Density profiles in pedestal region (H-mode) are measured in HL-2A and the characteristics of the density pedestal are described. Cold particle deposition by Supersonic Molecular Beam Injection (SMBI) within the pedestal is verified. ELM mitigation by SMBI into the H-mode pedestal is demonstrated and the relevant physics is elucidated. The sensitivity of the effect to SMBI pressure and duration are studied. Following SMBI, the ELM frequency increases and ELM amplitude decreases for a finite duration period. Increases in ELM frequency of SMBI ELM f / 0 ELM f 2-3.5 are achieved. This experiment argues that the ELM mitigation results from an increase in Page 2 higher frequency fluctuations and transport events in the pedestal, which are caused by SMBI. These inhibit the occurrence of large transport events which span the entire pedestal width. The observed change in the density pedestal profiles and edge particle flux spectrum with and without SMBI supports this interpretation. An analysis of the experiment and a model shows that ELMs can be mitigated by SMBI with shallow particle penetration into the pedestal.

Modulation of turbulent electron temperature fluctuations () and density fluctuations () by an m/n  =  1/1 tearing mode island was observed in the core plasma region of the HL-2A tokamak. High spatiotemporal resolution two-dimensional images of show the first evidence that the turbulence modulation occurs only when the island width exceeds a certain threshold value ( cm) and the modulation is localized merely in the inner area of the island due to significant alteration of local profiles and turbulence drives. Evidence also reveals that for large islands turbulence spreading takes place across the island region. The results are generally consistent with theories and simulations.

The dynamics of low–intermediate–high confinement transitions was studied using a four-step Langmuir probe in the HL-2A edge plasma. Two types (dubbed type-Y and type-J) of limit cycle oscillations (LCOs) with opposite temporal ordering between the radial electric field and turbulence were first observed. In type-Y, the turbulence grows first, followed by the localized electric field. In contrast, the electric field leads turbulence in type-J. In addition, the Reynolds stress gradient is found not enough to drive the LCO flow and the three-wave nonlinear coupling is weak there. The continuously increasing amplitude of magnetic fluctuations and the significant correlation between the magnetic fluctuation and the electron pressure gradient indicate an important role of diamagnetic drifts in the L–H transition. Mode numbers of magnetic fluctuations in the LCO frequency are identified to be m/n = 1/0.

The HL-2A tokamak has a very closed divertor geometry, and a new infrared camera has been installed for high resolution studies of edge-localized mode (ELM) heat load onto the outer divertor targets. The characteristics of power deposition patterns on the lower outer divertor target plates during ELMs are systematically analysed with infrared thermography. The ELM energy loss is in the range of 3%–8% of the total plasma stored energy. The peak heat flux on the outer divertor targets during ELMs currently achieved in HL-2A is about 1.5–3.2 MW m−2, the wetted area is about 0.5–0.7 m2, and the corresponding integrated power decay length at the midplane is about 25–40 mm. The rise time of the ELM power deposition is in the range of about 100 μs to 400 μs, and the decay time is typically 1.5 to 4 times longer than the corresponding rise time. Convective transport along open field lines during the ELM rise phase from the midplane towards the divertor targets is implied due to the correlation of parallel transport time in the scrape-off layer (SOL) and ELM power rise time. The peak ELM energy fluence is compared with those predicted by models and with experimental data from JET, ASDEX Upgrade, MAST, and COMPASS. The results, as a whole, show a good agreement.

Kinetic Alfvén and pressure gradient driven instabilities are very common in magnetized plasmas, both in space and the laboratory. These instabilities will be easily excited by energetic particles (EPs) and/or pressure gradients in present-day fusion and future burning plasmas. This will not only cause the loss and redistribution of the EPs, but also affect plasma confinement and transport. Alfvénic ion temperature gradient (AITG) instabilities with the frequency ω BAE < ω < ω TAE and the toroidal mode numbers n = 2−8 are found to be unstable in NBI internal transport barrier plasmas with weak shear and low pressure gradients, where ω BAE and ω TAE are the frequencies of the beta-and toroidicity-induced Alfvén eigenmodes, respectively. The measured results are consistent with the general fishbone-like dispersion relation and kinetic ballooning mode equation, and the modes become more unstable the smaller the magnetic shear is in low pressure gradient regions. The interaction between AITG activity and EPs also needs to be investigated with greater attention in fusion plasmas, such as ITER (Tomabechi and The ITER Team 1991 Nucl. Fusion 31 1135), since these fluctuations can be enhanced by weak magnetic shear and EPs.