Experimental Study Of Edge Localized Mode Mitigation With Supersonic Molecular Beam Injection On HL-2A Tokamak

The H-mode with type-? edge localized mode(ELM) instability has been selected as a candidate plan for ITER operation, for its high performance on confinement and maximum operating space. ELM makes the plasma pump out energy and particles, as well as the impurities, to the benefit of the long time discharge operating. But on the other hand, ELM could cause a large amount of plasma particle and energy loss, with some concomitant influence, such as damage to the plasma facing components and divertor, bringing of heavy impurities, and weakening of plasma confinement. In the next generation of large device ITER, it has been accepted that the divertor could not endure the heat exhaust due to the ELM,without any action to mitigate. The morphology features, physical mechanism, and controlling methods of ELM have become the important tasks in the physics of boundary plasmas. Experimental research have shown that supersonic molecular beam injection(SMBI)could feel the plasma to supplement the loss of particle and reach a higher density. Meanwhile,SMBI could well mitigate the ELM.In this work, the ELM mitigation induced by SMBI has been studied in the HL-2A H-mode plasmas. The morphology features and responses to different depositions of SMBI has been reported, and a sand-pile model has been used to simulate the process of the ELM mitigation. The radial turbulence measured by Doppler diagnosis during the mitigation has been reported as well.The probability density function(PDF) method has been used to analyze the morphology of the ELMs, and three types of distributions have been shown, corresponding to three phases of the burst of the ELMs. For the phases of ELM-free and pulse falling time, the PDFs are two kinds of Gaussian distribution, which means that plasma is in a steady state. The fluctuation of plasma is approximately stochastic. But for the phase of pulse ring, the PDF of ELM shows a form of power-law distribution, which indicates that the plasma is self-organized criticality.The ELM mitigation effects and its relationship with the deposition position of SMBI in the H-mode pedestal are reported for the first time, here the deposition of SMBI is the position of maximum electron density increment( t/ne??) after SMBI. We found that when the SMBI deposit 20% into the pedestal, the best ELM mitigation achieved, which is identified by a significant increase of the ELM frequency and decrease of ELM amplitude.A sand-pile model is used to simulate the ELM burst and explain the mitigation effect for different SMBI deposition positions. It is found that the gradient threshold is a key parameter in the process of the ELM mitigation, and there should be a local maximumgradient threshold in the middle of the pedestal. When the deposition of SMBI is close to this position, a best ELM mitigation effect can be observed.