Research of Circular Free Electron Laser

用相对论性旋转电子束对圆形自由电子激光器的相干微波和毫米波发射进行了实验研究。在圆形自由电子激光器中，电子束在均匀的轴向磁场中进行旋转运动。电子的运动由同轴波导中径向方向的方位周期性摆动磁场来改变。环形摆动器由两块导电墙（直径140*200mm）后面的永磁体组装而成。与传统的线性FEL相比，圆形FEL结构紧凑。在该装置中，典型的电子能量为250 ~ 500kev，电子束电流为50 ~ 200a，导流磁场为380G。典型的可调辐射频率为10-40GHz（脉冲持续时间500ns），最大辐射功率为1.8MW（电子束到辐射的能量转换效率为2.2%）。我们测量了振荡的模态纯度。因此，我们在11.77GHz （TM(8,1)）处获得了近单模振荡。寄生振荡TM (p, 1) （p=3、5、6、7、9、10）在TM（8、1）为优势模态时发生。然而，这些模态的幅度比TM（8,1）主导振荡模态小20-30dB。我们观察到在振荡时大约有5%的允许失谐。在这里，频率失谐delta被定义为bean模式频率omega_0和振荡频率omega_s之间的差。圆形FEL的线性理论表明，圆形FEL的允许频率失谐范围比线性FEL宽。宽允许失谐可能引起寄生模振荡。综上所述，我们实现了环形FEL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.