Active MHD Spectroscopy on the Resistive Wall Mode in DIII-D and JET

Active MHD spectroscopy measures the macroscopic plasma stability,while the plasma remains stable.The stability is probed by applying magnetic fields,which largely overlap with the structure of a mode.If the mode is only weakly damped,it amplifies the resonant component of the applied field,a phenomenon referred to as Resonant Field Amplification (RFA).This measurement technique is well suited to analyze the stability of the Resistive Wall Mode (RWM)in high-beta plasmas above the no-wall stability limit,where rapid toroidal plasma rotation in the order of a few percent of the Alfvén velocity is sufficient to stabilize the n = 1 RWM. Active MHD spectroscopy allows for a quantitative test of models of the underlying dissipative process,which is required for a reliable extrapolation of the stabilizing effect of plasma rotation in future experiments.We present recent measurements on the DIII-D and JET tokamaks,where non-axisymmetric control coils are used to apply a rotating or oscillating n 1 magnetic field at various frequencies.In both experiments the spectrum of the plasma response can be described by a single mode model and yields a measurement of the RWM damping rate and the mode rotation frequency.
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