HL-1M Tokamak Research Articles

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.

Observation of hydrogen pellet ablation material flow by tomography

An Hα emission measurement array with 25 detection channels has been built on HL-1M tokamak. Reconstruction maps of Hα emission distribution in plasma cross-section have been obtained by using tomography, with a hydrogen pellet injected into the plasma. The dimension, the shape and the rotation characteristic of the cross-section of the pellet ablation material flow tube have been observed. These results have been analysed and explained tentatively by using the `tail' model and the linear expansion model.

Disruption mitigation using laser ablation of high- Z impurities in HL-1M tokamak

A preliminary experiment triggering a plasma current quench by laser ablation of high-Z impurities has been performed in the HL-1M tokamak. The injection of impurities with higher electric charges into tokamak plasmas can increase the radiation cooling of the plasma. Resistive, highly radiating plasma formed prior to the thermal quench can dissipate both the thermal and magnetic energies, which is possibly a simple and potential approach to reducing significantly the plasma thermal energy and magnetic energy before a disruption thereby a safe plasma termination is obtained.

Scaling Properties of Runaway Electrons in HL-1M Tokamak

A simple one-dimensional numerical model including generation, acceleration and loss effects for runaway electrons are used to deduce the runaway energy εr. The simulation results are presented in a form of a scaling law of εr on plasma parameters. The scaling of εr and therefore the runaway confinement time τr and runaway electron diffusivity Dr have been studied in HL-1M tokamak, by measuring the hard-X ray spectra under different experimental conditions. A tentative explanation for the scaling of obtained data based on the effects from magnetic turbulence is presented.

Internal magnetic fluctuation in the HL-1M tokamak

The transport of runaway electrons in hot plasma can be comparatively easily measured by steady state or perturbation experiments, which provide runaway electron diffusivity Dr. The runaway diffusion coefficient has been obtained using four methods: (1) Dr is deduced from Plasma shift experiment; (2) Dr is deduced from its sawteeth oscillations behaviors of HXR flux and of SXR intensity; (3) Dr is deduced from microwave radiation intensity sawtoothing behaviors; (4) Dr is deduced from hard-X-ray bremsstrahlung spectra. The diffusivity can be interpreted in terms of a magnetic fluctuation level. The internal magnetic fluctuations level (b～r/BT) is estimated to be about (2—4)×10-4 in the HL-1M plasma. The results presented here demonstrate the effectiveness of using runaway transport techniques for determining internal magnetic fluctuations. A profile of the magnetic fluctuation level in the HL-1M can be estimated from Dr.

Fuelling and Diagnostics with Pellet Injection in the HL-1M Tokamak

The pellet injection experiments for fuelling and diagnostics have been carried out on the HL-1M tokamak. The eight-pellet injector was installed on HL-1M. A reliable monitor-detector and camera system was set up to take initial pellet photographs and measure the initial pellet speed and size. High fuelling efficiency of 60%-100% and a density profile with a peaking factor of 1.8-2.0 were obtained. The maximum density close to 1014/cm3 in HL-1M was achieved with newly optimized combined fuelling techniques. Two typical models of pellet ablation have been utilized for simulative calculation of the ablation rates in HL-1M. In comparison with the distribution of the measured Hα emission intensity from the digital data of the CCD camera, the experimental result seems more optimistic for core fuelling than theoretical predictions by the two models. The safety factor profile q(r) has been extracted from the information provided by the CCD camera during the pellet injection. The reliability of the measured results depends mainly on the calibration of the imaging space position. Based on the calibration, the measured q-profile becomes more reasonable than those published previously for the same shot number and same photograph.

Measurement of neutron radiation flux and dose on HL-1M tokamak

Five BF33 long counters are arranged in all around the HL_1M tokama k to mea sure the flux and dose of neutron radiation under the situations: the emission o f D_D fusion neutron being in deuterium plasma or the emission of photo_neutron being in hydrogen plasma. The results of measurement are compared with those of calculation, both agree roughly with each other. The dose equivalent of neutron radiation is very low, the measured value is far below the national and ministry issued standards of permissible dose equivalent.

Research of edge plasma structure on the HL-1M tokamak

In this paper, experimental results of plasma characteristics in several discharges have been measured in the plasma boundary region of the HL-1M tokamak using a multiarray of Mach/Langmuir 6 probes, electrostatic 4 probes and 20 group of 3 probes. In the experiments of Ohmic discharge, lower hybrid current drive, supersonic molecular beam injection, multi-shot pellet injection and neutral beam injection, etc., the correlation between the Reynolds stress and poloidal flow in the edge plasma is obtaimed. The experimental results suggest that the confinement improvement is related to the increasing rotation velocity and to the decreasing edge fluctuation. The results indicate that the sheared poloidal flow can be generated in tokamak plasma due to the radially varying Reynolds stress. The particle confinement with low-density discharge can increase 1—2 times in LHCD. The particle confinement time and poloidal rotation velocity can at least be doubled after MPI, and that the particle confinement time can increase one order of magnitude and can obtain high performance plasma during the experiment of SMBI.

Potential safe termination by laser ablation of high-Z impurity in the HL-1M tokamak

A preliminary experiment for triggering a plasma current quench by means of injection of high_Z impurities has been performed on the HL_1M before the major disruption.Using injection of impurity with higher charge of the nuclei allows u s to increase the radiation cooling on a 3?ms time scale and the electronic tem perature loss contract is 80% before current terminates.It can be a simple and p otential approach to decrease significantly the plasma thermal energy and the ma gnetic field energy before a disruption and to a safe plasma termination.

Simulation of Supersonic Molecular Beam Fueling on HL-1M Tokamak

The supersonic molecular beam (SMB) ablation and penetration processes in the HL-1M tokamak experiments are studied. The cluster formation and dissolution, SMB adiabatic expansion, shielding and cooling effect are all taken into account. An optimized numerical model is applied in the analysis and shown to be in good agreement with the experimental observations. The possibility of fueling large tokamak plasmas with SMB injection is explored.

Array of HgI2 detectors on the HL-1M Tokamak

An array of seven HgI2 detectors, which are suitable for detecting the time and space evolution of the x-ray radiation flux and energy spectrum in the range between 15–150 keV relevant to suprathermal electron and runaway electrons was installed on the HL-1M Tokamak (R0=1.02 m, a=0.26 m, BT=3.5 T, IP=350 kA, ne=3–6×1013 cm−3, Te=0.8–1.5 keV). Our iodine mercury (HgI2) semiconductor detectors possess the properties of low noise level, high detection efficiency, high counting rate, high energy resolution, and working at room temperature. Every detector is a high quality, high purity detector cut out of a big size HgI2 crystal. A new type of multichannel energy spectrometer with a high speed acquisition system was developed. Therefore, the spatial and temporal distribution of the x-ray energy spectrum induced by thermal and superthermal electrons can be observed.

The Singular Value Decomposition as a Tool of Investigating Central MHD Instabilities in the HL-1M Tokamak

A variety of strong MHD instabilities are always resulted from MHD activity of Tokamak plasmas. Central MHD instabilities can be observed with pinhole cameras to record soft x-ray (SXR) emission from the plasma along many chords with a high temporal resolution. The investigation of MHD instabilities often necessitates an analysis on spatial-temporal signals. The method of Singular Value Decomposition (SVD) can split such signals into orthogonal spatial and temporal vectors. By this means, the repetition time and the characteristic radius of various MHD phenomena such as sawteeth and snake-like perturbation can be obtained. Moreover, the (1,1) MHD mode is analyzed in great detail by SVD and used to determine the radius of the q = 1 surface.

Sawtooth-like electron density oscillations under laser blow-off and lower hybrid current drive in HL-1M tokamak

By using of a six-channel HCN laser interferometer, the sawtooth-like density oscillations without being exhibited in the ohmic phase plasma have been observed during lower hybrid current drive (LHCD) discharges in the HL-1M device. There are often accompanied by injected impurity with laser blow-off. Analyses show that this kind of density sawtooth-like oscillation comes from the mutual effect of LHCD and impurity. An available mechanism is the loss of superthermal electrons by the magnetic perturbation, which is caused by the crash of the central accumulated impurities.

Effect of Recycling in the HL-1M Tokamak

Tokamak plasma discharge disruption at high density is investigated. The instability analysis on model indicates that the disruption is resulted from the energy loss arising from hydrogen recycling on the edge of the plasma. This energy loss could lead to a contraction of the current channel and the production of a disruptively unstable configuration. Using a simple model we shall investigate the implications of recycling for disruptions. The critical high-density n ⩽ 1.6 × 1020 m-3 is reached in LH-1M.

Determination of q =1 surface by the variation of pellet ablation rate on the HL-1M

Strong drop of Hα emission has been observed on the HL-1M tokamak by means of a detector array while a pellet crosses the q=1 surface. In this article, the q=1 surface has been determined precisely by the interval and the shape of the Hα emission striations on the pellet trajectory due to the variation of pellet ablation rate. Theq-profile and current density distribution at the plasma centre region have been calculated according to the pellet ablation rate and the magnetic shear feature.

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.

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.

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.

Electron Heat Transport in an HL-1M Tokamak ECRH Plasma

Electron temperature profiles, the normalized temperature gradientlength and the electron heat diffusivity calculated by the heat pulsetransfer method in a plasma confinement region, are investigatedduring electron cyclotron resonance heating (ECRH) in an HL-1M tokamak.The electron temperature profiles remain almost constant. Thenormalized temperature gradient length is in the range of 7–13.Electron heat diffusivity in the ECRH plasmas is larger than that inthe ohmically heated plasmas. Electron heat diffusivity during off-axis(ρ<0.3) ECRH discharge is similar to that during on-axis ECRHdischarge.

Penetration and deposition of a supersonic molecular beam in the HL-1M tokamak

The supersonic molecular beam (SMB) ablation and penetration processes in HL-1M tokamak experiments are studied. The cluster formation and dissolution, and the SMB adiabatic expansion, shielding and cooling effect are all taken into account. An optimized numerical model is applied in the analysis and is shown to be in good agreement with the experimental observations. The possibility of fuelling large tokamak plasmas with SMB injection is explored.