Experimental characterization of non-Maxwellian electron energy distributions in a miniaturized microwave plasma neutralizer

The electron energy probability function (EEPF) is experimentally identified in a miniature microwave discharge neutralizer over wide operating conditions. A tiny Langmuir probe coupled with the analog differentiation technique offers precise time-averaged EEPFs over about three orders of magnitude, including high-energy, low-density electrons. The non-Maxwellian, concave EEPFs are observed in all experimental conditions. They approach Maxwellian with higher gas pressure, where they deviate from Maxwellian with the absorbed microwave power. Analyzing the measured EEPFs by the generalized EEPF theory, the index that determines the shape of EEPFs (Maxwellian corresponds to 1) is found to be 0.7 +/- 0.15. The ionization frequency is estimated using the measured non-Maxwellian EEPFs, and it is found that one would underestimate the ionization frequency with the Maxwellian approximation about up to ~& nbsp;20%, the value almost negatively correlating with the shape index. These results also suggest that the electron fluid model in microwave discharge neutralizers leads to the overestimation of the electron temperature and the Bohm velocity.