Summary of EM launcher experiments performed at LLNL

Performance results for three railguns are summarized. The system used a helium gas-driven injector and railgun launcher to accelerate 1- and 4-g polycarbonate projectiles intact up to 6.6 and 3.0 km/s, respectively. A 625 kJ capacitor bank powered the railgun, and an adjustable inductor provided pulse shaping and peak current control. Operation in hard and soft vacuum was reliably achieved. Projectiles were accelerated without blowby of injector gas or plasma. The diagnostic system measured the projectile position and launch velocity, verified that the projectile was launched intact in the desired direction, and identified system components where improvements could enhance performance. Flash x-ray radiography measured velocity and verified that projectiles were intact. Post-launch projectile travel along the axis of the launcher without tilt was recorded with flash radiographs and impact impressions or holes in witness plates. The system performed as expected up to 4-5 km/s but below expectations at higher velocities. Our diagnostics suggest that the decreased performance was probably caused by the restriking of a second arc in the breech of the railgun, which shunted the current from the propulsive arc. Estimates of ablated launcher mass, drag forces, methods of eliminating restrike, and suggestions for improving the performance of railguns are discussed.