Liu Yue-xiu

Effects of Plasma Density and Toroidal Magnetic Field on Lower Hybrid Current Drive Efficiency on HT-7 Tokamak

Lower hybrid current drive experiments on the HT-7 device have been carried out by scanning the following parameters: central line averaged electron density (ne = 0.6 - 2.0 × 1019 m-3) and toroidal magnetic field (Bt = 1.62 - 2.0 T). The dependence of current drive efficiency on these parameters has been studied and the experimental curves of current drive efficiency as a function of ne and Bt have also been obtained. From these experimental results, it can be seen that current drive efficiency rises with the increase of toroidal magnetic field. As plasma density increases, the current drive efficiency first increases to a certain value, then gradually decreases, that is, there exists an optimized density regime where a better drive efficiency can be obtained. The analysis shows that the current drive efficiency is mainly affected by wave accessibility and impurity concentration, and the competition of these two factors determines the current drive efficiency.

Composition of HT-7 LHCD and its Protection Systems

In this paper, the composition of HT-7 LHCD system along with its protection systems is introduced. It is very important for us to run the LHCD system safely and efficiently.

Sawtooth Activity during Lower Hybrid Current Drive Experiment in HT-7 Superconducting Tokamak

It has been found that lower hybrid waves excite various sawtooth oscillations during the HT-7 tokamak discharges. In this paper we present the results of lower hybrid current drive (LHCD) on sawtooth activity in a series of experiments with 2.45 GHz, 1 MW LH system on HT-7 superconducting tokamak. In these experiments, usually LH power is between 200 similar 400 kW, 2 ≤ N// p ≤ 3 (ΔN// = 0.8), and rf pulse in 500 ms. Sometimes, the sawtooth oscillation is accompanied by a wide range of magnetohydrodynamic activity in the plasma central region, normally with a m = 1/n =1 mode. A LHCD partial suppression of sawtooth, a lengthening sawtooth and a saturation of sawtooth oscillations are also observed during our experiments.

Enhanced Lower Hybrid Current Drive Experiments on HT-7 Tokamak

Effective Lower Hybrid Current Driving (LHCD) and improved confinement experiments in higher plasma parameters (Ip > 200 kA, ne > 2 × 1013 cm-3, Te ≥ 1 keV) have been curried out in optimized LH wave spectrum and plasma parameters in HT-7 superconducting tokamak. The dependence of current driving efficiency on LH power spectrum, plasma density ne and toroidal magnetic field BT has been obtained under optimal conditions. A good CD efficiency was obtained at higher plasma current and higher electron density. The improvement of the energy confinement time is accompanied with the increase in line averaged electron density, and in ion and electron temperatures. The highest current driving efficiency reached ηCD = IpeR/PRF ≈ 1.05 × 1019 Am-2/W. Wave-plasma coupling was sustained in a good state and the reflective coefficient was less than 5%. The experiments have also demonstrated the ability of LH wave in the start-up and ramp-up of the plasma current. The measurement of the temporal distribution of plasma parameter shows that lower hybrid leads to a broader profile in plasma parameter. The LH power deposition profile and the plasma current density profile were modeled with a 2D Fokker-Planck code corresponding to the evolution process of the hard x-ray detector array.

A Study of Energy Conversion Efficiency Versus Plasma Density by Lower Hybrid Current Drive in HT-7 Tokamak

Ramp-up experiments by means of lower hybrid wave on HT-7 superconducting tokamak have been performed and analyzed. A ramp-up rate of over 300 kA/s is obtained and a conversion efficiency of over 10% has been achieved during the ramp-up phase. The study of the dependence of conversion efficiency on plasma density shows that the conversion efficiency is affected by the driven current, which is mainly dominated by the competition of impurity concentration with wave accessibility condition. In addition, the effect of current profile may play an important role in determining the conversion efficiency.

Edge Plasma behavior during Improved Confinement by Lower Hybrid Wave Heating in HT-6M Tokamak

Lower hybrid heating (LHH) has been successfully carried out in the HT-6M tokamak. The H-mode has been obtained with a power threshold of 50 kW under a boronized wall condition. Both energy and particle confinements have been improved along with a dropped edge plasma density and an increase electron temperature during the LHH phase. A negative Er well plays a key role of triggering and sustaining the good confinement. Both electrostatic fluctuation of the plasma potential and the density fluctuations dropped to an ultra-low level. The observation of an enhanced Er shear before the reduction in turbulence level is consistent with an increased Er shear as the cause of turbulence suppression.

Evolution of Wave Energy Deposition Profile in HT-7 Lower Hybrid Current Drive Experiment

Lower hybrid waves (LHWs) with a selected n|| spectrum have been used to control the energy deposition profiles, and then the wave driven current profiles effectively in tokamak discharges. In our lower hybrid current drive experiment in the HT-7 tokamak, it was found that the set-up of the wave energy deposition profile is a graduation process. In the beginning phase of the wave injection duration, the waves (with different n|| spectra) deposit almost all their energy in the central region of the plasma column, even if their n|| are very different. Up to around one hundred milliseconds, the wave energy deposition profiles can only take their corresponding shapes according to the n|| spectra of LHWs. It also shown that this evolution process is affected obviously by the LHW driven current profile, which has been formed early.

Technology & Experiments of the Phase Adjustment of a Lower Hybrid Wave

In this paper, the technology of the phase adjustment of a lower hybrid wave on HT-7 superconducting tokamak is described and the calculating method involved in the technology is introduced. The design method of hardware and software is also presented. In addition, a series of experimental results are reported.