Design and construction of a two-stage opening switch

Abstract

Homopolar generators (HPGs) are modern day energy stores (megajoules) capable of large currents (megamps). The generators are typically low voltage and large capacitance, disallowing loads requiring fast rise times to be driven directly by the HPG. An opening switch must be provided to first, in the closed state, transfer energy from the high current source to an inductive energy store and then open to commutate the current to the load. To charge an inductive energy store with large currents for long times (hundreds of milliseconds) indicates a massive switch with many low resistance contacts. At the same time the switch must commutate current to the load in a short time interval (tens of microseconds) indicating a light, fast-acting device. A two-stage opening switch addresses these two conflicting requirements. The first stage is a massive mechanical switch of coaxial geometry with many low resistance galvanic contacts, which commutates through a low inductance to an explosive element second stage. The second-stage switch is then interrupted explosively to provide the microsecond commutation. The design, manufacture, and testing of the two-stage opening switch were performed at the Center for Electromechanics at The University of Texas at Austin (CEM-UT) Taylor Hall facility in Austin, TX, using a compact homopolar generator (CHPG) and a five-turn coaxial, cryogenic inductor as the pulsed power supply. Initial testing included mechanical actuations and electrical measurements to verify fundamental operation and proper assembly. Testing continued with the CHPG charging the inductor through the switch statically (no commutation) to characterize the current carrying capability of the primary galvanic contacts. Next, tests to check the ability of the first stage to commutate to the second stage by sliding the primary contacts onto the armature insulator were conducted. The fourth phase of testing was a series of two-stage (double) commutations to a fixed load to characterize performance and timing of triggers, voltage hold-off capability, and fixed load commutation times. The fifth and last phase of testing involved the two-stage switch commutating current to a variable impedance (railgun) load.