Resistive Wall Mode studies in JET

In advanced tokamak operation the ultimate performance limit is set by resistive wall modes (RWMs). The nature of the plasma damping term governing RWM stability is not unambiguously established. A model based on ion Landau damping represented through a parallel viscosity term has been used extensively, but recently a more accurate 'kinetic' model, based on drift-kinetic theory, has been implemented in the MARS-F stability code. The damping of stable RWMs may be determined experimentally by measuring the response to n=1 helical magnetic perturbations from coils external to the plasma, under conditions where rotational stabilisation suppresses RWM growth - in JET, saddle coil systems both internal and external to the vacuum vessel are available for such studies. The resonant field amplification (RFA) has been measured for both DC and AC applied magnetic perturbations. RFA is observed in JET as β increases, particularly beyond the no-wall limit, and good agreement with MARS-F is found for either the kinetic damping model or for strong ion Landau damping. The occurrence of a critical flow velocity below which the RWM becomes unstable can also be compared with modelling. Magnetic braking is used to slow the plasma until a naturally unstable mode occurs. Comparison of the critical velocity with MARS-F modelling again shows reasonable agreement with the kinetic damping model or strong ion Landau damping. The results presented provide a very important experimental validation of RWM damping models, allowing for extrapolation to ITER, where it is found that the observed strong damping leads to a requirement for a flow of ∼2 to 3% of the Alfven velocity at the plasma centre to stabilise RWMs. (author)