Response of a Resistive and Rotating Tokamak to External Magnetic Perturbations Below the Alfvenic Frequency

Full text: Plasma response to magnetic perturbations from the RWM to TAE frequency range is studied by using the MARS-F code to minimize the free energy of the plasma and excitation coils. MARS-F takes into account plasma toroidal flow, resistivity and/or various kinetic effects and an arbitrary external coil geometry. The present work was motivated by the discovery that magnetic perturbations can stabilize the ELMs. Previous systematic and useful studies on ELM suppression using the SURFMN code neglected the intrinsic plasma response. It led to the conclusion that outer ∼ 10% flux surfaces were found to be stochastic and posed difficulty to observations that plasma edge remains in the H-mode with good confinement. In this work, we verified SURFMN by checking it against analytic models and also MARS-F assuming vacuum plasma conditions. With inclusion of plasma, with flow profiles and various resistivity levels, MARS-F response deviates from vacuum significantly for the resonant components of perturbations. For ideal-plasma, the resonant components are completely suppressed at the mode resonant surfaces. With plasma resistivity, this suppression (or shielding) becomes imperfect. However, with experimentally measured rotation profile, and with a wide range of plasma resistivity (magnetic Reynolds numbers S = 106 to 108), the shielding remains substantial. In comparison to the vacuum response, the size of the magnetic island, which is proportional to √Bn, is much reduced and the field line stochasticity is limited to the outer ∼ 2% of the flux surfaces. This is consistent with the observation of only very minor modification to plasma transport. Similar results were found for different plasma shapes with various plasma elongation and triangularity. At higher frequencies, which are relevant for the TAE, RSAE etc, for plasma responses with large kinetic energy δK, its frequency is found independent of the geometry of the external coils, but the amplitude could be very different. The width in frequency of the response peaks is related to the continuum damping, which is also obtained by adding plasma resistivity. We conclude that the present formulation and results can be extended to study the perturbation of plasma by external coils in future devices, such as ITER. Work supported by USDOE under DE-FG03-95ER54309. (author)