Theoretical and Simulation Studies of Transverse Beam Size Effects on Optical Transition Radiation in Prewave Zone

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 tools have become indispensable for the OTR study. Moreover, the OTR theory, for the wave zone (far-field approximation), has been widely used in the literature. However, these theories for the prewave zone have 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 Kirchhoff’s method for studying the effect of beam transverse size on the angular distribution of the OTR in prewave zone. The proposed formalism involves complicated convolution integral of functions and the dimensions of integrand are not low. 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. Therefore, in order to get the radiation angular profile, we need to perform such integration for different observation angles that might be computationally intensive. Therefore, we parallelize the program with the Message Passing Interface (MPI) programming concepts. To verify the theoretical calculations, few two-dimensional FDTD simulations were carried out that show the validity of the proposed model. The proposed theory and numerical tool would be used to predict radiation properties of the NSRRC THz coherent transition radiation (CTR) in the prewave zone.