A Maximum Likelihood Tomographic Method applied to JET Current Quench Phase

In the last years, a new inversion method has been adapted to JET diagnostics. It is based on the Maximum Likelihood (ML) approach and has been applied to most systems of interest: the neutron, gamma ray and bolometric tomographies. In addition to its accuracy and reliability, the main competitive advantage of the ML inversion method is the fact that it can provide reliable estimates of the uncertainties in the reconstructions. The potential of this approach to analyse all the main emissivity types encountered on JET during the flat top phase of the discharges has been verified with both synthetic data and experimental measurements. Recently the same approach has been applied to the investigation of the current quench. Of course, during this phase an additional difficulty is posed by the errors in the magnetic topology; indeed given the ill-pose character of the tomographies on JET, a good reconstruction of the equilibrium is an essential input to any tomographic inversion on JET. In this perspective, particular attention has been devoted to producing good quality, high time resolution equilibria. Moreover, the effects of the uncertainties of the magnetic topologies on the ML tomographic inversions have been assessed. Application of the ML inversion technique to the signals of the hard X rays and the bolometric diagnostics has given very good results for example for the investigation of the runaways beam of electrons. The developed routines constitute a good complement to the other diagnostics, such as the cameras, typically used to investigate the current quench phase of the discharge. The ML tomographic technique is therefore becoming an important additional tool to investigate disruptions on JET, particularly the shatter pellet injector experiments, a crucial programme in support to ITER.