HL-1M Tokamak Research Articles

Neutral Beam Injection Experiments in the HL-1M Tokamak

Neutral beam injection (NBI) experiments have been carried out with twooperation modes of a bucket ion source in the HL-1M tokamak. During thefirst mode, more than 30% rise in ion temperature above the Ohmiclevel is routinely achieved after NBI power about 0.5 MW is injected.Ion temperature only increases 20-30% for the second operation mode,which is often limited by current termination. The heating effects ofthe NBI have been analysed experimentally and theoretically. Theperformance of the NBI system is well described.

The wall conditioning experiment by Li–Si coating in the HL-1M tokamak

The mixture coating of lithium and silicon, which is called ‘Li–Si coating’ for short, is formed on the plasma facing wall of HL-1M by means of direct current glow discharge with the mixture of lithium vapor and monosilane and helium. In HL-1M after Li–Si coating, the most favorable vacuum condition is obtained and, the impurities and recycling are restrained forcefully in plasma discharges. By once Li–Si coating, about two hundred quality shots are performed in HL-1M. The experimental results show that the Li–Si coating is of advantages of Li coating and siliconization but weakens their defects.

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.

Self-Organized Criticality Processes in HL-1M Tokamak Plasma

We study the dynamics of laminar time between successive bursts inanomalous particle flux measured at the HL-1M tokamak plasma edge. Theresults reveal that the flux fluctuations are self-similar in a narrowrange of time scales and that their probability distribution functionis not Gaussian. These properties are not consistent with thosepredicted by self-organized criticality (SOC) models as well as therunning sand-pile SOC model developed by Hwa and Kardar.

A Study on Helium Glow Discharge Cleaning in the HL-1M Tokamak

Based on the principle of ion-bombarded reemission and sputtering desorption, the Glow Discharge Cleaning with helium (GDC(He)) is an effective method for controlling the recycle of H on the chamber wall, Carbon (C), Oxygen (O) impurity and improving the wall conditioning in HL-1M tokamak. It is characterized by simplicity without magnet and safety, compared with Taylor Discharge Cleaning (TDC), Alternating current glow discharge Cleaning (AC), Electron Cyclotron Resonance-Discharge Cleaning (ECR-DC). Compared with bake-out degassing, the wall has a higher degassing rate during GDC(He) and a lower impurity concentration in vacuum chambers after GDC(He). Cleaning patterns have been developed dominantly for de-oxidization, decarbonization and de-hydrogenization. The cleaning parameters for H recycle on the wall are also presented. This paper mainly describes the GDC system along with its parameters, breakdown voltage, volt-ampere characteristic, the range of operation safe and suitable cleaning patterns in the HL-1M tokamak, finally concluding with some suggestions on HL-2A GDC.

Erosion and re-deposition behavior of plasma facing materials due to tokamak plasma disruption

Plasma irradiation experiments of several kinds of carbon materials and molybdenum were performed in the HL-1M tokamak. The samples emerged into plasma by a depth of 1.5 cm with an angle of 45° between sample surface and magnetic field. After ≈300 plasma discharges with ohmic and NBI heating, melting, microstructure of thin flakes, gas holes and bubbles were found on the surface of molybdenum. No obvious damages were found on the surface of carbon materials, only carbon fibers of 3D-C/C composite were preferentially eroded. Weight changes indicated that most of the samples had little weight loss but few of the samples had large weight increments in which some droplet-shape deposits were found on their surface. Most of the erosion and re-depositions occurred at random on the surface of samples implied that the dominant factor influenced the erosion and re-deposition of plasma facing materials was tokamak plasma disruption.

Role of Advanced Refuelling and Heating on EdgeReynolds Stress-Induced Poloidal Flow in HL-1M

The radial profile of electrostatic Reynolds stress, plasma poloidalrotations, radial and poloidal electric fields have been measured in theplasma boundary region of the HL-1M tokamak using a multiarray ofMach/Langmuir probes. In the experiments of ohmic discharge, lowerhybrid current drive, supersonic molecular beam injection (SMBI)and multi-shot pellet injection, the correlation between theReynolds stress and poloidal flow in the edge plasma is presented. Theradial profile changes of the Reynolds stress and poloidal flowvelocity Vpol with lower hybrid wave injection power and SMBIinjection are obtained. The results indicate that the sheared poloidalflow can be generated in tokamak plasma due to the radially varyingReynolds stress.

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.

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.

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.

Optic diagnosis of neutral beam injection on HL-1M

During the operation of a high-power neutral beam injection (NBI) system on the HL-1M tokamak, an optical diagnostic means using CCD camera was developed to characterize the NBI performance. The vacuum valve opening process and NBI period in the HL-1M experiment were displayed by a lot of photos taken with this means. Thus, the Hα emission profiles of the neutral beam (NB) and its interaction with plasma were given. Finally, the reason possible for plasma breakdown during NBI mode II discharge was investigated. Therefore, this in-situ diagnosis can provide more information of the NBI.

Studies of mode lock instability in the HL-1M tokamak

Locked mode instability in the HL-1M tokamak has been investigated in this paper. In the HL-1M tokamak the mode lock instability often appears in with low density discharges ( < 1 × 1019 m-3). The precursor perturbations on both soft-X rays and Mirnov signals simultaneously increase before mode locking. Their frequencies are the same and gradually decease, which indicate that the perturbation is global. Their existence time-scale is about 10ms. The plasma will suddenly stop rotating when the fluctuation amplitude or frequency reaches a threshold. Mode lock instability can often cause major disruptions. The rotation frequency is decreased to about 3kHz before mode locking in the HL-1M tokamak, which is consistent with the theoretical model.

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 witha CCD camera and an array of Hα emission detectors. It isobserved that the velocity of the pellet ablation cloud is obviouslyslowed down after it enters the plasma. The ablation clouds areelongated along the magnetic field direction. By means of multi-exposures with theCCD camera during pellet ablation, the safety factor profile isestimated with the ablation cloud inclination angles with respectto the torus on HL-1M. The pellet penetration depth is analysed withAbel inversion of the Hα emissions. The result is compared with theprediction of ablation models. Kuteev's model is more suitable fordescribing pellet ablation processes in the HL-1M plasma than the neutral gasshielding model.

Research on the penetration characteristics of three refuelling methods on HL-1M

In plasma physics experiments of HL-1M tokamak, three refuelling methods (gas puffing, pellet and supersonic molecular beam (SMB) injection) were used. The penetration characteristics and the average speed of the pellet and molecular beam were analysed and estimated by means of contour plots of Hα emissivity distribution. On HL-1M tokamak, the penetration depth of the pellet was up to 0.26 m, and the average speed of the pellet inside the plasma was less than that outside the plasma. The SMB was injected into the plasma from the low-field side and its average speed was more than 1 km s-1.

Effect of suprathermal electrons on central plasma relaxation oscillations during localized electron cyclotron heating on the HL-1M Tokamak

In this paper we discuss the typical behavior of HL-1M tokamak plasmas during ECRH. A large variety ofsawtooth types have been observed at different ECH power deposition locations. The observed central MHD activities detected by the soft x-ray diode array include saturated sawtooth, partially saturated sawtooth, double sawtooth, and strong m=1 bursts superimposed on the ramp phase of sawtooth. Complete suppression of sawtooth is achieved during ECRH, when the heating power is applied on the high-field side of low-density plasmas, and exceeds a threshold value of power. The m=1 bursts riding on the ramp phase of sawtooth can only be excited when the ECRH location is near the q=1 surface at the high field side. The conditions under which the various relaxation activities are produced, or suppressed, are described. Experimental results imply that the energetic electrons, generated during ECRH, are responsible for the modification or stabilization or excitation of the instability. Near the q=1 surface, while the passing electrons play a roleto reduce the shear and tend to stabilize the sawtooth activity, the barely-trapped electrons play a role to enhance or drive aninternal kink instability.

Influence of Superathermal Electrons on Central Plasma Relaxation Oscillations During Localized Electron Cyclotron Heating on the HL-1M Tokamak

During initial studies of ECRH in the HL-1M tokamak, non-standard central MHD activities, such as saturated sawtooth, partially saturated sawtooth, double sawtooth, and the strong m = 1 bursts have been observed while changing the heating location, the ECRH power, the plasma density. Complete suppression of sawtooth is achieved for the duration of the ECRH, when the heating power is applied on the high-field side of low-density plasma, and exceeds a threshold value of power. The m = 1 bursts riding on the ramp phase of sawtooth can only be excited when the ECRH location is near the q = 1 surface on the high field side. The conditions under which the various relaxation activities are produced or suppressed are described. Experimental results imply that the energetic electrons generated during ECRH are responsible for the modification/or stabilization/or excitation of the instability. Near the q = 1 surface, the passing electrons play the role of reducing the shear and tending to stabilize the sawtooth activity, while the barely-trapped electrons play the role of enhancing or driving an internal kink instability.

Velocity Measurement and Freezing Technology of Multi-Shot Pellet on HL-1M Tokamak

A measuring system for high-speed hydrogen ice pellet was introduced in this paper and the photographing of flying pellet taken therein. With the system, a pellet (minimum size of Φ 0.3 mm) velocity (ranging from 50 m/s to 2500 m/s) can be measured in the HL-1M tokamak fueling experiments. By analyzing photographs and the conditions of frozen pellets (including gas supply, gas replenishment, temperature controlling etc), the pellet-freezing technology is summarized in the paper.

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 anattempt to enhance the penetration depth and the fuelling efficiency, as well as to reduce boththe injected particle-wall surface interaction and the impurity content in the plasma.SMBI can be considered a significant improvement over conventional gas puffing. Witha penetration depth of hydrogen particles greater than 15 cm, the rate of increase ofelectron density, dn̄e/dt, was up to 7.2 × 1020m-3 s-1without disruption, and the highest plasma density was n̄e = 8.2 × 1019 m-3.The density profile peaking factor Qn reached a maximum value of more than 1.67 afterSMBI. The energy confinement time τE measured by diamagnetism is 10-30% longer thanthat with gas puffing with the other discharge conditions kept the same. SMBI hasrecently 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 (includingclusters) with the toroidal plasma, which indicates that hydrogen clusters may beproduced 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 thepresent experiment Γ* is about 127.

Investigation on Impurity Transport using LBO Technique

Impurity transport has been investigated on the HL-1M tokamak under the conditions of the ohmically heated discharge (OHD) and the lower hybrid current drive (LHCD). A small quantity of metallic impurity Al were injected into the plasma using the laser blow-off (LBO) technique. The ionized metallic impurity lines were measured using a vacuum ultra-violet (VUV) spectrometry. Al impurity transports in the plasma under the conditions of the OHD and LHCD were analyzed. The diffusion coefficient (D(r)) and inward convection velocity (V(r)) were obtained using numerical simulation with the impurity transport code (ITC). The simulation results showed that the transport coefficient during the LHCD decreased by fifty per cent compared with that during the OHD. Therefore the impurity-confined characteristics of the LHCD plasma was obviously improved.

Edge Structure of Reynolds Stress and Poloidal Flow on the HL-1M Tokamak

The measurement on radial profile of electrostatic Reynolds stress, plasma poloidal rotations, radial and poloidal electric field have been performed in the plasma boundary region of the HL-1M Tokamak using a multi-array of Mach/Langmuir probes. In the experiments of Lower Hybrid Current Drive (LHCD), Supersonic Molecular Beam Injection (SMBI), Multi-shot Pellet Injection (MPI) and Neutral Beam Injection (NBI), the correlation between the Reynolds stress and poloidal flow in the edge plasma is presented. The results indicate that a sheared poloidal flow can be generated in Tokamak plasma due to radially varying Reynolds stress.