The rise of electron temperature along the magnetic field line was clearly observed in divertor relevant recombining plasma, even though there was no additional electron heating source. Electron temperature obtained at recombination front was approximately 0.5 eV, whereas it increased up to greater than 1 eV at downstream of recombination front. Although this temperature rise is likely common in detached or recombining plasmas, the mechanism has not been understood yet. This report provides a reasonable interpretation of temperature rise along the magnetic field line, for the first time: depletion of low energy electrons due to volumetric recombination leads to deformation of electron energy distribution, resulting in an apparent increase in electron temperature. Our experiment supports this interpretation in that the experimentally observed electron temperature showed good agreement with calculated effective electron temperature.

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.

Validation of ion sensitive probe (ISP) measurements in radio-frequency plasma was investigated using Doppler spectroscopy. Ion temperature Ti obtained by an ISP was consistent with that obtained by Doppler spectroscopy when the recessed distance between two electrodes h satisfied 10ρe<h<10(ρe+λD). However, ISP yielded a larger ion temperature than that obtained by Doppler spectroscopy when the above relation was not fulfilled. The effective ion temperature calculated based on the ion kinetic motion matched well with the ISP measurement result, which indicates that the ISP could overestimate Ti owing to the ion Larmor motion when h is not optimized. The availability of the ISP technique for measuring the radial Ti profile was also investigated. Ti showed a rapid increase in the edge region of a cylindrical plasma and finally became larger than the electron temperature even though there was no direct ion heating power source. Calculations considering the ion kinetic effect well reproduced both the magnitude and the radial trend of ion temperature and ion density near the plasma edge. This result postulates that Ti evaluation could also be inadequate due to the ion kinetic effect when an ISP is placed at the edge and outside a finite boundary plasma.

The dependence of hydrogen plasma parameters on magnetic field configuration and neutral pressure was investigated in the radio-frequency (RF) plasma source DT-ALPHA. It was found that higher electron density was obtained when the lower hybrid resonance condition was satisfied near the RF antenna. It was also found that use of lower hydrogen neutral pressure yielded higher electron density plasma. By optimizing the resonance condition and neutral pressure, the hydrogen plasma of T e ∼ 10 eV and n e > 1 × 10 17 m −3 was achieved.