Author: Fujimoto, M.
Paper Title Page
TUP065
 Scanning Problems of FLARE, a THz-FEL Waveguide  
 
  • R.T. Jongma, D. Arslanov, M.A. Vermeulen, A.F.G. van der Meer
    Radboud University, Nijmegen, The Netherlands
  • M. Fujimoto
    ISIR, Osaka, Japan
  • V.O. Yatsyna
    University of Gothenburg, Gothenburg, Sweden
  • V. Zhaunerchyk
    Uppsala University, Uppsala, Sweden
  
  Funding: FLARE is part of the NCAS project funded through the “Big Facilities” programme of the Netherlands Organisation for Scientific Research (NWO).
The (0.2 – 3) THz free-elec­tron laser FLARE is equipped with a wave­guide ex­tend­ing over the full cav­ity length. There­fore, the tun­ing gaps ob­served in the long-wave­length range of FELIX, FELBE and CLIO, which were at­trib­uted to mode-con­ver­sion at the wave­guide free-space tran­si­tions, are avoided. Un­for­tu­nately, an even more se­vere scan­ning prob­lem is ob­served and con­tin­u­ous tun­ing of the pho­ton en­ergy is up to this mo­ment im­pos­si­ble. The ori­gin of this prob­lem is not yet un­der­stood and ex­per­i­ments to gain in­sight into the prob­lem are on­go­ing. We have in­ves­ti­gated the (co­her­ent) spon­ta­neous emis­sion as a func­tion of wave­length, the gain build-up in the vicin­ity of tun­ing gaps, and the op­er­a­tion at a mi­cro-pulse rep­e­ti­tion fre­quency at which only a sin­gle pho­ton bunch cir­cu­lates in the cav­ity. The lat­ter is ex­plored to in­ves­ti­gate if the low-fre­quency mode (the slow wave) that can also build up in a wave-guided cav­ity and trav­els at lower group ve­loc­ity than the elec­tron bunches, in­ter­feres with the ef­fi­cient power build-up of the de­sired high-fre­quency mode in the trail­ing bunches. Sta­tus and re­sults of the ex­per­i­ments will be dis­cussed. 		 
poster icon Poster TUP065 [4.287 MB]  
 
TUP073 High Power Operation of the THz FEL at ISIR, Osaka University 528
 
  • K. Kawase, M. Fujimoto, S. Funakoshi, K. Furukawa, A. Irizawa, G. Isoyama, R. Kato, K. Kubo, K. Miyazaki, S. Suemine, A. Tokuchi, R. Tsutsumi, M. Yaguchi
    ISIR, Osaka, Japan
  
  The THz FEL at Osaka Uni­ver­sity is based on the L-band linac that pro­vides a multi-bunch elec­tron beam with an 8 us du­ra­tion in the en­ergy range from 12.5 to 20 MeV. Al­though the RF fre­quency of the linac is 1.3 GHz, the bunch in­ter­vals are ex­panded to 9.2 ns for the FEL using a sub-har­monic buncher sys­tem that op­er­ates at 108 MHz, to en­hance the bunch charge to 1 nC/bunch. The FEL cov­ers the wave­length range from 30 to 150 um, and max­i­mum en­er­gies of the macropulse and the mi­cropulse are 3.7 mJ and 11 uJ, re­spec­tively, at ~70 um mea­sured at an ex­per­i­men­tal sta­tion. To en­hance the FEL power fur­ther, the elec­tron beam cur­rent can­not be in­creased sim­ply be­cause the beam load­ing in the ac­cel­er­a­tion tube is too high. To solve this prob­lem, we have de­vel­oped a 27 MHz grid pulser for the thermionic elec­tron gun that makes the bunch in­ter­vals 4 times longer and in­creases charge of the bunch 4 times higher whereas the beam load­ing is the same as that in the 108 MHz mode. In this new op­er­a­tion mode, where a sin­gle FEL pulse lases in the cav­ity, we have suc­ceeded in ob­tain­ing the mi­cropulse en­ergy ex­ceed­ing 100 uJ at a wave­length of 68 um.  
 
THP058 Solid-State Switch for a Klystron Modulator for Stable Operation of a THz- FEL 868
 
  • G. Isoyama, M. Fujimoto, S. Funakoshi, K. Furukawa, A. Irizawa, R. Kato, K. Kawase, K. Miyazaki, A. Tokuchi, R. Tsutsumi, M. Yaguchi
    ISIR, Osaka, Japan
  • F. Kamitsukasa
    Osaka University, Graduate School of Science, Osaka, Japan
  
  We have been con­duct­ing stud­ies on up­grade of the THz-FEL and its ap­pli­ca­tions, using the L-band elec­tron linac at ISIR, Osaka Uni­ver­sity. The sta­bil­ity of the FEL is cru­cial for these stud­ies and the op­er­a­tion of the FEL de­pends on char­ac­ter­is­tics of the elec­tron beam, es­pe­cially on sta­bil­ity of the elec­tron en­ergy, which is strongly af­fected by the RF power and its phase pro­vided to the linac. We uses a kly­stron mod­u­la­tor with the a highly sta­ble charg­ing sys­tem to the PFN with a frac­tional vari­a­tion of 8×10-5 (peak-to-peak), but the kly­stron volt­age varies by one order of mag­ni­tude larger due prob­a­bly to the thyra­tron used as a high volt­age and high cur­rent switch in the kly­stron mod­u­la­tor. In order to make the sta­bil­ity of the FEL higher, we have de­vel­oped a solid-state switch using sta­tic in­duc­tion thyris­tors. The per­for­mance of the switch is as fol­lows; the max­i­mum hold­ing volt­age is 25 kV, the max­i­mum cur­rent is 6 kA for the pulse du­ra­tion of 10 us, the switch­ing time is 270 ns, and the max­i­mum rep­e­ti­tion fre­quency is 10 Hz. The in­ten­sity fluc­tu­a­tion of the FEL macropulse is re­duced to a few per­cents using the solid state switch.