Charge-exchange measurements of high-energy fast ions in LHD using negative-ion neutral beam injection

TL;DR

This paper demonstrates a new diagnostic sightline on LHD that effectively measures high-energy fast ions produced by negative-ion neutral beam injection, validated through experiments and simulations, enhancing fast-ion diagnostics in fusion devices.

Contribution

The study introduces a novel tangential sightline geometry for FIDA diagnostics on LHD, enabling measurement of high-energy fast ions from negative-ion beams, which was not previously feasible.

Findings

New sightline confirms measurable FIDA signals from high-energy ions.

Validation against FIDASIM and GNET models supports diagnostic accuracy.

Tangential viewing angle improves detection of high-energy fast ions.

Abstract

A new sightline geometry for the fast-ion D-alpha (FIDA) diagnostic on the Large Helical Device (LHD) has been confirmed to measure signals for high-energy fast ions produced by negative-ion neutral beam injection. The newly installed sightline uses a 180 keV tangential negative-ion neutral beamline as the active source. Due to the small angle between the beamline and FIDA sightline, the relative velocity between fast ions and injected neutrals is small. This allows for high-energy fast ions just below the beam injection energy to produce measurable Doppler-shifted FIDA emission. Experiments were conducted at LHD in order to compare the new sightline, which views a high-energy negative-ion tangential beamline, and the old sightline, which views a low-energy perpendicular positive-ion neutral beamline. The measured FIDA signal is validated against predictions from the synthetic fast-ion diagnostic code FIDASIM with a distribution function modelled by the 5D transport code GNET. The results of the experiment confirm that reducing the viewing angle with a tangential active beam allows FIDA diagnostic to view high-energy fast ions with a sufficient signal-to-noise ratio.