A FEM-oscillator with a reversed guide magnetic field and a Bragg resonator as a RF radiation source for collider applications was studied. The configuration with a step of the corrugation phase is proved to be advantageous. It possesses such features as a high efficiency, precise tunability of the operating frequency and a narrow spectral band. It is demonstrated experimentally that such an oscillator is capable of operating at frequencies of ~30 GHz in single-mode regime with the frequency tuning in interval up to 6%. Frequency and spectrum measurements have been performed with precision of ~0.1%

Elzhov, A.V.

Ginzburg, N.S.

Kaminsky, A.K.

Kazacha, V.I.

Perelstein, E.A.

Peskov, N.Yu.

Sedykh, S.N.

Sergeev, A.P.

Sergeev, A.S.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

JINR-IAP FEM OSCILLATOR WITH BRAGG RESONATORN.S. Ginzburg1, A.V. Elzhov2, A.K. Kaminsky2, V.I. Kazacha2, E.A. Perelstein2,  N.Yu. Peskov1, S.N. Sedykh2, A.P. Sergeev2, A.S. Sergeev11 RAS Institute of Applied Physics, Nizhny Novgorod, Russia2 Joint Institute of Nuclear Research, Dubna, Russia E-mail: artel@sunse.jinr.ruA FEM-oscillator with a reversed guide magnetic field and a Bragg resonator as a RF radiation source for collider applications was studied. The configuration with a step of the corrugation phase is proved to be advantageous. It possesses such features as a high efficiency, precise tunability of the operating frequency and a narrow spectral band. It is demonstrated experimentally that such an oscillator is capable of operating at frequencies of ~30 GHz in single-mode regime with the frequency tuning in interval up to 6%. Frequency and spectrum measurements have been performed with precision of ~0.1%.PACS numbers: 41.60.Cr, 52.75.Ms, 84.40.Ik, 84.40.Fe1  INTRODUCTIONHigh-efficiency  narrow-band  free-electron  lasers (masers) (FEL, FEM) can be used for the application as pulse microwave power sources suitable for testing the high-gradient accelerating structures of linear colliders. Recent investigations at JINR on this subject are orient-ed at the frequencies of ~30 GHz which corresponds to the frequency of  the accelerating microwave field for the CLIC collider [1].2  FEATURES OF FEM-OSCILLATORS US-ING DIFFERENT TYPES OF BRAGG RES-ONATORSSince mid-1990s theoretical and experimental inves-tigations  of  the  FEM-oscillators  with  the  Bragg  res-onators  and reversed guide magnetic  field  are carried out at JINR in collaboration with RAS Institute of Ap-plied Physics (N.Novgorod) [2, 3]. The distributed feed-back in FEM is provided by Bragg reflectors – cylindri-cal waveguide sections with periodically corrugated in-ner surface.Several schemes of the Bragg resonators under in-vestigation are shown in Fig. 1. Initial experiments de-voted to the traditional Bragg resonator with a smooth tube between the reflectors (Fig. 1a) showed that it was possible to obtain both the single-mode and multi-mode regimes of generation in such FEM-oscillators at differ-ent resonator Q-factor values in the steady-state mode of operation.  The  applying  of  such  a  FEM-oscillator scheme  for  supplying  the  high-gradient  accelerating structures  is  coupled  with  certain  technical  problems. Particularly, it is difficult to provide fixing and precise tuning of the FEM frequency [4]. To solve these prob-lems we investigated a FEM scheme, where the feed-back was provided by Bragg resonators  with a  phase step of the mirror corrugation (Fig. 1b, c) [5, 6].For a symmetrical resonator (Fig. 1b) with the corru-gation phase shift equal to π there is the only high-quali-ty oscillation in the reflection zone of the Bragg struc-tures,  located  in  the  middle  of  this  band  (the  central mode). The Q-factors of other oscillations at the edges of the reflection zone (so-called side modes) are consid-erably  lower  than  that  of  the  central  mode.  Thus the electrodynamic selection in a resonator with a corruga-tion phase step results in the excitation of only the cen-tral mode and occurring of the single-mode regime of operation already at the linear stage of the process.Fig. 1. The schemes of the Bragg resonators (on the left): a) double-optical resonator with a section of  regular waveguide; b) symmetrical resonator with corrugation phase step; c) asymmetrical resonator with corrugation phase step. The reflection band and location of the frequencies of the resonator  eigenmodes (on the right).The equality of the microwave fluxes from the res-onator in the forward and in the backward directions is a drawback  of  the  symmetrical  Bragg  scheme.  To  en-hance the power radiated in the forward direction it is profitable to use the non-symmetrical resonator configu-ration (Fig. 1c). However in such a scheme the Q-fac-tors of the side modes grow, so the oscillator can be eas-ier excited at those parasitic oscillations. Optimizing the corrugation depth and lengths of the Bragg reflectors we obtained higher efficiency [7].3  PRECISE FREQUENCY TUNING IN FEM-OSCILLATORBesides the capability of providing the narrow-band RF radiation at a fixed frequency, a FEM-oscillator with a step in the corrugation phase also possesses the possi-bility of precise tuning of the operating frequency. For the corrugation phase shift between the Bragg reflectors ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 60-62.60equal to  π the fundamental  mode is  positioned at  the middle of the Bragg reflection zone. If the phase shift is varied from π to 2π (or to 0) the frequency of the funda-mental mode drifts to the lower edge (or to the higher edge) of the Bragg zone. It is important to note that only one high-Q eigenmode exists inside the Bragg zone at any value of phase shift (if the value of the phase shift does not closely approach the limiting values 0 or 2π). Therefore high selective properties of the resonator are maintained in the major part of the resonator reflection band.  Its width is  proportional to the wave coupling coefficient of the Bragg structures [5]. As a result, in-side this zone precise tuning of the oscillation frequency in an FEM is possible after mechanically changing the value of the phase shift between the Bragg structures.The frequency tuning of a FEM oscillator was inves-tigated by numerical simulations as well as in the exper-iments [4] for three different values of the phase shift between Bragg structures: π, π/2 and 3π/2. It was shown that the measured values of the central frequency and the frequency shift coincided, with an acceptable preci-sion, with the simulation results. The experimentally ob-tained spectral distributions were too broad due to a low accuracy of the measurements at kW power level due to RF breakdown in the RF detector setup. To prevent RF-breakdown in the last series of the experiment the out-put  RF signals  were  attenuated  along  the  waveguide, which transmitted the radiation for a distance of ~ 30 m into the measuring room.The FEM oscillator with a Bragg resonator is experi-mentally investigated at JINR using an induction linac LIU-3000  (electron  beam  energy  0.8 MeV,  current 200 A, pulse duration 350 ns).A schematic overview of the experimental setup is presented in Fig. 2.  The  electron  beam  was  injected from the linac (1) with a repetition rate of 0.5 Hz into the FEM oscillator (2) immersed in a solenoid. A wig-gler with a period of 6 cm producing the transverse hel-ical magnetic field was used to pump oscillating velo-city to the beam.Fig. 2. Scheme of the experiment: 1) accelerator LIU-3000 as the electron beam injector; 2) FEM os-cillator; 3) Rogowski coil; 4) isolator; 5) deflecting magnet; 6) calorimeter; 7) crystal detector of the microwave power; 8) attenuator; 9) wavemeter with the precise tuning of the resonant frequency;  10) insertion ring; 11) master oscillator; 12) RF frequency mixer; 13) digital oscilloscope.The Bragg resonator  was composed of  two wave-guide sections of equal lengths of 170 up to 197 mm, having a rectangular corrugation of period d = 5.64 mm and depth a1 = 0.5 mm. This corrugation provided a se-lective feedback at  an operating frequency range near 30 GHz  by  coupling  the  operating  H11-mode  and  the backward  E11-mode  of  a  circular  waveguide.  Precise tuning of the oscillation frequency in the FEM was per-formed by inserting short sections of a smooth wavegui-de between the reflectors. For an insertion ring (10) of length l the value of the phase shift is Δφ = 2πl/d.After the interaction region the beam was dumped onto the waveguide under the influence of the transverse magnetic field produced by the permanent magnet (5). The beam current was measured by Rogowski coils (3) at the FEM waveguide input and output. Measurements of the time dependence of the microwave power were carried out by calibrated semi-conductor crystalline de-tectors (6).Precise measurements of the radiation frequency and spectrum were carried out after transportation (7) of the radiation into a measuring room and after attenuation to the milliwatt level. Two measurement techniques were used: a narrow-band tunable band-pass waveguide filter (9) and heterodyne mixing (11, 12).The accuracy of the frequency measurement with the tunable filter was about 0.1%. To obtain the radiation spectrum using the filter consecutive measurements had to be done by adjusting the resonant frequency of the filter during a series of RF-pulses. In contrast, the het-erodyne technique allowed measuring the spectrum of a single RF-pulse. The accuracy of the heterodyne tech-ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 61-62.61nique was determined by the error in the master oscillat-or  frequency.  So,  combining  both  measurement  tech-niques, we can determine the absolute value of the oper-ating FEM frequency with a  precision of  about 0.1% and obtain a precise shape of a RF-pulse spectrum.0 50 100 150 200 250 3000,00,51,0Spectral amplitude, a.u.f - fm.o., MHzFig. 3. The RF pulse spectrum obtained by the heterodyne technique (fm.o. is the master oscillator  frequency).The operating frequency and spectrum of the FEM output radiation were measured for various lengths of the insertion ring in the resonator. Figure 3 presents the spectrum of the output heterodyne signal for a corruga-tion phase shift  Δφ = π. The FEM operating frequency is measured to be 29.98 GHz in this case. It is in a good agreement with the value measured using the resonant filter. The spectral band was found to be about 30 MHz (FWHM), i.e. close to the spectral resolution of the fil-ter.  This  value  corresponds  to  the  approximation Δf/f0 ≈ Q-1 where Q is the resonator quality factor which is equal to about 103 in our case.0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,828,529,029,530,030,5side modecentral modeFrequency, GHzCorrugation phase shift, in pi  unitsFig. 4. Dependence of the frequencies of the FEM oscillator modes on the value of the corrugation phase shift between Bragg reflectors.The dependence  of  the  frequency  of  the  “central” mode on the phase shift between the reflectors is shown in Fig. 4 (upper curve). The range of phase shift covers most of the Bragg reflection zone of the resonator, so the tuning range of about 6% has been achieved. The er-rors shown in Fig. 4 were defined mainly by mechanical tolerances in manufacturing and mounting the insertion ring. They increase the uncertainty of the frequency set-ting  up  to  ~0.15%.  The type  of  f(Δφ)  dependence  is close to linear which is in a good agreement with previ-ous  simulations  and  preliminary  experimental  results [4].For phase shift values deviating significantly from π, excitation  of  “side”  eigenmodes of  the  resonator  (i.e. positioned just outside the Bragg reflection zone) is also possible [7] and, indeed, was observed in some regimes at the proper initial mismatches from synchronism (i.e. proper wiggler and/or guide fields). The results of mea-surement for the low-frequency side-mode are also pre-sented in Fig. 4 (lower curve).4  CONCLUSIONSA possibility of creating a high-efficiency, precisely tunable, narrow-band (δf⁄f ~ 0.1%) FEM-oscillator using the reversed guide magnetic field and Bragg resonator has been proved. It has been demonstrated experimen-tally that an oscillator with the step of the corrugation phase is capable of operating at frequencies of ~30 GHz in single mode-regime with the frequency tuning in in-terval up to 6%. Frequency and spectrum measurements by the tunable band-pass waveguide filter and with the heterodyne technique have been performed with preci-sion of ~0.1%. The results on the precise frequency tun-ing are in a good agreement with the previous simula-tions and experiments.This work is supported by grants №№ 00-02-17519, 00-02-17232,  01-02-06249 of  Russian  Foundation  for Basic  Research  and  partially  by  INTAS  grant № 97-32041.REFERENCES1. J.-P.Delahaye et al. CLIC, a 0.5 to 5 TeV e± Com-pact  Linear Collider  //  Proc. of  EPAC’98,  Stock-holm, June 1998, p. 58-62.2. A.K.Kaminsky, A.A.Kaminsky, S.N.Sedykh et  al. Efficiency Optimization of the JINR Narrow-Band Millimeter Wavelength FEL-Oscillator //  Proc. of  the  Free  Electron  Lasers  Conference'96. Eds. G. Dattoly,  A. Renieri.  Elsevier  Science  B.V., 1997, v. II, p. 81-82.3. N.S.Ginzburg, A.K.Kaminsky, A.A.Kaminsky et al. Experimental  Observation  of  Mode  Competition and Single-Mode Operation in JINR-IAP Millime-ter-Wave FEM Oscillator //  Nuclear Instrum. and Meth. 1998, v. A407, p. 167-169.4. A.V.Elzhov,  I.N.Ivanov,  A.K.Kaminsky,  et  al. JINR  Activity  in  Microwave  Sources  for  TeV Range Linear Colliders. // Problems of Atomic Sci-ence and Technology.  Issue:  Nuclear-Physics  Re-search (36). 2000, № 2, p. 103-106.5. V.L.Bratman, G.G.Denisov, N.S.Ginzburg, M.I.Pe-telin. FEL’s with Bragg Reflection Resonators. Cy-clotron Autoresonance Masers  Versus  Ubitrons  // IEEE J.  of  Quant.  Electr.  1983,  v. QE-19,  №  3, p. 282.6. A.V.Elzhov,  N.S.Ginzburg,  I.N.Ivanov  et  al.  Re-cent Experiments on Free-Electron Maser for Two-Beam Accelerators //  Proc. of HEACC’98, Dubna, September 1998, p. 160-162.7. N.S.Ginzburg, A.A.Kaminsky, A.K.Kaminsky et al. High-Efficiency  Single  Mode  FEM-Oscillator Based on a Bragg Resonator with Step of Phase of 62Corrugation  //  Phys.  Rev.  Lett. 2000,  v. 84, p. 3574-3577.ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 63-62.63

JINR-IAP FEM oscillator with Bragg resonator

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JINR-IAP FEM oscillator with Bragg resonator

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