On the Influence of Ion Sheaths upon the Resonance Behaviour of a r. f. Plasma Probe

Abstract

The applicability of the r.f. resonance-probe method to the measurement of the electron density in a plasma is investigated in a thermal plasma of low density. The experiments show that in contrast with the investigations published so far, the resonance frequency determined by the r.f. probe does not agree with the plasma frequency but is found always to be lower than this. The difference between resonance frequency and plasma frequency is caused by the ion sheath in front of the probe; the thickness of the sheath determines the amount of the frequency shift. Therefore, the r.f. resonance-probe determination of the electron density is valid only if the geometrical dimensions of the probe environment are taken into account. By the r.f. probe, information on the plasma ion sheath, namely in front of the probe, can be obtained.