Results of theoretical and experimental studies of the microwave radiation sources on the base of the induction linac LIU-3000 (JINR, Dubna) are presented. In particular, a FEM-oscillator with the reversed guide magnetic field and Bragg resonator as well as an electron beam buncher in the two-beam accelerator (TBA) driver was studied

Denisov, G.G.

Elzhov, A.V.

Ginzburg, N.S.

Ivanov, I.N.

Kaminsky, A.K.

Kazacha, V.I.

Laziev, E.M.

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 ACTIVITY IN MICROWAVE SOURCESFOR TeV RANGE LINEAR COLLIDERSA.V. Elzhov, I.N. Ivanov, A.K. Kaminsky*, V.I Kazacha, E.A. Perelstein, S.N. Sedykh, A.P. SergeevJoint Institute for Nuclear Research, Dubna, RussiaG.G. Denisov, N.S. Ginzburg, N.Yu. Peskov, A.S. SergeevInstitute of Applied Physics of RAS, Nizhny Novgorod, RussiaE.M. LazievYerevan Physical Institute, Yerevan, ArmeniaResults of theoretical and experimental studies of the microwave radiation sources on the base of the in-duction linac LIU-3000 (JINR, Dubna) are presented. In particular, a FEM-oscillator with the reversed guide magnetic field and Bragg resonator as well as an electron beam buncher in the two-beam accelerator (TBA) driver was studied.PACS: 41.60.Cr, 52.75.Ms, 41.75.Lx, 41.75.Ht1. INTRODUCTIONResearch  works on  the  creation  of  powerful  pulse sources of the millimeter range coherent radiation were begun at  JINR in  1983  by V.P. Sarantsev's  initiative. Soon the applied implementation of these studies was formulated: development and creation of high-efficiency narrow-band free-electron lasers (masers) (FEL, FEM) on  the  base  of  JINR linear  induction  accelerators  for their application as pulse microwave power sources suit-able for tests (up to the nominal level) of the high-gradi-ent accelerating structures of linear colliders. In the in-vestigations another orientation has clarified itself. It is the creation of an electron beam buncher on the base of the FEM or travelling wave tube (TWT). Recently the frequency fixation (30 GHz) has been defined that cor-responds 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-ONATORS2.1. PROBLEMS OF OSCILLATOR FREQUENCY SETTINGIn 1994−1996 theoretical and experimental investi-gations of the FEM-oscillators with the Bragg resonators and reversed guide magnetic field were carried out at JINR in collaboration with IAP (N. Novgorod) [2,3]. At such magnetic field orientation the direction of the elec-tron cyclotron rotation is opposite to the direction of the electron rotation in a helical wiggler (this field direction is marked by the sign “−“ in the text).In the initial experiments we used mainly the tradi-tional  optical  Bragg resonator  with a smooth tube be-tween corrugated mirrors (Fig. 1,a).  Generation of  H11 mode radiation was carried out in the frequency interval of 30−40 GHz. _____________* Corresponding author: e-mail: alikk@sunse.jinr.ru The  computer  simulation  and  the  experiments showed that it  was possible to obtain both the single-mode and multi-mode regimes of the generation in such FEM-oscillators with different resonator Q-factor values in  the  steady-state  mode  of  operation.  With  the  res-onator Q-factor value about 300−400 the oscillator pro-vided the single-mode generation with the efficiency 20−25%. When the resonator Q-factor was increased up to 1000−1500, the radiation spectrum was multi-mode and the oscillator efficiency decreased down to 8−12%. The detailed description of the simulation results and experi-mental data contains in [3].Fig. 1. The schemes of the Bragg resonators (on the  left): a) double-optical resonator with the section of the  regular waveguide; b) symmetrical resonator with the  corrugation phase step; c) asymmetrical resonator with  the corrugation phase step. The reflection band and lo-cation of the frequencies of the resonator eigen modes (on the right).Comparison  of  the  obtained  experimental  results with the parameters of the microwave power source re-quired for the high-gradient accelerating structure of the CLIC collider is presented in Table. One can see that sa-tisfactory results  concerning the radiation  pulse  width and microwave pulse duration were obtained at the fre-quency close to the collider operating frequency. Low level of the output power was caused by the small elec-tron beam power in the accelerator LIU-3000 (0.8 MeV, 200 A).ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2000, № 2.Серия: Ядерно-физические исследования (36), с.103-106.103Comparison of the microwave power source charac-teristics required for supply of the CLIC accelerating  section with characteristics obtained for the FEM oscil-lator at JINR by 1997.CLIC de-mandsObtainedFrequency, GHz 30 31Spectrum width, % 0.3 0.25Power, MW 200 35Pulse duration, ns 140 40−100However, for the utilization of such FEM-oscillator scheme  for  supply  of  the  high-gradient  accelerating structure some complicated technical problems must be resolved.  First,  the  mode  of  operation  of  the  single-mode  generation  in  the  stated  FEM  configuration  is achieved at the final stage of interaction as a result of competition of several longitudinal modes being inside of the reflection band of the Bragg structures composing the  resonator.  These  modes  have  the  close  Q-factors. Relative shift between them is about 0.5%, and they all grow at the linear stage of the oscillator operation. Con-sequently, to obtain the generation at the precisely fixed frequency, it is necessary to provide the stability of the electron beam energy and the FEM magnetic fields bet-ter than a part of percent.Second, it is necessary to have a precise matching of the FEM-oscillator and mentioned structure frequencies for supply of the collider accelerating structure. The typ-ical Q-factor values of the structure are equal to 300. So the  upper  limit  of  the  admissible  frequency mismatch composes δω⁄ω ≈ 0.3%. The admissible error of the cor-rugation period must not exceed 0.1%, i.e. 5  µm if the corrugation period of the Bragg mirrors is about 5 mm. It is extremely difficult to meet so rigid requirements on the accuracy of the mirror fabrication, the magnetic field values and electron energy.To solve the problems of fixing and smooth tuning the  FEM  frequency  we  investigated  a  FEM  scheme, where the feedback was carried out with the help of the Bragg resonators with the phase step of the mirror corru-gation (Fig. 1,b,c) [4,5].  The resonator consists of two Bragg mirrors of lengths  L1 = L2 = L/2 (Fig. 1,b) (with-out the regular waveguide between them) with the corru-gation phase step between the mirrors. There is the only high-Q oscillation  in  a  resonator  with the  phase  step equal to pi in the reflection band of the Bragg structures, located in the middle of this band (the central mode). The Q-factors of the other oscillations,  being close to the reflection band boundary (so-called side modes), is much less the Q-factor of the central mode. Thus, the electrodynamic selection in a resonator with a corruga-tion phase step provides such conditions in a FEM-oscil-lator when only the central mode excites and the single-mode  mode  of  operation  occurs  already  at  the  linear stage of the process.The equality of the energy fluxes from the resonator in the forward (along the electron beam movement) and in  the  backward  directions  is  the  shortcoming of  the symmetrical  Bragg scheme (Fig. 1,b).  One-way output of the radiation in the collector direction can be provid-ed in the asymmetrical resonator with unequal lengths of the Bragg structures that is for shorter output structure (Fig. 1,c).As a result of optimization of the corrugation depth and lengths of the Bragg mirrors very high for  FEM-sources  efficiency  (up  to  35%)  was  obtained  in  this scheme of the FEM-oscillator [6].2.2. PRECISE FREQUENCY TUNING IN FEM-OSCILLATORIn  the  FEM-oscillator  with  the  corrugation  phase shift there is a possibility not only to fix the oscillation frequency but also provide the precise tuning of this fre-quency. The results of the computer simulation of the FEM-oscillators with three different values of the corru-gation  phase  steps  (on  pi,  pi/2  and  3pi/2)  between  the Bragg structures are presented in Fig. 2.a)b)c)0 100 200025500 100 2000255030 31 320130 31 320130 31 32010 100 20002550S P E C T R U M   o f   R A D I A T I O N  (a.u.)E L E C T R O N   E F F I C I E N C Y  (%)FREQUENCY (GHz)T I M E (ns)Fig. 2. Simulation results for dynamics in the FEM oscillator for different values of the corrugation phase step φ: a) φ = pi; b) φ = pi/2; c) φ = 3pi/2.Changing the step of the corrugation phase by pi (that is the value corresponding to the half of the corrugation period)  results  in  the oscillator  frequency shift  of  ~ 1 GHz. We have the frequency shift  δω⁄ω ~ 5−6% when the length of the Bragg structure changes on 5 mm. So the range of the frequency tuning in the resonator with the step of corrugation phase is sufficiently larger than for the traditional resonator scheme.It is important to note that under definite conditions the oscillator efficiency may be maintained practically constant when the phase step changes (see Fig. 2,a-c).The scheme of the experimental setup was similar to one used in [5,7]. The electron beam energy of the ac-celerator LIU-3000 was increased up to 1.0 MeV. The electron beam currents before the injection into the FEM waveguide and at its output were measured to be 200 A and 160−180 A respectively. The beam pulse duration was about 250 ns.The  Bragg  resonator  consisted  of  two  contiguous corrugated waveguides 26 cm and 13 cm long with the bore diameter 19 mm. The period of the corrugation and its depth were equal to 5.4 mm and 0.6 mm respectively. Changing the length of the initial section of the output Bragg structure we modified the value of the corrugation phase step. The power measurements were carried out with the help of two pulse crystal microwave detectors (measuring and monitor ones).  The narrow-band filter set in the measuring detector scheme was used for the RF frequency defining in contrast to the cut-off filters 104used in the earlier experiments. The transmission band-width  of  the  filter  at  the  level  of  3  dB  was  about 0.4 GHz. The  measured  output  RF power  in  the  fre-quency range 29−31 GHz (H11 mode) was at the level of 25−35 MW. The power level was not optimized. The ra-diation pulse duration was equal to 70−130 ns. The res-ults of the frequency tuning measurements are given in Fig. 3.29.0 29.5 30.0 30.5 31.0 31.5 32.00.00.20.40.60.81.0RF power, a.u.a) 29.0 29.5 30.0 30.50.00.20.40.60.81.0b)28.0 28.5 29.0 29.5 30.0 30.5 31.00.00.20.40.60.81.0RF power, a.u.c) 29.5 30.0 30.5 31.0 31.50.00.20.40.60.81.0d)Frequency, GHzFig. 3. Output  RF power versus  the frequency  for different values of the corrugation phase step φ: a, b)  φ = pi; c) φ = 3pi/2; d) φ = pi/2.Dependencies in Fig. 3,a and 3,b correspond to the phase step pi between the Bragg reflectors and were ob-tained at the wiggler field values of .1550 T and .146 T respectively. The guide magnetic field was equal to –.206  T.  Fig. 3,a  corresponds  to  the  excitation  of  the central mode of the Bragg resonator, and Fig. 3,b cor-responds to the side mode, i.e. mode shifted on the half of width of the resonator reflection band. Measurement of the frequency of the FEM-oscillator central mode in the case of the phase step 3pi/2 (Fig. 3,c) yields the fre-quency value  close  to  the  measured  one  for  the  side mode for the phase step  pi (Fig. 3,b).  When the phase step was equal to  pi/2 (Fig. 3,d)  the oscillator  was not started up at the central frequency though the generation on the shifted downward frequency was observed.From the comparison of the obtained results with the data of the computer simulations it follows:− measured values of the central frequency and the frequency shift coincide with a good precision with the simulation results;− the experimentally obtained spectral distributions have  the  broader  frequency  spectrum  in  comparison with the  simulations  results.  Reasons  of  the  spectrum broadening may be connected with the used method of measurement and demand a separate examination.It is worth noting that the oscillator parameters in the given results of the computer simulations were chosen so that the single-mode generation rose already at the linear stage of the oscillator operation. However, in the experiment  both  the  multi-mode  and  single-mode regimes  of  the  generation  were  possible  at  the  linear stage.The  electron  beam bunching  corresponding  to  the frequency of 30.6 GHz was registered with the help of a picosecond streak camera in these experiments as well as in the TWT experiments (see Section 3).3. INVESTIGATION OF THE BEAM BUNCHING IN THE TWT AMPLIFIERA novel scheme of  two-beam accelerator  driver  is being currently studied at  JINR [8].  In  this  scheme a synchronous microwave accompanies the electron beam along the whole driver. It should provide the high spatial phase stability of the output radiation. The scheme of the TWT amplifier is being studied for creation of the buncher for the low-energy electron beam.The  electrodynamic  structure  of  the  amplifier  had the length about 45 cm and main radius 9.1 mm. The scheme of the experiments is shown in Fig. 4.Fig. 4. Scheme of the experiment: 1−electron gun; 2−electron  beam;  3−accelerating  sections;  4−tuning  magnetic lens; 5−microwave transmission line; 6−mag-netron;  7−solenoid;  8−corrugated  cylindrical  waveg-uide;  9−quartz strip;  10−deflecting magnets;  11−opti-cal Cherenkov radiation; 12−microwave radiation; 13−RF  detector;  14−optical  mirror;  15−streak  camera IMACON-500.The  slow  electric  type  wave  E01 of  the  oversize cylindrical waveguide with corrugated wall was chosen as an operating wave. The entrance signal was transmit-ted into the operating waveguide (8) from the magnetron (6) with the help of the quasi-optical system previously used in [9]. The microwave losses from the magnetron to the TWT input were ~ 13 dB. The electron beam pa-rameters were as follows: 660 keV, 200 A, 200 ns. The uniform guide field of 0.1−0.2 T was used in these ex-periments and the input beam radius was tuned equal to 4−5 mm to reach the maximal output RF power.The phase wave velocity value βph = 0.86 at the op-erating magnetron frequency f = 36.4 GHz was close to the electron beam velocity. The measured maximum of the amplified microwave power was equal to ~ 5 MW with the gain ~ 30 dB. Coincidence with the correspond-ing calculations was rather good.We used a quartz strip with the thickness of 3 mm, width of  3  mm and refractive  index  n = 1.46  for  the beam bunching measurements. The target was placed on the accelerator axis at the distance ~ 80 mm from the 105TWT  output.  The  electron  bunches  moved  from  the TWT exit  to the quartz strip  accompanied by the mi-crowave,  passed  through  the  strip  and  generated Cherenkov radiation which was taken out through the vacuum window. Hereupon Cherenkov radiation was di-rected to the input slit of the streak camera with the help of the optical mirror.The  registered  set  of  bunches  is  shown in  Fig. 5. Sweep speed is equal to 13.3 ps/mm. Measured distance between bunches corresponded to the wavelength about 8.2  mm that  coincides  rather  well  with  the  working TWT frequency 36.4 GHz.0 40 80 120 160 200Time, ps04080120Intensity, a.u.0 40 80 120 160Frequency, GHz02004006008001000Fourier amplitude, a.u.Fig. 5. Temporal  profile  of  the  bunched  electron  beam recorded by the streak camera (top). Digitized in-tensity of streak-camera image (in arbitrary units) plot-ted versus time (bottom, left). The corresponding Fouri-er spectrum (bottom, right).The  experimental  bunching parameter  was defined as the ratio of the Fourier components at 36.4 GHz and zero frequency, respectively. The measured value was ~ 0.4.  It  is  close  to  the value obtained in  [10]  at  the beam energy of 2.2 MeV.The  numerical  simulation  shows  that  the  beam bunching  completely  disappears  at  ~ 4 cm  after  the TWT output if  the amplified microwave does not  ac-company the beam. At the same time, the fulfilled ex-periments confirmed that the electron bunches keep high level of the bunching parameter at the distance ~ 10 cm from the TWT exit being accompanied by the amplified microwave.4. CONCLUSIONSAs a result of the computer simulation and experi-mentally it was proved a possibility of creating a high-efficiency  (η ~ 25−30%),  precisely  tunable,  narrow-band  (δω⁄ω ~ 0.1−0.3%)  FEM-oscillator  using  the  re-versed guide magnetic field and Bragg resonator.The power of 5 MW in the E01 mode with the bunch-ing parameter about 0.4 was obtained in the first set of the experiments aimed at the development of the TWT amplifier used as the electron beam buncher in the new scheme of the TBA driver.The  electron  beam bunching was registered  with the help of Cherenkov radiation in the FEM oscillator and  TWT  amplifier  at  the  frequencies  of  30.7  and 36.4 GHz respectively.ACKNOLIGMENTThis work is supported by grants №№ 00-02-17519, 00-02-17232,  98-02-17685  of  Russian Foundation  for Basic Research and the 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. 58.2.A.K. Kaminsky et  al.  Efficiency Optimization  of the JINR Narrow-Band Millimeter Wavelength FEL-Oscillator // Proc. of the Free Electron Lasers Con-ference'96. Elsevier  Science.  B.  V. 1997.  V. II. P. 81.3.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. 167.4.V.L. Bratman  et  al.  FEL’s  with  Bragg  Reflection Resonators.  Cyclotron Autoresonance Masers Versus Ubitrons // IEEE J. of Quant. Electr. 1983. V. QE-19. No 3. P. 282.5.A.V. Elzhov et  al.  Recent  Experiments  on  Free-Electron Maser for Two-Beam Accelerators // Proc.  of HEACC’98, Dubna, September 1998. P. 160.6.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 Corrugation // Phys.Rev.Lett. 2000. V. 84. P. 3574.7.C.A. Goldenberg, A.A. Kaminsky, A.K. Kaminsky et al. FEL-oscillator for Feeding of High-Gradient Accel-erating Structure // Proc. of PAC'99, New York, 1999. P. 1055.8.A.V. Elzhov,  A.A. Kaminsky,  A.K. Kaminsky  et al.  New Scheme of Two Beam Accelerator  Driver Based on Linear  Induction Accelerator  //  Proc.  of  EPAC’98, Stockholm, June 1998. P. 472.9.Abubakirov E.B et al. Generation and Acceleration of  High-Current  Annular  Electron  Beam in Linear Induction Accelerator  and  Microwave Power  from Cherenkov TWT //  Proc. of EPAC’90, Nice, 1990. P. 34.10.J. Gardelle  et  al.  Experimental  Coupling  of  35 GHz RF-Cavities with an Intense Bunched Electron Beam //  Proc. of EPAC’98, Stockholm, June 1998. P. 463.106

JINR activity in microwave sources for TeV range linear colliders

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JINR activity in microwave sources for TeV range linear colliders

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