- Email: [email protected]
Soft X-ray generation experiment at the Tsinghua Thomson scattering X-ray source
Nuclear Instruments and Methods in Physics Research A 637 (2011) S168–S171
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Soft X-ray generation experiment at the Tsinghua Thomson scattering X-ray source Yingchao Du a,b,c,, Lixin Yan a,b,c, Jianfei Hua a,b,c, Qiang Du a,b,c, Renkai Li a,b,c, Jiaru Shi a,b,c, Wenhui Huang a,b,c, Huaibi Chen a,b,c, Chuanxiang Tang a,b,c a
Accelerator Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084, China Key Laboratory of Particle and Radiation Imaging, Tsinghua University, Ministry of Education, Beijing 100084, China c Key Laboratory of High Energy Radiation Imaging, Fundamental Science for National Defense, Beijing 100084, China b
a r t i c l e in f o
a b s t r a c t
Available online 10 February 2010
The development, design, and construction of the Tsinghua Thomson scattering X-ray (TTX) source have been ongoing since 2001. The TTX source is based on the Thomson scattering of a femtosecond laser pulse by a relativistic electron beam, which aims at an ultra-fast, high-flux, monochromatic, and tunable X-ray source for scientific, medical, and industrial applications. A recent experiment sought to generate a soft X-ray pulse through Thomson scattering. In this experiment, a 3-eV electron beam generated from a photocathode radio-frequency gun is focused by quadrupole magnets to several hundred microns and collided with a 120-mJ, 60-fs laser beam. The generated X-ray is detected by a micro-channel plate. The energy, pulse duration, and number of X-ray photons is estimated to be 290.4 eV, 1 ps, and 6.4 103, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: Thomson scattering X-ray source Photocathode RF gun Laser X-ray
1. Introduction For the development of the next generation of X-ray light sources, an important motivation is to achieve picosecond and sub-ps pulses of hard X-rays with a temporal resolution of several hundreds femtoseconds (fs), which is the fundamental time scale of atomic motions [1]. This type of X-ray source would be valuable for dynamic studies of a variety of physical, chemical, and biological processes. The development of femtosecond lasers has made it possible to study short time reactions in the femtosecond region. However, it is also limited by the optical probe due to the low photon energy for studying extended electronic levels in solids. The femtosecond time-scale hard X-ray is an important probe for studying the structure of solids through their interaction with core electronic levels in atoms. It can be used to observe the structural dynamic on the fundamental time scale. Such an X-ray source would be useful for industrial and medical imaging applications. Several concepts or methods have been proposed and demonstrated to generate a suitable X-ray pulse. These include the Ka Xray source with an intense laser [2], the X-ray free-electron laser (FEL) [3,4], synchrotron radiation with a slicing technique [5],
Corresponding author at: Accelerator Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084, China. Tel.: + 86 10 62795424. E-mail address: [email protected] (Y. Du).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.049
Thomson scattering of a femtosecond laser pulse by relativistic electron beams [6–8], and so forth. Among these sources, the X-ray source based on Thomson scattering (or inverse Compton scattering) between the laser photons and the relativistic electrons may lead to the novel femtosecond time light source facility. It can generate an ultra-short, high-flux monochromatic and tunable hard X-ray pulse with the short pulse laser, radiofrequency (RF) linac, and photocathode RF gun. It also provides a means of performing hard X-ray pump–probe experiments on a sub-picosecond time scale with a flux suitable for single-shot measurements, which enables experiments on samples undergoing irreversible damage such as shocks, plasma ablation, or ultra-fast melting. And, compared with the X-ray FEL facility and the synchrotron light source, it is more compact and affordable and can generate more hard X-rays. Enticed by these prospects, some laboratories have proposed and constructed such an X-ray source. The Accelerator Laboratory of Tsinghua University also proposed the Tsinghua Thomson scattering X-ray source (TTX) and began to study the concept in 2001 [9–12]. The scheme of the X-ray source, as shown in Fig. 1, consists of a linac system and a femtosecond terawatt laser system. The laser system generates both the 266-nm ultraviolet (UV) pulse for the photocathode and the 800-nm infrared (IR) pulse for the scattering interaction. The two pulses are derived from one 79.3-MHz Ti:sapphire oscillator in order to reduce the time jitter between the electron beam and the IR pulse. The linac system consists of a BNL/KEK/SHI-type 1.6-cell S-band photocathode RF gun, a 3-m
Y. Du et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S168–S171
IR
1
12
13
UV
18 10
2
3
4
5
6
7
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8
9
11 14
17 15
16 Fig. 1. Scheme of the TTX.
S-band SLAC-type traveling wave accelerating section, a four-dipole magnet compressor to compress the electron bunch to below 1 ps, and two X-band harmonic structures, thus enabling flexible manipulation of the phase space of the electron pulses and generating the 40–50-MeV ultra-short high-brightness electron pulse for the scattering interaction. The laser system is synchronized with the RF system through a timing circuit, with a timing jitter no greater than 0.5 ps. The electron and IR laser pulses collide in the interaction chamber with geometries ranging from 901 to 1801, and they generate a 20–50-keV X-ray pulse with fluxes from 106 to 108 photons/pulse and pulse durations from 200 fs to 1 ps. This source will serve as a tunable monochromatic X-ray source for advanced applications in science, industry, and medicine. Some main subsystems, such as the high-power system, the ultra-fast laser system with a 3-TW IR laser pulse, and the photocathode RF gun system, have been constructed and commissioned. The low-energy beam of 2–4 MeV is ready for experiments related to MeV ultra-fast electron diffraction (UED), bunch length measurement with a deflecting cavity, soft X-ray generation via Thomson scattering, THz generation, and so forth. The setup, results, and discussion of an experiment for soft X-ray generation via Thomson scattering are presented in this paper. In the experiment, the 3-MeV, 1-ps, 100-pC, normalized emittance en =6 mm mrad electron beam from the photocathode RF gun and the 800-nm, 60-fs, 120-mJ, linearly polarized laser are head-on collided.
2. Description of the experiment The schematic diagram of the experimental setup is shown in Fig. 2. The laser system generates both UV and IR laser pulses. These two laser pulses are from the same laser oscillator so that they are originally synchronized. The UV laser pulse irradiates on the cathode nearly perpendicularly, and it generates an electron pulse with a corresponding spatial distribution. The beam charge is influenced by the UV pulse energy, the electric field on the cathode, and the laser injected phase. In the experiment, the beam charge is about 105 pC/pulse, with an 30-mJ UV pulse energy and a 171 laser injected phase. This beam is accelerated by a high-amplitude electric field in the photocathode RF gun, and it reaches 2–4 MeV at the gun exit. A solenoid is used for emittance compensation and beam envelope control. Two beam diagnostic chambers with multi-silts and YAG screens are followed with the laser incident chamber and used for beam profile and emittance measurements. A triplet is used to focus the beam into the interaction chamber with a beam size of about several hundreds microns at the interaction region. The interaction chamber is also severed at the laser pulse compressor, where the IR laser pulse coming from the multi-pass amplifier is compressed from 400 ps to about 60 fs. The IR pulse
IR
1
12
13
UV
10
ICT 2
3
4 5 6
7
8
9 16
17 15
11
18
14
Fig. 2. Layout of the experiment (not to scale). The layout is as follows: (1) laser system (for UV and IR lasers); (2) photocathode RF gun; (3) solenoid; (4) integrated current transformer (ICT); (5) laser incidence chamber; (6) beam profile monitor chamber (with multi-slits and YAG screen); (7) Q-magnets; (8) parabolic mirror with a 4-mm center hole; (9) YAG screen; (10) permanent dipole; (11) collimator; (12) IR delay stage; (13) IR pulse compressor system; (14) MCP detector; (15) CCD camera; (16) IR laser dump; (17) electron dump; and (18) vacuum valves. (The RF system and timing system are not shown here.)
energy after the compressor is 120 mJ/pulse. It is then focused to the collision point by a parabolic mirror with a 17.5-mm focusing length and head-on scattered by the electron beam. Next it is introduced to the laser dump by another parabolic mirror placed upstream of the collision point. There is a 4-mm diameter hole on the center of the parabolic mirrors to enable the electron beam and the generated soft X-ray photons to pass through. A compact permanent dipole is used to introduce the electron beam to the beam dump and separate it from the generated X-ray photons. A Pb collimator with an 8-mm-diameter hole is placed before the detector to reduce the background due to bremsstrahlung. In order to adjust the time delay between the IR pulse and electron beam, a linear delay stage is used in the IR optical path. A circular micro-channel plate (MCP) (F4655-11: Hamamatsu Photonic K.K) detector is placed at the exit of the chamber to detect the X-ray signal. The MCP has very low sensitivity for the high-energy X-ray and a fast decay time constant in the ns region. It is selected to prevent the high-energy X-ray background from the dark current and electrons lost in the beam line, such as the parabolic mirror center hole and beam dump. The quantum detection efficiency is about 10% for a 200–300-eV X-ray photon, and the calibrated gain is about 4 mV/photoelectron [13]. The distance between the collision and the detector is about 890 mm, and the diameter of the sensitive area of the detector is 14.5 mm. Only the X-ray photons that pass through the parabolic mirror center hole and the collimator and that also fall in the detector sensitive area can be detected. The collection angle is limited to
S170
Y. Du et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S168–S171
Fig. 3. Beam emittance measurement with multi-slits. The figures depict: (a) an image on the YAG screen; (b) projections of the beamlets; and (c) phase space.
Table 1 Parameters of the electron beam and laser beam. Electron beam parameters
Laser beam parameters
Charge Energy Emittance Bunch length Beam size (x) Beam size (y) Repetition
Wavelength Energy Pulse duration Beam size (x) Beam size (y) Time jitter Repletion
105 pC 3.0 MeV 6p mm mrad 1 ps 0.80 mm 0.50 mm 10 Hz
800 nm 120 mJ/pulse 60 fs o 50 mm o 50 mm 500 fs 10 Hz
8 mrad by the detector diameter in the experiment. The signal is observed on a Tek 7404B oscilloscope with a 4-G bandwidth. The electron beam energy is measured by using the steering magnets combined in the solenoid. The magnet profile is measured and integrated over a long longitudinal range that extends into the fringe field region to give a more accurate prediction of steering versus beam energy and magnet current. A linear fit of magnet currents with a variation of the beam deflections gives the beam energy. The energy of the beam is 3.0 MeV, with a 171 laser injected phase at the current RF power level of 4.2 MW. The beam emittance is measured by using the multi-slits located in the first beam diagnostic chamber. The space-charge dominated beam is split into several beamlets, which are emittance dominated by the slits, and measured by the YAG screen located in the second beam diagnostic chamber. An image of the beamlets collected by the YAG screen is shown in Fig. 3. The emittance is a function of the beam launch parameters of beam charge, pulse length, beam radius on the photocathode, etc. The result for the experimental parameters (100-pC, 3-MeV, and 171 laser injected phase) is about 5.8 mm mrad. A three-cell p-mode standing wave RF deflecting cavity is used to measure the bunch length and the timing jitter between the bunch and the RF phase [14]. The results are 1-ps and 0.54-ps for the bunch length and timing jitter, respectively. The beam profiles at the collision point are obtained with a charge-coupled device (CCD) camera that captures the image on a 300-mm-thick YAG screen located at the collision point. The laser beam size is less than 50 mm, and the electron beam size is 800 mm in the x direction and 500 mm in the y direction. The electron beam size is three times larger than that of the simulation results with PARMELA, and the reasons for this are now under investigation. The typical parameters of the electron beam and laser beam are summarized in Table 1.
Fig. 4. Typical signals obtained by using the MCP. The signals are: (a) background signal from the electron beam without the IR laser; (b) signal with the electron beam and the IR laser; (c) X-ray signal generated by subtracting (a) from (b); and (d) photodiode signal.
The spatial overlap between the electron and laser is confirmed by observing the images on the screen with the CCD. The temporal overlap is confirmed with the MCP. When the IR laser pulse with a low pulse energy (~1 mJ) is focused on the YAG screen, plasma and X-ray photons will be produced. The MCP detector will ‘‘see’’ these photons, and the signal should be overlapped with the signal on the MCP, which comes from the X-ray generated by the electron beam passing through the YAG screen. The synchronization error with this method should be less than 100 ps. The IR linear delay stage is used to adjust the arrival time of the laser beam. When the two signals are overlapped on the oscilloscope, the YAG screen is removed, the IR laser pulse energy is increased to 120 mJ/pulse, and the delay stage is adjusted to find a weak X-ray signal on the MCP. After a weak response is found, the fine adjustment of the spatial and temporal overlap between the electron and laser beams is performed by optimizing the X-ray signal level.
3. Results and discussion The characteristic parameters of the scattered X-ray (i.e., energy of the scattered photons, total number of scattered X-ray photons, and number of photons detected with the MCP) is simulated with Cain. The electron and IR laser parameters are listed in Table 1. The maximum energy is 289.4 eV, and the total
Y. Du et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S168–S171
number of scattered photons is about 7.3 103. The number of detected photons is estimated to be about 51, with the correlation existing between the scattered angle and the energy of the scattered X-ray photons, and this number corresponds to about 0.7% of the total number of scattered photons. The pulse duration, which is the same as the bunch length of the electron beam with a head-on collision, is about 1 ps. Fig. 4 shows the typical MCP signal observed on the oscilloscope: (a) background signal from the electron beam without the IR laser, (b) signal with the electron beam and the IR laser, (c) Xray signal generated by subtracting (a) from (b), and (d) photodiode signal. The oscilloscope is trigged by the laser signal from the photodiode, and 40 times average is taken to reduce the pulse-to-pulse fluctuation. Two peaks are on the background signal because the electron beam is lost at two different positions along the beam line. The first peak, which is simultaneous with the scattered X-ray signal, is caused by the electron beam lost at the hole edge of the parabolic mirror. It will obviously increase while the beam propagation direction is slightly changed by the upstream steering magnets. The second peak is caused by the electrons lost in the beam dump, which is about 13 cm away from the beam line. The delay of the two peaks, ~800 ps, corresponds to the transport time for the electron beam and bremsstrahlung X-ray photons between the dump and beam line. The total number of detected photons is estimated from the gain and quantum efficiency of the detector. The maximum intensity of the X-ray signal, optimizing with the maximum overlap of the electron beam and laser beam in spatial and temporal, is about 18 mV. In this case, it corresponds to about 45 detected photons, and the total number of generated photons is estimated to be about 6.4 103.
4. Summary At the Tsinghua Thomson scattering X-ray source, a soft X-ray pulse is generated through Thomson scattering in the head-on
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collision between the 3-MeV electron beam from a photocathode RF gun and a terawatt femtosecond laser beam. The maximum energy, pulse duration, and number of generated X-rays are estimated to be 290.4-eV, 1-ps, and 6.4 103/pulse, respectively. In order to increase the energy and flux of the scattered photons, we will soon increase the electron energy to more than 40 MeV with a 3-m SLAC-type traveling wave accelerating section and also enhance the power of the laser for scattering up to 600 mJ/pulse. Experiments to utilize this X-ray pulse in practical applications will be undertaken next.
Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC), under Grant Nos. 10735050, 10805031, 10975088, and 10875070, and by the National Basic Research Program of China (973 Program), under Grant No. 2007CB815102.
References [1] SLAC, Scientific Needs for a Future X-ray Source in the US—A White Paper, SLAC-R-910, 2008. [2] C. Rischel, et al., Nature 390 (1997) 490. [3] SLAC, LCLS Conceptual Design Report, SLAC-R-593, 2002. [4] DESY, EXFEL Technical Design Report, TESLA FEL 2006-097, 2006. [5] R.W. Schoenlein, et al., Science 287 (2000) 2237. [6] K.J. Kim, et al., Nucl. Instr. and Meth. A 341 (1994) 351. [7] A. Ting, et al., Nucl. Instr. and Meth. A 375 (1996) ABS68. [8] R.W. Schoenlein, et al., Science 274 (1996) 236. [9] C.X. Tang, et al., in: Proceedings of LINAC 2006, Knoxville, TN, USA, 2006, p. 256. [10] Y.C. Du, et al., HEP & NP 32 (2008) 75. [11] C.X. Tang, et al., Chin. Phys. C 33 (Suppl. II) (2009) 146. [12] X.Zh. He, et al., HEP & NP 28 (2004) 446. [13] G.W. Fraser, Nucl. Instr. and Meth. Phys. Res. 195 (1982) 523. [14] J.R. Shi, et al., Chin. Phys. C 33 (Suppl. II) (2009) 161.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Soft X-ray generation experiment at the Tsinghua Thomson scattering X-ray source Yingchao Du a,b,c,, Lixin Yan a,b,c, Jianfei Hua a,b,c, Qiang Du a,b,c, Renkai Li a,b,c, Jiaru Shi a,b,c, Wenhui Huang a,b,c, Huaibi Chen a,b,c, Chuanxiang Tang a,b,c a
Accelerator Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084, China Key Laboratory of Particle and Radiation Imaging, Tsinghua University, Ministry of Education, Beijing 100084, China c Key Laboratory of High Energy Radiation Imaging, Fundamental Science for National Defense, Beijing 100084, China b
a r t i c l e in f o
a b s t r a c t
Available online 10 February 2010
The development, design, and construction of the Tsinghua Thomson scattering X-ray (TTX) source have been ongoing since 2001. The TTX source is based on the Thomson scattering of a femtosecond laser pulse by a relativistic electron beam, which aims at an ultra-fast, high-flux, monochromatic, and tunable X-ray source for scientific, medical, and industrial applications. A recent experiment sought to generate a soft X-ray pulse through Thomson scattering. In this experiment, a 3-eV electron beam generated from a photocathode radio-frequency gun is focused by quadrupole magnets to several hundred microns and collided with a 120-mJ, 60-fs laser beam. The generated X-ray is detected by a micro-channel plate. The energy, pulse duration, and number of X-ray photons is estimated to be 290.4 eV, 1 ps, and 6.4 103, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: Thomson scattering X-ray source Photocathode RF gun Laser X-ray
1. Introduction For the development of the next generation of X-ray light sources, an important motivation is to achieve picosecond and sub-ps pulses of hard X-rays with a temporal resolution of several hundreds femtoseconds (fs), which is the fundamental time scale of atomic motions [1]. This type of X-ray source would be valuable for dynamic studies of a variety of physical, chemical, and biological processes. The development of femtosecond lasers has made it possible to study short time reactions in the femtosecond region. However, it is also limited by the optical probe due to the low photon energy for studying extended electronic levels in solids. The femtosecond time-scale hard X-ray is an important probe for studying the structure of solids through their interaction with core electronic levels in atoms. It can be used to observe the structural dynamic on the fundamental time scale. Such an X-ray source would be useful for industrial and medical imaging applications. Several concepts or methods have been proposed and demonstrated to generate a suitable X-ray pulse. These include the Ka Xray source with an intense laser [2], the X-ray free-electron laser (FEL) [3,4], synchrotron radiation with a slicing technique [5],
Corresponding author at: Accelerator Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084, China. Tel.: + 86 10 62795424. E-mail address: [email protected] (Y. Du).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.049
Thomson scattering of a femtosecond laser pulse by relativistic electron beams [6–8], and so forth. Among these sources, the X-ray source based on Thomson scattering (or inverse Compton scattering) between the laser photons and the relativistic electrons may lead to the novel femtosecond time light source facility. It can generate an ultra-short, high-flux monochromatic and tunable hard X-ray pulse with the short pulse laser, radiofrequency (RF) linac, and photocathode RF gun. It also provides a means of performing hard X-ray pump–probe experiments on a sub-picosecond time scale with a flux suitable for single-shot measurements, which enables experiments on samples undergoing irreversible damage such as shocks, plasma ablation, or ultra-fast melting. And, compared with the X-ray FEL facility and the synchrotron light source, it is more compact and affordable and can generate more hard X-rays. Enticed by these prospects, some laboratories have proposed and constructed such an X-ray source. The Accelerator Laboratory of Tsinghua University also proposed the Tsinghua Thomson scattering X-ray source (TTX) and began to study the concept in 2001 [9–12]. The scheme of the X-ray source, as shown in Fig. 1, consists of a linac system and a femtosecond terawatt laser system. The laser system generates both the 266-nm ultraviolet (UV) pulse for the photocathode and the 800-nm infrared (IR) pulse for the scattering interaction. The two pulses are derived from one 79.3-MHz Ti:sapphire oscillator in order to reduce the time jitter between the electron beam and the IR pulse. The linac system consists of a BNL/KEK/SHI-type 1.6-cell S-band photocathode RF gun, a 3-m
Y. Du et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S168–S171
IR
1
12
13
UV
18 10
2
3
4
5
6
7
S169
8
9
11 14
17 15
16 Fig. 1. Scheme of the TTX.
S-band SLAC-type traveling wave accelerating section, a four-dipole magnet compressor to compress the electron bunch to below 1 ps, and two X-band harmonic structures, thus enabling flexible manipulation of the phase space of the electron pulses and generating the 40–50-MeV ultra-short high-brightness electron pulse for the scattering interaction. The laser system is synchronized with the RF system through a timing circuit, with a timing jitter no greater than 0.5 ps. The electron and IR laser pulses collide in the interaction chamber with geometries ranging from 901 to 1801, and they generate a 20–50-keV X-ray pulse with fluxes from 106 to 108 photons/pulse and pulse durations from 200 fs to 1 ps. This source will serve as a tunable monochromatic X-ray source for advanced applications in science, industry, and medicine. Some main subsystems, such as the high-power system, the ultra-fast laser system with a 3-TW IR laser pulse, and the photocathode RF gun system, have been constructed and commissioned. The low-energy beam of 2–4 MeV is ready for experiments related to MeV ultra-fast electron diffraction (UED), bunch length measurement with a deflecting cavity, soft X-ray generation via Thomson scattering, THz generation, and so forth. The setup, results, and discussion of an experiment for soft X-ray generation via Thomson scattering are presented in this paper. In the experiment, the 3-MeV, 1-ps, 100-pC, normalized emittance en =6 mm mrad electron beam from the photocathode RF gun and the 800-nm, 60-fs, 120-mJ, linearly polarized laser are head-on collided.
2. Description of the experiment The schematic diagram of the experimental setup is shown in Fig. 2. The laser system generates both UV and IR laser pulses. These two laser pulses are from the same laser oscillator so that they are originally synchronized. The UV laser pulse irradiates on the cathode nearly perpendicularly, and it generates an electron pulse with a corresponding spatial distribution. The beam charge is influenced by the UV pulse energy, the electric field on the cathode, and the laser injected phase. In the experiment, the beam charge is about 105 pC/pulse, with an 30-mJ UV pulse energy and a 171 laser injected phase. This beam is accelerated by a high-amplitude electric field in the photocathode RF gun, and it reaches 2–4 MeV at the gun exit. A solenoid is used for emittance compensation and beam envelope control. Two beam diagnostic chambers with multi-silts and YAG screens are followed with the laser incident chamber and used for beam profile and emittance measurements. A triplet is used to focus the beam into the interaction chamber with a beam size of about several hundreds microns at the interaction region. The interaction chamber is also severed at the laser pulse compressor, where the IR laser pulse coming from the multi-pass amplifier is compressed from 400 ps to about 60 fs. The IR pulse
IR
1
12
13
UV
10
ICT 2
3
4 5 6
7
8
9 16
17 15
11
18
14
Fig. 2. Layout of the experiment (not to scale). The layout is as follows: (1) laser system (for UV and IR lasers); (2) photocathode RF gun; (3) solenoid; (4) integrated current transformer (ICT); (5) laser incidence chamber; (6) beam profile monitor chamber (with multi-slits and YAG screen); (7) Q-magnets; (8) parabolic mirror with a 4-mm center hole; (9) YAG screen; (10) permanent dipole; (11) collimator; (12) IR delay stage; (13) IR pulse compressor system; (14) MCP detector; (15) CCD camera; (16) IR laser dump; (17) electron dump; and (18) vacuum valves. (The RF system and timing system are not shown here.)
energy after the compressor is 120 mJ/pulse. It is then focused to the collision point by a parabolic mirror with a 17.5-mm focusing length and head-on scattered by the electron beam. Next it is introduced to the laser dump by another parabolic mirror placed upstream of the collision point. There is a 4-mm diameter hole on the center of the parabolic mirrors to enable the electron beam and the generated soft X-ray photons to pass through. A compact permanent dipole is used to introduce the electron beam to the beam dump and separate it from the generated X-ray photons. A Pb collimator with an 8-mm-diameter hole is placed before the detector to reduce the background due to bremsstrahlung. In order to adjust the time delay between the IR pulse and electron beam, a linear delay stage is used in the IR optical path. A circular micro-channel plate (MCP) (F4655-11: Hamamatsu Photonic K.K) detector is placed at the exit of the chamber to detect the X-ray signal. The MCP has very low sensitivity for the high-energy X-ray and a fast decay time constant in the ns region. It is selected to prevent the high-energy X-ray background from the dark current and electrons lost in the beam line, such as the parabolic mirror center hole and beam dump. The quantum detection efficiency is about 10% for a 200–300-eV X-ray photon, and the calibrated gain is about 4 mV/photoelectron [13]. The distance between the collision and the detector is about 890 mm, and the diameter of the sensitive area of the detector is 14.5 mm. Only the X-ray photons that pass through the parabolic mirror center hole and the collimator and that also fall in the detector sensitive area can be detected. The collection angle is limited to
S170
Y. Du et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S168–S171
Fig. 3. Beam emittance measurement with multi-slits. The figures depict: (a) an image on the YAG screen; (b) projections of the beamlets; and (c) phase space.
Table 1 Parameters of the electron beam and laser beam. Electron beam parameters
Laser beam parameters
Charge Energy Emittance Bunch length Beam size (x) Beam size (y) Repetition
Wavelength Energy Pulse duration Beam size (x) Beam size (y) Time jitter Repletion
105 pC 3.0 MeV 6p mm mrad 1 ps 0.80 mm 0.50 mm 10 Hz
800 nm 120 mJ/pulse 60 fs o 50 mm o 50 mm 500 fs 10 Hz
8 mrad by the detector diameter in the experiment. The signal is observed on a Tek 7404B oscilloscope with a 4-G bandwidth. The electron beam energy is measured by using the steering magnets combined in the solenoid. The magnet profile is measured and integrated over a long longitudinal range that extends into the fringe field region to give a more accurate prediction of steering versus beam energy and magnet current. A linear fit of magnet currents with a variation of the beam deflections gives the beam energy. The energy of the beam is 3.0 MeV, with a 171 laser injected phase at the current RF power level of 4.2 MW. The beam emittance is measured by using the multi-slits located in the first beam diagnostic chamber. The space-charge dominated beam is split into several beamlets, which are emittance dominated by the slits, and measured by the YAG screen located in the second beam diagnostic chamber. An image of the beamlets collected by the YAG screen is shown in Fig. 3. The emittance is a function of the beam launch parameters of beam charge, pulse length, beam radius on the photocathode, etc. The result for the experimental parameters (100-pC, 3-MeV, and 171 laser injected phase) is about 5.8 mm mrad. A three-cell p-mode standing wave RF deflecting cavity is used to measure the bunch length and the timing jitter between the bunch and the RF phase [14]. The results are 1-ps and 0.54-ps for the bunch length and timing jitter, respectively. The beam profiles at the collision point are obtained with a charge-coupled device (CCD) camera that captures the image on a 300-mm-thick YAG screen located at the collision point. The laser beam size is less than 50 mm, and the electron beam size is 800 mm in the x direction and 500 mm in the y direction. The electron beam size is three times larger than that of the simulation results with PARMELA, and the reasons for this are now under investigation. The typical parameters of the electron beam and laser beam are summarized in Table 1.
Fig. 4. Typical signals obtained by using the MCP. The signals are: (a) background signal from the electron beam without the IR laser; (b) signal with the electron beam and the IR laser; (c) X-ray signal generated by subtracting (a) from (b); and (d) photodiode signal.
The spatial overlap between the electron and laser is confirmed by observing the images on the screen with the CCD. The temporal overlap is confirmed with the MCP. When the IR laser pulse with a low pulse energy (~1 mJ) is focused on the YAG screen, plasma and X-ray photons will be produced. The MCP detector will ‘‘see’’ these photons, and the signal should be overlapped with the signal on the MCP, which comes from the X-ray generated by the electron beam passing through the YAG screen. The synchronization error with this method should be less than 100 ps. The IR linear delay stage is used to adjust the arrival time of the laser beam. When the two signals are overlapped on the oscilloscope, the YAG screen is removed, the IR laser pulse energy is increased to 120 mJ/pulse, and the delay stage is adjusted to find a weak X-ray signal on the MCP. After a weak response is found, the fine adjustment of the spatial and temporal overlap between the electron and laser beams is performed by optimizing the X-ray signal level.
3. Results and discussion The characteristic parameters of the scattered X-ray (i.e., energy of the scattered photons, total number of scattered X-ray photons, and number of photons detected with the MCP) is simulated with Cain. The electron and IR laser parameters are listed in Table 1. The maximum energy is 289.4 eV, and the total
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number of scattered photons is about 7.3 103. The number of detected photons is estimated to be about 51, with the correlation existing between the scattered angle and the energy of the scattered X-ray photons, and this number corresponds to about 0.7% of the total number of scattered photons. The pulse duration, which is the same as the bunch length of the electron beam with a head-on collision, is about 1 ps. Fig. 4 shows the typical MCP signal observed on the oscilloscope: (a) background signal from the electron beam without the IR laser, (b) signal with the electron beam and the IR laser, (c) Xray signal generated by subtracting (a) from (b), and (d) photodiode signal. The oscilloscope is trigged by the laser signal from the photodiode, and 40 times average is taken to reduce the pulse-to-pulse fluctuation. Two peaks are on the background signal because the electron beam is lost at two different positions along the beam line. The first peak, which is simultaneous with the scattered X-ray signal, is caused by the electron beam lost at the hole edge of the parabolic mirror. It will obviously increase while the beam propagation direction is slightly changed by the upstream steering magnets. The second peak is caused by the electrons lost in the beam dump, which is about 13 cm away from the beam line. The delay of the two peaks, ~800 ps, corresponds to the transport time for the electron beam and bremsstrahlung X-ray photons between the dump and beam line. The total number of detected photons is estimated from the gain and quantum efficiency of the detector. The maximum intensity of the X-ray signal, optimizing with the maximum overlap of the electron beam and laser beam in spatial and temporal, is about 18 mV. In this case, it corresponds to about 45 detected photons, and the total number of generated photons is estimated to be about 6.4 103.
4. Summary At the Tsinghua Thomson scattering X-ray source, a soft X-ray pulse is generated through Thomson scattering in the head-on
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collision between the 3-MeV electron beam from a photocathode RF gun and a terawatt femtosecond laser beam. The maximum energy, pulse duration, and number of generated X-rays are estimated to be 290.4-eV, 1-ps, and 6.4 103/pulse, respectively. In order to increase the energy and flux of the scattered photons, we will soon increase the electron energy to more than 40 MeV with a 3-m SLAC-type traveling wave accelerating section and also enhance the power of the laser for scattering up to 600 mJ/pulse. Experiments to utilize this X-ray pulse in practical applications will be undertaken next.
Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC), under Grant Nos. 10735050, 10805031, 10975088, and 10875070, and by the National Basic Research Program of China (973 Program), under Grant No. 2007CB815102.
References [1] SLAC, Scientific Needs for a Future X-ray Source in the US—A White Paper, SLAC-R-910, 2008. [2] C. Rischel, et al., Nature 390 (1997) 490. [3] SLAC, LCLS Conceptual Design Report, SLAC-R-593, 2002. [4] DESY, EXFEL Technical Design Report, TESLA FEL 2006-097, 2006. [5] R.W. Schoenlein, et al., Science 287 (2000) 2237. [6] K.J. Kim, et al., Nucl. Instr. and Meth. A 341 (1994) 351. [7] A. Ting, et al., Nucl. Instr. and Meth. A 375 (1996) ABS68. [8] R.W. Schoenlein, et al., Science 274 (1996) 236. [9] C.X. Tang, et al., in: Proceedings of LINAC 2006, Knoxville, TN, USA, 2006, p. 256. [10] Y.C. Du, et al., HEP & NP 32 (2008) 75. [11] C.X. Tang, et al., Chin. Phys. C 33 (Suppl. II) (2009) 146. [12] X.Zh. He, et al., HEP & NP 28 (2004) 446. [13] G.W. Fraser, Nucl. Instr. and Meth. Phys. Res. 195 (1982) 523. [14] J.R. Shi, et al., Chin. Phys. C 33 (Suppl. II) (2009) 161.