Current Profile Measurements in JET Advanced Tokamak Scenarios

The evaluation of the spatial profile of the safety factor q is a key point in the analysis of Avanced Tokamak (AT) scenarios, hybrid or steady state. Determinations of q in JET are performed using various diagnostics and the equilibrium analysis constrained on the values of experimental data. In particular the constraints from Motional Stark Effect (MSE) and Polarimetry are routinely used in the EFIT code to optimize the equilibrium evaluation. In some cases the presence inside the plasma of a rational q surface is also evidenced by the occurrence of several types of MHD modes whose mode number ratio (m/n) can be compared with the q value of a particular flux surface. The analysis of signals from several diagnostics (magnetic probes, soft X, Charge exchange spectroscopy, ECE, interferometry and reflectometry) acquired on fast timescales can identify most of these modes and their radial location in the plasma. In particular Neo-classical Tearing Modes (NTM) with m<9, n<4, sawteeth (m/n=1), fishbones (1/1), (2/1) are frequently observed in JET; moreover it is often possible to mark quite precisely the crossing of minimum-q surfaces with rapid successions of modes excited by energetic ions (Alfven Cascades) which can appear only in q reversed discharges. The comparison of the MSE profiles with MHD markers can provide a useful tool for the validation of the measurements. In fact, recently in JET the correction of MSE angle calibrations to match Alfven Cascades and sawteeth, in reference shots, has enhanced the reliability of MSE q-profiles significantly. In this paper we report an extensive comparison of MSE profiles with MHD data for which identification and localization could be obtained.
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