Modelling of Resonant Field Amplification in JET

The effects of error fields on the stability and confinement of tokamak plasmas have been known for a long time. Passive or active error field correction, using magnetic coils, has been proved an efficient way to improve the plasma performance in many present tokamaks. It is also foreseen as an important design aspect in ITER. It was, however, only recently realised that the plasma response can play a crucial role in amplifying the external, static magnetic fields, due to the existence of meta-stable, low-n, low frequency MHD modes in the plasma. One example is the Resonant Field Amplification (RFA) caused by the Resistive Wall Mode (RWM), marginally stabilised by the plasma rotation or kinetic effects. In this work, we model the RFA response by solving the linearly perturbed, single fluid MHD equations as a forcing problem by using the MARS-F code, in which the source term is the coil currents flowing in the Error Field Correction Coils (EFCC) in JET. We investigate two types of the plasma response, observed in JET discharges, that occur before the plasma pressure reaches the no-wall limit for the ideal kink mode.
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