Modeling of active control of external magnetohydrodynamic instabilities

A general circuit formulation of resistive wall mode (RWM) feedback stabili zation developed by Boozer [Phys. Plasmas 5, 3350 (1998)] has been used as the basis for the VALEN computer code that calculates the performance of an active control system in arbitrary geometry. The code uses a finite elemen t representation of a thin shell structure in an integral formulation to mo del arbitrary conducting walls. This is combined with a circuit representat ion of stable and unstable plasma modes. Benchmark comparisons of VALEN res ults with large aspect ratio analytic model of the current driven kink mode are in very good agreement. VALEN also models arbitrary sensors, control c oils, and the feedback logic connecting these sensors and control coils to provide a complete simulation capability for feedback control of plasma ins tabilities. VALEN modeling is in good agreement with experimental results o n DIII-D [Garofalo , Nucl. Fusion 40, 1491 (2000)] and HBT-EP [Cates , Phys . Plasmas 7, 3133 (2000)]. VALEN feedback simulations have also been used t o evaluate and optimize the sensor/coil configurations for present and plan ned RWM experiments on DIII-D. These studies have shown a clear advantage f or the use of local poloidal field sensors driving a "mode control" feedbac k logic control loop and configurations which minimize the control coil cou pling to the stabilizing resistive wall. (C) 2001 American Institute of Phy sics.