Spectroscopic measurement of biasing effect on sheath electric field distribution in front of a metal plate inserted in a plasma flow

A model type experiment was made to study the biasing effect on localized electric field distribution in front of a negative DC biased metal disk in the He plasma flow from an ECR plasma source using the polarized laser-induced fluorescence (LIF) technique. A nonlinear decrease of the electric field strength was observed in distance from the electrode surface for the biasing voltage from 300 to 650 V. The sheath thickness variation was found to be proportional to the applied bias voltage to the 3/5 power. The experimental results are explained on the basis of a collisional sheath model. The applicability of this LIF technique to measurement of the electric field distribution in a biased divertor plasma is briefly discussed.