Stability of a Bulged Theta Pinch

An experimental and theoretical study has been made of the hydromagnetic instability of an axisymmetric bulged region in the midplane of a theta pinch 8 m long. The bulged region was generated adiabatically after the initial implosion in such a way that it fulfilled most of the assumptions of the theoretical model; in particular, it was free of end effects for the time of observation. At a filling pressure of 20 m Torr D2 and with a peak magnetic field of 20 kG the plasma, which was collision dominated, had values of temperature, density, and beta on the axis of 120 eV, 3 × 1016cm−3, and 0.7 ± 0.2. The predicted m = 1 instability was observed and for a range of bulge amplitudes its measured growth and propagation along the column agreed to within a factor of less than 2 with the theoretical values. It is concluded that the m = 1 instability of such bulged plasmas is well described by ideal hydro-magnetic theory. Modes with m ≥ 2 were not observed during the m = 1 growth and their absence is attributed to finite Larmor radius stabilization.