Abstract
A set of soft x-ray cameras provided measurements of high frequency instabilities as well as steady-state emission in the Mega Amp Spherical Tokamak (MAST). It is shown that Abel inversion can be readily applied to fluctuating soft x-ray emission from the MAST midplane associated with fast particle-driven ‘fishbone’ instabilities, characterised by toroidal mode number n = 1. Each fishbone burst had an early phase in which high amplitude fluctuating soft x-ray signals from the plasma core were close to being in phase with each other, and there was a region close to the outboard plasma edge in which the fluctuations were relatively weak and in antiphase with those in the core. The major radius of the ‘phase axis’ at which the mode amplitude changed sign Rp was initially outboard of the tokamak magnetic axis at R0, but moved inboard during the burst, eventually becoming close to R0, at which time the oscillations were of similar amplitude inboard and outboard of Rp. The fishbone radial structure early in the burst can be understood in part by recognising that the mode is supported by energetic ions with a high average toroidal rotation rate: in a co-rotating frame, the effective magnetic axis is shifted outboard by a distance that is comparable to the difference between the major radii of the phase axis early in the burst and the laboratory frame magnetic axis. It is conjectured that the transition to a mode with Rp ; R0 occurred because most of the energetic ions were expelled from the plasma core region where the mode amplitude peaked, so that the instability could no longer be characterised as an energetic particle mode. Abel inversion of fishbone soft x-ray emission thus provides useful insights into the nature of energetic particle modes in tokamak plasmas and their relationship with MHD modes.
| Original language | English |
|---|---|
| Article number | 034001 |
| Journal | Plasma Research Express |
| Volume | 3 |
| Issue number | 3 |
| DOIs | |
| Publication status | Published - 20 Sept 2021 |
Bibliographical note
Funding Information:The authors are grateful to Neal Crocker and Henry Wong (both University of California Los Angeles) for helpful conversations. This work was funded partly by the RCUK Energy Programme [grant number EP/ T012250/1]. It was also carried out partly within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement number 633 053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. To obtain further information on the data and models underlying this paper please contact [email protected].
Funding
The authors are grateful to Neal Crocker and Henry Wong (both University of California Los Angeles) for helpful conversations. This work was funded partly by the RCUK Energy Programme [grant number EP/ T012250/1]. It was also carried out partly within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement number 633 053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. To obtain further information on the data and models underlying this paper please contact [email protected].
Keywords
- Abel inversion
- Fast particles
- Instabilities
- Soft x-ray emission
- Spherical tokamaks
ASJC Scopus subject areas
- Nuclear and High Energy Physics
- Nuclear Energy and Engineering
- Condensed Matter Physics