Investigation of a radial transmission line transformer for high-gradient particle accelerators

This paper discusses the use of a photoconductively switched radial transmission line transformer as a high-gradient particle accelerator power system. In particular, a 1.8-m-radius radial line with a one-way transit time of 10 ns was fabricated and the voltage attenuation/amplification of a radially traveling wave measured. These results are compared with numerical solutions to the radial-wave equations using the method of characteristics. The line was operated as a step-down transformer by driving the inner radius with a pulse generator and measuring open circuit voltage at its outer radius. The voltage attenuation of the transformer agrees fairly well with an approximate theory for short-pulse propagation along the line (where pulse lengths are roughly one-tenth the one-way transit time of the line), and a more accurate computer solution using the method of characteristics has correctly predicted the voltage response of the line. These results imply that moderate voltage, short duration pulses (approximately 1 ns) can be amplified to very high voltages by uniformly driving the outside radius of this type of transmission line. The use of photoconductive switches to generate a radially converging, high-power wave transient required for high-gradient particle accelerators is suggested as an alternative to other methods because the switches can be synchronized to within several tens of picoseconds, and control of subsequent sections can be accomplished for relativistic systems