New developments in JET gamma emission tomography

JET neutron profile monitor is a unique instrument among neutron diagnostics available on large fusion research facilities [1-2]. The plasma coverage of the emissive region enables tomographic reconstruction of the spatial profiles of the ?-ray emission [3]. However, due to the availability of only two projection angles and to the coarse sampling, tomography is a highly limited data set problem. Information on plasma equilibrium from EFIT/EFTM is essential in order to compensate for the lack of experimental data and to obtain reconstruction with physical relevance. Several techniques have been developed to cope with this problem (see e.g. Refs 3-4). Among them the method based on the maximum likelihood principle, which incorporates a regularizing procedure that assumes smoothness on magnetic surfaces, given by plasma equilibrium proved to be a robust solution [5]. Recently several improvements have been performed in order to allow routinely analysis during the JET experimental campaigns. The method implementation has been optimized in order to allow extensive runs in inter-shot analysis. Integration with data acquisition software has been also accomplished. Recent developments have been dedicated to the problem of evaluating the errors associated with the reconstructed emissivity profile. A methodology for the numerical evaluation of the statistical properties of the uncertainties has been developed. Artefacts related to the experimental conditions for recording the projection data and to the restrictive measuring geometry have been identified and quantified. A detailed analysis of the main sources of artefacts, related to the neutron induced gamma-ray background and to the use of magnetic equilibrium profiles for compensating the lack of experimental data, has been performed.