Research of Circular Free Electron Laser

实验研究了使用相对论旋转电子束的圆形自由电子激光器(圆形自由电子激光器)的相干微波和毫米波辐射。在圆形自由电子激光器中，电子束在均匀的轴向磁场中作旋转运动。在同轴波导中，电子的运动被一个方向为径向的方位周期摇摆器磁场所改变，在两个导电壁(phi140*200 mm)后面组装永磁体形成一个圆形摇摆器。与传统的线性自由电子激光器相比，圆形自由电子激光器结构紧凑，典型的电子能量为250-500keV，束流为50-200A，引导磁场为380G。观察到典型的可调谐辐射频率为10-40 GHz(500 ns脉冲宽度)，最大辐射功率为1.8 mW(从电子束到辐射的能量转换效率为2.2%)。结果，我们在11.77 GHz(TM(8，1))获得了近乎单模的振荡。寄生振荡Tm(p，1)(p=3，5，6，7，9，10)以Tm(8，1)为主。然而，这些模式的幅度比TM(8，1)主导振荡模式的幅度小20-30dB。我们在振荡时观察到了大约5%的允许失谐量。本文将频率失谐量定义为豆模频率omega_0与振荡频率omega_S之差。圆形自由电子激光器的线性理论表明，圆形自由电子激光器的允许频率失谐量比线性自由电子激光器宽，这种宽的允许失谐量可能导致寄生模式振荡。因此，我们实现了圆形自由电子激光器的近单模和高功率(MW级)振荡，并明确了模式的基本特性。这些结果表明了圆形自由电子激光器作为毫米波和微波高功率振荡器的潜力。

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.