Modification of railgun plasma characteristics by spout holes on the bore

In a plasma armature railgun, spout holes made on the bore to exhaust some part of the plasma is one possible way to suppress the plasma armature broadening due to wall ablation. This paper describes how the railgun plasma is modified by the spout holes. Mach-Zehnder interferometry using a He-Ne laser enables the electron density distribution to be obtained over the plasma armature by a single shot. Electron density distribution in two cases of railguns without and with spout holes were compared. The distribution in case of the railgun without spout holes was mostly obtained and the value was 1.3/spl times/10/sup 19/ cm/sup 3/ at most. In the case of a railgun with spout holes, on the other hand, the distribution was partly missed since the laser beam was deflected due to high density gradient in the plasma. However, the obtained fractional distribution was good enough to see a difference from the other. The high density region was much narrower than that in the case without spout holes, namely, the spout holes made the plasma compact.