Advanced equilibrium reconstruction for JET with EFIT++

Many of the essential data analysis procedures for a tokamak experiment rely on the knowledge of the magnetic field structure obtained from MHD force balance. On JET, the code that is responsible for computing the magnetic equilibrium is called EFIT++ [1,2,3]. Interpretation of JET data has been challenging due to inconsistencies between diagnostic measurements and properties of the computed equilibrium [4,5]. In a recent code update, the most dominant discrepancy has been resolved, and a robust, automatic reconstruction mode has been enabled as part of the intershot analysis chain. This paper provides details of this successful run mode, its validation procedure and its limitations. A crucial improvement has been achieved by reviewing the calibration factors on the magnetic diagnostic system, in particular those of the poloidal field pick-up probes that are nearest to the plasma. The input magnetic data has been extended to include all available measurements around the tokamak. The numerical representation of the flux functions p' and ff' have been adjusted so that the EFIT algorithm is able to find an equilibrium that is consistent with the estimated input pressure profiles both in the core and the pedestal. The results have been validated against the High Resolution Thomson Scattering (HRTS) and Electron Cyclotron Emission (ECE) measurements of the electron temperature profiles, showing good agreement with the expected location of the magnetic separatrix at the midplane, and symmetry of the profiles around the magnetic axis. Infra-red (IR) camera images of the divertor show good agreement with the computed strike point locations, and consistency of total and soft X-ray (SXR) radiation analysis has also been improved. Using Motional Stark Effect (MSE) measurement of the magnetic pitch angle in the code provides a q profile that is consistent with basic MHD markers, such as the inversion radius during sawteeth activity.