Progress of HL -2A Experiments in 2005

HL -2A has been operated up to Ip = 400 kA, Bt = 2.6T, line average electron density ne=5 1013cm -3 and discharge duration Td > 2 s with both divertor and limiter configuration in 2005. FB control and siliconization have been used to improve long duration discharge reproducible and divertor configurations have been obtained. Several physics experimental results are as following:
1) Pellet injection 
The long time enhanced confinement has been realized after the central fuelling by the pellet injection on the HL -2A tokamak. The evolutions of some parameters shown in Fig. 1. are: plasma current Ip, loop voltage VL, line average electron density ne, electron temperature at plasma center (r = 1 cm ) and plasma edge (r = 35 cm ), intensity of the soft x ray, poloidal βp, energy confinement time τE separately from top to bottom. The hollow electron temperature profiles and peaked density profile are sustained about 200 ms. The energy confinement time increase from 50 ms to 120 ms. The electron heat diffusion coefficient χe decrease from 3.6m2 /s (before pellet injection) to 1.2m2 /s (after pellet injection).
Large sawteeth with long duration appear after the pellet injection. In general, the duration of  sawteeth is more then 30ms and the maximum is about 60ms. The magnitude of sawteeth decreases about 40% during the crash. The MHD modes with different frequencies have been observed using soft x ray array and Mirnov probes. The features of the MHD modes have been investigated. The mechanism of the enhanced confinement during the pellet injection may be related with both the peaked density profile and weak (or reversed) magnetic shear in plasma core.
2) Electron cyclotron heating
Two 68 GHz gyrotrons have been used during ECRH with total power of 1MW. The pulse duration is up to 1 s.Transmission system consists of oversized wave-guides with diameter 8 cm and some metallic reflectors. Microwave is launched into plasma in perpendicular to toroidal field at the low field side as an ordinary mode (o-mode). Both on-axis and off- axis ECRH have been carried out. The plasma center electron temperature increases to about 1300eV, as shown in Fig.2. The electron temperature profile can be changed effectively by off- axis ECRH.
3) Zonal Flow measurement
The 3-D structure for geodesic acoustic mode zonal flow (GAMZF) is measured by Three Step Langmuir Probes (TSLP) at the edge of HL -2A plasmas. Three arrays of TSLP are arranged poloidally and toroidally, and the corresponding symmetries (m = 0, n = 0) of the electric potential and field at GAM frequency (f= 7-15kHz) are obtained. The radial scale length of the flows is estimated as 2.4 -4.7 cm . The formation mechanism of the zonal flow is identified to be nonlinear three wave coupling between high frequency turbulent fluctuation and the flow. The estimated ambient fluctuation and zonal flow velocities are 1.0 km /s and 0.26 -0.65 km /s. The estimated shearing rate of the ZF is 8 104 m /s, which is lower than typical growth rate of drift instabilities of 105 m /s. The modulation effects of the flows on the ambient turbulent fluctuations are also demonstrated. Three dimensional feature of the GAMZF is in agreement with zonal flow theory.
4) Wall conditioning by siliconization
By using silicon as a material for wall coatings on HL -2A tokamak, the impurity fluxes released from the wall are reduced. Fig.3 shows some results measured by visible spectrometer. Besides the radiation from carbon ions and oxygen ions are suppressed significantly,, the continuum radiation in visible region is also decreased. It’s also found that a higher plasma density limit is achieved by means of siliconization.. During discharge with divertor configuration, plasma radiation in the measured spectral range decreases obviously.
5) Disruption prediction
A new method has been used for major disruption prediction by monitoring the characters of the MHD activity. According to the HL -2A experimental results, statistical analysis is applied to investigate the relationships between MHD activities and plasma major disruptions. Two typical features of MHD activity can finally cause the disruption on HL -2A . By introducing a new parameter, , as the criterion of disruption precursor, according to the statistical analysis, more than 95% major disruptions can be predicted. On the other hand, the >1 can be combined with other criterion parameters in neural network predictor to predict disruptions. Fig.4 shows statistical analysis of MHD activities and disruptions.
6) Condensate Molecular Beam injection
Both fuelling experiments with condensate supersonic molecular beam injection (CSMBI) and normal supersonic molecular beam injection (NMBI) were carried out on the HL -2A divertor plasma. The valve body was cooled to about 87 K by liquid nitrogen. The diameter of valve nozzle is 0.2 mm , the hydrogen gas pressure was 1.8 MPa. The pulse duration of the beam is less than 3 ???ms. The pressured hydrogen cooled to liquid nitrogen temperature provides condensated molecular beam which may become a cluster beam.Under the same operation parameter, the line averaged electron density increment after CSMBI with single hydrogen pulse is 1.35 1019m -3 as shown in Fig.5, which is three times higher than that for the NSMBI with the same pulse period.

 

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