Diffuse reflection model and noise stabilization for tangential image tomographic reconstruction (TITR) code

Abstract

A tomographic reconstruction code had been reported by us for inferring the poloidal emissivity of tokamak plasma from tangentially acquired images. Here we present modifications to the code that account for any diffuse reflections from the surfaces of walls enclosing the plasma. It is generally recognized that such reconstruction codes are highly susceptible to noise in the data. In this work we have analysed the sensitivity to noise for varying degrees of over-determinism in the set of equations; over-determinism is defined as the ratio of the number of detector signals available to the grid resolution of reconstruction. A tractable scheme for dividing the poloidal cross section into a finite number of unknown sub-tori and voids, while still keeping the over-determinism high, is incorporated. Finally it is shown that noise level >20% can be handled with over-determinism achievable from present day detector array/cameras. The singular value decomposition of the matrix, as used here, can be expected to converge even if any ill-conditioned matrix is encountered due to computational round-off errors in the estimation of chord lengths through sub-tori and voids.

Turbulence Velocimetry of Tangential Fast Imaging Data on QUEST

“…Fortunately, the absolute magnitudes of such artifacts are not substantially higher in this case. However, for more accurate estimation of velocity fields, the wall reflection needs to be modeled and compensated for in the raw images before applying ODP-PIV as demonstrated in the TITR code [11].…”

“…A quantitative evaluation of this effect is carried out under the framework of the TITR code [5,11]. It is seen that the effect is negligible in the edge and SOL of the plasma as compared to the plasma core as the density and hence the intensity is appreciably smaller in that region.…”

Plasma tomographic reconstruction from tangentially viewing camera with background subtraction

“…7,15 At the same time, the tangential tomography is based on rather basic methods; e.g., truncated singular value decomposition (SVD) or linear methods have been used. 2,5,6,8,9 One of the reasons are the higher computational demands of the tangential tomography.…”

“…Finally, reflections from the vessel components can disturb the reconstruction. [9][10][11][12][13] Particularly, in the case of the plasma shape reconstruction, reflections from the vessel wall and in-vessel components are of major importance unlike the case of fast fluctuations where the slowly evolving background can be subtracted. In particular, our data obtained from the tokamak COMPASS 14 with a specular steel vessel wall are significantly affected by light reflections.…”

Imaging of turbulent structures and tomographic reconstruction of TORPEX plasma emissivity

“…In previous works, circular plasma pixels were considered. 19 In this work, the poloidal cross-section of the vacuum vessel is divided into square plasma pixels with area of p ϫ p mm 2 . The lowest limit of p depends upon the robustness of the tomographic reconstruction technique against noise and quality of the images.…”