PIM: pre-pulse minimisation and considerations

Abstract

In 2002, an inductive voltage adder (IVA) machine called PIM (prototype IVA module) was commissioned, which had been designed and built at AWE. Originally its purpose was to assist the development of a multi-module X-ray machine for high-energy radiography but more recently it has been adapted for 1-3 MV single-module applications, the PIM machine consists of two inductive cavities (or cells) pulsed in parallel by a water dielectric Blumlein, pulse forming line (PFL), of impedance 10Ω, which is charged by a Marx generator consisting of 32 capacitors each of value 1.35 μF. The design incorporates a pre-pulse reduction system comprising a gas switch at the output end of the PFL and a 6 μF inductor which connects the inner Blumlein conductor to ground. The purpose of the gas switch is to isolate the inner Blumlein conductor from the induction cavities and e-beam diode during the charging phase. A fast output pulse from the Blumlein splits and appears across a Perspex-insulated accelerating gap in each cell. The inductively-isolated voltages add along a short vacuum transmission line and are applied to an e-beam diode. Additional inductive and resistive components were installed in the Marx generator, allowing the Blumlein charging rate to be varied and different electrical configurations to be investigated. Electrical modelling has been performed to determine the scope in the Marx characteristics based on the permissible range of output pre-pulse voltage from the Blumlein. This paper presents results which show that the pre-pulse can be kept within acceptable levels by suitable choice of electrical components in the Marx but that unacceptable levels arise if the choice is inappropriate.