Bojiang Ding

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.

Effect of local pressure on the crystallization product of amorphous alloys induced by mechanical milling

Abstract An Fe–N amorphous alloy was produced by mechanical milling a mixture of α-Fe and h-BN. Its crystallization process was investigated by mechanical milling (MM), annealing at ambient pressure and under high pressure. The crystallization product of the Fe–N amorphous alloy induced by MM is e-Fe x N alloy, which is different from its thermal crystallization product of γ ′ -Fe 4 N but is the same as its high pressure crystallization product obtained at temperatures between 690 and 800 K under pressures of 3–4 GPa. The difference between the mechanical crystallization product and thermal crystallization is attributed to the effects of local pressure and temperature on the crystallization thermodynamics.

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.

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.

High performance discharges near the operational limit in HT-7

Efforts have been made on the HT-7 tokamak to extend the stable operation boundaries. Extensive RF boronization and siliconization have been used and a wider operational Hugill diagram has been obtained. The transit density reached 1.3 times the Greenwald density limit in ohmic discharges. A stationary high performance discharge with qa = 2.1 has been obtained after siliconization. Confinement improvement was obtained as a result of the significant reduction of electron thermal diffusivity χe in the outer region of the plasma. An improved confinement phase was also observed with LHCD in the density range of 70-120% of the Greenwald density limit. Off-axis LH wave power deposition was attributed to the weak hollow current density profile. Code simulations and measurements showed good agreement with the off-axis LH wave deposition. Supersonic molecular beam injection has been successfully used to achieve stable high density operation in the region of the Greenwald density limit.

Lower Hybrid Current Drive Effects on HT-7 Superconducting Tokamak

Lower hybrid current drive (LHCD) experiments have been done on the superconducting Tokamak HT-7 by using a new developed long pulse megawatt lower hybrid wave system. An LHCD efficiency of 0.5 × 1019 Am-2/W in a pulse length of several hundreds of milliseconds has been achieved. After boronization of the tokamak vacuum vessel, considerable improvements of plasma energy confinement and particle confinement during LHCD phase have been achieved by adjusting wave spectra. An energy transfer from the lower hybrid wave to plasma bulk electrons has been demonstrated experimentally. A computer simulation of the LHCD experiments by using a ray tracing code combined with a two dimensional Fokker-Planck code is in good accordance with experimental measurements and suggests an important role of plasma current profile.

Improved confinement through internal transport barrier formation with lower hybrid current drive in the Hefei Tokamak-7

An improved confinement through internal transport barrier formation with lower hybrid current drive (LHCD) is obtained on the Hefei Tokamak-7 [J. K. Xie et al., Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. The experimental energy confinement time increases from 14.6 ms (Ohmic discharge phase) to 24.5 ms (LHCD phase), which is consistent with the value predicted by the high confinement mode scaling law of ITER93 (edge localized mode free). The confinement factor H (H=τE/τEITER89P) up to 1.42 is obtained during the improved confinement phase. The analysis shows that the modification of the plasma current profile and E×B are two possible reasons for the confinement improvement.

Sawtooth control experiments on HT-7 and EAST tokamak

Sawtooth control experiments were performed on HT-7 with a limiter configuration and on the Experimental Advanced Superconducting Tokamak (EAST) with a double null configuration. The sawtooth period can be modified by lower hybrid wave (LHW) and ion cyclotron resonance frequency (ICRF). Different sawtooth behavior was observed with the same plasma density, LHW power but different plasma currents on HT-7. There was a dwell time between LHW added and the sawtooth stabilization on HT-7 while the sawtooth stabilization immediately happened when LHW power was injected on EAST. The possible mechanism of the sawtooth control is discussed in this paper. All the experimental results have proved that power deposition of the radio frequency (RF) should be the key factor for sawtooth control.

The high pressure amorphization and the computer simulation of the pressure‐quenching process

High pressure was applied to the quenching process to produce the amorphous Cu‐Ti alloy bulk, which is confirmed more stable than that prepared by rapid quenching method. The amorphization by high pressure only occurs in those systems in which volume of the phase transformation is shrunk during heating process, including systems induced by high pressure. The conclusion was also confirmed by computer simulation, which was performed on Cs.

AMORPHIZATION MECHANISM FROM CRYSTALLINE ALLOYS UNDER HIGH PRESSURE

Pressure-induced amorphization is a new advanced research subject. It is found that the metastable crystalline phase was first formed for covalent compounds, such as Cd-Sb, then transformed into amorphous phase. But for metal alloys, such as Cr-Ti, amorphous phase was directly formed. Research results show that the metastable phase is formed only in the system which volume of phase transformation is expanded during cooling process because the pressure can accelerate nucleation, restrain atoms diffusion, decrease the critical cooling rate for the formation of metastable phase.