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Typical ELMy H-mode discharges have been achieved on the HL-2A tokamak with combined auxiliary heating of NBI and ECRH. The minimum power required is about 1.1 MW at a density of 1.6 × 10 19 m −3 and increases with a decrease in density, almost independent of the launching order of the ECRH and NBI heating. The energy loss by each edge localized mode (ELM) burst is estimated to be lower than 3% of the total stored energy. At a frequency of typically 400 Hz, the energy confinement time is only marginally reduced by the ELMs. The supersonic molecular beam injection fuelling is found to be beneficial for triggering an L-H transition due to less induced recycling and higher fuelling efficiency. The dwell time of the L-H transition is 20-200 ms, and tends to decrease as the power increases. The delay time of the H-L transition is 10-30 ms for most discharges and is comparable to the energy confinement time. The ELMs with a period of 1-3 ms are sustained for more than ten times the energy confinement time with enhanced confinement factor H 89 > 1.5, which tends to decrease with the total heating power. The confinement time in the H-mode discharges increases with plasma current approximately linearly.

Abstract. Significant experimental advances have been made on HL-2A tokamak along with substantial improvement and development of hardware. The three dimensional spectral structures of the low frequency zonal flow, the geodesic acoustic mode (GAM), and quasi-mode-like low frequency fluctuations have been observed simultaneously for the first time. In addition, the spectral structure of the density fluctuation at GAM frequency is also identified. A spontaneous particle transport barrier has been observed in Ohmic discharges without any external momentum input. The barrier is evidenced by particle perturbation study using modulated SMBI and microwave reflectometry. The non-local transport effect with new features induced by SMBI has been investigated. The e-fishbone instability excited by energetic electrons of non-Maxwellian distribution has been investigated via 10-channel CdTe hard x-ray detector. It is found that the e-fishbone is correlated with the existence of energetic electrons of 30-70 keV. The experiment shows that the suppression of m/n = 2/1 tearing modes can be sustained by ECRH with low modulation frequency of about 10 Hz. Extended confinement improvement is obtained after the mode suppression.

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Two experiment campaigns were conducted on HL-2A tokamak in 2003 and in 2004 after the first plasma was obtained at the end of 2002. Progresses in many aspects have been made, especially in the divertor discharge and feedback control of plasma configuration. Up to now, the following operation parameters have been achieved: Ip = 320 kA, Bt = 2.2 T and discharge duration T d = 1580 ms. With the feedback control of plasma current and horizontal position, an excellent repeatability of discharge has been achieved. The tokamak has been operated at both limitor configuration and single null (SN) divertor configuration. The HL-2A SN divertor configuration is simulated with the MHD equilibrium code SWEQU. The divertor experiment results were compared with the simulation results obtained with B2. When the divertor configuration is formed, the impurity radiation in main plasma decreases remarkably. The plasma performances are improved significantly after siliconization.

Recent experiment results from the HL-2A tokamak are presented in this paper. Supersonic molecular beam injection (SMBI) with liquid nitrogen temperature propellant is used. Low temperature SMBI can form hydrogen clusters that penetrate into the plasma more deeply and efficiently. Particle diffusion coefficient and convection velocity (D = 0.5–1.5 m2 s−1 and Vconv < 40 m s−1, respectively) are obtained at the plasma periphery using modulated SMBI. Multi-probe measurements reveal the m = 0–1, n = 0 symmetries of directly measured low frequency (7–9 kHz) electric potential and field are simultaneously observed for the first time. Impurity transport is determined with the laser blow-off system and transport code. A disruption predictor has been derived based on MHD activity observations and statistical analysis. Sawtooth characteristics during ECRH are investigated and coupling between m = 1 and m/n = 2/1 modes is studied. Detachment features of HL-2A divertor are numerically and experimentally studied using the code SOLPS5.0 and measured data. The long divertor legs and thin divertor throats in HL-2A pose MHD shaping problems resulting in momentum losses even at low densities and strongly enhanced main chamber losses.

Supersonic molecular beam injection (SMBI) was first proposed and demonstrated on the HL-1 tokamak and was successfully developed and used on HL-1M. Recently, new results of SMBI experiments were obtained by increasing the gas pressure from 0.5 to over 1.0 MPa. A stair-shaped density increment was obtained with high-pressure multipulse SMBI that was similar to the density evolution behaviour during multi-pellet injection. This demonstrated the effectiveness of SMBI as a promising fuelling tool for steady-state operation. The penetration depth and injection speed of the high-pressure SMBI were roughly measured from the contour plot of the H α emission intensity. It was shown that injected particles could penetrate into the core region of the plasma. The penetration speed of high-pressure SMBI particles in the plasma was estimated to be about 1200 m s −1 . In addition, clusters within the beam may play an important role in the deeper injection.

For the firsttime supersonic molecular beam injection (SMBI) and cluster jet injection (CJI) were applied to mitigate edge-localized modes (ELMs) in HL-2A successfully. The ELM frequency increased by a factor of 2-3 and the heat flux on the divertor target plates decreased by 50% on average after SMBI or CJI. Energetic particle induced modes were observed in different frequency ranges with high-power electron cyclotron resonance heating (ECRH). The high frequency (200-350kHz) of the modes with a relatively small amplitude was close to the gap frequency of the toroidicity-induced Alfven eigenmode. The coexistent multi-mode magnetic structures in the high temperature and low-collision plasma could affect the plasma transport dramatically. Long-lived saturated ideal magnetohydrodynamic instabilities during strong neutral beam injection heating could be suppressed by high-power ECRH. The absolute rate of nonlinear energy transfer between turbulence and zonal flows was measured and the secondary mode competition between low-frequency (LF) zonal flows (ZFs) and geodesic acoustic modes (GAMs) was identified, which demonstrated that ZFs played an important role in the L-H transition. The spontaneously generated E × Bshear flow was identified to be responsible for the generation of a large-scale coherent structure (LSCS), which provided unambiguous experimental evidence for the LSCS generation mechanism. New meso-scale electric potential fluctuations (MSEFs) at frequency f ∼ 10.5 kHz with two components of n = 0 and m/n = 6/2were also identified in the edge plasmas for the first time. The MSEFs coexisted and interacted with magnetic islandsof m/n = 6/2, turbulence and LF ZFs.