Study on an unmagnetized plasma-loaded two-stage backward wave oscillator and free electron laser

Abstract

DigestFree-electron laser pumped by electromagnetic wave (EM-FEL) is a kind of laser that uses a high-power electromagnetic wave as a wiggler to generate short wavelength radiation. It has potential to produce higher frequency radiation with low energy electron beams than a free-electron laser with a magnetostatic wiggler. In 1983 and 1984, Y. Carmel[l] et al. and G. G. Denisov[2] et al. observed the EM-FEL radiation at millimeter wavebands with a relativistic backward-wave oscillator (RBWO) at 3cm as the electromagnetic wiggler, respectively. In 1995, Z. Liang [3] reported the results of an electromagnetically pumped FEL with a separated FEL interaction region from the RBWO. Plasma-filled RBWO with a strong guiding magnetic field enhances the peak power over the vacuum case by a factor of up to 8[4], and peak power of EM-FEL with frequency from 12GHz to 26GHz was measured in the experiment simultaneously. In this paper, we report the experimental and simulation results of an unmagnetized plasma-loaded backward-wave oscillator and free-electron laser.The experimental system consists of pulse line accelerator, slow-wave structure of RBWO, interaction region of FEL, gas inlet equipment and signal diagnostic system. The accelerator produces a intense relativistic electron beam(REB) with 0.4MV,2.5KA,70ns. The slow wave structure of the RB WO is corrugated cylindrical waveguide, 1 10 1 10 Argon pressure (millitorr) Argon pressure ( million) Fig. 1 Output powers from relativistic BWO and FEL vs. the background gas pressure which the ripple of the inner wall varies as sinusoid, that is, R (2) = R , + h sin (2x2 / d ) , where R = 1 .26 cm is the average radius of the waveguide, h = 0.3cm is the ripple depth, d = 1.95 cm is the wavelength of the ripple.The interaction region of FEL is a smooth cylindrical waveguide with radius 1.26cm and length 6cm. The argon gas injection apparatus involves high-pressure gas bottle, pressure degrading system and air stream adjusting system etc. The gas was injected from the down-stream end. After passing through the high-frequency interaction region, the gas was pumped out by vacuum pump. Signals from RBWO and FEL were measured by dispersion delay line and by two different wavebands millimeter waveguide, respectively.When the REB goes through the background gas, it ionizes the gas to be plasma. As the plasma has a good focus on the electron beam[5], the electron beam can be transported without external magnetic field. The electron beam interacts with structure wave in slow-wave structure to excite 3-cm electromagnetic wave, and then with the electromagnetic wave to generate millimeter wavebands FEL. When the argon pressure changed from 0.6mTorr to 20.0mTon; we observed the following phenomenon:The parameters of REB seldom changed with the variation of the gas pressure. The current and voltage were 2.5KA and 400KV,respectivity.We observed output powers from the RBWO and FEL for argon pressure between 0.75 mTorr to 15 mTorr, 0-7803-6513-5/00$10.00