Development of wall-stabilized z discharges for intense ion-beam transport in inertial confinement fusion facilities

Abstract

The wall-stabilized z discharge has been scaled successfully to the parameters required for a light-ion-beam-driven inertial confinement fusion facility. The electrical behavior of discharges with various gas species, pressures, lengths, and currents has been investigated. These investigations identify the required dielectric strength of the discharge channel wall. A low-mass, low-Z wall construction with sufficient dielectric strength is demonstrated. The discharge internal dynamics have been studied using temporal and imaging interferometry, framing photography, magnetic-field measurement, and spectroscopy. The discharge current radial profile, and its dependence on discharge parameters, has been diagnosed. The discharge consists of a magnetohydrodynamically stable, imploding thick annulus. The observed radial profile explains data from previous transport experiments. Contamination of the discharge by wall material is found to be negligible during the times of interest. These observations motivate a zero-dimensional model of discharge behavior. This model reproduces approximately both the discharge dynamics and the electrical characteristics over a range of parameters. Calculations indicate that the beam ions will lose only 10% of their energy during transport through the discharge in a fusion facility. A conceptual design for a z-discharge transport system is presented. The results of this work confirm that wall-stabilized z-discharge transport is a viable, backup approach to transport in a light-ion-beam-driven inertial confinement fusion facility.