3D Tomography of MHD Fluctuations in the H-1NF Heliac

TL;DR

This paper presents a 3D tomographic method for reconstructing MHD eigenmodes in a stellarator device using limited-angle visible light emission data, overcoming challenges of toroidal geometry and restricted viewing access.

Contribution

It introduces a novel 3D tomographic reconstruction technique tailored for toroidal devices, utilizing Fourier mode representation and iterative inversion methods for high-resolution imaging.

Findings

High-quality reconstructions of radial fluctuation structures

Clear determination of poloidal mode numbers

Some differentiation of toroidal mode numbers

Abstract

A 3D tomographic reconstruction technique is described for inversion of a set of limited-angle high-resolution 2D visible light emission projections (extended in the vertical and toroidal directions) of global MHD eigenmodes in the H-1NF heliac. This paper deals with some of the features and challenges that arise in the application of tomographic imaging systems to toroidal devices, especially limited viewing access and the strong shaping of optimised stellarator/heliotron configurations. The fluctuations are represented as a finite sum of Fourier modes characterised by toroidal and poloidal mode numbers having fixed amplitude and phase in a set of nested cylindrical flux volumes in Boozer space. The complex amplitude is calculated using iterative tomographic inversion techniques such as ART, SIRT and standard linear least-squares methods. The tomography is applied to synchronous camera images of singly charged carbon impurity ion emission at 514nm obtained at three discrete poloidal viewing orientations. It is shown that the 2D amplitude and phase projections provide high quality reconstructions of the radial structure of the fluctuations that are compact in Boozer space and allow clear determination of the poloidal mode number as well as some degree of toroidal mode number differentiation.