Rayleigh-Taylor instabilities in railgun arcs

Plasma arcs used to accelerate projectiles in railguns are examined to determine their susceptibility to two types of Rayleigh-Taylor instabilities. The model used is similar to that developed by Tsai, Liskow and Wilcox (1981), although a number of modifications are made and discussed. It is found that the plasma is unstable to both the formation of current filaments and interchange motion, although the growth rates in the former case are usually too small to be of practical interest. A principal factor which affects the stability of the arc is shown to be the ratio of the arc to projectile mass, beta, with the growth rates of the instabilities increasing as beta increases and the plasma becoming stable as beta tends to zero. Physical reasons for this behavior are discussed. An attempt is made to reconcile the results with the existing experimental data, but the need for more work, both theoretical and experimental, is indicated.