HL-2A Experiments Scenarios for 2007


As one of the key research activities at the Center for Fusion Science, recent HL-2A tokamak programme is to address key physics issues relevant to ITER. In order to achieve the aim, HL-2A is equipped with extensive and advanced diagnostics, a digital control system, various fuelling systems pellet injection , gas puffing , SMBI, NBI        1 MW/45 keV, ECRH2 MW/68 GHzand LHCD 1 MW/ 2.45 GHzheating systems. The operation parameters of HL-2AR1.65 m, a0.4 mare as follows: toroidal magnetic field up to 2.8 T, plasma current of 450 kA, electron temperature up to 5 keV, ion temperature up to 1.5 keV, and electron density of181019 m3. The recent experi-  mental campaigns have been focused on studying and understanding the physics of turbulence, transport, MHD instabilities and energetic electron dynamics. In 2007 important experimental advances have been made on HL-2A tokamak, the achievement benefits a lot from the following hardware development and improvement. Diagnostics with high spatial and temporal resolution include: Thomson scattering, five soft-X ray arrays for MHD study, microwave reflectometer for electron density profile and Doppler reflectometry for plasma rotation and core turbulence measurements, specially designed Langmuir probes for 3-D zonal flow and turbulence measurement, 10-channel CdTe hard X-ray detectors providing energy distribution of energetic electrons. Besides, the supersonic molecular beam injection SMBI system has been improved. The modulation frequency of SMBI reaches  50 Hz, which is not only an advanced technology for fuelling, but also a powerful tool combined with microwave reflectometer for particle transport study. The NBI system with four ion sources power up to 1.5 MW has been commissioned and operated on HL-2A. In ECRH/ECCD system, the filament power supply and arc protection subsystems have been improved. The polarizer and wave dummy load have been developed. Single pulse and modulated ECRH/ECCD in X2 mode experiments have been realized as well.

In 2007 experimental campaign, HL-2A has made considerable progress in advancing the scientific understanding of the physics of turbulence, transport, MHD instabilities. The three dimensional spectral structures of 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 measurement. The non-local transport effect with new features induced by SMBI fuelling has been investigated. The e-fishbone instability excited by energetic electrons of non-Maxwellian distribution has been investigated with 10-channel CdTe hard X-ray detector. It is found that the e-fishbone is correlated with the existence of energetic electrons of 3070 keV. The experiment of ECRH shows that the suppression of m/n2/1 tearing modes can be sustained by ECRH with low modulation frequency of about 10 Hz. Continuous confinement improvement is obtained after the mode suppression.

Plasma theory and simulation also contributed to the physics study on HL-2A program, e.g., some hot theoretical subjects, such as the zonal flow and the formation mechanism of magnetohydrodynamics MHD flow and transport barriers, were investigated numerically. Relation between zonal flow and toroidity such as aspect ratio and elongation is studied. Collisionless residual zonal flow level was calculated for toroidally axisymmetric plasmas with arbitrary aspect ratio and large elongation. In 2007, the DMHD code has been improved from slab configuration into toroidal one to study the formation of shear flow. In addition, a minimum dynamics of electron temperature gradient was proposed to study confinement modes and their transition.