High-reliability 50-kV switch with 532-nm laser triggering

Model SG-182 is a trigatron switch of our commercial manufacture¹ that mounts directly onto the modified Scyllac terminal of capacitors from General Atomics. In this unique and original design, four Dielectric Sciences 2248 cables are driven through MultiContact® sockets in the upper switch plate. A close-fitting housing (shield canister) provides a low-inductance (27 nH) current return path as well as a mounting platform for the HN-88 re-entrant coaxial connectors. The maximum peak current is 160 kA with a charge transfer of 0.4 Coulombs. Twenty-four of these switches are installed in the single-turn magnet driver at the National High-Magnetic Field Laboratory². Redesign of the switch and implementation of laser triggering greatly reduces the probability of pre-firing and misfiring. Particular attention to details of the internal geometry, choice of electrode material and surface condition, and optimization of the purge-gas flow field results in a switch with greatly improved reliability. This paper describes the redesigned switch and laser triggering experiments with free-space beam transport. A companion paper describes the effort with fiber-optic beam transport and implications for multi-switch systems³. 532-nm laser light is chosen to reduce attenuation in the fiber and because of readily-available equipment. Self-breakdown voltages (V_sb) were measured for dry air, N₂, H₂, N₂: Ar and various attaching gas mixtures such as SF₆: Ar and SF₆:N₂. Breakdown probability data were fit to a Weibull distribution and compared with analogous data taken from a trigatron switch. Operational experience dictates that the “self-break fraction” (f = V_c/V_sb, where V_c is the charging voltage) must be reduced to f = 0.65-0.70 if N = 24 trigatron switches are to be operated with an ensemble pre-fire probability p &#x- 264; 0.01. This low value of f is accompanied by excessive jitter. For the laser-triggered SG-182L switch, f may be increased to 0.80 with superior runtime and jitter (e.g., t_sw = 16±0.8 ns for air at P = 2.5 bar and V_c = 48 kV). Only 1 pre-fire was experienced in >1500 shots under wide-ranging operating conditions, and that fault is likely attributable to an inadequate gas purge. Spectroscopic measurements of the spark give an equivalent blackbody temperature of ≈16 kK. Irradiation of the interior surfaces by copious UV radiation compromises the hold-off voltage of the insulator and may result in flashover. Modification of the insulator profile mitigates this problem.