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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 (Vsb) 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 = Vc/Vsb, where Vc 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 ≤ 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., tsw = 16±0.8 ns for air at P = 2.5 bar and Vc = 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.