High Current Transmission and Switching System for a Prototype, 20 Tesla, Toroidal Magnet

The Ignition Technology Demonstration (ITD) is a 0.06 scale prototype toroidal field magnet of the proposed full-scale IGNITEX (Ignition Experiment) tokamak. The goal of ITD is to achieve an on-axis magnetic confinement field of 20 T while demonstrating the magnet's ability to withstand high magnetic and thermal stresses [1,2]. To accomplish this task, a peak current of 9 MA must be transferred from six balanced homopolar generator (HPG)/busbar circuits to the liquid nitrogen (LN2) cooled magnet. HPGs are well suited for operation of single-turn coils because they are inherently high current, low voltage machines which can inertially store the energy required for a pulsed discharge. To date the system has delivered pulses of up to 8.14 MA to the toroidal magnet, producing an onaxis field of 18.1 T. In order to properly synchronize current transfer, an explosive closing switch is employed for each of the six independent HPG/busbar circuits. The switches operate by explosively driving a scalloped copper ring into a tapered annular gap made up of two copper alloy rings. With a jitter time of 10 μs, parallel circuit synchronization is better than 0.03% relative to the current rise time. The excellent performance of the switches during discharges of up to 8.14 MA is attributed to several design features which assure proper current distribution. Busbar design considerations have included electromagnetic loading, thermal gradients and magnet preloading effects. The switches and busbars have successfully operated at 82% of their rated action of 1.24 x 1011A2s per switch. Description of the ITD busbar/switching system, design improvements, and operational experience are presented.