Influence of the formation of a bi-Maxwellian distribution on volumetric recombining plasma spectroscopy

Abstract

Determination of the electron temperature Te and electron density ne is indispensable for evaluating the reaction rates of various atomic and molecular processes. In our numerical work, it was reported that a Boltzmann plot could yield considerably underestimated values of Te and ne when electrons form a bi-Maxwellian distribution [H. Takahashi et al., Contrib. Plasma Phys. 57, 322 (2017)]. To confirm this, helium volumetric recombining plasma spectroscopy was conducted with the DT-ALPHA device. It was found that the electron temperature TeB and electron density neB determined from the Boltzmann plot method were considerably smaller than those determined from continuum emission (TeC and neC). Langmuir probe measurements indicated that the electrons in the helium recombining plasma clearly form a bi-Maxwellian distribution. The collisional–radiative model for a plasma with a bi-Maxwellian distribution was then utilized to interpret the discrepancies in Te and ne. Using TeC, neC, and hot electron parameters, the model reproduced both TeB and neB well. This result indicates that the hot electron components are responsible for the discrepancy, even though their density is much smaller than that of bulk electrons. The present study experimentally confirms the influence of the formation of a bi-Maxwellian distribution on recombining plasma spectroscopy.