Chong Shik Park

Computational requirements for Green's function based photocathode source simulations

We demonstrate the computational requirements for a Greens function based photocathode simulation code called IRPSS. In particular, we show the necessary conditions, e.g. eigenmode number and integration time-step, for accurately computing the space-charge fields in IRPSS to less than 1 % error. We also illustrate how numerical filtering methods can be applied to IRPSS in conjunction with a multislice approach, for dramatically improving computational efficiency of electromagnetic field calculations.

Advanced Electromagnetic Analysis for Electron Source Geometries

One of the challenging issues for analytically modeling electron sources, such as rf photoinjectors, is calculating time-dependent electromagnetic fields generated by the electron beam within a complicated conductor boundary. This problem can be handled self-consistently using a time-dependent Green’s function method. We demonstrate this approach for a simplified electron source geometry, namely a semi-infinite circular pipe with a cathode. Following similar Lorentz gauge approaches that were used for computing electromagnetic potentials in low frequency (< 1 GHz) photoinjectors [1,2], we present a compact Green’s function solution for this simplified geometry. As an application, we show the electromagnetic potentials for a disk bunch of charge emanating from the cathode wall with acceleration parameters corresponding to the BNL 2.856 GHz 1.6 cell photocathode gun. [3]

A dispersion free method for modeling space-charge physics in a circular pipe

When modeling the physics of space-charge in high-power microwave sources, such as klystrons, it is necessary to have an accurate description of the space-charge fields which are generated by the time-dependent beam charge and current densities. In general, the method for computing these fields is nontrivial since the motion of charged particles in the beam can be in all three directions and includes the effects of image charges and currents on the surrounding conductor wall. We present a novel theoretical approach and corresponding numerical results for a dispersion free time-dependent Greens function method which can be utilized for calculating electromagnetic space-charge fields due to arbitrary circularly symmetric beam currents in a circular conducting pipe. Since the Greens function can be expanded in terms of solutions to the wave equation, the numerical solutions to the space-charge fields also satisfy the wave equation yielding a completely dispersion free numerical method. This technique is adequately suited for modeling bunched space-charge dominated beams, such as those found in high-power microwave sources, for which the effects of numerical grid dispersion and numerical Cherenkov radiation are typically found when using FDTD type methods. In addition, we also demonstrate how this new method can be used in a parallel computing framework.

Electromagnetic simulations of photocathode sources using green~s function methods

Summary form only given. The inclusion of space-charge fields into beam simulations of photocathode sources can be challenging since the beam is typically tightly bunched (implying rapid spatial and time variations of the fields) and fields are reflected by surrounding conducting structures, i.e. cathode, cavity walls, and irises. Since a Greens function by definition is generated by a Delta function source while satisfying an appropriate boundary conditions, i.e. a conducting boundaries, a Greens function method can be effectively utilized for computing the space-fields in a photocathode source simulation. We demonstrate how these methods are used in a newly developed code called IRPSS (Indiana RF photocathode source simulator) and show initial simulations using IRPSS