Stabilisation of resistive-wall modes in fusion plasmas

Doktorsavhandling, 2004

For the success of thermonuclear fusion by magnetic confinement it is important to improve on the stability limits imposed by large-scale magnetohydrodynamics (MHD) instabilities. This thesis focuses on the stabilisation of the "resistive-wall mode" (RWM), an instability which affects high-pressure tokamaks but also Reversed Field Pinches (RFP). In high-pressure tokamaks, one way to stabilise the RWM is to induce sufficient toroidal velocity of the bulk plasma. Numerical simulations with an ideal-MHD model have been used to study how the critical rotation velocity depends on different plasma parameters. Predictions of the model are compared with experimental data and are in reasonable agreement for the critical rotation required for the stabilisation of the RWM. The main stabilising effect comes from Alfvé resonances close to rational magnetic surfaces inside the plasma. A second method, adopted both in tokamaks and RFPs, uses external feedback coils which control the magnetic field at the plasma boundary. Active control of MHD instabilities has received growing attention in the latest years both for increasing the stability of high-pressure plasmas and to study fundamental MHD properties of RFPs. An investigation with a simple cylindrical model describes the main features of the RWM control problem and the analysis is combined with standard control theory. The results are encouraging, predicting the possibility to stabilise the RWM with moderate coil voltages. Favourable results of feedback control have already been obtained in tokamaks. The control problem for RFPs is more difficult and the predictions of the model will be tested in experiments planned for two devices, EXTRAP-T2R in Stockholm and RFX in Padua, Italy.