Plasma and Fusion Research

Volume 16, 2402072 (2021)

Regular Articles

Qualitative Improvement of Zeff Diagnostic Based on Visible Bremsstrahlung Profile Measurement by Changing Observation Cross Section in LHD
Yasuko KAWAMOTO1), Shigeru MORITA1), Motoshi GOTO1,2) and Tetsutarou OISHI1,2)
1)
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
2)
The Graduate University for Advanced Studies, SOKENDAI, Toki, Gifu 509-5292, Japan
(Received 26 November 2020 / Accepted 17 February 2021 / Published 7 June 2021)

Abstract

Full vertical profiles of visible bremsstrahlung continuum have been measured for diagnostics of effective ion charge, Zeff , with parallel optical fiber arrays at horizontally elongated plasma cross section in the Large Helical Device (LHD). Measured bremsstrahlung profiles were asymmetric due to non-uniform bremsstrahlung emissions located in the stochastic magnetic field layer. Then, the Zeff profile analysis has been done only for a magnetic configuration with relatively thin stochastic magnetic field layer, i.e. for plasmas at magnetic axis position of Rax = 3.60 m. The effect of the non-uniform bremsstrahlung emission disappears in the lower half profile at Rax = 3.60 m, while the Zeff profile analysis is extremely difficult in other magnetic configurations due to the presence of the non-uniform bremsstrahlung emission over the entire vertical profile from top to bottom. To improve the difficult situation in the Zeff diagnostic the fan array optical fiber system was newly installed at vertically elongated plasma cross section for full horizontal profile measurement. It is found that the non-uniform bremsstrahlung emissions disappear from the signal and the observed bremsstrahlung profile is almost symmetric between inboard and outboard radial profiles. The analysis on the Zeff profile has been carried out based on the plasma equilibrium database in LHD, TSMAP. A preliminary result is obtained in high-density discharges, which shows a flat Zeff profile and values of Zeff ∼ 1.

Keywords

effective ion charge, bremsstrahlung, visible spectrometer, Large Helical Device

DOI: 10.1585/pfr.16.2402072

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