Experimental characterization and modelling of the resistive wall mode response in a reversed field pinch

Abstract

Model-based control algorithms have potential advantages for Resistive Wall Mode (RWM) feedback control. In this study, a physics model of the RWM response to externally applied perturbation fields is validated against experiments in a reversed field pinch. The experimental characterization of the RWM plasma response is performed in the EXTRAP T2R device by excitation of non-axisymmetric perturbation magnetic fields utilizing an external array of saddle coils for RWM control. The modelling and experimental validation is carried out with an extended sensor array, resolving a wider spectrum of RWM compared to earlier studies, covering the relevant poloidal m=1 and toroidal -32<n<32 modes for this high aspect ratio RFP device. In addition to the non-resonant unstable modes, which are the primary target of RWM feedback control, this spectrum includes also a wide range of resonant modes. The validated resistive MHD model includes the passive stabilization effect on these modes from intrinsic plasma rotation. The inclusion of resistivity and plasma rotation in the present model provides a substantially better agreement between modelled and experimental growth rates than observed in earlier studies using the ideal MHD model. The present model provides a realistic description of the plasma response for both non-resonant and resonant modes, which is yet relatively simple and compatible with computing capabilities and latency limitations encountered in practical implementations of model based control algorithms.

Kinematic viscosity estimates in reversed-field pinch fusion plasmas

“…Also the resistivity can contribute to the difference. Despite in [41] and [42] a remarkable agreement is shown between the Spitzer estimate and the experimental measurements of the resistivity during magnetic reconnection experiments in collisional regime, a recent work [43] has pointed out the need for an order of magnitude anomaly of the resistivity in order to obtain a quantitative agreement between the linear resistive MHD model predictions and resonant modes growth rate measurements on the EXTRAP T2R reversed-field pinch device. The latter considerations motivate future studies regarding both resistivity and viscosity to understand the origin of the anomaly.…”