Abstract
Optical transition radiation (OTR) has been studied and applied on the beam diagnostics for decades. The potential implication of OTR also includes THz radiation sources. Therefore, the theoretical analysis and simulation tool become indispensable for the study of OTR. The OTR theory for the wave zone (far-field approximation) has been widely used, and the theory for the pre-wave zone has also been proposed on the basis of single electron approximation. In this study, we developed a theory with consideration of the electron beam structure based on the Kirchhoff’s method for studying the effect of beam transverse size on the angular distribution of OTR in pre-wave zone. The proposed formalism involves complicated convolution integral of functions and the dimension of the integrand is not the low dimension. To perform such integral, we developed a Fortran program for quasi-monte carlo method, which is robust and suitable for high dimensional integration. The disadvantage of this method is that a large amount of samplings may need to be employed to achieve good convergence. To get the radiation angular profile, we need to perform such integration for different observation angles. This is computing intensive, therefore we parallelize the program with MPI. Two-dimensional FDTD simulations were performed to verify the theoretical calculations. The proposed theory and numerical tool would be used to predict radiation properties of NSRRC THz coherent transition radiation (CTR) in the pre-wave zone.