- Email: [email protected]
In situ annealing of photocathodes using backside electron-beam irradiation
Nuclear Inst. and Methods in Physics Research B 462 (2020) 32–37
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
In situ annealing of photocathodes using backside electron-beam irradiation a,⁎
a
a,b
a,b
Daisuke Satoh , Tatsunori Shibuya , Hiroshi Ogawa , Masahito Tanaka Ryunosuke Kurodaa,b, Mitsuhiro Yoshidac, Hiroyuki Toyokawaa,b
,
T
a
Research Institute for Measurement and Analytical Instrumentation (RIMA), National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory (OPERANDO-OIL), National Institute of Advanced Industrial Science and Technology (AIST), 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8589, Japan c Accelerator Laboratory, High Energy Accelerator Research Organization KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Electron beam Photocathode RF gun Surface cleaning Quantum efficiency
Metal and metal-compound photocathodes are generally used as electron sources in radio frequency (RF) guns. These photocathodes are robust against laser irradiation and RF fields, are easy to handle, and have longevity. The surface cleaning of photocathodes is essential to improve and maintain their quantum efficiency (QE) in an RF gun cavity. In this paper, we propose a backside electron-beam irradiation heating technique as an in situ surface-cleaning procedure for photocathodes in an RF gun cavity. In the proposed technique, thermal electrons accelerated by a DC electric field are irradiated from the back of the photocathode, and their beam power is converted to heat, thereby heating the photocathode. Unlike other surface-cleaning techniques, surfaces of any cathode shape can be cleaned uniformly using this heating method. We designed and developed a cathode plug that includes the proposed heating system for large-diameter (~8 mm) photocathodes. Thermal simulation of the entire system, including the photocathode and its holding structure, and RF simulation of the choke structure for preventing the leakage of RF power were performed. The developed heating system was installed in an RF gun in High Energy Accelerator Research Organization KEK and subjected to RF aging and beam commissioning. From the results of beam commissioning, it was found that the QE of the photocathode increased to approximately 2.5 times the initial value due to the in situ annealing in the RF gun cavity.
1. Introduction Laser-driven photocathode RF gun systems are effective tools for generating MeV-order intense bright electron beams with low emittance and short pulse durations. These are used in a wide variety of applications such as MeV ultrafast electron-diffraction experiments [1,2], pulse radiolysis [3,4], X-ray generation using laser Compton scattering [5], and free-electron lasers [6]. These systems exhibit advantages such as excitation of high accelerating field, suppression of emittance growth caused by net space-charge effects, and the ability to control the time structures of electron beams by controlling the characteristics of laser pulses without using the RF bunch systems. In photocathode RF gun systems, the obtained beam parameters, such as bunch charge, intrinsic emittance, and bunch length, depend on the photoemission properties of the cathode (e.g., quantum efficiency, response time, etc.) and the characteristics of the laser pulse (e.g., pulse energy, wavelength, polarization, pulse length, etc.). Therefore, selecting the photocathode material is crucial.
⁎
Metal and metal-compound photocathodes [7–9], such as copper, magnesium, and lanthanum hexaboride, are generally used in RF guns. This is because they are robust against laser irradiation and RF fields, they are easy to handle, and can tolerate relatively poor vacuum conditions (~10−7 Pa) when compared to semiconductor photocathodes (e.g., K2CsSb and Cs2Te) [10–12]. These photocathodes, however, have much lower QE than the semiconductor photocathodes even with ultraviolet (UV) light. Additionally, they require surface treatments after installation in the RF gun to elicit and maintain superior photoemission properties. The photoemitting surface is generally cleaned using any of the following three methods: laser cleaning [13,14], ion-beam cleaning [15], and annealing [16]. In laser cleaning, which is the most popular method for improving the QE of photocathodes in an RF gun cavity, a UV laser pulse is focused to a small spot size. The fluence is close to the damage threshold and is scanned over the photoemitting surface to remove any contamination. Although the laser-cleaning method increases the QE of the photocathode, this technique can damage the
Corresponding author. E-mail address: [email protected] (D. Satoh).
https://doi.org/10.1016/j.nimb.2019.10.023 Received 12 June 2019; Received in revised form 2 October 2019; Accepted 22 October 2019 Available online 09 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 462 (2020) 32–37
D. Satoh, et al.
Fig. 1. Conceptual illustration of (a) the quasi-traveling-wave side-couple (QTWSC) type RF gun and (b) the developed backside electron-beam irradiation-type cathode plug.
RF input while maintaining the transverse emittance below 5.5 μm at the end of the RF gun [18]. These beam parameters were already achieved individually during RF gun commissioning.
cathode surface and generates a non-uniform photoelectron emission. In the ion-cleaning technique, ion beams are accelerated to a few kV and irradiated on to the cathode surface to eliminate contamination. This method also exhibits the risk of damaging the cathode surface, similar to laser cleaning. Also, it is particularly difficult to apply this technique to curved cathode surfaces. Annealing, on the other hand, is an excellent surface-treatment method since the surface of any cathode shape can be cleaned uniformly and repeatedly with high surfacetreatment accuracy. In this method, however, additional heat-isolation structures and RF shielding structures are required in the RF gun. Thus, annealing has hardly been applied for the surface-treatment of photocathodes in RF guns. In this paper, the development of a cathode plug equipped with an annealing system is described that can clean photocathode surfaces uniformly in the RF gun cavity. For annealing large-diameter cathodes efficiently in RF guns, a backside electron-beam irradiation-type heating system is proposed. A conceptual illustration of the RF gun cavity and the proposed heating system is shown in Fig. 1. The cathode plug for the photocathode RF gun, which adopted this heating system, was developed in collaboration with AIST-KEK. The cathode plug was installed into the RF gun of SuperKEKB electron linac in KEK and was commissioned.
2.2. Laser system A ytterbium (Yb) and neodymium (Nd) hybrid laser system [19] was used in the RF gun. The 114 MHz seed laser was generated using an Ybdoped mode-locked fibre oscillator. After the amplification of the seed pulse using a Yb-doped single-mode fibre amplifier, a transmission grating stretcher was used for the wavelength separation in each laser medium. The repetition rate was reduced to 25 Hz from 114 MHz using a semiconductor optical amplifier and electrical–optical module. At the end of the fibre part, the seed pulse was divided into two equal parts — one for the first Nd:YAG rod amplification line, which had a four-stage amplifier, and the other for the second line with a five-stage amplifier [19]. UV laser pulses were generated through two-stage wavelength conversions using barium borate crystals in the first and second laser lines separately; their pulse energies were 450 μJ and 800 μJ, respectively. Stable electron beam generation was achieved using these two laser lines. 2.3. Cathode material
2. RF gun system
Ce–Ir alloy photocathodes with a diameter of 8.0 mm were used as electron sources in this RF gun system. Ce–Ir alloys, which have good resistance to electron and ion bombardment [20], are suitable for photocathode RF gun systems. The 1/e lifetime of Ce–Ir alloy photocathodes is very long [21] and they exhibit QEs comparable to that of metal photocathodes such as copper and magnesium. They also require some form of surface cleaning to improve the QE after installation in the RF gun. In the Ce–Ir alloy photocathodes, the improvement of QE by laser cleaning [21] and annealing [22] has been confirmed. Also, their appropriate cleaning conditions were clarified. The photocathode surface was usually subjected to laser cleaning using UV laser pulses during previous RF gun commissioning.
In KEK, the SuperKEKB accelerator, which is an asymmetric-energy double-ring electron–positron collider, was constructed to search for new physics beyond the standard model. The electron injector linac provides the SuperKEKB rings with high-charge (~5 nC/bunch) and low-emittance (σ ~ 20 μm) electron beams to increase the luminosity [17]. To achieve these beam parameters, a photocathode RF gun system was adopted in the injector. This section describes the RF cavity, laser system, and photocathode material, which constitutes the photocathode RF gun system at KEK. 2.1. RF gun cavity A quasi-traveling-wave side-couple (QTWSC) type RF gun cavity was developed in order to achieve high bunch charges (> 5 nC/bunch) and low emittance (< 20 μm) simultaneously [18]. Two side-coupled cavities were arranged in a staggered manner in this cavity and a RF power with a π/2 phase difference was fed to each cavity [18]. The QTWSC-type RF gun had a sufficiently strong focusing field and short drift space such that the beam size could be small enough against the strong space-charge effect. According to the beam-tracking simulations, a 5 nC electron beam could be accelerated to 11.5 MeV with a 20 MW
3. Design and fabrication of the backside electron-beam irradiation-type heating system Annealing was used to clean the photocathode surface uniformly while preventing damage that could occur during laser cleaning. Surface treatment by annealing requires only temperature control, which is easier than laser cleaning. In this study, a backside electronbeam irradiation-type heating system is proposed that can anneal a large-diameter (~8.0 mm) photocathode, in situ, in the RF gun cavity. 33
Nuclear Inst. and Methods in Physics Research B 462 (2020) 32–37
D. Satoh, et al.
As shown in Fig. 1, thermal electrons accelerated by the DC electric field were irradiated from the back of the photocathode. Their beam power was converted to heat; thus, heating the photocathode. This method could heat large-diameter photocathodes more efficiently in comparison to direct or indirect heating. Furthermore, the thermionic emitter for photocathode heating could be operated at a temperature lower than direct and indirect heating methods. The heating power was determined by the product of the beam current and acceleration voltage. Thus, the proposed method exhibited an advantage in terms of longevity. Fig. 1(b) shows a conceptual illustration of the developed backside electron-beam irradiation-type cathode plug. This heating system was installed within the existing RF gun by assembling it inside an existing cathode plug. In this system, an RF choke structure, which provides RF shielding, was introduced. A cathode holding structure was designed to reduce the heat transfer from the photocathode.
approximately −90 dB. It was also confirmed that the stop bandwidth was sufficiently broad for practical use. 3.2. Thermal simulation The QE of the Ce–Ir alloy photocathode can be maximized by annealing over 1000 °C [22]. Therefore, this system was designed to be capable of annealing the photocathode above 1000 °C. The thermal simulations of the backside electron-beam irradiation-type heating system were calculated for the thermal steady state using CST MPHYSICS STUDIO. A heat source derived from the electron beam was virtually defined on the backside of the photocathode. The relationship between the heating power and surface temperature of the photocathode was calculated while assuming that all electron beam power was absorbed by the photocathode. For this thermal simulation, the physical properties of pure Ir were used to represent those of the Ce–Ir alloy photocathode. Additionally, the emissivity of the Ce–Ir alloy photocathode was defined as 0.3. Fig. 3(a) shows the relationship between the heating power and surface temperature of the Ce–Ir alloy photocathode. This figure indicates that the surface temperature of the photocathode can be over 1000 °C when using a heat source of approximately 35 W or more. Fig. 3(b) shows the thermal distribution around the cathode plug when a 50 W heat source is applied to the photocathode. This figure indicates that only the photocathode can be annealed efficiently without any unexpected heat accumulation in the system.
3.1. Design of the RF shielding structure The copper plug and photocathode cannot be in contact since the photocathode should be heated to a high temperature. As a result, RF power may leak from the gap between the photocathode and the plug. To prevent this RF leakage, an RF choke structure was introduced inside the cathode plug as shown in Fig. 1. This RF choke structure was designed to prevent the leakage of RF power, whose frequency is around 2.856 GHz. SUPERFISH code was used to calculate the eigenmode of the first cell of the RF gun cavity and choke structure. Fig. 2 shows the simulation results of the acceleration mode in the first cell of the QTWSC RF gun cavity, which includes the cathode plug. The electromagnetic field distribution shown in Fig. 2 indicates that this gap works as an RF choke structure. The transmission characteristics of this RF choke structure were calculated using the three-dimensional finite integration technique. From the simulation, the transmissivity of a 2.856 GHz microwave, in this RF choke structure, was calculated to be
3.3. Fabrication Photographs of the developed cathode plug based on the above design are shown in Fig. 4. A tungsten thermionic cathode was installed in the cathode plug. The thermionic cathode and extraction electrode were electrically isolated and the forming voltage could be applied according to the operating conditions. The cathode plug case made of oxygen-free copper was finished with an ultra-precision lathe in order to be installed in the RF gun cavity. 4. Experiments A preliminary test of the photocathode heating in the developed cathode plug without the copper cathode case was conducted on an offline test bench. The thermionic cathode, which was arranged at the backside of the photocathode, was biased with a negative voltage from a high-voltage DC power supply and the photocathode was grounded. The electron-beam power, used as the heat source, was mainly adjusted by changing the accelerating voltage of the thermionic cathode. The surface temperature of the photocathode was measured by a radiation thermometer. Table 1 shows the setting parameters and measured values at a condition when the surface temperature of the photocathode was over 1000 °C. The surface temperature of the photocathode exceeded 1000 °C by the irradiation with an electron beam having a beam power of about 50 W. After the operation test in the offline test bench, this cathode plug was introduced and tested in the RF gun cavity. The comparison among the simulation results (blue dots), the experimental results of the offline heating tests (red squares), and heating tests in the RF gun (green triangles) are shown in Fig. 5. In this figure, the horizontal axis indicates that the electron beam power used for photocathode heating while the vertical axis indicates the surface temperature of the Ce-Ir alloy photocathode. When comparing the simulation and experimental results of the offline heating test shown in Fig. 5, this indicates that approximately 40% more beam power is required when heating the photocathode to over 1000 °C. This difference is considered to be due to the reflection of the electron beams on the back surface of the photocathode and the beam losses at the electrode. These were not taken into account in the simulation. When comparing the experimental
Fig. 2. Simulation results of the acceleration mode in the first cell of the QTWSC RF gun cavity including the cathode plug. Red lines indicate the electric lines of the force. The circles indicate the axisymmetric magnetic field of the accelerating mode and the sizes of the circles represent the strength of the magnetic field.
34
Nuclear Inst. and Methods in Physics Research B 462 (2020) 32–37
D. Satoh, et al.
Fig. 3. (a) The relationship between the heating power and surface temperature of the Ce–Ir alloy photocathode. (b) The simulated thermal distribution around the cathode plug (heat source: 50 W).
Fig. 5. The relationship between the electron beam power used for heating and the surface temperature of the Ce-Ir alloy photocathode. The blue dots are the simulation results shown in Fig. 3(a). The red squares and green triangles are the experimental results of the offline heating tests and the heating tests in the RF gun, respectively.
Fig. 4. Photographs of the developed cathode plug. Table 1 The setting parameters and measured values of a preliminary heating test. Parameter
Value
Heater Current Heater Voltage Forming Voltage High Voltage Beam current Beam Power Surface Temperature
16 A 3.38 V −25 V 2.3 kV 23 mA 52.9 W 1029 °C
allowed continuous photocathode heating for up to six hours in the RF gun. There were no adverse effects on the RF gun cavity such as frequency shift and temperature increase of the RF gun cavity. Therefore, heat-isolation structure worked well in RF gun cavity. Next, the RF aging of the QTWSC RF gun cavity included in the cathode plug was conducted for two weeks. Fig. 6(a) shows a photograph of an RF aging test stand in KEK. Due to RF aging, the QTWSC RF gun cavity reached an input power level of 20 MW at a pulse width of 1.0 μs, which corresponds to the gun specifications. The maximum field strength of cathode surface corresponds to 120 MV/m for these operation conditions [23]. After the RF conditioning, the heating test of the photocathode was also performed in the RF cavity. The incandescence from the heated photocathode was visually confirmed from the view port and is shown in Fig. 6(b). Additionally, the surface temperature of the photocathode was confirmed to exceed 1000 °C by using the radiation thermometer. After the heating test, the RF power
results of the offline heating tests with the heating tests in the RF gun, a few extra tens of watts of beam power was required for heating the photocathode to 1000 °C in the RF gun. This could be due to the increase in the thermal dissipation by adding the copper cathode case and connecting the cathode plug and RF gun cavity. This experiment succeeded in heating the Ce-Ir alloy photocathode in the RF gun to over 1000 °C using the cathode plug. This system also 35
Nuclear Inst. and Methods in Physics Research B 462 (2020) 32–37
D. Satoh, et al.
example of the QE changes in the QTWSC RF gun for five months during SuperKEKB beam commissioning. The QE was calculated based on the pulse energy of the ultraviolet laser and the measured beam charge at the beam position monitor installed in the SuperKEKB injector linac. In this figure, the first three months of data are the QE before in situ annealing. Meanwhile, the latter two months of data are the QE after in situ annealing. Photocathode annealing was performed at 1000 °C for a total of 11 h in the RF gun. The results of the beam commissioning indicated that the QE of the Ce–Ir alloy photocathode was increased by in situ annealing in the RF gun cavity using the backside electron-beam irradiation-type heating system. The QE value increased up to 2.5 times in comparison to the initial QE. An electron beam with a maximum bunch charge of 3 nC was also obtained in this cathode plug. Compared with the experimental results of the photocathode annealing at the test bench [22] and the results of the beam commissioning in the RF gun, the effect of improving the QE by annealing in the RF gun was small. One hypothesis may be that the photocathode surface was not sufficiently cleaned in the RF gun. In this system, the amount of outgas during annealing was very large and the vacuum pressure at the time was more than 10−5 Pa. In order to overcome this problem, the vacuum condition needs to be improved during photocathode annealing in the RF gun. 5. Summary and conclusions
Fig. 6. (a) Photograph of the RF aging test stand. (b) Photograph of the heated photocathode inside the RF gun. (c) RF waveforms after a heating test of the photocathode for the RF gun system.
A backside electron-beam irradiation-type heating system, involving in situ annealing of a photocathode in the RF gun cavity, was proposed and developed to improve and maintain the QE. By introducing a choke structure inside the heating system, the leakage of RF power from the gap between the photocathode and cavity could be prevented while maintaining them without contact. An 8 mm diameter photocathode could be heated to above 1000 °C by irradiation with an electron beam having a beam power of approximately 50 W using this system. The cathode plug was equipped with this heating system and was introduced into the RF gun and RF conditioning was performed. No discharge occurred in the choke structure and the cathode plug worked as an RF shield. The QE of the Ce-Ir alloy photocathode was increased by in situ annealing in a RF gun cavity using this heating system. Therefore, a system capable of in situ surface treatment was successfully developed by inserting a cathode plug with a backside electron-beam irradiation-type heating system into an existing acceleration cavity. This heating system is expected to be applied to a wide variety of electron guns, such as high-current thermionic DC electron guns and thermionic RF guns, along with annealing photocathodes. 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. Acknowledgements
Fig. 7. QE changes in the QTWSC RF gun before and after the photocathode annealing.
The authors would like to thank T. Natsui and T. Noguchi for their helpful discussions for the development of the backside electron-beam irradiation-type heating system.
was supplied again to the RF gun cavity. Fig. 6(c) shows the RF waveforms after the heating test of the photocathode of this RF gun system. The forward (yellow line) and backward (red line) RF waveforms of the RF gun indicated that the gun was fed by a 1.0 μs long pulse. Also, a clean reflection waveform with a standard rabbit ear shape was obtained. These waveforms indicated that the RF power could be filled in the RF gun cavity without any problems. As a result, the RF shield structure worked without the leakage of RF power and multipactor discharge. Finally, this RF gun system was installed in the main electron linac in KEK and beam commissioning was performed. Fig. 7 shows an
References [1] [2] [3] [4] [5] [6] [7] [8]
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S.P. Weathersby, et al., Rev. Sci. Instrum. 86 (2015) 073702. P. Zhu, et al., New J. Phys. 17 (2015) 063004. J. Grodkowski, et al., J. Phys. Chem. A 107 (2003) 9794. J. Yang, et al., Nucl. Instrum. Methods Phys. Res. A 637 (2011) S24. S. Kashiwagi, et al., Nucl. Instrum. Methods Phys. Res. A 455 (2000) 36. A. Marinelli, et al., Nat. Commun. 6 (2015) 6369. T. Srinivasan-Rao, et al., J. Appl. Phys. 69 (1991) 3291. H.J. Qian, et al., Appl. Phys. Lett. 97 (2010) 253504.
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M. Boussoukaya, et al., Nucl. Instrum. Methods A 264 (1988) 131. D.H. Dowel, et al., Nucl. Instrum. Methods Phys. Res. A 356 (1995) 167. M. Ruiz-Osés, et al., Appl. Mater. 2 (2014) 121101. N. Terunuma, et al., Nucl. Instrum. Methods Phys. Res. A 613 (2010) 1. F. Zhou, et al., PRST-AB 15 (2012) 090703. F. Gontad, et al., PRST-AB 16 (2013) 093401. D.H. Dowell, PRST-AB 9 (2006) 063502. S. Mistry, Preparation and Evaluation of Metal Surfaces for Use as Photocathodes, Doctor thesis, Loughborough University (2018).
[17] Y. Ohnishi, et al., Prog. Theor. Exp. Phys. 2013 (2013) 03A011. [18] T. Natsui, et al., Proceedings of IPAC2014, Dresden, Germany, 2014, p. 667. [19] R. Zhang, et al., in: Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan, Nagaoka, Japan (2018) 763. [20] G.I. Kuznetsov, J. Phys. Conf. Ser. 2 (2004) 35. [21] D. Satoh, et al., Energy Procedia 131 (2017) 326. [22] D. Satoh, et al., in: Proceedings of IPAC2013, Shanghai, China, (2013) 327. [23] T. Natsui et al., in: Proceedings of IPAC2013, Shanghai, China (2013) 1117.
37
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
In situ annealing of photocathodes using backside electron-beam irradiation a,⁎
a
a,b
a,b
Daisuke Satoh , Tatsunori Shibuya , Hiroshi Ogawa , Masahito Tanaka Ryunosuke Kurodaa,b, Mitsuhiro Yoshidac, Hiroyuki Toyokawaa,b
,
T
a
Research Institute for Measurement and Analytical Instrumentation (RIMA), National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory (OPERANDO-OIL), National Institute of Advanced Industrial Science and Technology (AIST), 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8589, Japan c Accelerator Laboratory, High Energy Accelerator Research Organization KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Electron beam Photocathode RF gun Surface cleaning Quantum efficiency
Metal and metal-compound photocathodes are generally used as electron sources in radio frequency (RF) guns. These photocathodes are robust against laser irradiation and RF fields, are easy to handle, and have longevity. The surface cleaning of photocathodes is essential to improve and maintain their quantum efficiency (QE) in an RF gun cavity. In this paper, we propose a backside electron-beam irradiation heating technique as an in situ surface-cleaning procedure for photocathodes in an RF gun cavity. In the proposed technique, thermal electrons accelerated by a DC electric field are irradiated from the back of the photocathode, and their beam power is converted to heat, thereby heating the photocathode. Unlike other surface-cleaning techniques, surfaces of any cathode shape can be cleaned uniformly using this heating method. We designed and developed a cathode plug that includes the proposed heating system for large-diameter (~8 mm) photocathodes. Thermal simulation of the entire system, including the photocathode and its holding structure, and RF simulation of the choke structure for preventing the leakage of RF power were performed. The developed heating system was installed in an RF gun in High Energy Accelerator Research Organization KEK and subjected to RF aging and beam commissioning. From the results of beam commissioning, it was found that the QE of the photocathode increased to approximately 2.5 times the initial value due to the in situ annealing in the RF gun cavity.
1. Introduction Laser-driven photocathode RF gun systems are effective tools for generating MeV-order intense bright electron beams with low emittance and short pulse durations. These are used in a wide variety of applications such as MeV ultrafast electron-diffraction experiments [1,2], pulse radiolysis [3,4], X-ray generation using laser Compton scattering [5], and free-electron lasers [6]. These systems exhibit advantages such as excitation of high accelerating field, suppression of emittance growth caused by net space-charge effects, and the ability to control the time structures of electron beams by controlling the characteristics of laser pulses without using the RF bunch systems. In photocathode RF gun systems, the obtained beam parameters, such as bunch charge, intrinsic emittance, and bunch length, depend on the photoemission properties of the cathode (e.g., quantum efficiency, response time, etc.) and the characteristics of the laser pulse (e.g., pulse energy, wavelength, polarization, pulse length, etc.). Therefore, selecting the photocathode material is crucial.
⁎
Metal and metal-compound photocathodes [7–9], such as copper, magnesium, and lanthanum hexaboride, are generally used in RF guns. This is because they are robust against laser irradiation and RF fields, they are easy to handle, and can tolerate relatively poor vacuum conditions (~10−7 Pa) when compared to semiconductor photocathodes (e.g., K2CsSb and Cs2Te) [10–12]. These photocathodes, however, have much lower QE than the semiconductor photocathodes even with ultraviolet (UV) light. Additionally, they require surface treatments after installation in the RF gun to elicit and maintain superior photoemission properties. The photoemitting surface is generally cleaned using any of the following three methods: laser cleaning [13,14], ion-beam cleaning [15], and annealing [16]. In laser cleaning, which is the most popular method for improving the QE of photocathodes in an RF gun cavity, a UV laser pulse is focused to a small spot size. The fluence is close to the damage threshold and is scanned over the photoemitting surface to remove any contamination. Although the laser-cleaning method increases the QE of the photocathode, this technique can damage the
Corresponding author. E-mail address: [email protected] (D. Satoh).
https://doi.org/10.1016/j.nimb.2019.10.023 Received 12 June 2019; Received in revised form 2 October 2019; Accepted 22 October 2019 Available online 09 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 462 (2020) 32–37
D. Satoh, et al.
Fig. 1. Conceptual illustration of (a) the quasi-traveling-wave side-couple (QTWSC) type RF gun and (b) the developed backside electron-beam irradiation-type cathode plug.
RF input while maintaining the transverse emittance below 5.5 μm at the end of the RF gun [18]. These beam parameters were already achieved individually during RF gun commissioning.
cathode surface and generates a non-uniform photoelectron emission. In the ion-cleaning technique, ion beams are accelerated to a few kV and irradiated on to the cathode surface to eliminate contamination. This method also exhibits the risk of damaging the cathode surface, similar to laser cleaning. Also, it is particularly difficult to apply this technique to curved cathode surfaces. Annealing, on the other hand, is an excellent surface-treatment method since the surface of any cathode shape can be cleaned uniformly and repeatedly with high surfacetreatment accuracy. In this method, however, additional heat-isolation structures and RF shielding structures are required in the RF gun. Thus, annealing has hardly been applied for the surface-treatment of photocathodes in RF guns. In this paper, the development of a cathode plug equipped with an annealing system is described that can clean photocathode surfaces uniformly in the RF gun cavity. For annealing large-diameter cathodes efficiently in RF guns, a backside electron-beam irradiation-type heating system is proposed. A conceptual illustration of the RF gun cavity and the proposed heating system is shown in Fig. 1. The cathode plug for the photocathode RF gun, which adopted this heating system, was developed in collaboration with AIST-KEK. The cathode plug was installed into the RF gun of SuperKEKB electron linac in KEK and was commissioned.
2.2. Laser system A ytterbium (Yb) and neodymium (Nd) hybrid laser system [19] was used in the RF gun. The 114 MHz seed laser was generated using an Ybdoped mode-locked fibre oscillator. After the amplification of the seed pulse using a Yb-doped single-mode fibre amplifier, a transmission grating stretcher was used for the wavelength separation in each laser medium. The repetition rate was reduced to 25 Hz from 114 MHz using a semiconductor optical amplifier and electrical–optical module. At the end of the fibre part, the seed pulse was divided into two equal parts — one for the first Nd:YAG rod amplification line, which had a four-stage amplifier, and the other for the second line with a five-stage amplifier [19]. UV laser pulses were generated through two-stage wavelength conversions using barium borate crystals in the first and second laser lines separately; their pulse energies were 450 μJ and 800 μJ, respectively. Stable electron beam generation was achieved using these two laser lines. 2.3. Cathode material
2. RF gun system
Ce–Ir alloy photocathodes with a diameter of 8.0 mm were used as electron sources in this RF gun system. Ce–Ir alloys, which have good resistance to electron and ion bombardment [20], are suitable for photocathode RF gun systems. The 1/e lifetime of Ce–Ir alloy photocathodes is very long [21] and they exhibit QEs comparable to that of metal photocathodes such as copper and magnesium. They also require some form of surface cleaning to improve the QE after installation in the RF gun. In the Ce–Ir alloy photocathodes, the improvement of QE by laser cleaning [21] and annealing [22] has been confirmed. Also, their appropriate cleaning conditions were clarified. The photocathode surface was usually subjected to laser cleaning using UV laser pulses during previous RF gun commissioning.
In KEK, the SuperKEKB accelerator, which is an asymmetric-energy double-ring electron–positron collider, was constructed to search for new physics beyond the standard model. The electron injector linac provides the SuperKEKB rings with high-charge (~5 nC/bunch) and low-emittance (σ ~ 20 μm) electron beams to increase the luminosity [17]. To achieve these beam parameters, a photocathode RF gun system was adopted in the injector. This section describes the RF cavity, laser system, and photocathode material, which constitutes the photocathode RF gun system at KEK. 2.1. RF gun cavity A quasi-traveling-wave side-couple (QTWSC) type RF gun cavity was developed in order to achieve high bunch charges (> 5 nC/bunch) and low emittance (< 20 μm) simultaneously [18]. Two side-coupled cavities were arranged in a staggered manner in this cavity and a RF power with a π/2 phase difference was fed to each cavity [18]. The QTWSC-type RF gun had a sufficiently strong focusing field and short drift space such that the beam size could be small enough against the strong space-charge effect. According to the beam-tracking simulations, a 5 nC electron beam could be accelerated to 11.5 MeV with a 20 MW
3. Design and fabrication of the backside electron-beam irradiation-type heating system Annealing was used to clean the photocathode surface uniformly while preventing damage that could occur during laser cleaning. Surface treatment by annealing requires only temperature control, which is easier than laser cleaning. In this study, a backside electronbeam irradiation-type heating system is proposed that can anneal a large-diameter (~8.0 mm) photocathode, in situ, in the RF gun cavity. 33
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As shown in Fig. 1, thermal electrons accelerated by the DC electric field were irradiated from the back of the photocathode. Their beam power was converted to heat; thus, heating the photocathode. This method could heat large-diameter photocathodes more efficiently in comparison to direct or indirect heating. Furthermore, the thermionic emitter for photocathode heating could be operated at a temperature lower than direct and indirect heating methods. The heating power was determined by the product of the beam current and acceleration voltage. Thus, the proposed method exhibited an advantage in terms of longevity. Fig. 1(b) shows a conceptual illustration of the developed backside electron-beam irradiation-type cathode plug. This heating system was installed within the existing RF gun by assembling it inside an existing cathode plug. In this system, an RF choke structure, which provides RF shielding, was introduced. A cathode holding structure was designed to reduce the heat transfer from the photocathode.
approximately −90 dB. It was also confirmed that the stop bandwidth was sufficiently broad for practical use. 3.2. Thermal simulation The QE of the Ce–Ir alloy photocathode can be maximized by annealing over 1000 °C [22]. Therefore, this system was designed to be capable of annealing the photocathode above 1000 °C. The thermal simulations of the backside electron-beam irradiation-type heating system were calculated for the thermal steady state using CST MPHYSICS STUDIO. A heat source derived from the electron beam was virtually defined on the backside of the photocathode. The relationship between the heating power and surface temperature of the photocathode was calculated while assuming that all electron beam power was absorbed by the photocathode. For this thermal simulation, the physical properties of pure Ir were used to represent those of the Ce–Ir alloy photocathode. Additionally, the emissivity of the Ce–Ir alloy photocathode was defined as 0.3. Fig. 3(a) shows the relationship between the heating power and surface temperature of the Ce–Ir alloy photocathode. This figure indicates that the surface temperature of the photocathode can be over 1000 °C when using a heat source of approximately 35 W or more. Fig. 3(b) shows the thermal distribution around the cathode plug when a 50 W heat source is applied to the photocathode. This figure indicates that only the photocathode can be annealed efficiently without any unexpected heat accumulation in the system.
3.1. Design of the RF shielding structure The copper plug and photocathode cannot be in contact since the photocathode should be heated to a high temperature. As a result, RF power may leak from the gap between the photocathode and the plug. To prevent this RF leakage, an RF choke structure was introduced inside the cathode plug as shown in Fig. 1. This RF choke structure was designed to prevent the leakage of RF power, whose frequency is around 2.856 GHz. SUPERFISH code was used to calculate the eigenmode of the first cell of the RF gun cavity and choke structure. Fig. 2 shows the simulation results of the acceleration mode in the first cell of the QTWSC RF gun cavity, which includes the cathode plug. The electromagnetic field distribution shown in Fig. 2 indicates that this gap works as an RF choke structure. The transmission characteristics of this RF choke structure were calculated using the three-dimensional finite integration technique. From the simulation, the transmissivity of a 2.856 GHz microwave, in this RF choke structure, was calculated to be
3.3. Fabrication Photographs of the developed cathode plug based on the above design are shown in Fig. 4. A tungsten thermionic cathode was installed in the cathode plug. The thermionic cathode and extraction electrode were electrically isolated and the forming voltage could be applied according to the operating conditions. The cathode plug case made of oxygen-free copper was finished with an ultra-precision lathe in order to be installed in the RF gun cavity. 4. Experiments A preliminary test of the photocathode heating in the developed cathode plug without the copper cathode case was conducted on an offline test bench. The thermionic cathode, which was arranged at the backside of the photocathode, was biased with a negative voltage from a high-voltage DC power supply and the photocathode was grounded. The electron-beam power, used as the heat source, was mainly adjusted by changing the accelerating voltage of the thermionic cathode. The surface temperature of the photocathode was measured by a radiation thermometer. Table 1 shows the setting parameters and measured values at a condition when the surface temperature of the photocathode was over 1000 °C. The surface temperature of the photocathode exceeded 1000 °C by the irradiation with an electron beam having a beam power of about 50 W. After the operation test in the offline test bench, this cathode plug was introduced and tested in the RF gun cavity. The comparison among the simulation results (blue dots), the experimental results of the offline heating tests (red squares), and heating tests in the RF gun (green triangles) are shown in Fig. 5. In this figure, the horizontal axis indicates that the electron beam power used for photocathode heating while the vertical axis indicates the surface temperature of the Ce-Ir alloy photocathode. When comparing the simulation and experimental results of the offline heating test shown in Fig. 5, this indicates that approximately 40% more beam power is required when heating the photocathode to over 1000 °C. This difference is considered to be due to the reflection of the electron beams on the back surface of the photocathode and the beam losses at the electrode. These were not taken into account in the simulation. When comparing the experimental
Fig. 2. Simulation results of the acceleration mode in the first cell of the QTWSC RF gun cavity including the cathode plug. Red lines indicate the electric lines of the force. The circles indicate the axisymmetric magnetic field of the accelerating mode and the sizes of the circles represent the strength of the magnetic field.
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Fig. 3. (a) The relationship between the heating power and surface temperature of the Ce–Ir alloy photocathode. (b) The simulated thermal distribution around the cathode plug (heat source: 50 W).
Fig. 5. The relationship between the electron beam power used for heating and the surface temperature of the Ce-Ir alloy photocathode. The blue dots are the simulation results shown in Fig. 3(a). The red squares and green triangles are the experimental results of the offline heating tests and the heating tests in the RF gun, respectively.
Fig. 4. Photographs of the developed cathode plug. Table 1 The setting parameters and measured values of a preliminary heating test. Parameter
Value
Heater Current Heater Voltage Forming Voltage High Voltage Beam current Beam Power Surface Temperature
16 A 3.38 V −25 V 2.3 kV 23 mA 52.9 W 1029 °C
allowed continuous photocathode heating for up to six hours in the RF gun. There were no adverse effects on the RF gun cavity such as frequency shift and temperature increase of the RF gun cavity. Therefore, heat-isolation structure worked well in RF gun cavity. Next, the RF aging of the QTWSC RF gun cavity included in the cathode plug was conducted for two weeks. Fig. 6(a) shows a photograph of an RF aging test stand in KEK. Due to RF aging, the QTWSC RF gun cavity reached an input power level of 20 MW at a pulse width of 1.0 μs, which corresponds to the gun specifications. The maximum field strength of cathode surface corresponds to 120 MV/m for these operation conditions [23]. After the RF conditioning, the heating test of the photocathode was also performed in the RF cavity. The incandescence from the heated photocathode was visually confirmed from the view port and is shown in Fig. 6(b). Additionally, the surface temperature of the photocathode was confirmed to exceed 1000 °C by using the radiation thermometer. After the heating test, the RF power
results of the offline heating tests with the heating tests in the RF gun, a few extra tens of watts of beam power was required for heating the photocathode to 1000 °C in the RF gun. This could be due to the increase in the thermal dissipation by adding the copper cathode case and connecting the cathode plug and RF gun cavity. This experiment succeeded in heating the Ce-Ir alloy photocathode in the RF gun to over 1000 °C using the cathode plug. This system also 35
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example of the QE changes in the QTWSC RF gun for five months during SuperKEKB beam commissioning. The QE was calculated based on the pulse energy of the ultraviolet laser and the measured beam charge at the beam position monitor installed in the SuperKEKB injector linac. In this figure, the first three months of data are the QE before in situ annealing. Meanwhile, the latter two months of data are the QE after in situ annealing. Photocathode annealing was performed at 1000 °C for a total of 11 h in the RF gun. The results of the beam commissioning indicated that the QE of the Ce–Ir alloy photocathode was increased by in situ annealing in the RF gun cavity using the backside electron-beam irradiation-type heating system. The QE value increased up to 2.5 times in comparison to the initial QE. An electron beam with a maximum bunch charge of 3 nC was also obtained in this cathode plug. Compared with the experimental results of the photocathode annealing at the test bench [22] and the results of the beam commissioning in the RF gun, the effect of improving the QE by annealing in the RF gun was small. One hypothesis may be that the photocathode surface was not sufficiently cleaned in the RF gun. In this system, the amount of outgas during annealing was very large and the vacuum pressure at the time was more than 10−5 Pa. In order to overcome this problem, the vacuum condition needs to be improved during photocathode annealing in the RF gun. 5. Summary and conclusions
Fig. 6. (a) Photograph of the RF aging test stand. (b) Photograph of the heated photocathode inside the RF gun. (c) RF waveforms after a heating test of the photocathode for the RF gun system.
A backside electron-beam irradiation-type heating system, involving in situ annealing of a photocathode in the RF gun cavity, was proposed and developed to improve and maintain the QE. By introducing a choke structure inside the heating system, the leakage of RF power from the gap between the photocathode and cavity could be prevented while maintaining them without contact. An 8 mm diameter photocathode could be heated to above 1000 °C by irradiation with an electron beam having a beam power of approximately 50 W using this system. The cathode plug was equipped with this heating system and was introduced into the RF gun and RF conditioning was performed. No discharge occurred in the choke structure and the cathode plug worked as an RF shield. The QE of the Ce-Ir alloy photocathode was increased by in situ annealing in a RF gun cavity using this heating system. Therefore, a system capable of in situ surface treatment was successfully developed by inserting a cathode plug with a backside electron-beam irradiation-type heating system into an existing acceleration cavity. This heating system is expected to be applied to a wide variety of electron guns, such as high-current thermionic DC electron guns and thermionic RF guns, along with annealing photocathodes. 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. Acknowledgements
Fig. 7. QE changes in the QTWSC RF gun before and after the photocathode annealing.
The authors would like to thank T. Natsui and T. Noguchi for their helpful discussions for the development of the backside electron-beam irradiation-type heating system.
was supplied again to the RF gun cavity. Fig. 6(c) shows the RF waveforms after the heating test of the photocathode of this RF gun system. The forward (yellow line) and backward (red line) RF waveforms of the RF gun indicated that the gun was fed by a 1.0 μs long pulse. Also, a clean reflection waveform with a standard rabbit ear shape was obtained. These waveforms indicated that the RF power could be filled in the RF gun cavity without any problems. As a result, the RF shield structure worked without the leakage of RF power and multipactor discharge. Finally, this RF gun system was installed in the main electron linac in KEK and beam commissioning was performed. Fig. 7 shows an
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