Edge Plasma Characteristics Research Articles

First EMC3-EIRENE simulation of edge plasma behavior and heat flux deposition on divertor target on HL-2M

Abstract The EMC3-EIRENE code has been applied for the first time to simulate the edge plasma characteristics of the new tokamak device HL-2M. Impacts of the upstream electron density on the downstream plasma parameters and heat flux deposition on divertor targets are studied based on a parameter scan of upstream electron density from 1.0×1019 to 3.5×1019 m−3. The downstream plasma on HL-2M exhibits a good performance easily accessible to high-recycling regime. A prior detachment regime is obtained at the divertor region with the upstream electron density higher than 2.5×1019 m−3, which results from the stronger momentum loss for the high density scenarios. The heat flux depositions at the outboard divertor targets are higher than that at the inboard one for the same upstream electron density. Further, the enhanced upstream electron density can lead to an effective suppression in the peak value of heat loads on divertor targets.

Sizing up plasmas using dimensionless parameters

This paper reviews the application to magnetic-confinement fusion experiments of dimensional analysis, which holds that the behavior of physical systems can be determined from the scaling of phenomena with the set of dimensionless parameters that embody the governing physics. This paper begins by explaining the two most well-known approaches to dimensional analysis, and shows that the principle of similarity has been demonstrated in high-temperature plasmas of different physical size. Next, the measured dependences of cross-magnetic-field transport and edge plasma characteristics on dimensionless parameters are examined. These dimensionless parameter scans are generally in good agreement with drift wave models of turbulent transport (i.e., microturbulence), although some discrepancies remain. Finally, the benefits of incorporating dimensional analysis into the extrapolation of plasma behavior from present-day experiments to future burning plasma devices are discussed. The experiments reviewed in this paper have greatly improved our understanding of the underlying physics of many plasma phenomena.

Study of Edge Plasma Characteristics at H-mode Transition in Heliotron J

Characteristics of spontaneous transition to a high confinement mode (H-mode) have been studied in a helical-axis heliotron device, Heliotron J. The radiation profile measured with a silicon photodiode array shows that a interesting change in the profile occurs at the plasma edge region in the transition phase, forming a strong gradient near the last closed flux surface (LCFS). Relating to the transition, characteristics of bursty density and particle-flux fluctuations current in the scrape off layer (SOL) have been investigated. The probability distribution function for the fluctuations has a tail structure in the L-mode phase, while the tail is suppressed and its direction is reversed after the transition, which may be related to the suppression of edge-plasma turbulence.

Experimental Observation of the Edge and Divertor Plasma in HL-2A

Edge plasma characteristics were studied by a fast-scanning 4-probe array and a Mach/Reynolds stress/Langmuir 10-probe movable array in the boundary region. These probes could measure the edge plasma temperature, density, poloidal electric field, radial electric field, Reynolds stress, poloidal rotation velocities and their profiles, which could be obtained by changing the radial positions of the probe array shot by shot. The measured results were used to analyse plasma confinement, turbulent fluctuations and correlations. The fixed flush 3-probe arrays were mounted on the 4-divertor neutralization plates at the same toroidal cross-section in the divertor chamber. These probes were used to measure the profiles of the electron temperature, density and float potential in the divertor chamber. Edge plasma behaviours in both limiter configuration and divertor configuration are compared. The decay lengths of the edge temperature and density were measured and is emphasized for plasma behaviours of the supersonic molecular beam injection and lower hybrid current drive. The dependence of the radial gradient of Reynolds stress on the poloidal flow and the radial gradient of the electric field on turbulent loss are discussed.

Analyses of edge plasma characteristics in HL-2A

The edge plasma characteristics are studied by both a movable array of Mach/Reynolds stress/Langmuir 10-probes in the boundary region and the fixed flush probe arrays on the 4 divertor neutralization plates at the same toroidal cross-section in the HL-2A tokamak. The dependence of the Reynolds stress on poloidal flow in the edge plasma is analysed. The result indicates that the sheared poloidal flow in tokamak plasma can be induced by the radial gradient of Reynolds stress. In the divertor experiments of HL-2A, the profiles of the electron temperature, density and floating potential on divertor plates are measured by the flush probe arrays. The edge electron temperature in divertor configuration is higher than that in limiter configuration. The temperature asymmetry between outer and inner target plates is observed. The result of magnetic surface reconstructed from 18 Mirnov coils signals is presented. Both the particle recycling and the impurity flux in the bulk plasma during divertor discharges are discussed. Neutral gas pressure in divertor chamber, measured by fast ionization gauge during divertor discharge, is given.

H-mode confinement of Heliotron J

The L–H transition in a helical-axis heliotron, Heliotron J, is investigated. For electron cyclotron heating (ECH), neutral beam injection (NBI) heating and ECH + NBI combination heating plasmas, the confinement quality of the H-mode is examined with an emphasis on its magnetic configuration dependence. The vacuum edge rotational transform, ι(a)/2π, is chosen as a label for the magnetic configuration where ι/2π is the rotational transform and a is the average plasma minor radius in metres. The experimental ι(a)/2π dependence of the enhancement factor over the L-mode confinement reveals that specific configurations exist where high-quality H-modes (1.3 < HISS95 < 1.8) are attained. is the experimental global energy confinement time and is the confinement time scaling from the international stellarator database given as . R is the plasma major radius in metres, is the line-averaged plasma density in 1019 m−3, PL is the power loss in megawatts that accounts for the time derivative of the total plasma energy content and Bt is the toroidal magnetic field strength in tesla (Stroth U. et al 1996 Nucl. Fusion 36 1063). The ι (a)/2π ranges for these configurations are near values that are slightly less than those of the major natural resonances of Heliotron J, i.e. n/m = 4/8, 4/7 and 12/22. To better understand this configuration dependence, the geometrical poloidal viscous damping rate coefficient, Cp, is calculated for different values of ι(a)/2π and compared with the experimental results. The threshold line-averaged density of the H-mode, which depends on the configuration, is in the region of 0.7–2.0 × 1019 m−3 in ECH (0.29 MW) + NBI (0.57 MW) operation. As for the edge plasma characteristics, Langmuir probe measurements have shown a reduced fluctuation-induced transport in the region that begins inside the last closed flux surface (LCFS) and extends into the scrape-off layer. In addition, a negative radial electric field Er (or Er-shear) is simultaneously formed near the LCFS at the transition.

Fast neutral lithium beam probing of the edge region of the spherical tokamak ETE

The close relationship between the behavior of the edge plasma characteristics and the global confinement in discharges in tokamaks has been extensively explored in recent years. A 15 keV lithium beam with 80% neutralization efficiency, using a high ion current density emitter and a modified Pierce gun design for extraction of ions, was developed for edge studies on the spherical tokamak ETE—(major radius R=0.30 m, aspect ration A=1.5, designed plasma current of 200 kA with a torodial magnetic field up to 0.4 T). Already in the calibration phase this diagnostics confirmed to be an efficient experimental tool, allowing accurate measurements of the fast varying pressure of the discharges in ETE. The measurement of the temporal evolution of the density of the plasma edge with fast neutral lithium beam is in accordance with data from Langmuir probe for the region near the limiter and the one made by the Thomson scattering for the inner region.

Studies of high confinement plasma with lower hybrid current drive in the HT-7 superconducting tokamak

A high confinement plasma (including core plasma and edge plasma) produced by using lower hybrid current drive (LHCD) has been obtained on the HT-7 superconducting tokamak. An internal transport barrier in the core plasma was formed. The energy confinement time increases from 14.6 ms (Ohmic (OH) phase) to 24.5 ms (LHCD phase), which is close to the value calculated using the ITER93ELM free scaling law. The confinement factor H89 increases from 0.78 (OH phase) to 1.42 (LHCD phase). The experimental results were in good agreement with the simulations calculated with a ray tracing code and a two-dimensional Fokker–Planck equation. The edge plasma characteristics around the last closed flux surface were investigated using Langmuir probes. Turbulence and transport of the edge plasma were suppressed greatly by the lower hybrid wave. Studies show that the enhanced confinement plasma may be ascribed to a shear flow resulting from the shear of the radial electric field.

Edge plasma characteristics in the helicity injected torus (HIT-II) spherical tokamak

The helicity injected torus (HIT-II) device is a spherical tokamak capable of both inductive (Ohmic) and co-axial helicity injection (CHI) current drive. The HIT-II plasma edge, in both Ohmic and CHI discharges, has been characterized using a triple Langmuir probe. An Ohmic discharge develops in two phases, a slide-away phase followed by a normal Ohmic discharge. During the normal Ohmic discharge, the floating potential is negative, just as in a conventional large-aspect-ratio tokamak. The plasma density increases sharply from the plasma edge into the centre. The auto-power spectrum of Ohmic plasma edge fluctuations shows a nearly constant auto-power at low frequencies, with auto-power decreasing at higher frequencies, similar to observations in conventional large-aspect-ratio tokamaks. In HIT-II CHI discharges, the magnetic field lines at the plasma edge are clearly connected to the injector electrodes, as expected. However, the time-evolution of the floating potential in the core plasma is significantly different from that of the edge, which may indicate a decoupling of the core plasma from the CHI electrodes. Finally, the fluctuations at the edge of high-performance CHI discharges exhibit a coherent oscillation at a frequency similar to that of the observed n = 1 mode.

Numerical imaging of edge plasma characteristics in the high-recycling scrape-off layer of the KSTAR tokamak

Edge plasma characteristics of a high-recycling divertor operation regime of the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak are presented. The results were obtained by numerical simulation using a two-dimensional (2-D) two-fluid edge plasma transport code coupled with a 2-D Monte Carlo recycling neutral transport code. The simulations successfully display the two major characteristics of a scrapeoff layer in a high-recycling regime: a parallel temperature gradient and parallel pressure conservation. The results of the simulation can be utilized to provide guidelines for future operation of the KSTAR tokamak regarding the relation between upstream plasma properties and divertor operation regimes.

Edge plasma characteristics during ICRF heating on JFT-2M Tokamak

The roles of the edge plasma are investigated with respect to impurity release, antenna loading, and H-mode transition. Metal impurity release is considered to be caused by an RF electric field near the antennas, and can be reduced by the out-of-phase operation. Antenna loading with H-mode plasmas is dependent on the antenna-separatrix distance, and can be increased for sufficiently small separation. Electron temperature and electron density in the edge are the key parameters for H-mode transition, and they are the same, independent of the additional heating methods, and of the impurity level.