Electron energy distributions in plasmas. I

Abstract

The theory of the positive column is commonly based on the assumption of a Maxwellian energy distribution for the electrons, though in many cases it is far from clear how such a condition may be brought about. Moreover, past work has shown that the distribution is by no means always Maxwellian. Indeed, in any detailed consideration of the rates of inelastic collision processes occurring in the plasma it is desirable to be able to determine the distribution experimentally. This paper reports a method of carrying out a Druyvesteyn analysis electronically, and gives a critical account of its performance. A high-frequency voltage of small amplitude chopped with a certain lower frequency is applied to a spherical probe, and the second derivative of the current-voltage characteristic is found from the amplitude of the sine wave with the chopping frequency present in the probe current. An adequate signal-to-noise ratio is obtained by using narrow band amplification with phase-sensitive detection. A large amount of data has now been obtained with this method in a variety of striated discharges. It is found that in every case studied so far the energy distribution takes the form of two well-separated groups of electrons with sometimes a very small third group. The high-energy group is generated by the potential difference across the striation head and becomes progressively attenuated towards its tail. An effect of the varying distribution is a sudden increase in the net rate of loss of electrons from the low-energy group resulting from the fall in number of electrons capable of ionizing and the change in potential difference between wall and discharge at the end of the striation. This loss of electrons causes a fall in the local discharge conductivity and so gives rise to another potential step and striation head.