Implementing an X-ray tomography method for fusion devices

• Published in: European physical journal plus Vol. 136; no. 7; p. 706
• Main Authors: Jardin, A., Bielecki, J., Mazon, D., Peysson, Y., Król, K., Dworak, D., Scholz, M.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.07.2021; Springer Nature B.V
• Subjects: Algorithms; Applied and Technical Physics; Approximation; Atomic; Calibration; Complex Systems; Condensed Matter Physics; Emissivity; Focus Point on Modern Developments in Applications for Plasma Physics; Image reconstruction; Inverse problems; Iterative methods; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Parameter sensitivity; Physics; Physics and Astronomy; Plasma; Radiation; Redundancy; Regular Article; Regularization; Sensors; Soft x rays; Spatial resolution; Theoretical; Tokamak devices; Tomography
• ISSN: 2190-5444; 2190-5444
• DOI: 10.1140/epjp/s13360-021-01483-z

In fusion devices, the X-ray plasma emissivity contains essential information on the magnetohydrodynamic activity, the magnetic equilibrium and on the transport of impurities, in particular for tokamaks in the soft X-ray (SXR) energy range of 0.1–20 keV. In this context, tomography diagnostics are a key method to estimate the local plasma emissivity from a given set of line-integrated measurements. Unfortunately, the reconstruction problem is mathematically ill-posed, due to very sparse and noisy measurements, requiring an adequate regularization procedure. The goal of this paper is to introduce, with a didactic approach, some methodology and tools to develop an X-ray tomography algorithm. Based on a simple 1D tomography problem, the Tikhonov regularization is described in detail with a study of the optimal reconstruction parameters, such as the choice of the emissivity spatial resolution and the regularization parameter. A methodology is proposed to perform an in situ sensitivity and position cross-calibration of the detectors with an iterative procedure, by using the information redundancy and data variability in a given set of reconstructed profiles. Finally, the basic steps to build a synthetic tomography diagnostics in a more realistic tokamak environment are introduced, together with some tools to assess the capabilities of the 2D tomography algorithm.