Guangli Kuang

Plasma density behavior with new graphite limiters in the Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao et al., Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-qa was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Zeff1∕2fGW=0.9∼1.2, where fGW=n¯e∕nGW. (Here n¯e is the maximum line average electron density and nGW is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter.

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.

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.

Long pulse enhanced confinement discharges in the HT-7 superconducting tokamak by ion Bernstein wave heating and lower hybrid wave current drive

Significant progress has been made on the HT-7 superconducting tokamak (R=1.22 m, a=0.27 m) in the past year toward obtaining advanced operating modes in steady state. By combining ion Bernstein wave and lower hybrid current drive edge-localized-mode-free limiter H-mode discharges with H89∼2, βN*H89>3 have been obtained that lasted for 53 τE. A well-boronized wall prevented further impurity accumulation and the large pumping kept the recycling at a very low level. The density was kept almost constant with the feedback control. The noninductive driven current fraction is about 60% and bootstrap fraction is about 18%. The central electron temperature is about 2.6 keV and density above 2×1019 m−3. With reduced plasma parameters of central Te∼1.5 keV and ne0∼2×1019 m−3, H89∼1.45, βN∼1.1, the improved confinement phase lasts for more than 100 τE with the noninductive driven current fraction ∼70%.

Modulated toroidal current suppression of MHD activity on the HT-7 superconducting tokamak

Modulation of the plasma current has been used successfully to suppress MHD activity. This was achieved in discharges near the density limit where large MHD m = 2 tearing modes were suppressed by sufficiently large plasma current oscillations. The modulation of the plasma current must be large enough to move the resonance q = 2 surface outside the island width on a timescale faster than the growth time of the instability. These observations resemble those predicted in a previous study of non-linear effects on the m = 1 mode.

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.

Recent progress on steady-state high-performance plasma research in the Hefei Tokamak-7

High performance discharges under steady-state condition in the Hefei Tokamak-7 (HT-7) have been investigated. Lower hybrid current drive (LHCD) was used both for sustaining plasma current and current density profile control. The experiments demonstrated that features of ion Bernstein wave heating in controlling the electron pressure profile can be integrated into LHCD plasmas and the assist localization of LHCD. This local synergy effect was used to tailor the current density profile in a negative shear configuration and to control the internal transport barrier (ITB) under steady-state conditions. The high performance with a stationary ITB at the footprint of the minimum q and β N H 89 >2 was sustained for >220 τ E or >20 τ CR . The fraction of noninductive plasma current was larger than 80% in such discharges with considerable bootstrap current. The duration at H 89 >1.2 and β N ∼1 has been extended to nearly 8 s, longer than 400 τ E . More than 90% of the plasma current was sustained by LHCD and boots...

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.