A comparative study of sheath potential profile measurements with laser-heated and current-heated emissive probes.

Abstract

Emissive Langmuir probe is one of the most efficient diagnostic tools available for plasma potential measurements. Extensive studies have been carried out in designing different kinds of conventional (electrically heated) emissive probes (CEPs) to estimate the plasma potential. Laser heated emissive probe (LHEP) has been developed with certain advantages over the conventional probes such as low evaporation rate of the probe material, high lifetime, and high emission levels. Most importantly, the LHEP uses laser to heat the probe-tip and does not require electric current to heat the probe-tip like in CEP. The heating current in CEP substantially affects the plasma potential measurements, especially in the regions of plasma where high electric and magnetic field gradients are present. In this paper, we studied the plasma potential structures in sheath-presheath region using both LHEP and CEP in an unmagnetized dc-filament discharge plasma. Measurements of sheath spatial potential profile using laser heated emissive probe are compared with those obtained using conventional emissive probe.