J.H. Wu

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.

First results of LHCD experiments with 4.6 GHz system toward steady-state plasma in EAST

A 4.6 GHz lower-hybrid current drive (LHCD) system has been firstly commissioned in EAST in the 2014 campaign. The first LHCD results with 4.6 GHz show that LHW can be coupled to plasma with a low reflection coefficient, drive plasma current and plasma rotation, modify the plasma current profile, and heat plasma effectively. By means of configuration optimization and local gas puffing near the LHW antenna, good LHW–plasma coupling with a reflection coefficient less than 5% is obtained. The maximum LHW power coupled to plasma is up to 3.5 MW. The current drive (CD) efficiency is up to 1.1 × 1019 A m−2 W−1 and the central electron temperature is above 4 keV, suggesting that LH power could be mainly deposited in the core region, which is in agreement with code simulation. Experiments show that the current profile is effectively modified and toroidal rotation in the co-current direction is driven by the LHCD. Also, the CD efficiency and current profile depend on the launched wave spectrum, suggesting the possibility of controlling the current profile by changing the phase difference. Repeatable H-mode plasma is obtained by either the 4.6 GHz LHCD system alone, or together with a 2.45 GHz LHCD system, the NBI (neutral beam injection) system. The different ELM features of H-mode between the different heating methods are under investigation.

Effect of gas puffing from different side on lower hybrid wave-plasma coupling in experimental advanced superconductive tokamak

Effect of gas puffing from electron-side and ion-side on lower hybrid wave (LHW)-plasma is investigated in experimental advanced superconductive tokamak for the first time. Experimental results with different gas flow rates show that electron density at the grill is higher in the case of gas puffing from electron-side; consequently, a lower reflection coefficient is observed, suggesting better effect of puffing from electron-side on LHW-plasma. The difference in edge density between electron- and ion-side cases suggests that local ionization of puffed gas plays a dominant role in affecting the density at the grill due to different movement direction of ionized electrons and that part of gas has been locally ionized near the gas pipe before diffusing into the grill region. Such difference could be enlarged and important in ITER due to the improvement of plasma parameters and LHW power.

Effort of lower hybrid current drive experiments toward to H-mode in EAST

Lower hybrid current drive (LHCD) is an effective tool to achieve high confinement (H-mode) plasma in EAST. To utilize LHCD for accessing H-mode plasma, efforts have been made to improve LHW (lower hybrid wave)-plasma coupling and current drive capability at high density. Improved LHW-plasma coupling by means of local gas puffing and gas puffing from the electron side is routinely used during EAST operation with LHCD. High density experiments suggest that low recycling and high LH frequency are preferred for LHCD experiments at high density, consistent with previous results in other machines. The effect of LHCD on the current profile in EAST demonstrates that it is possible to control the plasma profile by optimizing the LHW spectrum. Repeatable H-mode plasma was obtained by LHCD and the maximum density during H-mode with the combination of 2.45 GHz and 4.6 GHz LH waves was up to 4.5 × 1019 m−3.

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.

NUCLEATE POOL BOILING HEAT TRANSFER OF BINARY MIXTURES WITH DIFFERENT BOILING RANGES

The nucleate pool boiling heat transfer data on a smooth flat surface were measured for three binary mixtures of HC600a/HFC134a, HC600a/HC290, and HC600a/HFC23. Much effort was made to investigate the influence of the boiling range on the pool-boiling heat transfer performance. From the experimental results, the HC600a/HFC23 mixture with a wide boiling range showed lower heat transfer coefficients (HTCs) than the mixture with a narrow boiling range such as HC600a/HFC134a and HC600a/HC290 systems. The measured data were also compared with the results predicted by five well-known correlations. It can be found that the average deviation is less than 25% for mixtures with narrow boiling ranges, but a larger deviation for mixtures with wide boiling ranges.

CFD research on hydrodynamic gas bearings with different styles of grooves

The gas bearing is an appealing technology which is widely used in turbo-machine in large-scale cryogenic systems for its inherent characteristic of oil-free and high-speed capability. The hydrodynamic gas bearings are more effective on cryogenic system but have less load capacity and dynamic stability by comparing with externally pressurized gas bearings. Etching some grooves on shaft or bearing is proved to be effective on improving the static and dynamic performance by experiments. The performance parameters of hydrodynamic gas bearing, such as load capacity and stiffness, are dominated by the styles and the geometric parameters of groove. In this paper, the effect of groove styles and geometric parameters on the performance parameters of hydrodynamic gas bearings is presented. And a novel style of groove is designed which can effectively improve the static and dynamic performance. The results based on the calculation of CFD show that: geometric parameters of the groove do have some influence on static and dynamic performance of hydrodynamic gas bearings, and there is an optimal objective value for each parameter of groove which makes load capacity and stiffness maximum. Compared with the spiral-style and π-style groove, the novel style of groove presented in this paper has better static performance.

Boronization during the first plasma operation on EAST

Both ion cyclotron rf and glow discharge boronization have been successfully used for wall conditioning on EAST tokamak device. The whole process is monitored continuously by residual gas analyzer and film thickness monitor. These diagnostics provide detailed information about the boronization. High hydrogen inventory level observed after boronization maybe due to the boronization material used (C2B10H12). Ion cyclontron rf conditioning is proved to be an efficient wall conditioning method for superconducting device because it could be carried out under toroidal magnetic field. In this paper, the procedure of boronization is described, and subsequently sample analysis and the effect on plasma operation are introduced. Conclusion is given at the end.