Speaker
Description
A new perpendicular FIDA view for MAST-U has been implemented, which restores the dual view capability (including toroidal and vertical) possessed by the FIDA diagnostic on MAST [1], giving a view of the trapped particle regions of phase space and allowing for broader energy resolution because the Doppler shift of the beam emission is close to zero. The original vertical views were removed when the MAST tokamak was upgraded, as the Super-X divertor displaced them. The new system involves lenses in a re-entrant port looking through windows protected by shutters, alleviating many of the problems of the original system where the in-vessel lenses were coated, degrading the throughput. However, owing to the constraints of the existing port infrastructure and the required viewing direction towards the beam, the view is highly oblique through to the viewing window, requiring careful design of the new lens and collection fibre mounts.
It is expected that the first data from this new diagnostic will be presented, and comparisons will be made with predictions from the FIDASIM code. The system will also be described, including details about the passive view, which is mirrored with respect to the active view but originates from the same window. As such, the passive FIDA has a reverse Doppler shift whilst background radiation remains the same. A comparison of the weight functions of the toroidal and perpendicular views will be carried out with a view to orbit tomography [2]. Also, the Solid-State Neutral Particle analyser [3] views deeply trapped particles in the same region.
In lower-density MAST-U discharges, the beam penetrates towards the core, and the toroidal FIDA system observes fast ion transport induced by low-frequency instabilities, including fishbones and long-lived modes [1], as well as TAEs (R Blyth, this conference). The original vertical FIDA system had too weak a signal to detect these effects. Still, many fast ion resonances are expected to affect trapped particles and, hence, the perpendicular FIDA signal more predominantly. The existing toroidal system is of limited utility for interpreting the active when the off-axis beam used or low-field side gas-puffing is employed because charge exchange with edge neutrals [4] produces a bright and toroidally asymmetric passive signal, thought to be due to prompt loss effects. On the other hand, the new perpendicular view is expected to have less passive FIDA signal but views the bright divertor strike-point regions.
- Work supported by DE-SC0019007
[1] CA Michael et al 2013 Plasma Phys. Control. Fusion 55 095007
[2] B. Madsen et al Rev. Sci. Ins. 89 (2018) 10D125-1–5
[3] G. Prechel et al, Rev. Sci. Ins. 93, 113517 (2022)
[4] P Ollus et al 2024 Plasma Phys. Control. Fusion 66 025009
| Presentation type | Poster |
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