Fiber-optic triggering of a 50-kV switch with implications for large pulsed power systems

Laser-triggered switching in pulsed power systems is a mature technology, however attempts at beam transport with fiber optics yielded limited success until now. The objective of this work is conversion of the 24 SG-182 switches¹ that drive the single-turn pulsed magnet at the National High-Magnetic Field Laboratory² from electrically to laser triggered devices with a concomitant reduction in the probability of pre-firing and misfiring. Electrical triggering also generates troublesome noise that interferes with the proper functioning of diagnostics at the magnet; laser triggering removes the source of this noise. A companion paper describes laser triggering experiments with the newly redesigned SG-182L switch and free-space propagation of the 532-nm, 5.4-ns laser beam³. Here, fiber-optic transport is much more desirable to alleviate frequent and tedious beam alignment of the multiplexed laser system. A “plug-and-play” topology is developed that requires no realignment following routine switch disassembly and maintenance. The system is optimized to be relatively insensitive to small differences in the laser energy between trigger channels and fiber-optic cable routing. Beam launch, transport, collimation, and focusing criteria are defined in an effort to minimize switch runtime and jitter (t_sw + σ). A best-practice compromise between minimum runtime and minimum pre-fire probability is found for f = V_c/V_sb ≈ 0.80, where V_c and V_sb are charging and self-breakdown voltages, respectively. Using thick-clad, step-index fiber with a 200-μm core, switch runtimes as low as 44+3 ns were demonstrated with 2.3 mJ of laser energy (dry air operation at 2.7 bar). Substantially faster breakdown (t_sw = 9.3+0.6 ns) is possible with N₂: Ar mixtures at lower voltages. Reliable triggering with a little as 0.8 mJ of laser energy is possible- 10-m fiber lengths are required because the laser and launch optics must be isolated from the pulsed power platform due to vibration. Because light exiting the fiber cannot be focused as tightly as with free-space beams, the switch breakdown mechanism is different. With free-space beams, optimum breakdown conditions occur when a tenuous plasma filament completely bridges the anode-cathode gap. Here, a spark is initiated at or near the anode surface and the plasma is swept across the gap by the applied electric field, resulting in complete breakdown.