Results from recent hydrogen pellet acceleration studies with a 2-M railgun

A 3.2-mm-diameter, two-stage, fuseless, plasma-arc-driven electromagnetic railgun has been designed, constructed, and successfully operated to achieve a record velocity of 2.67 km/s for a 3.2-mmD × 4-mmL solid hydrogen pellet. The first stage of this hydrogen pellet injector is a combination of a hydrogen-pellet generator and a gas gun. The second stage is a 2-m-long railgun which serves as a booster accelerator. The gas gun accelerates a frozen hydrogen pellet to a medium velocity and injects it into the railgun through a perforated coupling piece, which also serves as a pressure-relieving mechanism. An electrical breakdown of the propellant gas, which follows the pellet from the gas gun into the railgun, forms a conducting plasma-arc armature immediately behind the pellet, allowing for fuseless operation of the railgun. Studying the pressure profile and the behavior of the plasma-arc armature inside the railgun bore led to eliminating spurious arcing, which prevents operation of the railgun at high voltages (and, therefore, at high currents). A timing circuit that can automatically measure the pellet input velocity and allows for accurate control of arc initiation behind the pellet helps prevent pellet disintegration and mistriggering of the arc initiation circuit. Results from the recent cryogenic operation of the two-stage pellet acceleration system are reported.