Yuanxi Wan

Overview of steady state operation of HT-7 and present status of the HT-7U project

Significant progress has been made on the HT-7 superconducting tokamak recently. Repeatable plasma discharges for up to 10.7 s have been routinely obtained, which nearly reached the present limit of the PF system. LHCD experiments have been successfully carried out. A full LH wave current drive for up to 3 s has been achieved. A quasi-steady-state H mode was observed with a very high plasma density for LHCD. ICRF waves were used for wall conditioning, startup, and fast wave and ion Bernstein wave heating. A new RF conditioning technique is shown to be a very powerful means of conditioning future large superconducting tokamaks. The physical design of HT-7U has been nearly finished. The engineering design and some R&D have started. A 2:3 model of the TF coil and a 1:4 model of the central solenoid with a cable in conduit conductor cable have been finished. Testing of the coils will start soon. The engineering design is under way in accordance with the schedule.

A lower hybrid current drive system for ITER

A 20 MW/5 GHz lower hybrid current drive (LHCD) system was initially due to be commissioned and used for the second mission of ITER, i.e. the Q = 5 steady state target. Though not part of the currently planned procurement phase, it is now under consideration for an earlier delivery. In this paper, both physics and technology conceptual designs are reviewed. Furthermore, an appropriate work plan is also developed. This work plan for design, R&D, procurement and installation of a 20 MW LHCD system on ITER follows the ITER Scientific and Technical Advisory Committee (STAC) T13-05 task instructions. It gives more details on the various scientific and technical implications of the system, without presuming on any work or procurement sharing amongst the possible ITER partners(b). This document does not commit the Institutions or Domestic Agencies of the various authors in that respect.

Lower hybrid current drive experiments and improved performance on the HT-7 superconducting tokamak

The feedback control system to control plasma current and position on the HT-7 superconducting tokamak was greatly improved in early 1998. Lower hybrid current drive (LHCD) experiments with the improved control system were performed to sustain long pulse discharges and to improve plasma confinement. Partial non-inductive current drive and full non-inductive current drive for several seconds by means of LHCD were demonstrated. It was observed that plasma confinement could be considerably improved by LHCD. Experimental evidence suggests that this improvement during the LHCD phase could be due to the modification of the current profile in the outer region of the plasma. MHD modes (especially m = 2) seem unstable with such a current profile. The EFIT code was modified for the reconstruction of the magnetic surfaces in HT-7 and a test computation was performed.

Plasma density behavior in the Hefei tokamak-7

The density profiles were measured in the Hefei tokamak-7 (HT-7) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1997), p. 61] ohmic discharges by means of a new multichannel far-infrared (FIR) laser interferometer. The progress on the extension of the HT-7 ohmic discharge operation region was introduced. The experiment results at the density limit, the multifaceted asymmetric radiation from the edge (MARFE) phenomena, the rf (radio frequency) boronization experiments, and the fueling efficiency studies were reported. The plasma physics in the molecular beam injection (MBI), the pellet injection (PI), and the gas puffing (GP) fueling experiments was studied and discussed.

High density operation on the HT-7 superconducting tokamak

The structure of the operation region has been studied in the HT-7 superconducting tokamak, and progress on the extension of the HT-7 ohmic discharge operation region is reported. A density corresponding to 1.2 times the Greenwald limit was achieved by RF boronization. The density limit appears to be connected to the impurity content and the edge parameters, so the best results are obtained with very clean plasmas and peaked electron density profiles. The peaking factors of electron density profiles for different current and line averaged densities were observed. The density behaviour and the fuelling efficiency for gas puffing (20-30%), pellet injection (70-80%) and molecular beam injection (40-50%) were studied. The core crash sawteeth and MHD behaviour, which were induced by an injected pellet, were observed and the events correlated with the change of current profile and reversed magnetic shear. The MARFE phenomena on HT-7 are summarized. The best correlation has been found between the total input ohmic power and the product of the edge line averaged density and Zeff. HT-7 could be easily operated in the high density region MARFE-free using RF boronization.

FAR-INFRARED LASER DIAGNOSTICS ON THE HT-6M TOKAMAK

A multichannel far‐infrared (FIR) hydrogen cyanide (HCN) laser interferometer was developed to measure plasma electron density profile on the HT‐6M tokamak. The structure of the seven‐channel FIR laser interferometer is described. The laser source used in the interferometer was a continuous‐wave glow discharge HCN laser with a cavity length of 3.4 m and power output of about 100 mW at 337 μm. The detection sensitivity was 1/15 fringe with a temporal resolution of 0.1 ms. Experimental results were measured by the seven‐channel FIR HCN laser interferometer with edge Ohmic heating, a pumping limiter, and ion cyclotron resonant heating on the HT‐6M tokamak are reported.

Electron heating by ion Bernstein wave in the HT-7 tokamak

Ion Bernstein wave (IBW) heating has been investigated in the HT-7 superconducting tokamak. Electron heating mode was concentrated on deuterium plasma with an injecting power to 320?kW. The bulk electron temperature showed a significant rise with a heating factor, ?Te?ne/PRF, up to 9.4?eV?1013?cm-3?kW-1. The direct electron heating via electron Landau damping from IBW has been demonstrated after intensive RF wall conditioning. Both on-axis and off-axis electron heating were obtained. Electron temperature profile was modified by IBW under different plasma conditions. Edge density showed no sharp drop during IBW heating. Plasma density and temperature remained almost unchanged in front of antenna. But edge temperature and density inside of plasma visibly increased and their profiles became steeper. Particle confinement improvement was observed by IBW.

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.

MARFE phenomena in the HT-7 tokamak

Abstract Multifaceted asymmetric radiation from the edge (MARFE) phenomena on the HT-7 superconducting tokamak are summarized in this paper. The best correlation has been found between the total input ohmic power and the product of the edge line average density and Z eff . The occurrence of MARFEs are strongly correlated with impurity density, and always occur at Z eff =3–8 ohmic discharges. In the HT-7 tokamak high Z eff discharges, it is found that the MARFEs usually occur at values of Z eff 1/2 ρ in the range of 0.5–0.7, where ρ =π a 2 n e / I p (10 20 MA −1 m −1 ). In the case of good wall condition ( Z eff =1–2), the onset of MARFEs have not been observed before reaching the Greenwald density limit on the HT-7. Improved confinement mode induced by a MARFE is observed, and it is maintained for about 40 ms. MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked.

Operational region and sawteeth oscillation in the EAST tokamak

The first plasma discharges were successfully achieved on the experimental advanced superconducting tokamak (EAST) in 2006. The sawteeth behaviours were observed by means of soft x-ray diagnostics and ECE signals in the EAST. The displacement and radius of the q = 1 surface was studied and compared with the result of equilibrium calculation. The density sawtooth oscillation was also observed by the HCN laser interferometer diagnostics. The structure of the EAST operational region was studied in detail. Plasma performance was obviously improved by the boronization and wall conditioning. It was observed that lower qa and a wider stable operating region is extended by the GDC boronization.