The motion of a theta pinch plasma in a perturbed magnetic field

Abstract

The motion of a theta pinch hydrogen plasma in the presence of a local perturbing magnetic field is described. A small coil outside the plasma was excited by capacitors to produce a dipole field. Experiments were performed on a 3.5m long, collisional, high-beta plasma with a temperature of 60 eV, a radius of 1 cm and a density of 1016 particles cm -3. Sinusoidal currents of 20kA and frequency 106 rad s-1 in a 3-turn perturbing field coil produced oscillating plasma displacements of 1 cm, which propagated along the plasma with a velocity of 20 cm mus-1. The displacement was proportional to the coil current, inversely proportional to the axial magnetic field outside the plasma, and was spatiplly damped with an e-folding length of 20 cm. Using a step current waveform the plasma could be moved to an equilibrium position which was displaced from the initial unperturbed equilibrium. Experiments on a 2 m long low-beta plasma with a temperature of 10 eV showed that propagating plasma displacements of 0.5 cm could be obtained, which were damped in 5 cm.

Predictions of the plasma displacement and velocity are made by considering the motion as long wavelength, m = 1 perturbations about an equilibrium position. Damping mechanisms are introduced by making an analogy between the excited waves and Alfven waves in a dissipative medium.

Feedback stabilization of long wavelength gross modes in theta-pinch plasmas is shown to be possible, using a system of coils similar to those used to excite plasma motion.

Experiments were performed to determine the effect of the initial conditions on the 3.5m theta-pinch parameters, and the results compared with computations. A computed heating rate was obtained; including partial ionization of the initial gas revealed a value of the initial rate of change of axial magnetic field below which no heating occurred. This cut-off was overcome experimentally by using bias fields.