X-ray tomography on the TCV tokamak

Abstract

The TCV tokamak offers outstanding variability of the plasma shape. Using x-ray tomography, the shape of the inner flux surfaces of a poloidal cross section of the plasma can be reconstructed, including fast variations due to MHD activity. Both the hardware and the software of the 200 channel system developed for TCV are described.A new 'dynamical' calibration takes actual plasma parameters into account to determine the spectrum-dependent detector efficiency, resulting in an enhanced quality of reconstructions.Tomographic inversions are obtained using a variety of methods such as maximum entropy, linear regularization and a newly developed method based on the Fisher information. The merits of the different algorithms, which have been implemented as MATLAB functions, are compared.Inversion results are analysed with the help of singular-value decomposition, allowing, for example, identification of MHD modes without using any a priori information on the poloidal mode structure.Recent results on the dependence of sawtooth activity on the plasma triangularity are presented to demonstrate the performance of the soft x-ray tomography system.

Characterisation of coherent rotating modes in a magnetised plasma column using a mono-sensor tomography diagnostic

“…Several inversion methods were tested on arbitrary signals and on ghost images, having in mind future analyses on non-coherent modes using an upgraded tomography set-up being installed on Mistral. Among zero, first and second order, minimum Fisher information and SVD regularisations 19 , the second order spatial derivative proved to have the most consistent results and a low sensitivity to noise in our case. Figure 4 shows a ghost image (left), its discretisation (top-right) and the tomography result associated (bottom-right).…”

Radiation distributions in TCV

“…Tomographic reconstructions of foil bolometer and AXUV diode signals were obtained with first order linear regularization on a rectangular grid covering the whole plasma chamber [10]. Based on these images, both the spatial distributions of the radiative power and the time dependent total radiative power emitted by the plasma may be determined.…”

Test ion transport in a collisional, field-reversed configuration

“…There are a variety of methods which lead to the 2D image. Some are analytical and others rely on numerical, matrix-inversion techniques [20]. The analytical methods involve Fourier analysis of the mode structure of the image.…”

“…It was developed by Anton et al [20] as a hybrid of linear regularization (LR) and minimum Fisher information (MFI). The basic idea is to solve the following set of equations:…”