Potential Safe Plasma Termination Using Laser Ablation of High-Z Impurity in Tokamak

Major plasma disruptions are considered to be a serious problem for development of tokamak fusion reactors. The preliminary experiment results in HL-1M and HL-2A presented here describe the methods of amelioration of plasma current quench in major disruptions using laser ablation of high-Z impurities, which support the design of next generation large tokomaks like ITER. Using injection of impurity with higher electric charge allows us to increase the radiation cooling. Resistive, highly radiating plasma formed prior to the thermal quench, can dissipate both the thermal and magnetic energy. It can be possibly a simple and potential approach to decrease significantly the plasma thermal energy and magnetic energy before a disruption thereby a safe plasma termination is obtained. 1. Instruction In contemporary large tokamaks, the disruptive termination of a discharge can reduce the lifetime of the first wall materials because they are irradiated by the intense heat flux at the energy quench and bombarded by energetic runaway electrons during the current quench, and generate high electromagnetic forces acting on vacuum vessel components due to the intense eddy current and halo current at the current quench. Thus, it is necessary to avoid or mitigate the energy quench and the current quench, and to control an anticipated disruption or emergency shutdown in the large tokamak machines. The thermal energy and the magnetic field energy associated with the plasma current must be dissipated safely when a discharge is terminated in disruption. (1) The magnetic energy can be dissipated by impurity radiation if plasma position control is maintained. (2) The thermal energy is usually conducted to the plasma contact points in about 0.2-0.5 ms time period for a thermal quench prior to the dissipation of magnetic energy. Therefore, resistive highly-radiating plasma formed prior to the thermal quench could dissipate both the thermal and magnetic energies and minimize the risk of local deposition. Using the injection of impurities with higher electric charges can produce resistive highly radiating plasma and increase the radiation cooling of plasma to make a safe termination of the disruption. There appears an approach to reducing significantly the plasma thermal energy before a disruption. A high-Z impurity (KCl) pellet injection has been used in such an application on T-10 (3) and noble gas jet injection has been used for disruption mitigation on DⅢ-D. In this paper it is reported that a preliminary experiments are performed in HL-1M,HL-2A using the injection of Al or Ti by laser ablation to investigate this approach. By analyzing features of the MHD perturbations, a new criterion is introduced to predict the major disruptions. In addition, a primary off-line neural network is developed to cross-check disruption prediction. 2. Experimental Results