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

Determination of the electron temperature T_e and electron density n_e 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 T_e and n_e 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 T e B and electron density n e B determined from the Boltzmann plot method were considerably smaller than those determined from continuum emission (T e C and n e C). 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 T_e and n_e. Using T e C, n e C, and hot electron parameters, the model reproduced both T e B and n e B 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.