- Email: [email protected]
Development of high-power S-band load
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Development of high-power S-band load Xiangcong Meng a,b , Jiaru Shi a,b ,∗, Hao Zha a,b , Qiang Gao a,b , Zening Liu a,b , Jiayang Liu a,b , Yuliang Jiang a,b , Ping Wang a,b , Huaibi Chen a,b , Jiaqi Qiu c a
Department of Engineering Physics, Tsinghua University, Beijing CN-100084, China Key Laboratory of Particle and Radiation Imaging, Tsinghua University, Ministry of Education, Beijing CN-100084, China c Nuctech Company Limited, Beijing CN-100084, China b
ARTICLE Keywords: High-power load Microwave device S-band High-power test stand
INFO
ABSTRACT Many prototypes of all stainless steel high-power loads have been developed in Tsinghua University to meet the needs of high-power test systems for different microwave bands. The loads, which work at the S-band with a center frequency of 2856 MHz, are based on a waveguide structure with regular grooves. These loads are tested at 290 MW peak power successfully. This study presents the design, simulation, fabrication, high-power test results and analysis of multipactor phenomenon.
1. Introduction Radio frequency (RF) load is an important device in microwave circuit that absorbs residual microwave energy. If the residual microwave energy cannot be sufficiently absorbed, then the energy will be reflected back to the power source and may cause damage. RF load can be classified into two types, namely, water and dry loads, on the basis of its absorption material. Water load is unsafe and unstable in high power because of the risk of frangibility and water leakage compared with the dry load. At present, many accelerator systems demand dry loads that have high-power capacity. Several prototypes of dry loads with different RF absorbers, such as an array of SiC cylinders, porcelain doped by SiC powder, or lossy ceramic, have been developed in SLAC. The designed and fabricated SLAC loads employ thermal coated technology using FeCrAl layer on the grooves in the S-Band waveguide. The installed linac loads have been operated at more than 30 MW peak and 5 kW average RF power without multipactor signatures [1]. Another type of all stainless steel load had been developed in CERN, and the high-power test was conducted in the X-band facility of KEK. The geometry of this load was based on the rectangular WR90 waveguide and had a grooved surface where the RF wall current was efficiently damped. The highpower test in KEK shows that the load can be operated at 50 MW peak or 5 kW average power [2,3]. An S-band high-power test facility with multi-hundred MW levels of pulse power has been constructed in Tsinghua University for the study of pulse compressor [4]. However, the present dry loads can hardly satisfy the needs of this test facility. Thus, several types of highpower S-band loads have been proposed on the basis of the X-band loads designed at BINP (Protvino, Russia) in 1997 and at CERN in 2010 [2,3]. The loads are made of pure stainless steel with regular grooves in the
waveguide [5]. The models have been proposed and simulated in the electromagnetic simulation code CST. Some improvements, such as the structure of match section and the number of stainless steel slices, have been made in the model to absorb high pulse power. The sizes of the loads have been optimized in CST, followed by fabrication and bench test. The load has been successfully operated at 290 MW peak power with 300 ns pulse width and 10 Hz repetition frequency in the test. Furthermore, similar loads in the C- and X-bands have been developed and used in accelerator systems. 2. Theory and design In a conventional load, RF power is usually absorbed by a dielectric or magnetic material, such as SiC or ferrite. However, for such special materials, welding is complicated and expensive. In addition, the absorber material is unstable in terms of temperature rise due to RF power dissipation. A type of pure metal RF load based on rectangular waveguide with good thermostability and high-power tolerance was developed in Tsinghua University. This S-band load is made of magnetic stainless steel (type: SS430, 𝜇 = 6), and the dimensions are based on BJ32 waveguide (72.14 × 34.04 mm), with length of approximately 1 m. Conceptually, this type of load will have strong absorption on RF frequencies that are close to the cut-off frequency of the waveguide. A waveguide cross-section geometry has been designed to achieve the sufficient absorption, as shown in Fig. 1. Regular grooves with gradually changing depth can be found on the H-plane surface where the RF wall current is efficiently damped. The load is divided into two parts at the middle of the E-plane for the convenient processing of the grooves.
∗ Corresponding author at: Department of Engineering Physics, Tsinghua University, Beijing CN-100084, China. E-mail address: [email protected] (J. Shi).
https://doi.org/10.1016/j.nima.2019.02.002 Received 5 July 2018; Received in revised form 24 January 2019; Accepted 1 February 2019 Available online 23 February 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
X. Meng, J. Shi, H. Zha et al.
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Fig. 3. Model for the vacuum area of a quarter of the load in CST. Fig. 1. Cross-section of the prototype. Table 1 Load parameters.
These two parts are welded together to the full load after manufacturing and bench test. The load is composed of four longitudinal sections, namely, the taper, match absorption, regular absorption, and end box sections (Fig. 2). The taper section connects the absorption section and the Sband flange in the accelerator. The absorption section consists of two parts, that is, the match section with parallel stainless steel slices of gradual height and the regular section with slices of constant height. As the name suggests, the match section is used to minimize the reflection due to geometry change, whereas the regular section is used to ensure sufficient damping to the RF power. The end box section is reserved for tool backlash movement, and a vacuum port is placed at the end of this section to achieve a high vacuum environment. Moreover, a water channel at the bottom of both half loads maintains a relatively stable temperature. Two slopes are adopted in the match absorption section, and additional stainless steel slices are used to achieve improved absorption effect. We model the vacuum part in the CST to obtain the optimized parameters of the load. The model is divided into three parts that are simulated separately, as shown in Fig. 3. The full model is simulated after the three parts are matched. This method is called ‘‘segmented matching’’ simulation. The model represents a quarter of the load geometry. Many parameters, such as the slope in the match section and the length of the regular section, are scanned for matching. These parameters play an important role in the matched absorption. Meanwhile, the width and number of the stainless steel slices are considered to reach a high power. The parameters are optimized to 1.5 mm width, 21 slices in comparison with the 1 mm, and 15 slices in CERN to achieve high power with acceptable absorption. We acquire several optimized designs of prototypes with different dimensions in the S-band. Similar simulations are performed in the C- and X-bands, and the results show promise. The distribution of the electromagnetic field in the load is simulated with 1 W power input, as shown in Fig. 4. The complex magnitude of electric and magnetic fields is shown in the left and right sides of Fig. 4, respectively. From the figure, the electromagnetic field damps along the longitudinal direction. Moreover, the maximum surface electric field is approximately 28.2 MV/m, and the magnetic field is 66.5 kA/m with 200 MW input power, which is relatively safe for the stainless steel in terms of RF break down. Furthermore, the electromagnetic field becomes zero at the end of the load, which indicates excellent absorption.
Parameter
Value
Frequency Dimensions of waveguide port Length of the load Vacuum level Cooling temperature Cooling flow rate
2.856 GHz 72.14×34.04 mm (BJ32) 1.067 m 8×10−7 Pa 28 ± 0.2 15 L/min
Thermal analysis on the load is necessary because RF heating will change the geometry and increase the reflection. The boundary between the stainless steel and the background is set as a constant temperature. The heat source is loaded from the computed electromagnetic field. The calculated distribution of temperature on the surface of the stainless steel for 12 kW average input power is shown in Fig. 5. The maximum temperature is 462 K (188 ◦ C), which is acceptable for stainless steel because the thermal deformation is small at this temperature. In conclusion, a type of all stainless steel load is designed and optimized with extremely low reflection, large bandwidth, and good thermostability. The mechanic design is shown in Fig. 6.
3. Bench test and weld The dimensions of the mechanic design of the load are slightly larger than those of the simulated model due to welding deformation. After fabrication, the load is cleaned and assembled for the bench test. The S-parameter is tested by the vector network analyzer (VNA) for the reflection. Tuning is conducted to obtain the minimum reflection on the working frequency. The two half parts of the load are welded by an Argon arc weld after tuning. Finally, the assembled load is tested prior to the high-power test (Fig. 7). The measured S11 of the load after assembly and the simulated S11 are shown in Fig. 8. The load after assembly performs well with the 1.058 VSWR in the working frequency (2.856 GHz), and the bandwidth below −20 dB is 129 MHz. The simulated result shows good absorption capacity due to the errors of the processing and welding technique. Nevertheless, the test shows good agreement between the measured and simulated results.
Fig. 2. Longitudinal section of the half load.
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Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Fig. 4. Complex magnitude distribution of the electric field (left) and magnetic field (right) in the load.
Fig. 5. Distribution of temperature on the surface.
4. High-power test Fig. 7. Assembled load during bench test.
The high-power test of the load is conducted at the test stand, as shown in Fig. 9. The test stand combines the power of two klystrons, each of which generates 50 MW peak power with 4 μs pulse width. An RF pulse compressor is used in the test stand to achieve more than 500 MW peak power [4]. The power is divided equally to the two loads. Several directional couplers are used at the output of the pulse compressor and the input of the load, in which the RF signal can be detected by a diode detector. Some of the parameters of the test stand is shown in Table 1. The vacuum level can reach 10−6 Pa when a breakdown occurs. During the high-power test, the repetition frequency and pulse width are changed to obtain a high peak power. After the three consecutive conditioning periods, the output power of the pulse compressor can reach the peak value of 580 MW. Table 2 lists the breakdown rates during the conditioning. The power absorbed by each load is 290 MW with 300 ns pulse width. The observed breakdown rate is small, which
Fig. 8. Measured S11 of the assembled load.
Fig. 6. Assembly drawing of the entire load (left) and separate parts of the load (right).
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X. Meng, J. Shi, H. Zha et al.
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Table 2 Number of breakdowns while conditioning. Round
Repetition frequency/Hz
Pulse width/ns
Peak power to each load/MW
Conditioning time/h
Number of breakdowns
Breakdown rate
1 2 3
10 10 5
500 300 300
190 290 290
3.25 3.67 8.3
4 1 8
3.42×10−5 /pulse 0.75×10−5 /pulse 5.33×10−5 /pulse
Fig. 9. Layout of the high-power test system.
Fig. 11. Reflected waveforms in different incident powers (A) and the comparison in 78 MW input (B).
Fig. 10. Input and output power of the load.
indicates that the load can be operated on the high-power system stably. Fig. 10 shows the input and reflected powers in the load. The test results show that the S11 of the load is nearly −29 dB, and the VSWR is 1.07. The results agree with the bench test and prove the good high-power performance of the load. The difference of the shape of the waveform between the incident and reflected RF power is indicated in the peer review. Consequently, additional data are recorded to study this phenomenon. The incident power of the load has increased from 50 MW to 250 MW, and the waveforms and phase of incident and reflected powers are analyzed carefully. The waveform shapes of the incident power remain the same. The waveforms of reflected power and one of them compared with the incident power are shown in Fig. 11. A few bumps are observed at both sides of the peak. The bump on the right appears at the farther distance from the peak with the increase in input power. Further analysis shows that the incident power is nearly the same when the bump appears. Therefore, the reflection of the load will reach the maximum when the incident power reaches a certain value. This inference can be
Fig. 12. Relationship of the reflection (A) and phase difference (B) against the incident power.
verified by comparing the incident and reflected powers in Fig. 11(B). A possible explanation might be the occurrence of multipactor between the stainless steel slices at a certain level of incident power [6]. 212
X. Meng, J. Shi, H. Zha et al.
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
To prove the preceding inference, we derive the reflection coefficient at different pulse times. The data points of the reflection coefficient at a certain incident power in all the experimental situations are then plotted in one figure. The mean value and the standard deviation in every small interval are calculated, and the final figure is shown in Fig. 12(A). The reflection coefficient reaches the maximum at the input power of 60 dBW (1 MW). At this power level, the calculated voltage between the slices is lower than 1 kV, which indicates a high probability for the occurrence of multipactor phenomenon. The phase information is measured using an oscilloscope (Teledyne LeCroy WaveMaster 8 ZiB Oscilloscope) with high sampling rate and processed in a similar manner. The phase difference between the reflected and incident RF against the input power is shown in Fig. 12(B). The phase information indicates the difference of the plasma and stainless steel material absorption. The change of phase difference is below 10 degrees during the absorption of high power (>70 dBW). At high RF power until the maximum level during our test experiment, the load shows excellent absorption, and the reflection coefficient agrees with the simulation and VNA measurement results.
geometry of the load is optimized to achieve strong absorption and thermostability. The simulations and bench tests are in good agreement. The load can be operated stably at 290 MW with 300 ns pulse width and 10 Hz repetition frequency. Moreover, the high-power test imply that the load can operate with the absorption of stainless steel material and plasma medium due to multipactor phenomenon. Similar loads can be designed and fabricated in the S-, C-, and X-bands to meet the broad requirements on high-power RF loads. Acknowledgments The authors are grateful to Igor Syratchev for useful discussions during the design stage and Anatoly Krasnykh for his insightful comments which improved this work to a great extent. This work was supported by the National Natural Science Foundation of China (No. 11635006). References [1] A. Krasnykh, Overview of High Power Vacuum Dry RF Load Designs, SLAC-TN-15037. [2] Matsumoto Shuji, et al., High Power Evaluation of X-Band High Power Loads, 2011, MOP074. [3] E. Montesinos, et al., High temperature radio frequency loads, in: Conf. Proc., vol. 110904, 2011, No. IPAC-2011-TUPS103. [4] Wang Ping, et al., Development of an S-band spherical pulse compressor, Nucl. Instrum. Methods Phys. Res. A 901 (2018) 84–91. [5] Meng Xiangcong, et al., Development of S-band High Power Load. 7th Int. Particle Accelerator Conf., IPAC’16, Busan, Korea, May 8–13, 2016. JACOW, Geneva, Switzerland, 2016. [6] V. Baglin, et al., The Secondary Electron Yield of Technical Materials and Its Variation with Surface Treatments, No. LHC-Project-Report-433, 2000.
5. Conclusion A new type of high-power S-band load is designed and fabricated at Tsinghua University. The load is made of all stainless steel with a gradually varied grooved structure. Two slopes in the match section and several stainless steel slices are adopted for good absorption. The
213
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Development of high-power S-band load Xiangcong Meng a,b , Jiaru Shi a,b ,∗, Hao Zha a,b , Qiang Gao a,b , Zening Liu a,b , Jiayang Liu a,b , Yuliang Jiang a,b , Ping Wang a,b , Huaibi Chen a,b , Jiaqi Qiu c a
Department of Engineering Physics, Tsinghua University, Beijing CN-100084, China Key Laboratory of Particle and Radiation Imaging, Tsinghua University, Ministry of Education, Beijing CN-100084, China c Nuctech Company Limited, Beijing CN-100084, China b
ARTICLE Keywords: High-power load Microwave device S-band High-power test stand
INFO
ABSTRACT Many prototypes of all stainless steel high-power loads have been developed in Tsinghua University to meet the needs of high-power test systems for different microwave bands. The loads, which work at the S-band with a center frequency of 2856 MHz, are based on a waveguide structure with regular grooves. These loads are tested at 290 MW peak power successfully. This study presents the design, simulation, fabrication, high-power test results and analysis of multipactor phenomenon.
1. Introduction Radio frequency (RF) load is an important device in microwave circuit that absorbs residual microwave energy. If the residual microwave energy cannot be sufficiently absorbed, then the energy will be reflected back to the power source and may cause damage. RF load can be classified into two types, namely, water and dry loads, on the basis of its absorption material. Water load is unsafe and unstable in high power because of the risk of frangibility and water leakage compared with the dry load. At present, many accelerator systems demand dry loads that have high-power capacity. Several prototypes of dry loads with different RF absorbers, such as an array of SiC cylinders, porcelain doped by SiC powder, or lossy ceramic, have been developed in SLAC. The designed and fabricated SLAC loads employ thermal coated technology using FeCrAl layer on the grooves in the S-Band waveguide. The installed linac loads have been operated at more than 30 MW peak and 5 kW average RF power without multipactor signatures [1]. Another type of all stainless steel load had been developed in CERN, and the high-power test was conducted in the X-band facility of KEK. The geometry of this load was based on the rectangular WR90 waveguide and had a grooved surface where the RF wall current was efficiently damped. The highpower test in KEK shows that the load can be operated at 50 MW peak or 5 kW average power [2,3]. An S-band high-power test facility with multi-hundred MW levels of pulse power has been constructed in Tsinghua University for the study of pulse compressor [4]. However, the present dry loads can hardly satisfy the needs of this test facility. Thus, several types of highpower S-band loads have been proposed on the basis of the X-band loads designed at BINP (Protvino, Russia) in 1997 and at CERN in 2010 [2,3]. The loads are made of pure stainless steel with regular grooves in the
waveguide [5]. The models have been proposed and simulated in the electromagnetic simulation code CST. Some improvements, such as the structure of match section and the number of stainless steel slices, have been made in the model to absorb high pulse power. The sizes of the loads have been optimized in CST, followed by fabrication and bench test. The load has been successfully operated at 290 MW peak power with 300 ns pulse width and 10 Hz repetition frequency in the test. Furthermore, similar loads in the C- and X-bands have been developed and used in accelerator systems. 2. Theory and design In a conventional load, RF power is usually absorbed by a dielectric or magnetic material, such as SiC or ferrite. However, for such special materials, welding is complicated and expensive. In addition, the absorber material is unstable in terms of temperature rise due to RF power dissipation. A type of pure metal RF load based on rectangular waveguide with good thermostability and high-power tolerance was developed in Tsinghua University. This S-band load is made of magnetic stainless steel (type: SS430, 𝜇 = 6), and the dimensions are based on BJ32 waveguide (72.14 × 34.04 mm), with length of approximately 1 m. Conceptually, this type of load will have strong absorption on RF frequencies that are close to the cut-off frequency of the waveguide. A waveguide cross-section geometry has been designed to achieve the sufficient absorption, as shown in Fig. 1. Regular grooves with gradually changing depth can be found on the H-plane surface where the RF wall current is efficiently damped. The load is divided into two parts at the middle of the E-plane for the convenient processing of the grooves.
∗ Corresponding author at: Department of Engineering Physics, Tsinghua University, Beijing CN-100084, China. E-mail address: [email protected] (J. Shi).
https://doi.org/10.1016/j.nima.2019.02.002 Received 5 July 2018; Received in revised form 24 January 2019; Accepted 1 February 2019 Available online 23 February 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
X. Meng, J. Shi, H. Zha et al.
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Fig. 3. Model for the vacuum area of a quarter of the load in CST. Fig. 1. Cross-section of the prototype. Table 1 Load parameters.
These two parts are welded together to the full load after manufacturing and bench test. The load is composed of four longitudinal sections, namely, the taper, match absorption, regular absorption, and end box sections (Fig. 2). The taper section connects the absorption section and the Sband flange in the accelerator. The absorption section consists of two parts, that is, the match section with parallel stainless steel slices of gradual height and the regular section with slices of constant height. As the name suggests, the match section is used to minimize the reflection due to geometry change, whereas the regular section is used to ensure sufficient damping to the RF power. The end box section is reserved for tool backlash movement, and a vacuum port is placed at the end of this section to achieve a high vacuum environment. Moreover, a water channel at the bottom of both half loads maintains a relatively stable temperature. Two slopes are adopted in the match absorption section, and additional stainless steel slices are used to achieve improved absorption effect. We model the vacuum part in the CST to obtain the optimized parameters of the load. The model is divided into three parts that are simulated separately, as shown in Fig. 3. The full model is simulated after the three parts are matched. This method is called ‘‘segmented matching’’ simulation. The model represents a quarter of the load geometry. Many parameters, such as the slope in the match section and the length of the regular section, are scanned for matching. These parameters play an important role in the matched absorption. Meanwhile, the width and number of the stainless steel slices are considered to reach a high power. The parameters are optimized to 1.5 mm width, 21 slices in comparison with the 1 mm, and 15 slices in CERN to achieve high power with acceptable absorption. We acquire several optimized designs of prototypes with different dimensions in the S-band. Similar simulations are performed in the C- and X-bands, and the results show promise. The distribution of the electromagnetic field in the load is simulated with 1 W power input, as shown in Fig. 4. The complex magnitude of electric and magnetic fields is shown in the left and right sides of Fig. 4, respectively. From the figure, the electromagnetic field damps along the longitudinal direction. Moreover, the maximum surface electric field is approximately 28.2 MV/m, and the magnetic field is 66.5 kA/m with 200 MW input power, which is relatively safe for the stainless steel in terms of RF break down. Furthermore, the electromagnetic field becomes zero at the end of the load, which indicates excellent absorption.
Parameter
Value
Frequency Dimensions of waveguide port Length of the load Vacuum level Cooling temperature Cooling flow rate
2.856 GHz 72.14×34.04 mm (BJ32) 1.067 m 8×10−7 Pa 28 ± 0.2 15 L/min
Thermal analysis on the load is necessary because RF heating will change the geometry and increase the reflection. The boundary between the stainless steel and the background is set as a constant temperature. The heat source is loaded from the computed electromagnetic field. The calculated distribution of temperature on the surface of the stainless steel for 12 kW average input power is shown in Fig. 5. The maximum temperature is 462 K (188 ◦ C), which is acceptable for stainless steel because the thermal deformation is small at this temperature. In conclusion, a type of all stainless steel load is designed and optimized with extremely low reflection, large bandwidth, and good thermostability. The mechanic design is shown in Fig. 6.
3. Bench test and weld The dimensions of the mechanic design of the load are slightly larger than those of the simulated model due to welding deformation. After fabrication, the load is cleaned and assembled for the bench test. The S-parameter is tested by the vector network analyzer (VNA) for the reflection. Tuning is conducted to obtain the minimum reflection on the working frequency. The two half parts of the load are welded by an Argon arc weld after tuning. Finally, the assembled load is tested prior to the high-power test (Fig. 7). The measured S11 of the load after assembly and the simulated S11 are shown in Fig. 8. The load after assembly performs well with the 1.058 VSWR in the working frequency (2.856 GHz), and the bandwidth below −20 dB is 129 MHz. The simulated result shows good absorption capacity due to the errors of the processing and welding technique. Nevertheless, the test shows good agreement between the measured and simulated results.
Fig. 2. Longitudinal section of the half load.
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Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Fig. 4. Complex magnitude distribution of the electric field (left) and magnetic field (right) in the load.
Fig. 5. Distribution of temperature on the surface.
4. High-power test Fig. 7. Assembled load during bench test.
The high-power test of the load is conducted at the test stand, as shown in Fig. 9. The test stand combines the power of two klystrons, each of which generates 50 MW peak power with 4 μs pulse width. An RF pulse compressor is used in the test stand to achieve more than 500 MW peak power [4]. The power is divided equally to the two loads. Several directional couplers are used at the output of the pulse compressor and the input of the load, in which the RF signal can be detected by a diode detector. Some of the parameters of the test stand is shown in Table 1. The vacuum level can reach 10−6 Pa when a breakdown occurs. During the high-power test, the repetition frequency and pulse width are changed to obtain a high peak power. After the three consecutive conditioning periods, the output power of the pulse compressor can reach the peak value of 580 MW. Table 2 lists the breakdown rates during the conditioning. The power absorbed by each load is 290 MW with 300 ns pulse width. The observed breakdown rate is small, which
Fig. 8. Measured S11 of the assembled load.
Fig. 6. Assembly drawing of the entire load (left) and separate parts of the load (right).
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X. Meng, J. Shi, H. Zha et al.
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
Table 2 Number of breakdowns while conditioning. Round
Repetition frequency/Hz
Pulse width/ns
Peak power to each load/MW
Conditioning time/h
Number of breakdowns
Breakdown rate
1 2 3
10 10 5
500 300 300
190 290 290
3.25 3.67 8.3
4 1 8
3.42×10−5 /pulse 0.75×10−5 /pulse 5.33×10−5 /pulse
Fig. 9. Layout of the high-power test system.
Fig. 11. Reflected waveforms in different incident powers (A) and the comparison in 78 MW input (B).
Fig. 10. Input and output power of the load.
indicates that the load can be operated on the high-power system stably. Fig. 10 shows the input and reflected powers in the load. The test results show that the S11 of the load is nearly −29 dB, and the VSWR is 1.07. The results agree with the bench test and prove the good high-power performance of the load. The difference of the shape of the waveform between the incident and reflected RF power is indicated in the peer review. Consequently, additional data are recorded to study this phenomenon. The incident power of the load has increased from 50 MW to 250 MW, and the waveforms and phase of incident and reflected powers are analyzed carefully. The waveform shapes of the incident power remain the same. The waveforms of reflected power and one of them compared with the incident power are shown in Fig. 11. A few bumps are observed at both sides of the peak. The bump on the right appears at the farther distance from the peak with the increase in input power. Further analysis shows that the incident power is nearly the same when the bump appears. Therefore, the reflection of the load will reach the maximum when the incident power reaches a certain value. This inference can be
Fig. 12. Relationship of the reflection (A) and phase difference (B) against the incident power.
verified by comparing the incident and reflected powers in Fig. 11(B). A possible explanation might be the occurrence of multipactor between the stainless steel slices at a certain level of incident power [6]. 212
X. Meng, J. Shi, H. Zha et al.
Nuclear Inst. and Methods in Physics Research, A 927 (2019) 209–213
To prove the preceding inference, we derive the reflection coefficient at different pulse times. The data points of the reflection coefficient at a certain incident power in all the experimental situations are then plotted in one figure. The mean value and the standard deviation in every small interval are calculated, and the final figure is shown in Fig. 12(A). The reflection coefficient reaches the maximum at the input power of 60 dBW (1 MW). At this power level, the calculated voltage between the slices is lower than 1 kV, which indicates a high probability for the occurrence of multipactor phenomenon. The phase information is measured using an oscilloscope (Teledyne LeCroy WaveMaster 8 ZiB Oscilloscope) with high sampling rate and processed in a similar manner. The phase difference between the reflected and incident RF against the input power is shown in Fig. 12(B). The phase information indicates the difference of the plasma and stainless steel material absorption. The change of phase difference is below 10 degrees during the absorption of high power (>70 dBW). At high RF power until the maximum level during our test experiment, the load shows excellent absorption, and the reflection coefficient agrees with the simulation and VNA measurement results.
geometry of the load is optimized to achieve strong absorption and thermostability. The simulations and bench tests are in good agreement. The load can be operated stably at 290 MW with 300 ns pulse width and 10 Hz repetition frequency. Moreover, the high-power test imply that the load can operate with the absorption of stainless steel material and plasma medium due to multipactor phenomenon. Similar loads can be designed and fabricated in the S-, C-, and X-bands to meet the broad requirements on high-power RF loads. Acknowledgments The authors are grateful to Igor Syratchev for useful discussions during the design stage and Anatoly Krasnykh for his insightful comments which improved this work to a great extent. This work was supported by the National Natural Science Foundation of China (No. 11635006). References [1] A. Krasnykh, Overview of High Power Vacuum Dry RF Load Designs, SLAC-TN-15037. [2] Matsumoto Shuji, et al., High Power Evaluation of X-Band High Power Loads, 2011, MOP074. [3] E. Montesinos, et al., High temperature radio frequency loads, in: Conf. Proc., vol. 110904, 2011, No. IPAC-2011-TUPS103. [4] Wang Ping, et al., Development of an S-band spherical pulse compressor, Nucl. Instrum. Methods Phys. Res. A 901 (2018) 84–91. [5] Meng Xiangcong, et al., Development of S-band High Power Load. 7th Int. Particle Accelerator Conf., IPAC’16, Busan, Korea, May 8–13, 2016. JACOW, Geneva, Switzerland, 2016. [6] V. Baglin, et al., The Secondary Electron Yield of Technical Materials and Its Variation with Surface Treatments, No. LHC-Project-Report-433, 2000.
5. Conclusion A new type of high-power S-band load is designed and fabricated at Tsinghua University. The load is made of all stainless steel with a gradually varied grooved structure. Two slopes in the match section and several stainless steel slices are adopted for good absorption. The
213