Lianghua Yao

Plasma behaviour with hydrogen supersonic molecular beam and cluster jet injection in the HL-2A tokamak

The experimental results of low pressure supersonic molecular beam injection (SMBI) fuelling on the HL-2A closed divertor indicate that during the period of pulsed SMBI the power density convected at the target plate surfaces was 0.4 times of that before or after the beam injection. An empirical scaling law used for the SMBI penetration depth for the HL-2A plasma was obtained. The cluster jet injection (CJI) is a new fuelling method which is based on and developed from the experiments of SMBI in the HL-1M tokamak. The hydrogen clusters are produced at liquid nitrogen temperature in a supersonic adiabatic expansion of moderate backing pressure gases into vacuum through a Laval nozzle and are measured by Rayleigh scattering. The measurement results have shown that the averaged cluster size of as large as hundreds of atoms was found at the backing pressures of more than 0.1 MPa. Multifold diagnostics gave coincidental evidence that when there was hydrogen CJI in the HL-2A plasma, a great deal of particles from the jet were deposited at a terminal area rather than uniformly ablated along the injecting path. SMB with clusters, which are like micro-pellets, will be of benefit for deeper fuelling, and its injection behaviour was somewhat similar to that of pellet injection. Both the particle penetration depth and the fuelling efficiency of the CJI were distinctly better than that of the normal SMBI under similar discharge operation. During hydrogen CJI or high-pressure SMBI, a combination of collision and radiative stopping forced the runaway electrons to cool down to thermal velocity due to such a massive fuelling.

Plasma behaviour with molecular beam injection in the HL-1M tokamak

A new method of gas fuelling has been introduced in the HL-1M tokamak. The method consists of a pulsed high speed molecular beam formed by a Laval type nozzle. The velocity of the well collimated hydrogen beam is about 500 m/s. About 6 × 1019 molecules pass through the nozzle and into the vacuum chamber in each pulse. A series of helium pulses was injected into the HL-1M low density (e = 4 × 1018 m-3) hydrogen plasma. With penetration depth up to 12 cm, the ramp-up rate of the electron density, de/dt, was as high as 3.1 × 1020 m-3s-1 at steady state, and the resulting plasma density reached e = 5.6 × 1019 m-3. The profile peaking factor of the electron density, Qn = ne(0)/ne of about 100 ms after helium molecular beam injection (MBI) reached a maximum value of more than 1.51. The energy confinement time τE measured by diamagnetism is 26 ms, which is over 30% longer than that of the gas puffing (GP) results under the same operational conditions. The improvement of τE and increase of Qn for MBI were comparable to those of small pellet injection (PI) in HL-1M, as well as those of slow PI in ASDEX (Kaufmann, M., et al., Nucl. Fusion 28 (1988) 827). It is argued that the peaked density profile induced by the deepened particle injection is a factor essential for the confinement improvement apart from the isotope effect of helium particles, because the density peaking factor Qn is normally less than 1.4 for GP plasma in HL-1M. The particle confinement time with MBI increased sixfold in comparison with that before injection.

Hydrogen cluster-like behaviour during supersonic molecular beam injection on the HL-1M tokamak

A new method of gas fuelling, pulsed supersonic molecular beam injection (SMBI), has been successfully developed and used in the HL-1M tokamak. SMBI is an attempt to enhance the penetration depth and the fuelling efficiency, as well as to reduce both the injected particle-wall surface interaction and the impurity content in the plasma. SMBI can be considered a significant improvement over conventional gas puffing. With a penetration depth of hydrogen particles greater than 15 cm, the rate of increase of electron density, de/dt, was up to 7.2 × 1020m-3 s-1 without disruption, and the highest plasma density was e = 8.2 × 1019 m-3. The density profile peaking factor Qn reached a maximum value of more than 1.67 after SMBI. The energy confinement time τE measured by diamagnetism is 10-30% longer than that with gas puffing with the other discharge conditions kept the same. SMBI has recently been improved to enhance the flux of the beam and to allow a survey of the cluster effect within the beam. A series of new phenomena show the interaction of the beam (including clusters) with the toroidal plasma, which indicates that hydrogen clusters may be produced in the beam according to the Hagena empirical scaling law of clustering onset, Γ* = kd0.85P0/T02.29. If Γ* > 100, clusters will form. In the present experiment Γ* is about 127.

Self-Assembly of Micro-Parts onto Si Substrates at Liquid--Liquid Interface

We report a new approach for the self-assembly of cuboid micro-parts onto Si substrates to construct three-dimensional microstructures. To perform assembly, the Si substrates are prepared with a deep cavity array as binding sites. An aggregate composed of hundreds of uniformly aligned micro-parts is formed at the C10F18–H2O interface. The micro-parts are arranged by passing the substrate through the aggregate of micro-parts, thus the micro-parts are left on the substrate, and then the substrate is vibrated ultrasonically in the solution, making it possible for the micro-parts to fall into the cavities on the substrate. Finally the substrate is pulled out of the solution after assembly. This technique could give a high yield of up to 70%, providing a new method for micro-assembly.

High performance experiments on high pressure supersonic molecular beam injection in the HL-1M tokamak

Supersonic molecular beam injection (SMBI) was first proposed and demonstrated on the HL-1 tokamak and was successfully developed and used on HL-1M. Recently, new results of SMBI experiments were obtained by increasing the gas pressure from 0.5 to over 1.0 MPa. A stair-shaped density increment was obtained with high-pressure multi-pulse SMBI that was similar to the density evolution behaviour during multi-pellet injection. This demonstrated the effectiveness of SMBI as a promising fuelling tool for steady-state operation. The penetration depth and injection speed of the high-pressure SMBI were roughly measured from the contour plot of the Hα emission intensity. It was shown that injected particles could penetrate into the core region of the plasma. The penetration speed of high-pressure SMBI particles in the plasma was estimated to be about 1200 m s−1. In addition, clusters within the beam may play an important role in the deeper injection.

Investigation of Supersonic Molecular Beam Injection into the HL-1M Tokamak

As a new fueling method, supersonic molecular beam injection (SMBI) has been successfully developed and used in the HL-1M tokamak and HT-7 superconducting tokamak. SMBI can enhance penetration depth and fueling efficiency. It can be considered a significant improvement over conventional gas puffing. In recent experiments, hydrogen clusters have been found in the beam produced by high working gas pressure. The hydrogen particles of the beam have penetrated into the plasma center region, in which the average velocity of the injected beam is >1200 m/s. The rate of increase of electron density for SMBI, d[bar]ne/dt, approaches that of small ice pellet injection (PI). The plasma density increases step by step after multipulse SMBI, just as with the effects of multipellet fueling. Comparison of fueling effects was made between SMBI and small ice PI in the same shot of ohmic discharge in HL-1M.

Pellet and Molecular Beam Injection Fueling on the HL-1M Tokamak

The Eight-shot Pellet Injector (EPI) and Molecular Beam Injector (MBI) as new plasma fueling methods have been developed and installed on the HL-1M tokamak for fueling experiments. The main structures and characteristics of the fueling device and the typical fueling experimental results with EPI and the MBI are reported. In these experiments, typical responses of plasma in discharges with PI and MBI are the peaked density profile Qn = ne(0)/〈ne〉 of >1.65 for MBI and of 2 for PI. The improvement of confinement time τE is usually better than 10 to 30% of Gas Puffing (GP) discharge in the same operation condition. In addition, the penetration depth and deposition region of fueling particles, the variance of soft X-ray sawteeth, the rotation and flow of plasma in edge region as well as the photographing of ablation clouds with PI and MBI are compared and presented in this paper.