Observations of large-amplitude helical kink instabilities and field reversal in a fast pinch experiment (HBTX-1)

Abstract

Amplification of toroidal flux in compressed stabilized z-pinches in HBTX-1 (R = 100 cm, a = 6 cm) has been observed during MHD instabilities. The increase in flux leading to reversal of longitudinal magnetic field in the outside region when the toroidal flux is conserved is attributed to the large-amplitude stage of an m = 1 helical kink instability which redistributes the magnetic field. The measured values of wavelength, amplitude and growth rate in the early stages of the m = 1 instability observed during self-reversal are predicted with linear MHD stability computations which include dissipative effects. The observed radial expansion of the plasma is interpreted as a non-linear effect associated with the large-amplitude stage of the instability. After the process of self-reversal an approximately constant value of the pinch ratio (defined as the ratio of the average value of Bθ at the wall to Bz, averaged over the plasma) is obtained, which is independent of the initial parameters of the pinch.