Research of Circular Free Electron Laser

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Coherent microwave and millimeter wave emission from a circular free electron laser (circular FEL) using a relativistic roatating electron beam has been experimentally studied. In the circular FEL,an electron beam is subject to rotating motion in a uniform axial magnetic field. The motion of electron is modified by an azimuthally periodic wiggler magnetic field whose direction is radial in the coaxial wave guide.A circular wiggler is created by assembly of permanent magnets behind two conducting walls (phi140*200mm). The circular FEL is compact as compared with a conventional liner FEL.In this device, typical electron energy, beam current and guide magnetic field are 250-500keV,50-200A and 380G,respectively. Tunable radiation frequency was observed to be typically 10-40GHz (500ns pulse duration), and the 1.8MW maximum radiation power (2.2% energy conversion efficiency from the electron beam to the radiation).We measured mode purity of the oscillation. As a result, we obtained nearly single mode oscillation at 11.77GHz (TM(8,1)). The parasitic oscillation TM (p, 1) (p=3,5,6,7,9,10) occurred while TM (8,1) were dominant mode. However, the magnitudes of these modes were 20-30dB smaller than TM (8,1) dominant oscillation mode.We observed about 5% permissible detuning DELTAomega at the oscillation. Here, the frequency detuning DELTAomega is defined as difference between the bean mode frequency omega_0 and oscillation frequency omega_s. The linear theory of the circular FEL suggests that the permissible frequency detuning of the circular FEL is relatively broader than linear FEL.The broad permissible detuning may cause in the parasitic mode oscillation.For conclusion, we achieved the nearly single mode and high power (MW class) oscillation of the circular FEL and mode fundamental characteristics were made clear. These results indicate the potentiality of the circular FEL as a high power oscillator for millimeter wave and microwave.