Properties of a fluctuating double layer in a magnetised plasma column

Abstract

Wave phenomena at a strong electric double layer are investigated experimentally in a magnetised plasma column where the electron gyro frequency is smaller than the plasma frequency. Fluctuations with frequencies below the ion plasma frequency and high frequency fluctuations, peaked close to the electron plasma frequency, are found to be dominating. The low and high frequency fields are shown to have characteristic spatial distributions with the maximum fields occurring in localised regions at the double layer. Spatial correlation measurements show that the high frequency field is associated with axially propagating waves. The phase velocity is nearly constant over a wide range of frequencies and 10 to 20% smaller than the velocity of the electron beam that is formed in the layer. The double layer is characterised by a steep potential gradient in a central region with a thickness of about 20 Debye lengths. This region is surrounded by 'presheaths' where free electrons and ions are accelerated towards the layer. The role of the ionisation in the anode plasma is also discussed.