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"Ill-posed inverse problems in the controlled nuclear fusion neutron measurements", dr Katarzyna Mikszuta-Michalik, IFPiLM

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Similar algorithms can be used to solve inverse problems related to the measurement by different neutron diagnostics on fusion devices. The tomography reconstruction of the neutron emissivity is the tool for studying plasma temperature and density or interactions and transport of the ions during the tokamak operation. An algorithm based on the Minimum Fisher Information Regularisation (MFR) has been implemented to analyse the data obtained by neutron cameras on ITER and JET tokamaks. The MFR algorithm was applied to optimise the Radial Neutron Camera for ITER architecture and detectors type during its design. Investigation of various operation regimes and issues that can appear during the experiment allows for preparing the efficient too for thermal plasma analysis during ITER operation. The total neutron yield was applied to the MFR algorithm as the additional constraint and improved the tomography reconstruction precision. The developed method was used to analyse the experimental data from the neutron camera on JET tokamak collected during the deuterium-tritium campaign in 2021. The tomography calculations provide information about fuel isotopes ratio influence on the plasma stability and MHD activity impact on neutron production. The combination of Tikhonov Regularization, Minimum Fisher Information, Maximum Entropy, and Maximum Likelihood methods was applied to analyse the energy spectra emitted by two kinds of the 14 MeV portable neutron generators based on deuterium-tritium reaction. The activities induced in the Al, Zn, Fe, Ni, Zr, Au by Gradel-Fusion NSD 35 2-DT-C-W gas-plasma target neutron generator were measured by the BrilLanCe 380 scintillator with the lowest uncertainty equal to 10%. The reconstruction results show the domination of the 14 MeV neutrons in the energy spectrum. The impact of the deuterium-deuterium and tritium-tritium reactions is not demonstrated. The lowest energy spectrum reconstruction uncertainty, equal to 18%, was obtained by calculating the mean spectrum from all the considered methods. The neutron emission from the sealed tube neutron generator Genie 16C manufactured by Sodern was monitored by Al, Fe, Ni, Zr, Au, Mg, and Nb foils. The HPGe spectrometer measured their activity with a precision of better than 12%. The best spectrum reconstruction uncertainty was obtained for mean energy spectra and varied from 11% to 15%. A significant impact from the scattered neutrons on the measured neutron intensity makes it impossible to distinguish the presence of the deuterium-deuterium reaction. It is planned to implement the developed methodology on fusion devices like tokamaks.

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Research projects carried out at the IPPLM are funded by the Polish Ministry of Education and Science, the National Science Centre and by the European Commission within the framework of EUROfusion Consortium under grant agreement No 633053. Financial support comes also from the International Atomic Energy Agency, European Space Agency and LaserLab Consortium as well as from the Fusion for Energy Agency.

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