361. [19] Zhou Li Wei, Electron Optics with Wide Beam Focusing, Beijing Institute of Technology Press, Beijing, China, 1993, p. 133.
A large detection area and high spatiotemporal resolution X-ray streak camera with a planar photocathode was developed to meet the requirements of ICF experiments and high density physics research. First, the electron-optical system of the streak tube was designed via numerical simulation. Further, we developed the streak tube and experimentally tested the parameters of the streak tube, including the dynamic spatiotemporal resolution and the dynamic deflect sensitivity. The test results indicate that the optimal diameter of the planar photocathode is at least 40 mm, and the temporal resolution is approximately 4 ps. In addition, the dynamic spatial resolution is 20 lp/mm, whereas the deflect sensitivity is approximately 64 kV/mm. Further work will develop a vastly superior streak camera that can achieve a spatial resolution of 1500 line pairs in a planar photocathode of 50 mm in diameter. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Zhang Jing-Jin: Funding acquisition, Software, Methodology, Writing — original draft. Liu Ai-lin: Resources, Data curation, Formal analysis. Yang Qin-lao: Project administration, Investigation, Validation. Zong Fang-ke: Writing — review & editing. Guo Bao-ping: Conceptualization, Supervision. Acknowledgment We would like to thank Editage (www.editage.cn) for English language editing. Funding This paper was granted by National Natural Science Foundations of China (NSFC) (No. 11805137); Science and Technology Program of Shenzhen (JCYJ 20170818102618203, JCYJ 20170818141616714). References [1] Y.P. Opachich, P.M. Bell, D.K. Bradley, N. Chen, J. Feng, A. Gopal, B. Hatch, T.J. Hilsabeck, E. Huffman, J.A. Koch1, O.L. Landen, A.G. MacPhee, S.R. Nagel, S. Udin, Structured photocathodes for improved high-energy x-ray efficiency in streak cameras, Rev. Sci. Instrum. 87 (11) (2016) S228. 11
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.
Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
[20] Zhang Jing-jin, Lei Bao-guo, Yang Qin-lao, Improvement of imaging performance for X-ray image tube, J. Shenzhen Univ. (Sci. Eng.) 34 (1) (2017) 14–19.
[21] T.J. Hilsabeck, J.D. Hares, J.D. Kilkenny, P.M. Bell, A.K.L. Dymoke-Bradshaw, J.A. Koch, P.M. Celliers, D.K. Bradley, T. McCarville, M. Pivovaroff, R. Soufli, R. Bionta, Pulse-dilation enhanced gated optical imager with 5 ps resolution, Rev. Sci. Instrum. 81 (10) (2010) E317.
(a) P.M. Bell, J.D. Kilkenny, R.L. Hanks, O.L. Landen, Measurements with a 35 psec gate timemicrochannel plate camera, Proc. SPIE 1346 (1991) 456–464.
12
Please cite this article as: Z. Jing-Jin, L. Ai-lin, Y. Qin-lao et al., Design of a large-format high-resolution streak camera with a planar photocathode, Nuclear Inst. and Methods in Physics Research, A (2019) 163076, https://doi.org/10.1016/j.nima.2019.163076.