In order to study impurity transport and their progression in the plasma, small amount of high-Z impurities were injected into HL-2A ohmic heated plasmas by laser blow-off and traced with good spatial temporal resolution by a soft X-ray camera. An analysis of the evolution of the impurities＇ radiation distribution in the plasma revealed that impurity transport in the central region was profoundly different from that outside of it. It was found that the sawteeth tended to be inverted on the central soft X-ray chord signals after the impurity injection and the soft X- ray profiles exhibited discontinuities during the movement of the impurities in the plasma central region. Detailed 2D visualization of the evolution of impurities＇ radiation using computer tomography has provided further understanding of the properties of impurity transport in the HL-2A plasma.
This paper describes the behaviour of impurity transport in HL-2A ohmic discharges. In 2005, small quantities of metallic impurities （A1, Ni and Ti） were successfully injected into HL-2A plasmas by laser blow-off technique, and their progression was followed by the soft x-ray cameras with good spatial and temporal resolutions. The impurity confinement time is estimated from the characteristic decay time of the soft x-ray signal of the injected impurities, and it is about 30-60 ms. The transport coefficients of impurities （including diffusion coefficient and convection velocity） in radial different region have been derived by using a one-dimenslonal impurity transport code, the results present that diffusion coefficient is much smaller in the central region of plasmas than the outside of it, and it is much larger than that of neoclassical theory predictions; namely, it is anomalous.
Energy confinement time taken from 135 discharges of the 2006 campaign in HL-2A is studied. The data obtained from the measurements are verified by comparing diamagnetic energy with the electronic kinetic energy calculated from both the electron temperature and density profiles. Two data sets for supporting the ITER L-Mode confinement database are generated from the 2006 campaign. The dependence of TE on the line-averaged electron density during ohmic phases is analysed. The comparison of TE in electron cyclotron resonance heating （ECRH） plasma as well as the existing ITER L-mode scalings is made. The results show that the energy confinement time is consistent with the ITER L-mode scalings.
HL-2A tokamak with two close divertors has been operated since 2003. In the experimental campaign of 2004 the divertor configuration has been successfully formed and the sillconization as a wall conditioning has been firstly done in this device. The divertor configuration can be reconstructed by the CFc code. Impurity behavior has been investigated during the experiment with divertor configuration and wall conditioning. The reduction of impurity is clear under both conditions of divertor configuration and siliconization.
Transient perturbation methods are most appropriate to study particle transport in tokamaks. Two most commonly used techniques of impurity injection are laser blow-off and gas puffing. The laser blow-off injection technique is undoubtedly the best one to study impurity transport because the injection time and the alnount of injected material can be controlled in a certain phase of the discharge with a minimum perturbation of the plasma parameters. The information of the impurity transport is obtained from the detection of emission of injected impurity ions located in different spatial regions. The radiation includes line emission, soft X-ray emission and plasma global radiation.
CUI Zhengying HUANG Yuan SUN Ping ZHENG Yongzhen SHI Peilan LU Jie FU Bingzhong ZHANG Peng PAN Yudong DONG Yunbo DENG Wei YANG Qingwei DING Xuantong
In order to avoid destroying the vessel wall and poisoning the plasma, at the same time to efficiently extract the waste product of the fusion process from the system, divertors were adopted in some machines just at the beginning of dealing with this problems. Up to now, the remarkable progress has been made in this respect field . In order to understand the physics behind divertors, HL-2A tokamak with two closed divertors was successfully built in 2002. Its parameters are as follows：
CUI Zhengying SUN Ping PAN Yudong LI Wei WANG Quanming CAO Zeng WANG Mingxu
A 1 m vacuum ultraviolet （VUV） spectrometer with temporal and spatial resolution was developed for impurity study of HL-2A tokamak. The instrument is equipped with two concave gratings blazed at 80 nm and 150 nm, respectively, and a windowless back-illuminated charge coupled device （CCD） detector of 256 × 1024 pixels. Tile total wavelength coverage of spectrometer is 30～ 320 nm with a spectral resolution of 0.015 nm at a width of entrance slit of 10 μm. A portion of this range is observed during a plasma discharge with a spectral range of 20 nm. The minimum integration time of the detector system is about 6.7 ms for each frame in a full binning mode. Using a space-resolved slit located between the entrance slit and the grating a radial profile on the vertical direction with a range of 400 mm can be obtained. The primary results were successfully obtained with high signal-to-noise ratio and good spectral resolution, which demonstrated the instrument functions very well.
The divertor configuration was successfully formed and the siliconization as a wall conditioning was first achieved on HL-2A tokamak experimentally in 2004. The divertor configuration is reconstructed by the use of the CFC code. Impurity as an important issue is investigated in the experiments with divertor configuration and wall conditioning. Impurities dramatically decrease after both the divertor configuration is formed and silicon is coated on the surface of the vacuum vessel.
The electron density profile peaking and the impurity accumulation in the HL-2A tokamak plasma are observed when three kinds of fuelling methods are separately used at different fuelling particle locations. The density profile becomes more peaked when the line-averaged electron density approaches the Greenwald density limit nG and, consequently, impurity accumulation is often observed. A linear increase regime in the density range ne 〈 0.6nG and a saturation regime in ne 〉 0.6nG are obtained. There is no significant difference in achieved density peaking factor fne between the supersonic molecular beam injection （SMBI） and gas puffing into the plasma main chamber. However, the achieved fne is relatively low, in particular, in the case of density below 0.7nG, when the working gas is puffed into the divertor chamber. A discharge with a density as high as 1.2nG, i.e. ne ： 1.2nG, can be achieved by SMBI just after siliconization as a wall conditioning. The metallic impurities, such as iron and chromium, also increase remarkably when the impurity accumulation happens. The mechanism behind the density peaking and impurity accumulation is studied by investigating both the density peaking factor versus the effective collisionality and the radiation peaking versus density peaking.
