Diffusion and low frequency oscillation of plasma in a magnetic field

The diffusion rate of ions in a plasma is studied with-respect to a longitudinal magnetic field in the range 0∼1,000 gauss (lower than the critical field for screw instability) from measurement of the diffusion current on the wall of the discharge tube. Comparing with the classical ambipolar diffusion theory, a small deviation is found when the magnetic field comes to a certain value. In addition, we investigated an oscillation of the plasma in a magnetic field. As a result of these studies, it is turned out that the plasma becomes unstable suddenly to produce low frequency oscillations at a certain magnetic field where enhancement of the diffusion coefficient appears. It seems to suggest that there is another kind of critical magnetic field for instability under the critical field for screw instability. (author)