Visible light tomography of MHD eigenmodes in the H-1NF stellarator using magnetic coordinates

A tomographic reconstruction technique is described for the inversion of a set of limited-angle high-resolution 2D visible light emission projections of global MHD eigenmodes in the H-1NF heliac. The technique is well suited to limited viewing access in toroidal devices and the strong shaping of optimized stellarator/heliotron configurations. Fluctuations are represented as a finite sum of Fourier modes characterized by toroidal and poloidal mode numbers having fixed amplitude and phase in a set of nested flux volumes in Boozer coordinates (Boozer 1980 Phys. Fluids 23 904–8). Iterative tomographic inversion techniques and standard linear least-squares methods are used to solve for the complex amplitudes. The method is applied to synchronous camera images of singly charged carbon impurity ion emission at 514 nm obtained at three discrete poloidal viewing orientations (Haskey et al 2014 Rev. Sci. Instrum. 85 033505). 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.