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Evaluation of the Electron Temperature of the Ablation Cloud for the Small-Size Hydrogen Pellet Using Paschen Series in Heliotron J

Akihiro IWATA, Shinichiro KADO¹⁾, Gen MOTOJIMA²⁾, Taiichi SHIKAMA³⁾, Minato MURAKUMO³⁾, Atsuki MORI, Hiroyuki OKADA¹⁾, Takashi MINAMI¹⁾, Shinsuke OHSHIMA¹⁾, Shigeru INAGAKI¹⁾, Fumiyoshi KIN¹⁾, Shinji KOBAYASHI¹⁾, Akihiro ISHIZAWA, Yuji NAKAMURA, Shigeru KONOSHIMA¹⁾, Tohru MIZUUCHI¹⁾ and Kazunobu NAGASAKI¹⁾

Graduate School of Energy Science, Kyoto University, Uji 611-0011, Japan

1)

Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan

2)

National Institute for Fusion Science, Toki 509-5292, Japan

3)

Graduate School of Engineering, Kyoto University, Kyoto 615-8530, Japan

(Received 4 September 2023 / Accepted 16 February 2024 / Published 25 March 2024)

Abstract

We simultaneously measured the electron temperature (T_e) and electron density (n_e) using a low-dispersion near-infrared spectrometer in a small-size pellet ablation cloud in Heliotron J, a medium-sized helical-axis heliotron device. We applied the intensity ratio of the Paschen-α, β, and γ to determine T_e based on the collisional-radiative model, which was fairly consistent with the partial local thermodynamic equilibrium (LTE) in the upper principal quantum numbers of 4, 5, and 6. For a typical pellet injection discharge, T_e and n_e were determined to be 0.9 eV and 4 × 10²¹ m⁻³, respectively. Our derived generalized empirical calibration curve demonstrates a weak influence of T_e on n_e evaluation, particularly in the range of 0.4 - 2.0 eV. Subsequently, we determined the region where the LTE is achieved for the Paschen series.

Copyright (c) 2024 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

pellet ablation cloud, near-infrared spectroscopy, collisional-radiative model, partial LTE, electron temperature measurement, Heliotron J

DOI: 10.1585/pfr.19.1402017

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