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 location of the main chamber. Tomographic reconstructions are also crucial to perform power balance analysis and to estimate heat fluxes for turbulence studies. One of the issues, associated to 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 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 specific hypothesis, however, cannot be guaranteed in 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 bolometers array is located, can be rectified acceptably well by a physically based correction. The sensible improvements of the bolometric Maximum Likelihood tomography’s outputs are also documented and the possible physical explanation for such 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 not negligible relevance.

On the potential of physics-informed neural networks to solve inverse problems in tokamaks

“…To provide an independent validation, the reconstructions of the most sophisticated tomographic technique deployed on JET, based on the maximum likelihood (ML) approach, are also reported [16][17][18][19]. The ML method has been chosen not only for its accuracy but also because one of its main characteristics is the capability of providing a statistically sound estimate of the confidence interval for each pixel [21,22]. As can be derived from the tomograms reported and the discussion in the following, the reconstructions obtained with the PINN are of more than satisfactory quality.…”

“…For example, most equilibrium reconstruction codes use the one-fluid approximation, assume that the resistivity is zero, that the plasma is in a steadystate toroidally symmetric regime, while bolometer tomography usually assumes that emissivity is toroidally symmetric (this assumption is required when cameras see the plasma in different toroidal positions). Unfortunately, these hypotheses are true as first approximations at best, while if one is interested in extracting more physically meaningful information, more detailed reconstruction should be performed [18][19][20][21][22].…”