THEORY AND CALCULATION OF CURRENT PROFILE CONTROL NEAR RATIONAL SURFACES IN A TOKAMAK BY ELECTRON-CYCLOTRON WAVES

The effect of electron cyclotron current drive (ECCD) on nonlinear res istive magnetohydrodynamic (MHD) perturbations in a tokamak is analyze d. Along the ray trajectories of electron cyclotron wave (ECW) beams i njected from the low-field side in the ordinary mode (O-mode) polariza tion, the power deposition, and current drive efficiencies are calcula ted using fully relativistic analytical expressions. The driven curren t is inserted in a consistent theory of nonlinear magnetic island evol ution, which provides the scaling of the radio-frequency (RF) driven c urrent required to control magnetic reconnection in terms of the plasm a equilibrium parameters and optical properties. It is shown that the threshold for error field driven islands can be increased by ECCD and that the current profile around the q=2 rational surface can be shaped to control the evolution of rotating magnetic islands by modulation o f the gyrotron power source. (C) 1996 American Institute of Physics.