Study of the Blocking Effect of a Langmuir Probe in Medium Pressure Plasma Measurement

Abstract

The blocking effect of a probe in plasma measurement has been studied theoretically for the cylindrical type probe of finite length and the disk type probe with finite thickness. For a disk probe, the blocking effect increases with increasing probe thickness. In obtaining the electron density Ne, if the probe thickness is not taken into account, the determined Ne will be smaller than its true value. A maximum percentage error of 85% may result with a disk probe if zero thickness is assumed. For a cylindrical probe, the blocking effect increases with increasing probe length. Zakharova's theory assuming an infinite length is not valid at d(=t/rν)≦3,where 2t=L, L is the probe length and rνis the probe radius. This theory is also applicable for obtaining Ne using a large cylinder type sampling probe. Some experiments are presented and discussed.