- Email: [email protected]
Four-channel planar FEM for high-power mm-wave generation (theoretical and experimental problems)
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
Four-channel planar FEM for high-power mm-wave generation (theoretical and experimental problems) A.V. Arzhannikova,*, V.T. Astrelina, V.B. Bobyleva, N.S. Ginzburgb, V.G. Ivanenkoa, P.V. Kalinina, S.A. Kuznetsova, N.Yu. Peskovb, P.V. Petrovc, A.S. Sergeevb, S.L. Sinitskya, V.D. Stepanova a
Russian Academy of Sciences (RAS), Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia b Institute of Applied Physics RAS, Nizhny Novgorod 603600, Russia c RFNC-VNIITF, Snezhinsk 456770, Russia
Abstract Theoretical and experimental problems in the realization of a concept of a multi-beam generator are studied for the case of a four-channel planar FEM oscillator at 75 GHz. Computer simulations and ‘‘cold’’ and ‘‘hot’’ experiments have demonstrated clear possibility for construction of this oscillator based on the ELMI-device. r 2003 Elsevier Science B.V. All rights reserved. PACS: 84.40.Ik Keywords: FEL; Spatial coherence; Distributed feedback; Bragg resonator; High-power microwaves
1. Introduction In Refs. [1,2] we proposed a novel concept to achieve a few GW power level in the mm-wave band that is based on the creation of a multichannel planar FEM. Such a generator consists of several planar masers with a moderate (B20 cm) width, which operate as oscillators connected together as modules in a single device. The device has a multi-layer electrodynamic structure using 2D Bragg gratings to produce 2-D distributed feedback in each layer and special waveguides to connect feedback between all layers. In this case, *Corresponding author. Tel.: +7-3832-394-912; fax: +73832-342-163. E-mail address: [email protected] (A.V. Arzhannikov).
the space of the beam–wave interaction can be developed in two transverse directions. For experimental testing of this idea we have chosen a four-channel planar oscillator for the ELMI-device experiment [3]. There are a lot of problems in the path to the realization of this project and a few are discussed in this short paper. First of all we have theoretically studied the key problem of the influence of the parameters of the connecting waveguides on the co-phase operation of the module-oscillators in the multi-channel generator. The other problem is computer simulations on the generation of several sheet beams in order to obtain appropriate beam parameters. Undoubtedly, these problems will also be solved in experiments in the future. But, for that production of new technical units and an essential reconstruction of the ELMI-device are needed. Now, on the
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00855-6
ARTICLE IN PRESS 130
A.V. Arzhannikov et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
first step of our experimental investigations, we only test a new concept of an electrodynamic system, the so-called Bragg RF deflector, which outputs the generated mm-radiation from the channel that the E-beam passes in.
2. Theoretical studies and computer simulations 2.1. General arrangement of four-channel FEM The general arrangement of a four-channel FEM is shown in Fig. 1. The geometry of a single module was chosen close to the conditions of the experiments which were done at the ELMI-device. Each FEM-module is driven by a sheet beam generated by a magnetically insulated diode. The transverse velocity of electrons (b> E0:2) is pumped by the magnetic field of a planar undulator. The undulator field is created by five parallel planes of coils. Each plane consists of coils connected in sequence but with alternating polarity. The number of turns in the coils at the beginning and at the end of each plane is smoothly varied from six turns up to 20 in a length of 12 cm. For a 4-cm undulator period the maximum value of the transverse magnetic field is 0.2 T and the longitudinal field reaches 1.5 T. According to computer calculations, the difference in the magnetic field strength between various slit channels is less than 3% in the regions with passing E-beam
Fig. 1. Schematic of four-channel FEM-generator.
sheets. This value is enough to provide synchronous pumping of the waves. The width of the sheet beams is 15 cm at a current per unit width of B200 A/cm. The geometry of the resonators permits use of different combinations of 1-D and 2-D structures for the upstream and downstream reflectors. Synchronization of the modules is provided by transverse microwave energy flow produced by scattering off the 2-D Bragg gratings and transported from module to module through bent waveguides located in the places of the 2-D reflectors. To form a circle of feedback the first module should be connected to the fourth one by a special waveguide. 2.2. Synchronization of wave generation In the first series of computer simulations the radiation synchronization process considered was based on the solution of averaged equations of the beam–wave interaction for six modules of the multi-channel FEM-generator (see Ref. [2]). These simulations have shown that for an appropriate choice of parameters the processes of the wave excitation with strong synchronization in the modules are realized, and good coherence of the radiation at the maser exit is achieved. We then carried out simulations for a four-module FEMgenerator with the geometry presented in Fig. 1 and parameters close to the ones required for the experiments, which are planned at the ELMIdevice (see above). The beam parameters were the following: the electron energy 1 MeV, the current per unit of width B200 A/cm, the electron transverse velocity B(0.2–0.25)c: At a generated wave frequency of 75 GHz the Pierce parameter was CE4 103 : The width of the interaction space was lx ¼ 20 cm and the length of regular part of the resonators was lz ¼ 35 cm. The entrance 2-D Bragg reflector had a length of 16 cm and the exit 1-D Bragg reflector was 10 cm long. The coefficient of Ohmic losses in the waveguides connecting the channels was varied in intervals of 5–50%. The computer simulations show that the frequency of oscillation in a steady-state mode is close to the frequency of a precise Bragg resonance of the reflectors and a stationary regime of generation is
ARTICLE IN PRESS A.V. Arzhannikov et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
131
established in all modules simultaneously at time 300 ns. Moreover, the amplitudes and the phases of the wave in the modules are practically the same (see Fig. 2), which means that the radiation generated in all modules is strongly synchronized. It should also be noted that synchronization takes place even with a spread in the resonance condition in different modules (for example, due to a difference in the electron energies) exceeding the halfwidth of the self-excitation band. Further auto-oscillations were broken. Situations in which the synchronization state were disturbed and the separate modules operated on miscellaneous frequencies was not observed in simulations. 2.3. Generation of E-beams Four sheet electron beams are generated by a magnetically insulated diode with four parallel Fig. 3. Results of simulation for current distribution and electron trajectories in four-beam diode.
strongly elongated cathodes (there is a picture for a half of the diode in Fig. 3). The geometry and location of the cathodes and the anode slits are chosen, from a self-consistent simulation of beam generation and propagation in such a diode, to provide similar current densities in all channels. The example of the results obtained in one of the simulation runs is presented in Fig. 3. The current density distribution on the beam thickness for two channels is shown for the case after passing the beams through the anode slits. It should be pointed out that graphite formers additionally cut the thickness of these beams in the channels to 4 mm.
3. Experimental investigations
Fig. 2. Structure of partial waves in four-channel FEM.
In order to test units of the multi-channel FEM we made an electrodynamic system for the present experiments at the ELMI-device and with geometry like that of a single channel of the multichannel maser. Operation of the Bragg RFdeflector in the 75 GHz band was preliminarily tested in ‘‘cold’’ measurements at a special test
ARTICLE IN PRESS 132
A.V. Arzhannikov et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
Fig. 4. Signals of the diode voltage, the beam current (left graph) and the radiation power (right one).
bench. Results of these measurements were in good agreement with those of the computer simulations. This deflector was then used in the experiments on the generation of 75 GHz radiation at the ELMI-device. In a series of experiments the strength of the transverse component of the undulator magnetic field was varied at a fixed longitudinal field value of 10 kG. The results of one typical shot for the amplitude of the undulator field 0.8 kG at which the maximum of the radiation power is obtained, are shown in Fig. 4. If one compares these results and those of the previous experiments, two factors, the application of the RF-deflector and the remoteness of the beam collector from the resonator, have permitted one to increase the pulse duration of the radiation power up to 200–300 ns. We characterize the power distribution over the radiation flow cross-section by the visual picture of neon bulb lighting. One hundred neon bulbs were homogeneously distributed on a square panel with sizes 20 20 cm2. The picture of their lighting for the case when the panel was mounted in the shot at the distance 0.5 m from the output window, is presented by the right photo in Fig. 5. The left photo shows the geometry of the panel. The distribution of the light intensity in the right photo is qualitatively close to the one for the radiation power, which is obtained in the ‘‘cold’’ measurements and in numerical simulations for excitation of an H10 wave in the resonator. If the undulator field was equal to zero, no lighting was registered.
Fig. 5. Geometry (left) and picture of lighting (right) of the neon bulb panel under the microwave radiation pulse.
4. Conclusion Theoretical and numerical considerations have shown that a planar FEM with 2-D distributed feedback is suitable for use as a module for constructing multi-channel devices in order to produce microwave radiation of GW level. Computer simulations for the operation of a fourmodule FEM-generator have demonstrated a perfect synchronization of all modules even with a little bit of different energies of the electron beams. The project of a four-channel FEM– generator is designed on the basis of the ELMIdevice.
Acknowledgements The authors would like to thank the Russian Foundation for Basic Research (grant No 01-0216749) and INTAS (grant No 2192) for partially supporting this work.
References [1] A.V. Arzhannikov, et al., Digest of Technical Papers of PPPS-2001, Las Vegas, USA, June 2001, pp. 561–564. [2] N.S. Ginzburg, et al., Nucl. Instr. and Meth. A 475 (2001) 173. [3] N.V. Agarin, et al., Nucl. Instr. and Meth. A 445 (2000) 222.
Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
Four-channel planar FEM for high-power mm-wave generation (theoretical and experimental problems) A.V. Arzhannikova,*, V.T. Astrelina, V.B. Bobyleva, N.S. Ginzburgb, V.G. Ivanenkoa, P.V. Kalinina, S.A. Kuznetsova, N.Yu. Peskovb, P.V. Petrovc, A.S. Sergeevb, S.L. Sinitskya, V.D. Stepanova a
Russian Academy of Sciences (RAS), Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia b Institute of Applied Physics RAS, Nizhny Novgorod 603600, Russia c RFNC-VNIITF, Snezhinsk 456770, Russia
Abstract Theoretical and experimental problems in the realization of a concept of a multi-beam generator are studied for the case of a four-channel planar FEM oscillator at 75 GHz. Computer simulations and ‘‘cold’’ and ‘‘hot’’ experiments have demonstrated clear possibility for construction of this oscillator based on the ELMI-device. r 2003 Elsevier Science B.V. All rights reserved. PACS: 84.40.Ik Keywords: FEL; Spatial coherence; Distributed feedback; Bragg resonator; High-power microwaves
1. Introduction In Refs. [1,2] we proposed a novel concept to achieve a few GW power level in the mm-wave band that is based on the creation of a multichannel planar FEM. Such a generator consists of several planar masers with a moderate (B20 cm) width, which operate as oscillators connected together as modules in a single device. The device has a multi-layer electrodynamic structure using 2D Bragg gratings to produce 2-D distributed feedback in each layer and special waveguides to connect feedback between all layers. In this case, *Corresponding author. Tel.: +7-3832-394-912; fax: +73832-342-163. E-mail address: [email protected] (A.V. Arzhannikov).
the space of the beam–wave interaction can be developed in two transverse directions. For experimental testing of this idea we have chosen a four-channel planar oscillator for the ELMI-device experiment [3]. There are a lot of problems in the path to the realization of this project and a few are discussed in this short paper. First of all we have theoretically studied the key problem of the influence of the parameters of the connecting waveguides on the co-phase operation of the module-oscillators in the multi-channel generator. The other problem is computer simulations on the generation of several sheet beams in order to obtain appropriate beam parameters. Undoubtedly, these problems will also be solved in experiments in the future. But, for that production of new technical units and an essential reconstruction of the ELMI-device are needed. Now, on the
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00855-6
ARTICLE IN PRESS 130
A.V. Arzhannikov et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
first step of our experimental investigations, we only test a new concept of an electrodynamic system, the so-called Bragg RF deflector, which outputs the generated mm-radiation from the channel that the E-beam passes in.
2. Theoretical studies and computer simulations 2.1. General arrangement of four-channel FEM The general arrangement of a four-channel FEM is shown in Fig. 1. The geometry of a single module was chosen close to the conditions of the experiments which were done at the ELMI-device. Each FEM-module is driven by a sheet beam generated by a magnetically insulated diode. The transverse velocity of electrons (b> E0:2) is pumped by the magnetic field of a planar undulator. The undulator field is created by five parallel planes of coils. Each plane consists of coils connected in sequence but with alternating polarity. The number of turns in the coils at the beginning and at the end of each plane is smoothly varied from six turns up to 20 in a length of 12 cm. For a 4-cm undulator period the maximum value of the transverse magnetic field is 0.2 T and the longitudinal field reaches 1.5 T. According to computer calculations, the difference in the magnetic field strength between various slit channels is less than 3% in the regions with passing E-beam
Fig. 1. Schematic of four-channel FEM-generator.
sheets. This value is enough to provide synchronous pumping of the waves. The width of the sheet beams is 15 cm at a current per unit width of B200 A/cm. The geometry of the resonators permits use of different combinations of 1-D and 2-D structures for the upstream and downstream reflectors. Synchronization of the modules is provided by transverse microwave energy flow produced by scattering off the 2-D Bragg gratings and transported from module to module through bent waveguides located in the places of the 2-D reflectors. To form a circle of feedback the first module should be connected to the fourth one by a special waveguide. 2.2. Synchronization of wave generation In the first series of computer simulations the radiation synchronization process considered was based on the solution of averaged equations of the beam–wave interaction for six modules of the multi-channel FEM-generator (see Ref. [2]). These simulations have shown that for an appropriate choice of parameters the processes of the wave excitation with strong synchronization in the modules are realized, and good coherence of the radiation at the maser exit is achieved. We then carried out simulations for a four-module FEMgenerator with the geometry presented in Fig. 1 and parameters close to the ones required for the experiments, which are planned at the ELMIdevice (see above). The beam parameters were the following: the electron energy 1 MeV, the current per unit of width B200 A/cm, the electron transverse velocity B(0.2–0.25)c: At a generated wave frequency of 75 GHz the Pierce parameter was CE4 103 : The width of the interaction space was lx ¼ 20 cm and the length of regular part of the resonators was lz ¼ 35 cm. The entrance 2-D Bragg reflector had a length of 16 cm and the exit 1-D Bragg reflector was 10 cm long. The coefficient of Ohmic losses in the waveguides connecting the channels was varied in intervals of 5–50%. The computer simulations show that the frequency of oscillation in a steady-state mode is close to the frequency of a precise Bragg resonance of the reflectors and a stationary regime of generation is
ARTICLE IN PRESS A.V. Arzhannikov et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
131
established in all modules simultaneously at time 300 ns. Moreover, the amplitudes and the phases of the wave in the modules are practically the same (see Fig. 2), which means that the radiation generated in all modules is strongly synchronized. It should also be noted that synchronization takes place even with a spread in the resonance condition in different modules (for example, due to a difference in the electron energies) exceeding the halfwidth of the self-excitation band. Further auto-oscillations were broken. Situations in which the synchronization state were disturbed and the separate modules operated on miscellaneous frequencies was not observed in simulations. 2.3. Generation of E-beams Four sheet electron beams are generated by a magnetically insulated diode with four parallel Fig. 3. Results of simulation for current distribution and electron trajectories in four-beam diode.
strongly elongated cathodes (there is a picture for a half of the diode in Fig. 3). The geometry and location of the cathodes and the anode slits are chosen, from a self-consistent simulation of beam generation and propagation in such a diode, to provide similar current densities in all channels. The example of the results obtained in one of the simulation runs is presented in Fig. 3. The current density distribution on the beam thickness for two channels is shown for the case after passing the beams through the anode slits. It should be pointed out that graphite formers additionally cut the thickness of these beams in the channels to 4 mm.
3. Experimental investigations
Fig. 2. Structure of partial waves in four-channel FEM.
In order to test units of the multi-channel FEM we made an electrodynamic system for the present experiments at the ELMI-device and with geometry like that of a single channel of the multichannel maser. Operation of the Bragg RFdeflector in the 75 GHz band was preliminarily tested in ‘‘cold’’ measurements at a special test
ARTICLE IN PRESS 132
A.V. Arzhannikov et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 129–132
Fig. 4. Signals of the diode voltage, the beam current (left graph) and the radiation power (right one).
bench. Results of these measurements were in good agreement with those of the computer simulations. This deflector was then used in the experiments on the generation of 75 GHz radiation at the ELMI-device. In a series of experiments the strength of the transverse component of the undulator magnetic field was varied at a fixed longitudinal field value of 10 kG. The results of one typical shot for the amplitude of the undulator field 0.8 kG at which the maximum of the radiation power is obtained, are shown in Fig. 4. If one compares these results and those of the previous experiments, two factors, the application of the RF-deflector and the remoteness of the beam collector from the resonator, have permitted one to increase the pulse duration of the radiation power up to 200–300 ns. We characterize the power distribution over the radiation flow cross-section by the visual picture of neon bulb lighting. One hundred neon bulbs were homogeneously distributed on a square panel with sizes 20 20 cm2. The picture of their lighting for the case when the panel was mounted in the shot at the distance 0.5 m from the output window, is presented by the right photo in Fig. 5. The left photo shows the geometry of the panel. The distribution of the light intensity in the right photo is qualitatively close to the one for the radiation power, which is obtained in the ‘‘cold’’ measurements and in numerical simulations for excitation of an H10 wave in the resonator. If the undulator field was equal to zero, no lighting was registered.
Fig. 5. Geometry (left) and picture of lighting (right) of the neon bulb panel under the microwave radiation pulse.
4. Conclusion Theoretical and numerical considerations have shown that a planar FEM with 2-D distributed feedback is suitable for use as a module for constructing multi-channel devices in order to produce microwave radiation of GW level. Computer simulations for the operation of a fourmodule FEM-generator have demonstrated a perfect synchronization of all modules even with a little bit of different energies of the electron beams. The project of a four-channel FEM– generator is designed on the basis of the ELMIdevice.
Acknowledgements The authors would like to thank the Russian Foundation for Basic Research (grant No 01-0216749) and INTAS (grant No 2192) for partially supporting this work.
References [1] A.V. Arzhannikov, et al., Digest of Technical Papers of PPPS-2001, Las Vegas, USA, June 2001, pp. 561–564. [2] N.S. Ginzburg, et al., Nucl. Instr. and Meth. A 475 (2001) 173. [3] N.V. Agarin, et al., Nucl. Instr. and Meth. A 445 (2000) 222.