Analysis of NTM (de)stabilization by ECH in ASDEX Upgrade

摘要

Stabilization/destabilization of the major resistive magnetohydrodynamic instabilities such as the Neoclassical Tearing Modes (NTMs) is a crucial issue for plasma confinement. The uncontrolled growth of such rotating helical modes can lead to loss of plasma energy and to disruptions. This is commonly observed for the m/n=2/l mode on ASDEX Upgrade tokamak [!.]. The aim of this work is to investigate how the NTMs can be controlled'by the Electron Cyclotron Heating (ECH) and current drive (ECCD), which are powerful tools for this purpose. The local change of resistivity due to ECH affects the plasma equilibrium current density and, consequently, the safety factor q and also the helical current perturbation parallel to the magnetic field within the magnetic island. The usual tearing stability parameter ?'_0 can be modified by moving the EC power deposition inside/outside the r_s position of the rational q=m/n surface of the mode. This crucial parameter can be calculated starting from the modification of the electron temperature T_e by ECH evaluating the favourable or unfavourable changes of plasma current density gradient, J'=J_0[(T_c/T_(c0))~(3/2)}]'. The ?'_0 calculation is obtained by using the following analytic expression [2] which embodies toroidal and shaping effects like the Shafranov's shift and elongation of magnetic surfaces