- Email: [email protected]
Performance of the 2 × 4-cell superconducting linac module for the THz-FEL facility
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Performance of the 2 × 4-cell superconducting linac module for the THz-FEL facility ✩ Zhou Kui a , Lao Chenglong a , Wu Dai a , Luo Xing a, *, Wang Jianxin a , Xiao Dexin a , Shan Lijun a , He Tianhui a , Shen Xuming a , Lin Sifen a , Yang Linde a , Wang Hanbin a , Yang Xingfan a , Li Ming a , Lu Xiangyang b a b
Institution of Applied Electronics, Chinese Academy of Engineering Physics, Mianyang 621900, China State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
ARTICLE
INFO
Keywords: Superconducting linac module Cryomodule test Field gradient
ABSTRACT A high average power THz radiation facility has been developed by the China Academy of Engineering Physics. It is the first CW THz user facility based on superconducting accelerator technology in China. The superconducting linac module, which contains two 4-cell 1.3 GHz TESLA-like superconducting radio frequency cavities, is a major component of this facility. The expected electron energy gain is 6–8 MeV with a field gradient of 8–10 MV/m. The design and fabrication of the linac module is complete. This paper discusses its assembly and results from cyromodule tests and beam commissioning. At 2 K, the cryomodule works smoothly and stably. Both cavities have achieved effective field gradients of 10 MV/m. In beam loading experiments, 8 MeV, 5 mA electron beams with an energy spread less than 0.2% have been produced, which satisfies our requirements.
1. Introduction
2. Processing and preparation
The China Academy of Engineering Physics (CAEP) is currently developing a THz radiation facility (THz-FEL). This is the first high average power THz user facility based on superconducting accelerator technology in China [1]. The THz-FEL facility includes a high-brilliance electron gun [2,3], a superconducting linac module [4] and a highperformance undulator [5] as shown in Fig. 1. The goal is to produce 1–3 THz radiation with an average output power of greater than 10 W. The effective cavity gradient is 8–10 MV/m and the electron energy after acceleration is 6–8 MeV. Table 1 lists some of the design parameters of the 2 × 4-cell superconducting linac module [4]. The superconducting linac module is a critical component for this facility. As shown in Fig. 2, it contains the following subsystems: a cryostat, two 4-cell TESLA-like SRF cavities, each with a tuner and power coupler, and auxiliary systems including RF sources, cryogenic system and low level RF control system. The design and fabrication of these subsystems have been completed [4]. This paper discusses the assembly of the cryomodule and results from cryomodule tests and beam commissioning.
The operation of the superconducting accelerator has challenging requirements: very low contamination, low vacuum, cryogenic temperatures and low magnetic fields. Thus, care must be taken in the preparation and assembly of its components to achieve good performance. The first step was the conditioning of the power couplers, which was done in Chengdu. The conditioning reduces the outgassing on the inner surfaces. Fig. 3 shows the setup used to condition the power couplers. Two power couplers are connected in series with a test box so both couplers could be processed at the same time. The microwave power was provided by an RF amplifier (an IOT), and was increased in small increments during the conditioning. The vacuum and arc state in the couplers were monitored as well as the temperatures of the coupler surfaces. If one of the monitored values exceeded the safety threshold range, a protective interlock turned off the RF. After the monitored values returned to normal, the microwave power was increased gradually again. Fig. 4 shows a photo of the conditioning setup. Since the inner surfaces of the power couplers were not as clean
✩ Work supported by China National Key Scientific Instrument and Equipment Development Project (2011YQ130018), National Natural Science Foundation of China with grant (11475159, 11505173, 11505174, 11575264 and 11605190). * Corresponding author. E-mail address: [email protected] (L. Xing).
https://doi.org/10.1016/j.nima.2018.03.054 Received 17 August 2017; Received in revised form 15 March 2018; Accepted 15 March 2018 Available online 23 March 2018 0168-9002/© 2018 Elsevier B.V. All rights reserved.
Z. Kui et al.
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 1. General layout of the CAEP THz-FEL facility. Table 1 Design parameters of the 2 × 4-cell superconducting linac module. Parameters
Designed value
Frequency Repetition rate Beam current (𝐼𝑏 ) Unloaded quality factor (𝑄0 ) External quality factor (𝑄𝑒 ) Effective field gradient (𝐸𝑎𝑐𝑐 ) Energy gain 2 K heat loss Magnetic field @ central axis
1.300 GHz 54.17 MHz 1–5 mA ≥5 × 109 8 × 105 –5 × 106 8–10 MV/m 6–8 MeV ≤20 W ≤20 mG
ultrapure water. The assembly of the cavity string was completed in a 100-class clean room as shown in Fig. 5. The cavity string was next shipped to our lab in Chengdu after vacuum leak checks and back-filling with nitrogen. The final step was the assembly in the local experimental hall. Fig. 6 shows the cryomodule (in yellow) after assembly with the cryogenic and microwave systems attached.
Fig. 2. The cross-sectional view of the superconducting linac module [4].
as expected, it took about 2 months to condition the couplers to 20 kW with full reflection in pulsed mode and 30 kW in CW traveling wave mode. Since a 100-class clean room was not available at our laboratory, we did the cavity string assembly at Peking University after the conditioning of the power couplers. The SRF cavities with the cold coupler sections attached were sent back-filled with nitrogen to Peking University. Prior to this, buffered chemistry polishing (BCP) and high-pressure rinsing (HPR) of the 4-cell cavities had been done. Both cavities had been lightly polished, removing a 10 um layer, and rinsed for 4 h with high-pressure
3. Cryomodule test After the assembly of the cryomodule, a test platform was set up. Each 4-cell cavity had its own RF source and control system. As shown in Fig. 7, the signal from a weakly-coupled pickup antenna at the end of each cavity was sent to a low-level RF (LLRF) control system. The amplitude and phase of the cavity field could be regulated by the LLRF control system, which generated a low power output signal to drive the IOT. High power RF from the IOT was transmitted through a waveguide, directional coupler and power coupler into the cavity. To
Fig. 3. Diagram illustrating the features of the coupler conditioning setup. 30
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Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 4. The coupler conditioning setup.
Fig. 5. The cavity string of the linac module after assembly.
protect the cavities and couplers, vacuum, temperature and arc signals were monitored by an interlock system. If anything abnormal occurred, a protective switch would zero the drive signal immediately. In addition, the forward, reflected and pickup signals were measured by a power meter and oscilloscope simultaneously in the control room. It took about 10 days to cool down the cavities from room temperature to 2 K. To prevent deformation of the inner mechanical structure, precooling with liquid nitrogen was done slowly, and it took about a week for the thermal shields to reach 80 K. However, the temperature of the cavity string was still about 200 K. The cooldown with liquid helium to 4 K processed much faster, within about one day to avoid hydrogen poisoning [6]. Fig. 8 shows the resulting temperatures within the cryomodule in its 2 K state. The cryomodule has an 80 K shield outside of the 2 K cavity He vessels [4]. The temperatures are detected via silicon diode temperature sensors, which have an accuracy of ± 0.2 K from 1.5 K to 30 K and ±
Fig. 6. The superconducting cryomodule after assembly.
0.4 K from 30 K to 100 K. The lowest temperature measured was at the bottom of the liquid helium vessel. Since the cryogenic temperature sensor is attached to the outside of the vessel, not directly immersed in the superfluid helium, the measured temperature is somewhat higher than 2 K. During the cool down process, the vacuum levels in the cryostat, cavities and couplers were recorded, as shown in Fig. 9. The vacuum of the cryostat was measured by a compact full-range gauge (measurement uncertainty of ± 30%) at the bottom of the cryostat, while the vacuum in the cavities and couplers were monitored by ionization gauges 31
Z. Kui et al.
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 7. Block diagram of the RF power and control system for each cavity.
Fig. 8. The temperature distribution of the cryomodule with the cavities at 2 K. Table 2 Resonant frequencies at different temperatures. Temperature
𝑓𝑐𝑎𝑣𝑖𝑡𝑦#1 /MHz
𝑓𝑐𝑎𝑣𝑖𝑡𝑦#2 /MHz
300 K 4K 2K
1297.894 1300.025 1299.943
1297.959 1300.089 1300.013
thermal contraction, leading to resonant frequency increases of 2.131 MHz and 2.130 MHz for Cavity 1 and 2, respectively. In comparison, the cavity frequencies changed by about 1.8 MHz from 300 K to 80 K [7]. From 4 K to 2 K, the resonant frequencies decreased 0.082 MHz and 0.076 MHz, respectively. This change was caused by the reduced pressure in the helium vessel, from about 1065 mbar at 4 K to about 29 mbar at 2 K. Because the pre-tuning at room temperature was done accurately, the resonant frequencies at 2 K were very close to 1.300 GHz for both cavities. There are two pressure sensors (nonlinearity within ± 1%) installed symmetrically on each tuner to monitor the tuning force acting on the cavity flanges. More details of the tuner measurements are presented in Ref. [7]. At 4 K, the pre-tuning forces acting on Cavity 1 and 2 are 3.93 kN and 4.40 kN, respectively. We changed the tuner force by operating the associated stepper motor and measured the resulting change in the cavity resonant frequency using a network analyzer (measurement
Fig. 9. The vacuum variations of the cryomodule during the cooling down process.
(measurement uncertainty of ± 7%) along the vacuum pipe. As expected, the decrease in temperature lowered the vacuum pressure levels. The resonant frequencies of both cavities were also recorded during cooldown (see Table 2). From 300 K to 4 K, the cavities shrank due to 32
Z. Kui et al.
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 12. Forward (blue for Cavity 1 and black for Cavity 2), reflected (cyan for Cavity 1 and red for Cavity 2) and pickup (dark yellow for Cavity 1 and purple for Cavity 2) powers measured by a power meter in pulsed mode.
Fig. 10. Cavity frequency at 4 K versus tuner force.
precision of <1 kHz, which depends on the measurement bandwidth). The results in Fig. 10 show that the tuning range for both cavities is larger than 200 kHz, which is enough to adjust the resonant frequency to 1.300 GHz. There exists some hysteresis in the force exerted by the motor tuner for Cavity 2. When the stepper-motor reverses direction, the force acting on this cavity is reduced by up to 25% due to play in the support system. Better tightening of the support components would have reduced this. The relationship between the effective field gradient, 𝐸𝑎𝑐𝑐 , and the voltage of the cavity pickup signal, 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 , was measured at 4 K. The LLRF system was operated in self-exited loop (SEL) mode to establish the cavity field, which means the drive frequency was always matched to the cavity resonant frequency. The coupling coefficient of the pickup is fixed, so the pickup power, 𝑃𝑝𝑖𝑐𝑘𝑢𝑝 , is proportional to the dissipated power in the cavity, which is proportional to 𝐸𝑎𝑐𝑐 squared. Thus, 𝐸𝑎𝑐𝑐 is proportional to 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 , which can be expressed as, 𝐸𝑎𝑐𝑐 = 𝛼 ⋅ 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 . The purpose of the calibration was to compute the coefficient 𝛼 for both cavities so that 𝐸𝑎𝑐𝑐 can be measured directly from the pickup signals at 2 K. The quantity 𝐸𝑎𝑐𝑐 is independently computed from the following equation, √ 4 × 𝑃𝑓 × 𝑄𝑒 × 𝑅∕𝑄0 𝐸𝑎𝑐𝑐 = (1) 𝐿𝑐𝑎𝑣
quality factor of the power coupler, which is derived from the decay time of the pickup signal as measured on an oscilloscope. 𝑅∕𝑄0 for both cavities is 444 Ω and the effective cavity length, 𝐿𝑐𝑎𝑣 , is 484 mm [4]. 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 is calculated from the pickup power measured by a power meter. At 4 K, we varied 𝑃𝑓 and measured 𝑃𝑝𝑖𝑐𝑘𝑢𝑝 . From these data, we computed 𝐸𝑎𝑐𝑐 and 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 and then did a linear fit to obtain the coefficients 0.0545 and 0.0827, as shown in Fig. 11(a). At 2 K, the superconducting linac module worked smoothly and stably. The effective field gradients of both cavities reached 10 MV/m, as shown in Fig. 11(b), which satisfies our design goal. 4. Beam commissioning With the cryomodule performance established, beam loading experiments were performed using the electron gun and beam diagnostic system. The beam loading effect was observed successfully in pulsed mode, as shown in Fig. 12. The repetition rate and macro pulse length of the electron beam were 1 Hz and 1 ms, respectively, and the beam current was about 4.5 mA. The fluctuations at the top of the forward and reflected pulses indicate the compensation process of the lowlevel RF system. The electron beams take away microwave energy when traveling through the cavity, potentially leading to a decrease in the stored energy and the field gradient. To compensate, the lowlevel RF system increases the forward power to maintain a constant gradient. With the beam loading, the cavity match to the forward power
where 𝑃𝑓 is the forward power measured from the directional coupler near the power coupler using a power meter, and 𝑄𝑒 is the external
Fig. 11. (a) 𝐸𝑎𝑐𝑐 versus 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 measurements for both cavities at 4 K together with linear fit results, (b) 𝐸𝑎𝑐𝑐 computed from the pickup signal at 2 K. 33
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Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 13. (a) Color-enhanced beam spot on a YaG screen after the spectrometer magnet and (b) the corresponding beam energy distribution.
Acknowledgments
improves, decreasing the reflected power. From the differences between the forward power and the reflected power, the powers flowing into Cavity 1 and 2 are estimated to be 14.6 kW and 20.4 kW, respectively. So, the power transfer to the beam is almost 35 kW. The energy and energy spread of the electron beam was measured from the beam image generated on a YAG screen after the spectrometer magnet, which has a nominal 300 mm radius of curvature. Fig. 13(a) shows an example of the image for a 400 ns long 5 mA beam and Fig. 13(b) shows the corresponding transverse energy distribution. The beam energy was about 8.2 MeV and the rms energy spread was 0.19%, which satisfies our requirements [1].
The authors would like to thank Peking University for the help with cryomodule assembly and the Institute of High Energy Physics Chinese Academy of Sciences (IHEP) for their help with the coupler conditioning. References [1] Xu Zhou, et al., Design of a high average power terahertz-FEL facility, J. Terahertz Sci. Electron. Inf. Technol. 11 (1) (2013) 1–6. [2] H. Wang, M. Li, X. Yang, et al., DC high voltage photoemission electron gun for CAEP FEL, in: FEL 2011 - 33th International Free Electron Laser Conference, 2011, pp. 598– 600. [3] H. Wang, K. Li, M. Li, et al., A GaAs photoemission DC gun for CAEP high-average power THz FEL, in: FEL 2014 - 36th International Free Electron Laser Conference, 2014, pp. 318–321. [4] Luo Xing, et al., Design and fabrication of the 2×4-cell superconducting linac module for the free-electron laser, Nucl. Instrum. Methods Phys. Res. A 871 (2017) 30–34. http://dx.doi.org/10.1016/j.nima.2017.06.058. [5] L. Yang, D. Deng, et al., Physics design of undulator in high average power free electronic laser terahertz source, High Power Laser Part. Beams 25 (2013) 153–157. [6] J. Knobloch, The Q disease in superconducting niobium RF cavities, AIP Conf. Proc. 671 (2003) 133–150. http://dx.doi.org/10.1063/1.1597364. [7] Mi Zhenghui, et al., Design and test of frequency tuner for a CAEP high power THz free-electron laser, Chin. Phys. C 39 (2) (2015) 95–100.
5. Conclusion A 2 × 4-cell superconducting linac module for the THz-FEL facility has been assembled, cooled to 2 K and operated successfully, meeting the beam gradient design goal (up to 10 MV/m). In pulsed-mode beam commissioning experiments, 5 mA, 8.2 MeV beams with <0.2% rms energy spreads were generated. Further beam loading experiments are in progress.
34
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Performance of the 2 × 4-cell superconducting linac module for the THz-FEL facility ✩ Zhou Kui a , Lao Chenglong a , Wu Dai a , Luo Xing a, *, Wang Jianxin a , Xiao Dexin a , Shan Lijun a , He Tianhui a , Shen Xuming a , Lin Sifen a , Yang Linde a , Wang Hanbin a , Yang Xingfan a , Li Ming a , Lu Xiangyang b a b
Institution of Applied Electronics, Chinese Academy of Engineering Physics, Mianyang 621900, China State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
ARTICLE
INFO
Keywords: Superconducting linac module Cryomodule test Field gradient
ABSTRACT A high average power THz radiation facility has been developed by the China Academy of Engineering Physics. It is the first CW THz user facility based on superconducting accelerator technology in China. The superconducting linac module, which contains two 4-cell 1.3 GHz TESLA-like superconducting radio frequency cavities, is a major component of this facility. The expected electron energy gain is 6–8 MeV with a field gradient of 8–10 MV/m. The design and fabrication of the linac module is complete. This paper discusses its assembly and results from cyromodule tests and beam commissioning. At 2 K, the cryomodule works smoothly and stably. Both cavities have achieved effective field gradients of 10 MV/m. In beam loading experiments, 8 MeV, 5 mA electron beams with an energy spread less than 0.2% have been produced, which satisfies our requirements.
1. Introduction
2. Processing and preparation
The China Academy of Engineering Physics (CAEP) is currently developing a THz radiation facility (THz-FEL). This is the first high average power THz user facility based on superconducting accelerator technology in China [1]. The THz-FEL facility includes a high-brilliance electron gun [2,3], a superconducting linac module [4] and a highperformance undulator [5] as shown in Fig. 1. The goal is to produce 1–3 THz radiation with an average output power of greater than 10 W. The effective cavity gradient is 8–10 MV/m and the electron energy after acceleration is 6–8 MeV. Table 1 lists some of the design parameters of the 2 × 4-cell superconducting linac module [4]. The superconducting linac module is a critical component for this facility. As shown in Fig. 2, it contains the following subsystems: a cryostat, two 4-cell TESLA-like SRF cavities, each with a tuner and power coupler, and auxiliary systems including RF sources, cryogenic system and low level RF control system. The design and fabrication of these subsystems have been completed [4]. This paper discusses the assembly of the cryomodule and results from cryomodule tests and beam commissioning.
The operation of the superconducting accelerator has challenging requirements: very low contamination, low vacuum, cryogenic temperatures and low magnetic fields. Thus, care must be taken in the preparation and assembly of its components to achieve good performance. The first step was the conditioning of the power couplers, which was done in Chengdu. The conditioning reduces the outgassing on the inner surfaces. Fig. 3 shows the setup used to condition the power couplers. Two power couplers are connected in series with a test box so both couplers could be processed at the same time. The microwave power was provided by an RF amplifier (an IOT), and was increased in small increments during the conditioning. The vacuum and arc state in the couplers were monitored as well as the temperatures of the coupler surfaces. If one of the monitored values exceeded the safety threshold range, a protective interlock turned off the RF. After the monitored values returned to normal, the microwave power was increased gradually again. Fig. 4 shows a photo of the conditioning setup. Since the inner surfaces of the power couplers were not as clean
✩ Work supported by China National Key Scientific Instrument and Equipment Development Project (2011YQ130018), National Natural Science Foundation of China with grant (11475159, 11505173, 11505174, 11575264 and 11605190). * Corresponding author. E-mail address: [email protected] (L. Xing).
https://doi.org/10.1016/j.nima.2018.03.054 Received 17 August 2017; Received in revised form 15 March 2018; Accepted 15 March 2018 Available online 23 March 2018 0168-9002/© 2018 Elsevier B.V. All rights reserved.
Z. Kui et al.
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 1. General layout of the CAEP THz-FEL facility. Table 1 Design parameters of the 2 × 4-cell superconducting linac module. Parameters
Designed value
Frequency Repetition rate Beam current (𝐼𝑏 ) Unloaded quality factor (𝑄0 ) External quality factor (𝑄𝑒 ) Effective field gradient (𝐸𝑎𝑐𝑐 ) Energy gain 2 K heat loss Magnetic field @ central axis
1.300 GHz 54.17 MHz 1–5 mA ≥5 × 109 8 × 105 –5 × 106 8–10 MV/m 6–8 MeV ≤20 W ≤20 mG
ultrapure water. The assembly of the cavity string was completed in a 100-class clean room as shown in Fig. 5. The cavity string was next shipped to our lab in Chengdu after vacuum leak checks and back-filling with nitrogen. The final step was the assembly in the local experimental hall. Fig. 6 shows the cryomodule (in yellow) after assembly with the cryogenic and microwave systems attached.
Fig. 2. The cross-sectional view of the superconducting linac module [4].
as expected, it took about 2 months to condition the couplers to 20 kW with full reflection in pulsed mode and 30 kW in CW traveling wave mode. Since a 100-class clean room was not available at our laboratory, we did the cavity string assembly at Peking University after the conditioning of the power couplers. The SRF cavities with the cold coupler sections attached were sent back-filled with nitrogen to Peking University. Prior to this, buffered chemistry polishing (BCP) and high-pressure rinsing (HPR) of the 4-cell cavities had been done. Both cavities had been lightly polished, removing a 10 um layer, and rinsed for 4 h with high-pressure
3. Cryomodule test After the assembly of the cryomodule, a test platform was set up. Each 4-cell cavity had its own RF source and control system. As shown in Fig. 7, the signal from a weakly-coupled pickup antenna at the end of each cavity was sent to a low-level RF (LLRF) control system. The amplitude and phase of the cavity field could be regulated by the LLRF control system, which generated a low power output signal to drive the IOT. High power RF from the IOT was transmitted through a waveguide, directional coupler and power coupler into the cavity. To
Fig. 3. Diagram illustrating the features of the coupler conditioning setup. 30
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Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 4. The coupler conditioning setup.
Fig. 5. The cavity string of the linac module after assembly.
protect the cavities and couplers, vacuum, temperature and arc signals were monitored by an interlock system. If anything abnormal occurred, a protective switch would zero the drive signal immediately. In addition, the forward, reflected and pickup signals were measured by a power meter and oscilloscope simultaneously in the control room. It took about 10 days to cool down the cavities from room temperature to 2 K. To prevent deformation of the inner mechanical structure, precooling with liquid nitrogen was done slowly, and it took about a week for the thermal shields to reach 80 K. However, the temperature of the cavity string was still about 200 K. The cooldown with liquid helium to 4 K processed much faster, within about one day to avoid hydrogen poisoning [6]. Fig. 8 shows the resulting temperatures within the cryomodule in its 2 K state. The cryomodule has an 80 K shield outside of the 2 K cavity He vessels [4]. The temperatures are detected via silicon diode temperature sensors, which have an accuracy of ± 0.2 K from 1.5 K to 30 K and ±
Fig. 6. The superconducting cryomodule after assembly.
0.4 K from 30 K to 100 K. The lowest temperature measured was at the bottom of the liquid helium vessel. Since the cryogenic temperature sensor is attached to the outside of the vessel, not directly immersed in the superfluid helium, the measured temperature is somewhat higher than 2 K. During the cool down process, the vacuum levels in the cryostat, cavities and couplers were recorded, as shown in Fig. 9. The vacuum of the cryostat was measured by a compact full-range gauge (measurement uncertainty of ± 30%) at the bottom of the cryostat, while the vacuum in the cavities and couplers were monitored by ionization gauges 31
Z. Kui et al.
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 7. Block diagram of the RF power and control system for each cavity.
Fig. 8. The temperature distribution of the cryomodule with the cavities at 2 K. Table 2 Resonant frequencies at different temperatures. Temperature
𝑓𝑐𝑎𝑣𝑖𝑡𝑦#1 /MHz
𝑓𝑐𝑎𝑣𝑖𝑡𝑦#2 /MHz
300 K 4K 2K
1297.894 1300.025 1299.943
1297.959 1300.089 1300.013
thermal contraction, leading to resonant frequency increases of 2.131 MHz and 2.130 MHz for Cavity 1 and 2, respectively. In comparison, the cavity frequencies changed by about 1.8 MHz from 300 K to 80 K [7]. From 4 K to 2 K, the resonant frequencies decreased 0.082 MHz and 0.076 MHz, respectively. This change was caused by the reduced pressure in the helium vessel, from about 1065 mbar at 4 K to about 29 mbar at 2 K. Because the pre-tuning at room temperature was done accurately, the resonant frequencies at 2 K were very close to 1.300 GHz for both cavities. There are two pressure sensors (nonlinearity within ± 1%) installed symmetrically on each tuner to monitor the tuning force acting on the cavity flanges. More details of the tuner measurements are presented in Ref. [7]. At 4 K, the pre-tuning forces acting on Cavity 1 and 2 are 3.93 kN and 4.40 kN, respectively. We changed the tuner force by operating the associated stepper motor and measured the resulting change in the cavity resonant frequency using a network analyzer (measurement
Fig. 9. The vacuum variations of the cryomodule during the cooling down process.
(measurement uncertainty of ± 7%) along the vacuum pipe. As expected, the decrease in temperature lowered the vacuum pressure levels. The resonant frequencies of both cavities were also recorded during cooldown (see Table 2). From 300 K to 4 K, the cavities shrank due to 32
Z. Kui et al.
Nuclear Inst. and Methods in Physics Research, A 895 (2018) 29–34
Fig. 12. Forward (blue for Cavity 1 and black for Cavity 2), reflected (cyan for Cavity 1 and red for Cavity 2) and pickup (dark yellow for Cavity 1 and purple for Cavity 2) powers measured by a power meter in pulsed mode.
Fig. 10. Cavity frequency at 4 K versus tuner force.
precision of <1 kHz, which depends on the measurement bandwidth). The results in Fig. 10 show that the tuning range for both cavities is larger than 200 kHz, which is enough to adjust the resonant frequency to 1.300 GHz. There exists some hysteresis in the force exerted by the motor tuner for Cavity 2. When the stepper-motor reverses direction, the force acting on this cavity is reduced by up to 25% due to play in the support system. Better tightening of the support components would have reduced this. The relationship between the effective field gradient, 𝐸𝑎𝑐𝑐 , and the voltage of the cavity pickup signal, 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 , was measured at 4 K. The LLRF system was operated in self-exited loop (SEL) mode to establish the cavity field, which means the drive frequency was always matched to the cavity resonant frequency. The coupling coefficient of the pickup is fixed, so the pickup power, 𝑃𝑝𝑖𝑐𝑘𝑢𝑝 , is proportional to the dissipated power in the cavity, which is proportional to 𝐸𝑎𝑐𝑐 squared. Thus, 𝐸𝑎𝑐𝑐 is proportional to 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 , which can be expressed as, 𝐸𝑎𝑐𝑐 = 𝛼 ⋅ 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 . The purpose of the calibration was to compute the coefficient 𝛼 for both cavities so that 𝐸𝑎𝑐𝑐 can be measured directly from the pickup signals at 2 K. The quantity 𝐸𝑎𝑐𝑐 is independently computed from the following equation, √ 4 × 𝑃𝑓 × 𝑄𝑒 × 𝑅∕𝑄0 𝐸𝑎𝑐𝑐 = (1) 𝐿𝑐𝑎𝑣
quality factor of the power coupler, which is derived from the decay time of the pickup signal as measured on an oscilloscope. 𝑅∕𝑄0 for both cavities is 444 Ω and the effective cavity length, 𝐿𝑐𝑎𝑣 , is 484 mm [4]. 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 is calculated from the pickup power measured by a power meter. At 4 K, we varied 𝑃𝑓 and measured 𝑃𝑝𝑖𝑐𝑘𝑢𝑝 . From these data, we computed 𝐸𝑎𝑐𝑐 and 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 and then did a linear fit to obtain the coefficients 0.0545 and 0.0827, as shown in Fig. 11(a). At 2 K, the superconducting linac module worked smoothly and stably. The effective field gradients of both cavities reached 10 MV/m, as shown in Fig. 11(b), which satisfies our design goal. 4. Beam commissioning With the cryomodule performance established, beam loading experiments were performed using the electron gun and beam diagnostic system. The beam loading effect was observed successfully in pulsed mode, as shown in Fig. 12. The repetition rate and macro pulse length of the electron beam were 1 Hz and 1 ms, respectively, and the beam current was about 4.5 mA. The fluctuations at the top of the forward and reflected pulses indicate the compensation process of the lowlevel RF system. The electron beams take away microwave energy when traveling through the cavity, potentially leading to a decrease in the stored energy and the field gradient. To compensate, the lowlevel RF system increases the forward power to maintain a constant gradient. With the beam loading, the cavity match to the forward power
where 𝑃𝑓 is the forward power measured from the directional coupler near the power coupler using a power meter, and 𝑄𝑒 is the external
Fig. 11. (a) 𝐸𝑎𝑐𝑐 versus 𝑉𝑝𝑖𝑐𝑘𝑢𝑝 measurements for both cavities at 4 K together with linear fit results, (b) 𝐸𝑎𝑐𝑐 computed from the pickup signal at 2 K. 33
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Fig. 13. (a) Color-enhanced beam spot on a YaG screen after the spectrometer magnet and (b) the corresponding beam energy distribution.
Acknowledgments
improves, decreasing the reflected power. From the differences between the forward power and the reflected power, the powers flowing into Cavity 1 and 2 are estimated to be 14.6 kW and 20.4 kW, respectively. So, the power transfer to the beam is almost 35 kW. The energy and energy spread of the electron beam was measured from the beam image generated on a YAG screen after the spectrometer magnet, which has a nominal 300 mm radius of curvature. Fig. 13(a) shows an example of the image for a 400 ns long 5 mA beam and Fig. 13(b) shows the corresponding transverse energy distribution. The beam energy was about 8.2 MeV and the rms energy spread was 0.19%, which satisfies our requirements [1].
The authors would like to thank Peking University for the help with cryomodule assembly and the Institute of High Energy Physics Chinese Academy of Sciences (IHEP) for their help with the coupler conditioning. References [1] Xu Zhou, et al., Design of a high average power terahertz-FEL facility, J. Terahertz Sci. Electron. Inf. Technol. 11 (1) (2013) 1–6. [2] H. Wang, M. Li, X. Yang, et al., DC high voltage photoemission electron gun for CAEP FEL, in: FEL 2011 - 33th International Free Electron Laser Conference, 2011, pp. 598– 600. [3] H. Wang, K. Li, M. Li, et al., A GaAs photoemission DC gun for CAEP high-average power THz FEL, in: FEL 2014 - 36th International Free Electron Laser Conference, 2014, pp. 318–321. [4] Luo Xing, et al., Design and fabrication of the 2×4-cell superconducting linac module for the free-electron laser, Nucl. Instrum. Methods Phys. Res. A 871 (2017) 30–34. http://dx.doi.org/10.1016/j.nima.2017.06.058. [5] L. Yang, D. Deng, et al., Physics design of undulator in high average power free electronic laser terahertz source, High Power Laser Part. Beams 25 (2013) 153–157. [6] J. Knobloch, The Q disease in superconducting niobium RF cavities, AIP Conf. Proc. 671 (2003) 133–150. http://dx.doi.org/10.1063/1.1597364. [7] Mi Zhenghui, et al., Design and test of frequency tuner for a CAEP high power THz free-electron laser, Chin. Phys. C 39 (2) (2015) 95–100.
5. Conclusion A 2 × 4-cell superconducting linac module for the THz-FEL facility has been assembled, cooled to 2 K and operated successfully, meeting the beam gradient design goal (up to 10 MV/m). In pulsed-mode beam commissioning experiments, 5 mA, 8.2 MeV beams with <0.2% rms energy spreads were generated. Further beam loading experiments are in progress.
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