Ht Team

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.

Quasi-steady-state ac plasma current operation in HT-7 tokamak

A quasi-steady-state alternating current operation assisted by lower hybrid wave (LHW) was achieved on a HT-7 superconducting tokamak with plasma current of Ip = 125 kA, line-averaged density of 1.5 × 1019 m−3, electron temperature of Te = 500 eV and 30–50 s plasma duration. Plasma current was sustained and smoothly transferred from one direction to the other without loss of ionization. Plasma position control, LHW assistance, strong gas puffing and good wall condition are the key issues to have a smooth transition of plasma current. Our modelling results show that current reversal equilibrium configuration with two oppositely flowing currents in the high-field-side and the low-field-side during current reversal exists. This is in agreement with experimental measurements.

Turbulence and transport studies in the edge plasma of the HT-7 tokamak

The edge fluctuations and transport in the HT-7 tokamak are investigated using a Langmuir probe array in ohmic and IBW heated discharges. The normalized fluctuation levels are in the range 15-60% and 5-20% for electron density and temperature, respectively, and have the non-Boltzmann relation ef/Te > e/ne > e/Te in the SOL of the ohmically heated plasmas. The fluctuation spectra show typical broadband features as observed in other tokamaks. In IBW heated plasmas, the particle confinement is greatly improved and the poloidal velocity shear in the SOL is strongly modified in a manner that is equivalent to an additional poloidal velocity in the electron diamagnetic direction produced by the IBW. A decorrelation in the fluctuations and the suppression of turbulent transport by the effect of the poloidal shear are found to be common mechanisms that are observed from the radial profiles of the fluctuating quantities in all these discharges. Electrostatically driven turbulent transport can account for a significant part of global particle losses in both ohmic and IBW heated plasmas.

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%.

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...

Runaway Electron Beam Instability in Slide-Away Discharges in the HT-7 Tokamak

Slide-away discharges are achieved by decreasing the plasma density or ramping down the plasma current in runaway discharges in the HT-7 tokamak. In the case of plasma current ramp down, the ratio of the electron plasma frequency to the electron cyclotron frequency is higher than in the stationary pulses when the discharge goes into a slide-away regime. The instability regime is characterized by relaxations in the electron cyclotron emission due to relativistic anomalous Doppler effect which transfers energy from parallel to perpendicular motion. The triggering of relativistic anomalous Doppler effect at higher density by ramping down of plasma current may provide a alternative runaway energy control scenario.

Studies of high confinement plasma with lower hybrid current drive in the HT-7 superconducting tokamak

A high confinement plasma (including core plasma and edge plasma) produced by using lower hybrid current drive (LHCD) has been obtained on the HT-7 superconducting tokamak. An internal transport barrier in the core plasma was formed. The energy confinement time increases from 14.6 ms (Ohmic (OH) phase) to 24.5 ms (LHCD phase), which is close to the value calculated using the ITER93ELM free scaling law. The confinement factor H89 increases from 0.78 (OH phase) to 1.42 (LHCD phase). The experimental results were in good agreement with the simulations calculated with a ray tracing code and a two-dimensional Fokker–Planck equation. The edge plasma characteristics around the last closed flux surface were investigated using Langmuir probes. Turbulence and transport of the edge plasma were suppressed greatly by the lower hybrid wave. Studies show that the enhanced confinement plasma may be ascribed to a shear flow resulting from the shear of the radial electric field.

A fast-scanning heterodyne radiometer for electron cyclotron emission measurements in HT-7 superconducting tokamak

A fast-scanning heterodyne radiometer employing a backward-wave oscillator (BWO) in 78–118 GHz was developed and installed for electron cyclotron emission (ECE) measurements in HT-7 superconducting tokamak. The radiometer measures 16 ECE frequency points with a scanning time period of 0.65 ms, each of the frequency points can be preset by the program to meet specific interests in physics. The high scanning speed is achieved by carefully choosing a BWO, very finely adjusting the radiometer over the full waveband, and by eliminating some elements that are routinely included in a fast-scanning radiometer system. A horizontal view of the ECE optics was installed to measure electron temperature profiles. The spatial resolution is about 1 cm (Bt=2 T) in the center of the HT-7 minor cross section, determined by the intermediate frequency of 0.1–0.5 GHz in the radiometer. Vertically viewing optics along a perpendicular chord was also installed to study nonthermal ECE spectra. Preliminary measurement results in pe...

Overview on the progress of tokamak experimental research in China

Tokamak experimental research in China has made important progress. The main efforts were related to quasi-steady-state operation, LHCD, plasma heating with ICRF, IBW, NBI and ECRH, fuelling with pellets and supersonic molecular beams, and first wall conditioning techniques. Plasma parameters in the experiments were much improved, for example ne = 8 × 1019m-3 and a plasma pulse length of >10 s were achieved. ICRF boronization and conditioning resulted in Zeff close to unity. Steady state full LH wave current drive has been achieved for more than 3 s. LHCD ramp-up and recharge have also been demonstrated. The best ηCDexp ≈ 0.5(1+0.085exp(4.8(BT-1.45)))neICDRp/PLH = 1019 m-2 A W-1. Quasi-steady-state H-mode-like plasmas with a density close to the Greenwald limit were obtained by LHCD, where the energy confinement time was nearly five times longer than in the ohmic case. The synergy between IBW, pellet and LHCD was tested. Research on the mechanism of macroturbulence has been extensively carried out experimentally. AC tokamak operation has been successfully demonstrated.

Suppression of MHD instabilities by ion Bernstein wave (IBW) heating in the HT-7 tokamak

In the HT-7 tokamak, the m = 2/n = 1 tearing mode can be effectively suppressed by the ion Bernstein wave (IBW) when the location of power deposition is near the q = 2 rational surface. Off-axis electron heating and electron density broadening have been observed. In the meantime, the particle confinement is improved with an increase in electron density and a drop in Dα emission. The induced large electron density gradients and electron pressure gradients can be spatially correlated with the IBW deposition profile via theoretical calculation. It is suggested that off-axis IBW heating modified the electron pressure profile and the current density profile can be redistributed, thus resulting in the suppression of the magnetohydrodynamics (MHD) instability. It provides an integrated way for both the stabilization of tearing modes and controlling pressure profiles.