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A multi-electron-beam Cherenkov oscillator operating at mm wavelengths
Nuclear Instruments and Methods in Physics Research A 349 (1994) 299-301 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Sect,on A
A multi-electron-beam Cherenkov oscillator operating at mm wavelengths Qingyuan Wang a,*, Kui Zhao b, Chia-erh Chen b, Kesong Hu c Yutao Chen c, Pingshan Wang c Beijing Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, China b Department of Technical Physics, Peking University, Beijing 100871, China c Institute of Applied Electronics, China Academy of Engineering Physics, Chengdu 610003, China a
Received 28 March 1994
A multi-electron-beam Cherenkov maser has the potential of using an electron beam with large spot and an interaction cavity of large transverse dimensions with respect to its operation wavelength to generate high-power microwave radiation. This paper describes a joint experimental research on such a device in which a multi-dielectric cavity of transverse dimensions of 20 mm× 34 mm and a total current of 1036 A at 374 kV, in four sheet beams, were employed to generate 9.1 MW microwave radiation at 36.05 GHz. The conversion efficiency was 2.3%.
I. Introduction A dielectric Cherenkov maser consists of a cylindrical waveguide partially filled with an annular dielectric liner in which a relativistic electron beam propagates. In the past years [1-3], dielectric Cherenkov masers have received considerable attention due to its advantages of relative simplicity and its demonstrated ability to operate from cm wavelengths to submillimeter wavelengths. However, as a slow-wave device, for good wave-beam coupling the diameter of the waveguide must be approximately equal to the wavelength to be generated. This will impose a limitation on the drive beam power and power-handling capacity of the interaction region, and high-power operation of the device will be difficult. To solve this problem [4,5], a multi-electron-beam Cherenkov maser was proposed and an experimental demonstration of the physical mechanism underlying it was carried out at the end of 1990. Four sheet beams of total current 280 A at 500 kV w e r e introduced into a multi-dielectric cavity and a stimulated Cherenkov radiation of 1.7 M W at 33.4 GHz was generated, yielding an efficiency of about 1.2%. In August through December of 1993, we have performed an improved experimental research on such a device at Chengdu, under the collaboration between Department of Technical physics, Peking University and Institute of Applied Electronics, China Academy of Engineering Physics. In this paper, the experiment will be described in detail.
* Corresponding author.
2. The experimental setup and the measured result The experiment setup is shown schematically in Fig. 1. The electron beam was provided by the pulse line accelerator EPA-74 at the Chinese Academy of Engineering Physics. Its diode consisted of a square graphite cathode with a planar head of 20 m m × 20 mm and a stainless-steel disk anode with four parallel slits of 2.8 m m x 20 m m spaced by 2.2 mm, which also served as the first mirror of the interaction cavity. The diode and the waveguide system were immersed in an axial guide magnetic field of 10 kG produced by a solenoid wound on a circular stainless-steel cylinder, 123 m m ID, 4 mm thick. The interaction region was a rectangular waveguide of inner transverse dimensions 20 mm × 34 mm and a length of 470 mm. On the inner side walls of the waveguide were a couple of opposing polyethylene plates of length 456 m m and thickness 3 mm. In each of them there were five parallel groves of depth 1 mm. In the grooves five parallel ceramic (95% A120 3, ~ = 10) plates of length 456 m m and width 30 m m were mounted. The top and bottom ceramic plates had a thickness of 0.6 m m while the others a thickness of 1.2 mm, yielding beam channel widths between ceramic plates of 3.8 mm. The ceramic plates and the polyethylene plates were tapered off in 5 m m at their rears to decrease the wave reflection. Behind the waveguide was a rectangular pyramidal horn of length 250 mm and flared dimensions of 77 m m × 77 mm. The cavity was finally completed with a quartz window of diameter 128 mm and a thickness of 8 mm, through which the microwave power radiated into free space. The mismatch for the radiation at the output window provided wave reflection for the oscillator.
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 1 6 8 - 9 0 0 2 ( 9 4 ) 0 0 6 1 1 -A
300
Q. Wang et aL /NucL Instr. and Meth. in Phys. Res. A 349 (1994) 299-301
1.4 1.2
The parameters of the accelerator and of the device were fixed throughout the experiment. The generated microwave power was received by standard horn 1, guided through a B J-320 waveguide, and split into two branches. The signal of the first branch, after enough attention, was sent through one arm of a magic T circuit to crystal detector 1. The signal of the second branch is sent through a B J-320 waveguide dispersive delay line of length L = 37.3 m to the same crystal detector, through another arm of the magic T circuit. The two waveforms of the detector were simultaneously displayed on an oscilloscope. The frequency was decided by time interval ~- between the two waveforms
1
0.8 0.6 0.4 0.2
°_3o
-20
-10 0 10 0(degree)
20
30
1.2
/f=fl0/V'l
-- ( L / c r ) 2 ,
(1)
/
where flo = 21.1 GHz was the cutoff frequency of the BJ-320 waveguide and c the light speed in free space. The microwave power was also received by standard horn 2, sampled by a group of directional couplers and detected by crystal detector 2 to perform power measurement. The normalized horizontal angular distribution of the power density E l 0 ) was measured by horizontally moving horn 2 step by step, with each step of 3.75 ° in angle at a semi-circle of radius R = 116.5 cm around the output window. At each position of the machine measurements were repeated for five times. The whole oscillator was then turened 90 ° in the stainless steel cylinder so that the normalized vertical angular distribution of the power density F(~p) could be measured in the horizontal direction. The normalization was done with respect to the powers received when horn 2 was at the axis of the output system E(O) and F ( ~ ) were measured and shown in Fig. 2. The total output power could be decided by
Foul --
~
15
14
3
13
o
lo
20:30
~(degree) Fig. 2. Normalized horizontal (a) and vertical (b) angular distributions of the power density of the output system.
horn 2, K = 0.93 the attenuation factor of the transmission line between detector 2 and the oscilloscope, /3 the coupling coefficient of the directional couplers in decibels. In order to measure the beam current, a rectangle 1 S2 Faraday cup was located at different positions in the waveguide and measurements with the machine were repeated for five shots. The front of the Faraday cup was a stainless-steel plate, which had four slits of 20 m m × 3.8 mm spaced by 1.2 mm. The Faraday cup configuration could be well matched with the multi-dielectric waveguide system. When the Faraday cup was located after the dielectric plates, the averaged beam current was 545 A. After removing the dielectric system from the waveguide and locating the Faraday cup right behind the anode, an
where v is the voltage response of crystal detector 2 when horn 2 was placed at the axis of the output system, d the sensitivity of the crystal detector, A the effective area of
2
'
-ao-2o-lo
(2)
1
0.6 0.4
dAK10 fl/l° ~ - w / 2 J - 7 / 2 × F ( q o ) cos 0 dO dq~,
(b)
k
4
To voltage
12
11
10
9
8
7
6
5
Fig. 1. Experimental setup arrangement: 1-cathode, 2-anode, 3-stainless-steel cylinder, 4-output horn, 5-oscilloscope, 6-crystal detector 1, 7-dispersive delay line, 8-crystal detector 2, 9-horn 1, 10-directional couplers, 11-horn 2, 12-window, 13-rectangular waveguide, 14-dielectric plates, 15-solenoid.
Q. Wang et al. /NucL Instr. and Meth. in Phys. Res. A 349 (1994) 299-301
301
3. Conclusions
Fig. 3. Waveforms of beam voltage, voltage responses of crystal detector 1 and 2 of a typical shot. averaged beam current of 1036 A was received. The electron beam was transported with an efficiency of 53%. The transportation of the electron beam was not satisfactory. Replacing the Faraday cup with lucite witness plates and changing their position in the waveguide, we found the multi-electron-beam to be twisted anticlockwise downstream, which was responsible for the spoiling of the transportation of the electron beam in the multi-dielectric waveguide. We believe that the twisting of the electron beam was due to the motion of the electrons in a cross-field composed of a radial space-charge electric field and the axial guide magnetic field. This problem will be studied in our future work. Fig. 3 shows the waveforms of the beam voltage, voltage responses of crystal detector 1 and 2 of a typical shot. From the first waveform the beam voltage could be calculated to be 374 kV and from the second waveform the time interval could be read ~-= 156 ns, suggesting a frequency of 36.05 GHz, at which the sensitivity of crystal detector 2 was d = 4.48 m V / m W , the effective area of the receiving horn A = 464.6 mm 2 and the coupling coefficient of the directional couplers /3 = - 61.98 dB. The third waveform indicated that the voltage response of detector 2 was 80 mV. Performing the integration in Eq. (2), the output power was 9.1 MW. Taking into consideration that the beam current which entered the cavity was 1036 A, the conversion efficiency was 2.3%.
An improved experimental research on a multi-electron-beam Cherenkov free-electron laser oscillator was described. A total current of 1036 A at 374 kV was introduced into a multi-dielectric system and the output power was increased in magnitude compared with that of the previous experimental research to 9.1 MW. The operation frequency was 36.05 GHz and the conversion efficiency was 2.3%. The beam transportation in the waveguide was not very satisfactory and the operating conditions of the accelerator, which had a long risetime in its beam voltage waveform, needed to be adjusted. Also, we were not able to optimize the length of the interaction region of the cavity. These problems will be studied in detail in our future experimental and theoretical work, and the operation of the device could be further improved.
Acknowledgements The authors would like to thank Mr. Tiancai Chen, Mr. Yanqing Gan, Ms. Shaoying Yu and Mr. Zichuan Li for their substantial contributions to the experiment. This work was supported by the National Fund of Natural Science under grant 69380001.
References [1] K.L. Felch, K.O. Busby, R.W. Layman, D.K. Kapilow and J.E. Walsh, Appl. Phys. Lett. 39 (1981) 601. [2] A.N. Didenko, A.R. Borisov, G.P. Fomenko and Yu.G. Shtein, Sov. Phys. Lett. 9 (1983) 26. [3] E.P. Garate, J. Walsh, C. Shaughnessy and B. Johnson, Nucl. Instr. and Meth. A 259 (1987) 125. [4] Qingyuan Wang, Shanfu Yu and Shenggang Liu, Int. J. IR& MM Wave. 10 (1989) 889. [5] Qingyuan Wang, Shanfu Yu, Sheuggang Liu, Kesong Hu, Yutao Chen and Pingshan Wang, Appl. Phys. Lett. 59 (1991) 2378.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Sect,on A
A multi-electron-beam Cherenkov oscillator operating at mm wavelengths Qingyuan Wang a,*, Kui Zhao b, Chia-erh Chen b, Kesong Hu c Yutao Chen c, Pingshan Wang c Beijing Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, China b Department of Technical Physics, Peking University, Beijing 100871, China c Institute of Applied Electronics, China Academy of Engineering Physics, Chengdu 610003, China a
Received 28 March 1994
A multi-electron-beam Cherenkov maser has the potential of using an electron beam with large spot and an interaction cavity of large transverse dimensions with respect to its operation wavelength to generate high-power microwave radiation. This paper describes a joint experimental research on such a device in which a multi-dielectric cavity of transverse dimensions of 20 mm× 34 mm and a total current of 1036 A at 374 kV, in four sheet beams, were employed to generate 9.1 MW microwave radiation at 36.05 GHz. The conversion efficiency was 2.3%.
I. Introduction A dielectric Cherenkov maser consists of a cylindrical waveguide partially filled with an annular dielectric liner in which a relativistic electron beam propagates. In the past years [1-3], dielectric Cherenkov masers have received considerable attention due to its advantages of relative simplicity and its demonstrated ability to operate from cm wavelengths to submillimeter wavelengths. However, as a slow-wave device, for good wave-beam coupling the diameter of the waveguide must be approximately equal to the wavelength to be generated. This will impose a limitation on the drive beam power and power-handling capacity of the interaction region, and high-power operation of the device will be difficult. To solve this problem [4,5], a multi-electron-beam Cherenkov maser was proposed and an experimental demonstration of the physical mechanism underlying it was carried out at the end of 1990. Four sheet beams of total current 280 A at 500 kV w e r e introduced into a multi-dielectric cavity and a stimulated Cherenkov radiation of 1.7 M W at 33.4 GHz was generated, yielding an efficiency of about 1.2%. In August through December of 1993, we have performed an improved experimental research on such a device at Chengdu, under the collaboration between Department of Technical physics, Peking University and Institute of Applied Electronics, China Academy of Engineering Physics. In this paper, the experiment will be described in detail.
* Corresponding author.
2. The experimental setup and the measured result The experiment setup is shown schematically in Fig. 1. The electron beam was provided by the pulse line accelerator EPA-74 at the Chinese Academy of Engineering Physics. Its diode consisted of a square graphite cathode with a planar head of 20 m m × 20 mm and a stainless-steel disk anode with four parallel slits of 2.8 m m x 20 m m spaced by 2.2 mm, which also served as the first mirror of the interaction cavity. The diode and the waveguide system were immersed in an axial guide magnetic field of 10 kG produced by a solenoid wound on a circular stainless-steel cylinder, 123 m m ID, 4 mm thick. The interaction region was a rectangular waveguide of inner transverse dimensions 20 mm × 34 mm and a length of 470 mm. On the inner side walls of the waveguide were a couple of opposing polyethylene plates of length 456 m m and thickness 3 mm. In each of them there were five parallel groves of depth 1 mm. In the grooves five parallel ceramic (95% A120 3, ~ = 10) plates of length 456 m m and width 30 m m were mounted. The top and bottom ceramic plates had a thickness of 0.6 m m while the others a thickness of 1.2 mm, yielding beam channel widths between ceramic plates of 3.8 mm. The ceramic plates and the polyethylene plates were tapered off in 5 m m at their rears to decrease the wave reflection. Behind the waveguide was a rectangular pyramidal horn of length 250 mm and flared dimensions of 77 m m × 77 mm. The cavity was finally completed with a quartz window of diameter 128 mm and a thickness of 8 mm, through which the microwave power radiated into free space. The mismatch for the radiation at the output window provided wave reflection for the oscillator.
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 1 6 8 - 9 0 0 2 ( 9 4 ) 0 0 6 1 1 -A
300
Q. Wang et aL /NucL Instr. and Meth. in Phys. Res. A 349 (1994) 299-301
1.4 1.2
The parameters of the accelerator and of the device were fixed throughout the experiment. The generated microwave power was received by standard horn 1, guided through a B J-320 waveguide, and split into two branches. The signal of the first branch, after enough attention, was sent through one arm of a magic T circuit to crystal detector 1. The signal of the second branch is sent through a B J-320 waveguide dispersive delay line of length L = 37.3 m to the same crystal detector, through another arm of the magic T circuit. The two waveforms of the detector were simultaneously displayed on an oscilloscope. The frequency was decided by time interval ~- between the two waveforms
1
0.8 0.6 0.4 0.2
°_3o
-20
-10 0 10 0(degree)
20
30
1.2
/f=fl0/V'l
-- ( L / c r ) 2 ,
(1)
/
where flo = 21.1 GHz was the cutoff frequency of the BJ-320 waveguide and c the light speed in free space. The microwave power was also received by standard horn 2, sampled by a group of directional couplers and detected by crystal detector 2 to perform power measurement. The normalized horizontal angular distribution of the power density E l 0 ) was measured by horizontally moving horn 2 step by step, with each step of 3.75 ° in angle at a semi-circle of radius R = 116.5 cm around the output window. At each position of the machine measurements were repeated for five times. The whole oscillator was then turened 90 ° in the stainless steel cylinder so that the normalized vertical angular distribution of the power density F(~p) could be measured in the horizontal direction. The normalization was done with respect to the powers received when horn 2 was at the axis of the output system E(O) and F ( ~ ) were measured and shown in Fig. 2. The total output power could be decided by
Foul --
~
15
14
3
13
o
lo
20:30
~(degree) Fig. 2. Normalized horizontal (a) and vertical (b) angular distributions of the power density of the output system.
horn 2, K = 0.93 the attenuation factor of the transmission line between detector 2 and the oscilloscope, /3 the coupling coefficient of the directional couplers in decibels. In order to measure the beam current, a rectangle 1 S2 Faraday cup was located at different positions in the waveguide and measurements with the machine were repeated for five shots. The front of the Faraday cup was a stainless-steel plate, which had four slits of 20 m m × 3.8 mm spaced by 1.2 mm. The Faraday cup configuration could be well matched with the multi-dielectric waveguide system. When the Faraday cup was located after the dielectric plates, the averaged beam current was 545 A. After removing the dielectric system from the waveguide and locating the Faraday cup right behind the anode, an
where v is the voltage response of crystal detector 2 when horn 2 was placed at the axis of the output system, d the sensitivity of the crystal detector, A the effective area of
2
'
-ao-2o-lo
(2)
1
0.6 0.4
dAK10 fl/l° ~ - w / 2 J - 7 / 2 × F ( q o ) cos 0 dO dq~,
(b)
k
4
To voltage
12
11
10
9
8
7
6
5
Fig. 1. Experimental setup arrangement: 1-cathode, 2-anode, 3-stainless-steel cylinder, 4-output horn, 5-oscilloscope, 6-crystal detector 1, 7-dispersive delay line, 8-crystal detector 2, 9-horn 1, 10-directional couplers, 11-horn 2, 12-window, 13-rectangular waveguide, 14-dielectric plates, 15-solenoid.
Q. Wang et al. /NucL Instr. and Meth. in Phys. Res. A 349 (1994) 299-301
301
3. Conclusions
Fig. 3. Waveforms of beam voltage, voltage responses of crystal detector 1 and 2 of a typical shot. averaged beam current of 1036 A was received. The electron beam was transported with an efficiency of 53%. The transportation of the electron beam was not satisfactory. Replacing the Faraday cup with lucite witness plates and changing their position in the waveguide, we found the multi-electron-beam to be twisted anticlockwise downstream, which was responsible for the spoiling of the transportation of the electron beam in the multi-dielectric waveguide. We believe that the twisting of the electron beam was due to the motion of the electrons in a cross-field composed of a radial space-charge electric field and the axial guide magnetic field. This problem will be studied in our future work. Fig. 3 shows the waveforms of the beam voltage, voltage responses of crystal detector 1 and 2 of a typical shot. From the first waveform the beam voltage could be calculated to be 374 kV and from the second waveform the time interval could be read ~-= 156 ns, suggesting a frequency of 36.05 GHz, at which the sensitivity of crystal detector 2 was d = 4.48 m V / m W , the effective area of the receiving horn A = 464.6 mm 2 and the coupling coefficient of the directional couplers /3 = - 61.98 dB. The third waveform indicated that the voltage response of detector 2 was 80 mV. Performing the integration in Eq. (2), the output power was 9.1 MW. Taking into consideration that the beam current which entered the cavity was 1036 A, the conversion efficiency was 2.3%.
An improved experimental research on a multi-electron-beam Cherenkov free-electron laser oscillator was described. A total current of 1036 A at 374 kV was introduced into a multi-dielectric system and the output power was increased in magnitude compared with that of the previous experimental research to 9.1 MW. The operation frequency was 36.05 GHz and the conversion efficiency was 2.3%. The beam transportation in the waveguide was not very satisfactory and the operating conditions of the accelerator, which had a long risetime in its beam voltage waveform, needed to be adjusted. Also, we were not able to optimize the length of the interaction region of the cavity. These problems will be studied in detail in our future experimental and theoretical work, and the operation of the device could be further improved.
Acknowledgements The authors would like to thank Mr. Tiancai Chen, Mr. Yanqing Gan, Ms. Shaoying Yu and Mr. Zichuan Li for their substantial contributions to the experiment. This work was supported by the National Fund of Natural Science under grant 69380001.
References [1] K.L. Felch, K.O. Busby, R.W. Layman, D.K. Kapilow and J.E. Walsh, Appl. Phys. Lett. 39 (1981) 601. [2] A.N. Didenko, A.R. Borisov, G.P. Fomenko and Yu.G. Shtein, Sov. Phys. Lett. 9 (1983) 26. [3] E.P. Garate, J. Walsh, C. Shaughnessy and B. Johnson, Nucl. Instr. and Meth. A 259 (1987) 125. [4] Qingyuan Wang, Shanfu Yu and Shenggang Liu, Int. J. IR& MM Wave. 10 (1989) 889. [5] Qingyuan Wang, Shanfu Yu, Sheuggang Liu, Kesong Hu, Yutao Chen and Pingshan Wang, Appl. Phys. Lett. 59 (1991) 2378.