Properties of a fluctuating double layer in a magnetized plasma column

Wave phenomena at a strong electric double layer were investigated in a magnetized 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 dominated. The low and high frequency fields have characteristic spatial distributions in the form of single humps with the position of the maxima related to the double layer position. 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 characterized 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 ionization in the anode plasma is discussed.