Influence of the gas density on the motion of a free plasma piston in the railgun channel

The influence of the gas density on the acceleration of a plasma armature inside the railgun channel filled with various gases (xenon, air, or helium) under atmospheric pressure is investigated experimentally and theoretically. It is shown that, after the discharge current has reached a steady value, the velocity of the glowing plasma front ceases to grow and remains constant as long as so does the current. The length over which the velocity saturates is equal to a few centimeters, i.e., is much shorter than the railgun channel length. The maximum velocity of the plasma piston meets a predicted limit, which is determined by the drag of the medium and a decrease in the acceleration of the plasma armature when a fraction of the material evaporated from the rails is involved into motion. The plasma composition depends on the electrode material. The velocities measured when the channel is filled with helium (V = 17.5 km/s) or air (V = 9.8 km/s) noticeably exceed the sound speed inside the plasma piston (5–7 km/s).