Analysis of the effect of external gas flow on helical instabilities of arc plasmas

The effect of external gas flow on helical instability of arcs is studied b y using an analytical method. The magnetohydrodynamic equations in an elect rostatic approximation serve as the starting point of the theory. Using a l inear time-dependent perturbation theory, the analytic expression that corr esponds to the term of a gas flow stabilizing effect is deduced, and the gr owth rate of the helical instability is given. It is found that in the shor t wavelength perturbation case, the effect of external gas flow is large, s o it is beneficial for stabilizing arcs. However, its effect is very small for intermediate and long wavelength perturbations. At the same time, numer ical results show that the stabilizing effect of the flow on helical instab ility increases with increasing shear viscosity of external gas flow. (C) 2 001 American Institute of Physics.