Anomalous Diffusion and Instabilities of an Argon Plasma in a Strong Magnetic Field

The electron density decay with time in an argon discharge tube energized with a capacitor bank of energy up to 3000 J in a magnetic field up to 30 kG parallel to the axis of the tube is measured by using electron continuum intensity, ion current probe, and microwave techniques. The electron temperature decay is obtained by the spectral line intensity ratio method. The oscillation or instability in the plasma is monitored with electrostatic and magnetic probes. The coefficient of ambipolar diffusion is evaluated from the decay data obtained in the diffusion dominated region. The data thus obtained indicate that the coefficient of ambipolar diffusion agrees with the prediction of the collision theory up to a critical magnetic field of about 1000 G. Beyond 1000 G, the onset of oscillations is detected and the coefficient of diffusion is much higher than the collision theory prediction. For B>1000 G, the coefficient of diffusion varies as B−1 and the absolute values are quite close to Bohm's. It is thought that the mechanism responsible for the anomalous diffusion is the onset of instabilities in the plasma, ion oscillations dominating at low magnetic fields, and the universal drift instability at high magnetic fields.