Instability of an Alternating-Current Positive Column

Abstract

The positive column of a glow discharge in a magnetic field is known to become macroscopically unstable at some critical magnetic field. The driving mechanism proposed by Kadomtsev and Nedospasov for the growth of a helical (m = 1) perturbation is such that the periodic reversal of the axial electric field at a frequency greater than a critical frequency would be expected to lead to stability. This critical frequency is expected to be approximately the e-folding rate. An experimental study has been made in dc and half-wave rectified-current discharges and in square and sinusoidal ac glow discharges in H2, D2, He, Ne, and Ar at frequencies as high as 70 kcps. Over a wide range of frequencies the half-wave rectified discharges yielded instabilities whose onset and helical nature duplicated those of the dc cases. However, the instability in the ac discharges was indeed suppressed as the frequency of electric field reversal was increased. The frequency-dependent suppression of the instability is compared with theoretical growth rates. The effect of including several m modes is calculated and good qualitative agreement is obtained with the Kadomtsev and Nedospasov model. However, better quantitative agreement is obtained when the Johnson and Jerde modification is employed.