The improved energy confinement has been observed in many tokamaks with center pellet injection since 1980＇s. The pellet enhanced performance （ PEP ） was achieved with high power additional heating in JET and other large tokamaks. The mechanism of the PEP mode has been analyzed theoretically . The analysis shows that a few mechanisms are involved in the reduction of anomalous transport and the relative weight of these mechanisms depends on the experimental conditions. In this paper we report the pellet injection experiment results without auxiliary heating on HL-2A tokamak. Our works focus on the investigation of the electron heat transport in ohmic discharges with center pellet fuelling.
DING Xuantong YANG Qingwei YAN Longwen LIU Dequan XIAO Zhenggui ZHU Genliang LIU Yi ZHOU Yan PAN Yudong CUI Zhengying LIU Zetian SHI Zhongbing JI Xiaoquan XIAO Weiwen LIU Yong
An analytical algorithm for fast calculation of the Abel inversion for density profile measurement in tokamak is developed. Based upon the assumptions that the particle source is negligibly small in the plasma core region, density profiles can be approximated by an asymmetrical Gaussian distribution controlled only by one parameter VoID and VoID is constant along the radial direction, the analytical algorithm is presented and examined against a testing profile. The validity is confirmed by benchmark with the standard Abel inversion method and the theoretical profile. The scope of application as well as the error analysis is also discussed in detail.
Three spectroscopic systems have been developed for the study of light impurity particle transport in the HT-7 tokamak. A visible multi-channel spectroscopic system （VIS） is used to obtain the brightness distribution of the line emission from ionized light impurities. The profile of Zeff（r） has been obtained from the visible multi-channel bremsstrahlung measurement （VB）. The system with a rotating hexahedral mirror for space-time resolved spectroscopy measurement from ultraviolet to visible （UV） can provide the brightness distribution of two different emission lines of the light impurities simultaneously. The emissivities by these multi-channel measurements can be obtained by Abel inversion. The measurement was performed in typical OH discharges in the HT-7 tokamak. The carbon particle transport was analyzed. The feasibility of these diagnostic systems for the impurity particle transport study is clearly demonstrated.
O-mode Doppler reflectometer has been successfully developed as an important diagnostic system on HL-2A. It can be used to measure the turbulence propagation in both plasma edge and confinement zone. The Doppler reflectometer system consists of two fixed frequency homodyne receivers： 15 GHz （corresponding to cutoff density of 0.3×10^19 m^-3） and 33 GHz （corresponding to cutoff density of 1.35× 10^19 m^-3）. The Doppler reflectometry principle and the experimental arrangements on HL-2A are presented. Meanwhile, the experimental Doppler reflectometric spectra under different discharge conditions, with and without ECRH, were obtained. Furthermore, the turbulence propagation velocity change and the profile were also observed in different discharge conditions.
Edge plasma characteristics were studied by a fast-scanning 4-probe array and a Much/Reynolds stress/Langmuir 10-probe movable array in the boundary region. These probes could measure the edge plasma temperature, density, poloidal electric field, radial electric field, Reynolds stress, poloidal rotation velocities and their profiles, which could be obtained by changing the radial positions of the probe array shot by shot. The measured results were used to analyse plasma confinement, turbulent fluctuations and correlations. The fixed flush 3-probe arrays were mounted on the 4-divertor neutralization plates at the same toroidal cross-section in the divertor chamber. These probes were used to measure the profiles of the electron temperature, density and float potential in the divertor chamber. Edge plasma behaviours in both limiter configuration and divertor configuration are compared. The decay lengths of the edge temperature and density were measured and is emphasized for plasma behaviours of the supersonic molecular beam injection and lower hybrid current drive. The dependence of the radial gradient of Reynolds stress on the poloidal flow and the radial gradient of the electric field on turbulent loss are discussed.
