Determination of Plasma Stability using Resonant Field Amplification in JET
The Resonant Field Amplification has been systematically measured on JET using active MHD spectroscopy to probe plasma stability at high and low beta and compared with theoretical predictions. At high beta, RFA has been used to identify the ideal no-wall beta limit in several advanced regimes: the hybrid regime, with low magnetic shear and q(0) close to 1, and high beta plasmas being developed for steady-state application, with low or reversed magnetic shear and 1.0 < qmin < 2.5, including regimes with and without Internal Transport Barriers (ITBs). RFA has been measured as a plasma response to applied helical fields with toroidal number n = 1, and the diagnostic has been extended to the n = 2 probing. 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 q(0) value for monotonic q-profiles or qmin for reversed shear profiles, and on details of the current density profile near the plasma edge. At low beta, RFA has been observed prior to onset of a fast rotating n = 1 mode, and during 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 be not connected with the no-wall beta limit associated with the RWM, but may reflect a proximity to other stability thresholds. A model retaining information about the plasma response in comparison with the kinetic damping model is presented to describe the resonant field amplification in the presence of a stable RWM.