Study Of Tearing Modes Controlled By Radio Frequency Current Drive

The tearing modes(TMs)or neoclassical tearing modes(NTMs)are found to degrade the plasma energy confinement.It is well known that TMs/NTMs can be suppressed by localized radio frequency(rf)current drive,by modifying the local plasma current density profiles and/or compensating the missing bootstrap current inside the island.The electron cyclotron wave(ECW)has a good localization of power deposition because of its narrow beams and strong absorption,so that electron cyclotron current drive(ECCD)is suitable for mode stabilization.The stabilization of TMs/NTMs by ECCD has been carried out in many tokamak experiments.Control of tearing modes by rf current drive is studied in this dissertation.The physics of TMs and NTMs and their stabilization by rf current drive are reviewed in chapter 1.Numerical studies on the stabilization of NTMs by ECCD have been carried out in chapter 2 based on reduced MHD equations,focusing on the amount of the required driven current for mode stabilization.The dependence of the minimum driven current required for NTMs stabilization on some parameters,including the local bootstrap current density,radial width of the driven current,radial deviation of the driven current from the resonant surface,and the island width when applying ECCD,are studied.By fitting the numerical results,simple expressions for these dependences are obtained.Based on these fitting expressions,the required modulated ECW power for NTM stabilization in ITER is found.Plasma turbulence,in particular edge density fluctuations,can broaden the deposition width of injected microwave beams in ITER.The required modulated ECW power for mode stabilization is numerically found to be 28-79MW,which is greater or far greater than the designed ECW power in ITER(20 MW),if the density preturbations broaden the ECW deposition width to the range 0.05a to 0.1a(a is the plasma minor radius),indicating that the only possibility for NTMs stabilization in ITER is to inject modulated ECCD when the island is not too large.Analysis based on the modified Rutherford equation(MRE)has also been carried out in chapter 3.The comparison with numerical results is studied,and the corresponding results have the same trend as numerical ones,while a quantitative difference between them exists.This difference becomes smaller when the applied rf current is smaller.In chapter 4 numerical modelling on tearing mode stabilization by ECCD has been carried out for the purpose of disruption avoidance,focusing on stabilizing the magnetic island which can grow to a large width and therefore might cause plasma disruption.When the island has become large,a threshold in driven current for fully stabilizing the mode is found,and below this threshold the island width only slightly decreases.The island's O-point shifts radially towards the magnetic axis as the mode grows,so that applying ECCD at the minor radius of island's O-point has a stronger effect than that at the original equilibrium rational surface for stabilizing a large island.During the island growth the required driven current for mode stabilization increases with the island width,indicating that it is more effective to apply ECCD as early as possible for disruption avoidance,as observed in experiments.The numerical results have been compared with those obtained from the modified Rutherford equation.