Zero bias emission current in laser heated emissive probe and proper choice of probe-tip material

Abstract

Emissive probes are considered as one of the best diagnostic tools for the direct measurement of plasma potential in low temperature plasma systems. Laser-heated emissive probes (LHEPs), due to their higher lifetime, are becoming more popular than their current-heated conventional counterparts. Graphite due to its higher absorption coefficient at CO2 laser wavelength and LaB6 due to its lower work function are commonly used as the probe materials for laser heated probes. Experiments with a probe-tip made of a graphite material have revealed that the emission processes other than thermionic emission also occur on heating the material with the laser, which can affect the plasma potential measurements and change the I-V characteristics of the LHEP. This additional emission due to the laser irradiation on the graphite-tip explains the measurements of lower values of plasma potentials and also describes the observation of an increase in electron saturation current with the increase in thermionic emission current due to heating. The possible mechanisms of this negatively charged-emission other than the thermionic emission from the graphite-tip have been thoroughly characterized in the absence of plasma. This extra emission is not observed in the probe-tip made up of LaB6, and a good comparison of I-V characteristics could be established with a conventional filament emissive probe in the case of LaB6.