The electrical properties of a spherical plasma probe near the plasma frequency

An experimental investigation has been made to find out how a conductor in a plasma behaves when an alternating potential is applied to it with a frequency in the neighbourhood of the plasma frequency. Attention was paid to the way in which the mean current to the probe (the `rectified' current) depended upon the frequency, particularly when it approached the plasma frequency and when the current became abnormally large (`resonance rectification'). The probe-to-plasma admittance was also investigated as a function of frequency. The experimental results on rectified current and admittance are compared with values derived from the theory of Buckley for a spherical plasma probe. This theory gives a detailed description of the way in which these quantities depend on the electron-neutral collision frequency and on the frequency of the applied alternating potential. In many respects previous theories are unrealistic because separate sheath, radio-frequency penetration and unperturbed plasma regions, each having different physical characteristics, have been considered. Buckley, using values of sheath potential computed by Bernstein and Rabinowitz from Poisson's equation for spherical symmetry, has shown that it is not necessary to separate the various regions. The effect of collisions between electrons and neutrals is also taken into account in a realistic way by including a collision term in the Boltzmann equation. The experimental data on rectified current and probe-to-plasma admittance are found to agree with the theoretical data which refer to particular values of the ratio of the electron-neutral collision frequency to angular plasma frequency.