Determination of plasma stability using resonant field amplification in JET

Resonant field amplification (RFA) has been systematically measured on JET, using active MHD spectroscopy to probe plasma stability at high and low beta, and compared with theoretical predictions. RFA has been measured as a plasma response to externally applied fields. At high beta, RFA has been used to identify the ideal no-wall beta limit. It was found experimentally and explained theoretically that the beta limit strongly depends on the current density and q profiles, and in particular on the qmin value, and the current density profile near the plasma edge. At low beta, RFA has been observed and analysed in detail during edge-localized mode (ELM)-free periods prior to the first ELM either after L–H transition or after long ELM-free periods during a pulse. These observations confirm that the measured increase in the RFA in some cases (e.g. at low beta) may not be connected with the no-wall beta limit associated with the RWM, but may reflect a proximity to other stability thresholds. Reduction in RFA is observed during an outer mode for the first time. The first results on n = 2 probing on JET are presented. (paper)