Yuejiang Shi

Imaging x-ray crystal spectrometer on EAST

A high-resolution imaging x-ray crystal spectrometer is described for implementation on the EAST tokamak to provide spatially and temporally resolved data on the ion temperature, electron temperature and poloidal plasma rotation. These data are derived from observations of the satellite spectra of helium-like argon, Ar XVII, which is the dominant charge state for electron temperatures in the range from 0.4 to 3.0 keV and which is accessible to EAST. Employing a novel design, which is based on the imaging properties of spherically bent crystals, the spectrometers will provide spectrally and spatially resolved images of the plasma for all experimental conditions, which include ohmically heated discharges as well as plasmas with rf and neutral-beam heating. The experimental setup and initial experimental results are presented.

The first experimental results from x-ray imaging crystal spectrometer for KSTAR

The x-ray imaging crystal spectrometer (XICS) for the Korea Superconducting Tokamak Advanced Research has been first applied for the experimental campaign in 2009. The XICS was designed to provide measurements of the profiles of the ion and electron temperatures from the heliumlike argon (Ar XVII) spectra. The basic functions of the XICS are properly working although some satellites lines are not well matched with the expected theoretical values. The initial experimental results from the XICS are briefly described.

Dynamics of runaway electrons in lower hybrid current drive plasmas in the HT-7 tokamak

In lower hybrid current drive (LHCD) plasmas with non-zero loop voltage, the fast electron tail can act as a seed population of runaways. The runaways are found to be enhanced in the presence of lower hybrid (LH) waves when the fast electron tail extends above the critical velocity for electrons to run away. In runaway discharges, the LH waves may suppress runaway electrons due to the drop in the electric field linked to the non-inductive current drive. In this paper, the behaviour of runaway electrons in an extensive range of LHCD plasmas is presented.

Soft x-ray pulse height analyzer in the HT-7 tokamak

A new soft x-ray pulse height analyzer (PHA), based on a Silicon Drift Detector (SDD) linear array consisting of 15 SDD detectors, has been installed in the HT-7 tokamak. The energy resolution of 150–180 eV at photon energy of 6 keV is achieved for the SDD with Peltier cooling. The effective time response of the SDD PHA is 50 ms. The profiles of electron temperature and the intensity of metallic impurities can be obtained with a spatial resolution of 3 cm. The performance and first experimental results from the new PHA system are presented.

Manufacturing of 50 kA superconducting transformer for ITER correction coil conductor test

To meet the specifications of International Thermonuclear Experimental Reactor correction coil (CC) conductor, a 50 kA superconducting transformer has been designed and manufactured to provide the short sample of the CC conductor the current. The transformer consists of two concentric layer-wound superconducting solenoids with the primary inside the secondary coil. In order to test the transformer, the two legs of the secondary coil were directly connected by superconducting cables. A 500 W/4.5 K refrigerator was used to provide the supercritical helium. The maximum current of 56.3 kA in the secondary coil loop was obtained.

Experimental investigation of the interaction of IBW and LHCD in the HT-7 tokamak

Experiments of simultaneous injection of LHWs and IBWs into plasmas have been conducted in the HT-7 tokamak. Evidence that IBW can control the plasma pressure profile was observed in LHCD plasmas. It was found that electron Landau damping (ELD) of IBWs occurred around the maxima of parallel refractive index, n||, and that the ion cyclotron resonant layer can enhance off-axis LHW power deposition and create a localized fast electron current channel. But the global LHCD efficiency during an IBW pulse was not improved obviously. As a result of the localization of LHCD during an IBW pulse, the global current density profile was modified, which was indicated by the internal inductance, li. Optimization of this operation mode utilizing the features of the interaction of IBWs and LHCD provides a new way of obtaining a high performance plasma under steady-state conditions.

Enhancement of runaway production in lower hybrid current driven plasma with IBW heating in the HT-7 tokamak

Runaway production is observed to be enhanced in lower hybrid current driven (LHCD) plasmas during ion Bernstein wave (IBW) heating as compared with the LHCD only plasma in the HT-7 tokamak. The distortion of the electron distribution function is the effect of the quasilinear diffusions of two types of waves. IBWs modified the distribution function of the electrons by helping to fill the so-called lower hybrid wave (LHW) spectral gap for low parallel velocity. Thus the LHW was significantly coupled to the fast electrons produced by the IBW, and partial LHW power was absorbed on the first pass without significant n?-upshift. The synergy interaction of two types of waves results in high electron parallel energy and enhanced quasilinear diffusion, which is favorable for the production of runaway electrons. This is directly related to the improvement of plasma performance in the operation mode of simultaneous injection of LHW and IBW power.

Digital lock-in technique for the motional Stark effect diagnostic

The fast data acquisition system (FAS) has been developed for the motional Stark effect (MSE) diagnostic in many machines. The FAS digitizes the signals coming directly out of the photomultiplier tube. The software solution of digital lock-in technique to derive the MSE signals from the data of FAS is presented in this article. The calculated frequency and phase of the photoelastic modulators (PEMs) with this digital lock-in technique can reach so high precision that the first, second, and fourth harmonic frequency signals of PEMs can be perfectly recovered.

Extension of operational limits on EAST

The first plasma has been achieved successfully in the Experimental Advanced Superconducting Tokamak (EAST). Boronization by the glow discharge (GDC) method was studied in experiments. The plasma performance was obviously improved by GDC boronization. Extension of the operational region and improvement in the plasma performance were obtained. Sawtooth discharges were observed by means of soft x-ray signals, electron cyclotron emission signals and line averaged electron density after boronization. Lower qa and more stable operation were also achieved following GDC boronization. The plasma current ramp-up rate was also improved as a result of decreased impurity content and low averaged loop voltage due to boronization. PLEASE NOTE: THERE HAS BEEN A RETRACTION PUBLISHED FOR THIS ARTICLE.

Overview of the latest HT-7 experiments

An overview of the progress with experiments in the HT-7 during 2003?4 is presented. The H-mode, negative reversed shear and high li operational scenarios were investigated for quasi-steady-state high performance plasma discharges. Ion Bernstein wave (IBW) heating at 30?MHz produced a typical edge H-mode plasma. Transport in both electron and ion channels were reduced in the outer half portion of the plasma. H-mode was produced in off-axis lower hybrid current drive (LHCD) plasmas by launching a lower hybrid wave (LHW) with a greater parallel refractive index, , of 3.1. The improved confinement status in such plasma discharges could be sustained for more than 400?E. A high inductance, li, plasma was created using a fast plasma current ramp down and sustained by central LHCD and IBW heating for a duration of >1?s with a strongly peaked electron temperature profile. The highest central electron temperature obtained was 4.5?keV. An increase in the energy confinement time with li was observed. It was found that the IBW heating could improve the plasma confinement at the same li if part of the LHW power was replaced by IBW. Stationary internal transport barriers with normalized performance H89?N > 1?3 were obtained with combined injection of LHW and IBW for a duration of several hundred energy confinement times in the weak negative magnetic shear (RS). Increasing the total injection power to 1?MW did not degrade the plasma confinement significantly in the RS operational scenario. Long pulse discharges were performed, using three feedback controls for the plasma current and position, the central line-averaged electron density and the magnetic swing flux of the transformer. The longest plasma discharge, with a duration of 240?s, Te(0) ~ 1?keV and the central electron density > 0.8 ? 1019?m?3 was achieved in 2004. The wall saturation and refreshment (wall pumping) were first observed in such long pulse discharges. A fully LHW current driven plasma without using ohmic current in the central solenoid coils was sustained for 80?s. The main limitation for the pulse length was due to the recycling, which caused an uncontrollable rise in the electron density.