Investigations of electromagnetic space-charge effects in beam physics

• Main Author: Park, Chong Shik
• Format: Dissertation
• Language: English
• Subjects: philosophy of science; physics, theory
• ISBN: 9781109586688; 110958668X

In order to achieve high quality charged particle beams for electron sources, such as rf photoinjectors, and high-power microwave sources, such as klystrons, the effects of space-charge need to be addressed. In these systems, the beam energy is relatively low while the beam charge is relatively high leading to important space-charge physics. While there have been a number of previous works which have analyzed the physics of space-charge, most of these studies have looked at the problem assuming that the space-charge physics was either purely electrostatic so that analytical techniques could be applied or fully electromagnetic using only computational methods, such as the Yee-PIC algorithm, for simulating these effects. The present study is unique in that it incorporates novel theoretical techniques using time-dependent Green's functions for computing the fields and modeling the space-charge physics analytically and numerically.
This work presents the calculations of time-dependent electromagnetic spacecharge fields for a perfectly conducting pipe with and without a cathode. Using a Lorenz gauge formalism within Maxwell's equations, the beam space-charge fields are computed from a time-dependent Green's function method. In this method, the correct conductor boundary conditions are implemented such that the effects of image charges and image currents due to the cathode and cavity walls are included.
In order to simulate the beam dynamics and electromagnetic space-charge effects in the rf photocathode gun, a new code called IRPSS (Indiana Rf Photocathode Source Simulator), has been developed. The results of IRPSS are compared with electrostatic codes, such as a time-independent Green's function based space-charge algorithm, as well as known beam physics codes, such as PARMELA. We highlight the important numerical challenges and computational limits which we have analyzed during the development of IRPSS. In addition, we compare IRPSS results to an experimental beam loss measurement performed on the Argonne Wakefield Accelerator rf photocathode gun.
We also show how the code can be extended to include important physics for beams with non-negligible transverse currents, such as those found in high-power microwave sources.