Enyao Wang

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.

Observation of internal kink instability purely driven by suprathermal electrons in the HL-1M tokamak

Strong m = 1 MHD activities are observed in the HL-1M tokamak during off-axis electron cyclotron resonance heating (ECRH) when the cyclotron resonance location is placed just outside the q = 1 surface at the high-magnetic-field side of the magnetic surface. Addition of lower-hybrid waves to ECRH significantly enhances the MHD excitation, but lower-hybrid waves alone cannot excite or sustain the mode. This result is a clear demonstration of the suprathermal trapped electron effect on the instability because of the absence of energetic ions in the plasma.

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.

Improvement of Plasma Performance with Wall Conditioning in the HL-1M Tokamak

Studies and selection of plasma facing materials continue to be a concern for future fusion devices, and ongoing efforts are being made in the HL-1M tokamak. The advanced methods of wall modification adopted on HL-1M are surveyed. Significant improvements in tokamak plasma performance have been obtained by using boron, silicon or lithium containing substances as a material for wall coatings. The siliconization technique is highlighted. Lithiumization, as the newest technique, will be investigated further.

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.

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project

Recent experimental results from the HL-1M tokamak and progress in the HL-2A project are presented in this paper. In HL-1M, strong fishbone instability was observed during off-axis electron cyclotron resonance heating (ECRH). This is the first observation of fishbone instability purely driven by energetic electrons produced by ECRH. The molecular beam injection (MBI) was first proposed and demonstrated in HL-1M. Recently, new results of the MBI experiment were obtained by increasing the pressure of the gas. A stair-shaped density increment was obtained with high-pressure multi-pulse MBI just like the density evolution behaviour during multi-pellet injection. It is shown that injected particles penetrated into the core region of the plasma. HL-2A is a divertor tokamak constructed at SWIP based on the original ASDEX main components. The mission of the HL-2A project is to explore the physics issues involved in an advanced tokamak. For the first phase, divertor (edge plasma) and confinement research will be emphasized. The major parameters of HL-2A are R = 1.65 m, a = 0.4 m, Bt = 2.8 T, IP = 0.48 MA. The main parameters and characteristics of the subsystems such as power supply, pumping, diagnostics and auxiliary heating are presented in this paper. The first plasma of HL-2A was obtained at the end of 2002.

Snake perturbations during pellet injection and LHCD in the HL-1M tokamak

Excitation of snake perturbations has been observed in the core region of pellet-fuelled HL-1M plasmas when the pellets cross the surface with a q value of 1. It is observed that the snake oscillations have an m = 1, n = 1 helicity with quite a long lifetime. A detailed comparison has been made between the locations of the q = 1 surface and the snake oscillation. Through measurements of the plasma q-profile by means of multi-exposures with a CCD camera during pellet ablation, and investigation of the pellet ablation process, possible mechanisms for the formation of the snake oscillation are discussed. In addition, a large, long-lived snake-like oscillation is frequently observed in lower-hybrid current driven (LHCD) discharge in which the sawtooth has been stabilized early in the discharge. There is evidence that such a perturbation is due to impurity accumulation during sawtooth-stabilization, and good performance with peaking profiles after LHCD is limited by magnetohydrodynamic instabilities including sawtooth and snake activities in HL-1M plasmas.

Pellet fuelling experiments in the HL-1M tokamak

An eight shot pellet injector is used to fuel the HL-1M tokamak from the low field side. The pellet ablation process is investigated with a CCD camera and an array of Hα emission detectors. It is observed that the velocity of the pellet ablation cloud is obviously slowed down after it enters the plasma. The ablation clouds are elongated along the magnetic field direction. By means of multi-exposures with the CCD camera during pellet ablation, the safety factor profile is estimated with the ablation cloud inclination angles with respect to the torus on HL-1M. The pellet penetration depth is analysed with Abel inversion of the Hα emissions. The result is compared with the prediction of ablation models. Kuteev’s model is more suitable for describing pellet ablation processes in the HL-1M plasma than the neutral gas shielding model.

Overview of the HL-1M Tokamak Experiments

Experimental progress with the HL-1M tokamak has been made in many areas including confinement improvement, auxiliary heating, plasma fueling, and wall conditionings. An H-mode induced by a biased electrode was obtained with the formation of an internal transport barrier at the region of r/a ~ 0.4 to 0.5. Confinement improvement by lower-hybrid current drive (LHCD) was extensively investigated. Confinement improvement seems to be related to the production of the radial electron field during LHCD. In off-axis electron cyclotron resonance heating (ECRH), double sawteeth in soft X-ray radiation were observed, which implies that reversed magnetic shear could be formed during ECRH. At higher ECRH power, when the resonance position is near the q = 1 surface, fishbone instability was observed and investigated. An eight-shot pellet injector was used for the experiments. The pellet ablation process was investigated with a charge-coupled device (CCD) camera and an Hα emission detector array. Clearly, asymmetry in the pellet cloud was observed in both the toroidal and poloidal directions. It has been found that the pellet velocity slows down clearly after the pellet enters the plasma. The density limit has been investigated on HL-1M at different wall conditionings with three kinds of fueling methods. It was found that a higher density limit could be achieved under the following conditions: (a) a strong reduction of the impurity content after siliconization and (b) a peaked density profile with pellet injection and/or supersonic molecular beam injection. With a neutral beam injection (NBI) system of 1 MW, preliminary results of NBI experiments were obtained with an increase of ion temperature from 450 to 700 eV.

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.