The ELM (edge localized mode) mitigation by SMBI (supersonic molecular beam injection) has been studied in the HL-2A H-mode plasmas. The ELM mitigation effect and its relationship with the deposition position of SMBI in the H-mode pedestal are reported for the first time experimentally. We found that when the deposition of SMBI is about 20% into the pedestal, the best ELM mitigation effect is achieved, which is identified by a significant increase of the ELM frequency and also a decrease of the ELM amplitude. The theoretical inference that no deep injection is needed is confirmed. The sand-pile model is used to simulate the ELM burst and explain the mitigation effect for different SMBI deposited 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 gradient threshold in the middle of the pedestal. When the deposition of SMBI is close to the edge of this region, the best ELM mitigation effect can be observed.

No abstract

No abstract

A 7-channel motional Stark effect polarimeter based on four polarizers and a spectrometer has been developed in the HL-2A tokamak, which is the first time successful utilizing this kind of polarimeter on a tokamak. The accuracy of the angle can reach ±0.25° in the calibration experiments. Pilot experiments of measuring the magnetic pitch angle have been successfully carried out in the weak motional Stark effect plasma discharge with toroidal magnetic field of ~1.3 T and beam energy of ~25 keV/amu. The pitch angles of magnetic field are obtained for 7 spatial points covering 24 cm along major radius with time resolution of 40 ms; the profiles of safety factor are obtained by combining with the Equilibrium and Reconstruction Fitting Code. The core value of safety factor (q) is less than 1 during the sawtooth oscillation and the position of q = 1 surface is well consistent with the results measured by soft X-ray array.

No abstract