Correction of JET bolometric maximum likelihood tomography for local gas puffing

Abstract

Tomography is applied daily to bolometric data in magnetically controlled nuclear fusion devices to infer important quantities of the studied plasmas, such as the emissivity profiles or the radiated power in different locations of the main chamber. Tomographic reconstructions are also crucial for power balance analysis and to estimate heat fluxes for turbulence studies. One of the issues, associated with any tomographic technique, resides in the ill-posed nature of the mathematical problem, meaning that more than one possible emissivity profile is compatible with the measurements within the estimated uncertainties. Therefore, a careful evaluation of the quality of the obtained reconstructions is an important step of the analysis. On Joint European Torus (JET), since the two cameras are located in different toroidal locations, axial symmetry of the plasma radiation must be assumed to perform traditional tomographic reconstructions. Such a specific hypothesis, however, cannot be guaranteed in the case of symmetry-breaking mechanisms, such as strong local gas puffing for fuelling. The present contribution addresses this problem of local plasma fuelling, focusing on the observed significant mismatch between measured and back-calculated projections in specific pulses, including the last JET DT campaign (DTE2) carried out in 2021. The analysis indicates that the observed discrepancy, due to the gas injected primarily from valves located in the same octant where one of the bolometer arrays is located, can be rectified acceptably well by a physically-based correction. The sensible improvements in the bolometric maximum likelihood tomography’s outputs are also documented, and the possible physical explanation for such a phenomenon is discussed. The study performed in this article is expected to have a significant impact on the analysis of JET data and on the physical interpretation of the results obtained during the recent DTE2 experiments, for which a proper evaluation of the radiated power estimates is of non-negligible relevance.