毫米波自由电子激光的数值模拟和实验的比较

COMPARISION OF MICROWAVE FEL NUMERICAL SIMULATIONS WITH EXPERIMENTAL DATA

从波导管毫米波自由电子激光器的设计要求出发,根据Livermore实验室FRED程序的物理思想,编制了空间三维的数值模拟程序(WAGFEL)。为了检验程序的可靠程度,结合ELF装置的实际参数,进行了数值模拟并和实验结果进行了比较。结果表明,把Wiggler磁场B_w增大300 Gs后,WAGFEL程序的模拟结果和Livermore实验室的实验结果基本符合。模拟使用的全部参数,除B_w增大300Gs外,都是ELF的实际参数。模拟时峰值磁场B_w=4050Gs,实验测量峰值磁场B_w=3720Gs,相差在8％左右。WAGFEL程序可以用来从事毫米波自由电子激光器的设计以及基本物理问题的研究。

In this paper, we compare microwave FEL numerical simulations using our code named WAGFEL which is a three-dimensional code, with the experimental data of Livermore Laboratory. WAGFEL evolves the electrons' energies and ponderomotive phase according to the averaged single -particle equations derived by KMR. and the fields according to the paraxial wave equation with the source term as derived byColson. The particle motion is fully three-dimensional. The field solver is two-dimensional, because only the lowest TE01 mode is considered in the rectangular waveguide. In order to obtain good agreement between experiment and theory, the code must include the effect of the longitudinal space-charge force in the KMR equations, the effect of v2<c(v2 is the longitudinal velocity of electron in the wiggler ). and the effect of the shift in signal phase velocity due to the presence of the waveguide. The comparision includes microwave power output as a function of wiggler magnetic field for 1,2, and 3m long wigglers microwave power output as a function of wiggler length: the phase of TE01 mode as a function of wiggler length: the output power as a function of input microwave power: and output power as a function of wiggler length for tapered wiggler. Comparision between numerical simulations using WAGFFL and the experimental data of Livermore shows that the experimental detuning curves for 1,2, and 3m long wigglers agree very well with our simulations except for wiggler magnetic field Bw which is 8 % higher in the simulation. WAGFFL uses all the experimental data without free parrameters. The experimental postsaturation oscillation in power, named synchrotron oscillation, is in agreement with our simulation, but has a difference of about 25% from the analytical value. Through the above comparision it can be concluded that our numerical simulation by WAGFFL is reasonable. Therefore. WAGFFL code can be used to design microwave FEL.