Ohmic Discharges Research Articles

X-Point-Position-Dependent Intrinsic Toroidal Rotation in the Edge of the TCV Tokamak.

Edge intrinsic rotation was investigated in Ohmic L-mode discharges on the Tokamak à Configuration Variable, scanning the major radial position of the X point, R(X). Edge rotation decreased linearly with increasing R(X), vanishing or becoming countercurrent for an outboard X point, in agreement with theoretical expectations. The core rotation profile shifted fairly rigidly with the edge rotation, changing the central rotation speed by more than a factor of two. Core rotation reversals had little effect on the edge rotation velocity. Edge rotation was modestly more countercurrent in unfavorable than favorable ∇B shots.

Core impurity transport in Alcator C-Mod L-, I- and H-mode plasmas

Core impurity transport has been investigated for a variety of confinement regimes in Alcator C-Mod plasmas from x-ray emission following injection of medium and high Z materials. In ohmic L-mode discharges, impurity transport is anomalous (Deff ≫ Dnc) and changes very little across the LOC/SOC boundary. In ion cyclotron range of frequencies (ICRF) heated L-mode plasmas, the core impurity confinement time decreases with increasing ICRF input power (and subsequent increasing electron temperature) and increases with plasma current. Nearly identical impurity confinement characteristics are observed in I-mode plasmas. In enhanced Dα H-mode discharges the core impurity confinement times are much longer. There is a strong connection between core impurity confinement time and the edge density gradient across all confinement regimes studied here. Deduced central impurity density profiles in stationary plasmas are generally flat, in spite of large amplitude sawtooth oscillations, and there is little evidence of impurity convection inside of r/a = 0.3 when averaged over sawteeth.

Density impact on toroidal rotation in Tore Supra: experimental observations and theoretical investigation

The effect of plasma density on toroidal rotation was examined in density ramp-up experiments at a plasma current of 1 MA and a toroidal magnetic field of 3.6 T in Tore Supra ohmic discharges. Experimental measurements have shown that the toroidal rotation amplitude is reduced at all radii when increasing the plasma density, although it remains in the counter-current direction. Neoclassical predictions including ripple-induced toroidal friction are in qualitative agreement with such observations. However, in the core (r/a < 0.5) the plasma accelerates at nl ∼ 3 × 1019 m−2 and breaks again above nl ∼ 3.5 × 1019 m−2. This rotation bifurcation did not appear to be correlated to a possible influence of the sawtooth activity. Nonetheless, quasi-linear gyrokinetic simulations have shown that the transition of the nature of linear microinstabilities from ITG-TEM to pure ITG occurs at nl ∼ 3 × 1019 m−2, which could indicate a change on the turbulence driven contributions and explain the deviation from neoclassical predictions in the plasma core.

Density modulation experiment to determine transport coefficients on Joint-TEXT Tokamak.

Density modulation experiments have been conducted on Joint-TEXT (J-TEXT) Tokamak Ohmic discharge to investigate particle transport based on a model with constant diffusion plus inward convection. Like the HCN interferometer, the newly developed three-wave polarimeter-interferometer system (POLARIS) is used to measure the perturbed density. The comparison of results between the HCN interferometer and POLARIS is given. The consistent results indicate the validity of the analysis scheme. At lower densities, the typical particle confinement time τp is found to increase with electron density, while it saturates at higher densities.

Measurements of the toroidal torque balance of error field penetration locked modes

Detailed measurements from the DIII-D tokamak of the toroidal dynamics of error field penetration locked modes under the influence of slowly evolving external fields, enable study of the toroidal torques on the mode, including interaction with the intrinsic error field. The error field in these low density Ohmic discharges is well known based on the mode penetration threshold, allowing resonant and non-resonant torque effects to be distinguished. These m/n = 2/1 locked modes are found to be well described by a toroidal torque balance between the resonant interaction with n = 1 error fields, and a viscous torque in the electron diamagnetic drift direction which is observed to scale as the square of the perturbed field due to the island. Fitting to this empirical torque balance allows a time-resolved measurement of the intrinsic error field of the device, providing evidence for a time-dependent error field in DIII-D due to ramping of the Ohmic coil current.

Radial homogeneity of geodesic acoustic modes in ohmic discharges with low B in the T-10 tokamak

The electric potential oscillations in the hot plasma zone have been measured directly using a heavy ion beam probe at the frequencies of geodesic acoustic modes in the T-10 tokamak (the major and minor radii are R = 1.5 m and a = 0.3 m, respectively, and the toroidal magnetic field is B = 1.5–2.5 T). In discharges with the lowered magnetic field B = 1.55 T, the diagnostic beam can probe a rather wide radial plasma region (0.06 < r < 0.28 m). This made it possible to study the radial structure of geodesic acoustic modes. It has been shown that the frequency and amplitude of geodesic acoustic modes in the region under study are constant over the radius in the whole region of observation. Thus, it has been shown experimentally that the observed frequency of geodesic acoustic modes may not correspond to the predictions of the local theory (f∼ √Te) in a wide radial range comparable with the plasma minor radius. The numerical simulation of the turbulence dynamics using the solution of Braginskii magnetohydrodynamic equations for a peripheral plasma has showed the formation of geodesic acoustic modes in the observed frequency range owing to the nonlinear interaction between broadband turbulence modes.

СПЕКТР ИЗЛУЧЕНИЯ ПЛАЗМЫ ТОКАМАКА Т-10 В ВИДИМОМ ДИАПАЗОНЕ

One of the issues in studies of the modern plasma devices is linked with the impurities behavior and its entrance to the central plasma from the walls of the vacuum chamber. High density of the impurities affects the plasma discharge characteristics and mainly determines the efficiency of the plasma confinement. This article presents the impurity components diagnostics of the T-10 tokamak plasma, which is based on spectroscopic measurements in visible range of wavelengths with low spectral resolution. The radiation in range 390— 710 nm of steady-state ohmic discharges is collected in section «A» of the tokamak equipped with graphite limiters. Monochromator MDR-206 without the output gap is used for the light detection in combination with the Proscan HS 101H CCD camera. Measured spectra shows the working gas lines of deuterium and well visible lines of the impurities: carbon (the main T-10 plasma contaminant), lithium, nitrogen and oxygen. Of particular interest are the oxygen and nitrogen lines, which shall not be observed under normal conditions and which indicate poor preparation of the vacuum chamber for operation mode. The diagnostics described in this article is required for normal operation on the tokamak for monitoring the plasma impurity components behavior during the experimental period and for rapid response in case of significant change in the lines intensities.

Experimental investigation of neon seeding in the snowflake configuration in TCV

Recent TCV experiments have examined the effect of the poloidal field strength in the vicinity of the x-point of diverted configurations on their ability to radiate a large fraction of the exhaust power. A larger region of low poloidal field is a key characteristic of the “snowflake” configuration, which has been proposed as an alternative divertor solution that decreases the power flux to the targets in a DEMO-size tokamak. In the investigated Ohmic discharges, increasing the plasma density and seeding neon both increased the radiated exhaust fraction up to 60–70%. In all cases, the highest radiation fraction was determined by the onset of MHD rather than a radiation instability. The experiments indicate that, while the conventional single-null configuration leads to more radiation (+10%) at higher densities, the snowflake configuration radiates more when seeding neon impurities (+15%). Extrapolation of these modest, but systematic, dependencies on the divertor geometry to reactor-relevant higher heating power and larger device size must be based on a physics model.

SOL parameters, and particle and heat fluxes at divertor targets from electric probe measurements in KSTAR

The plasma parameters in the scrape-off layer (SOL) region and the particle flux at the divertor target are measured by using a fast reciprocating Langmuir probe assembly (FRLPA) and a fixed edge Langmuir probe array (ELPA) in the KSTAR. The e-folding lengths of the plasma density (n e ) and the electron temperature (T e ), λ ne and λ Te , are obtained from the radial profile measurement using the FRLPA at the outboard mid-plane during ohmic and H-mode discharges. Particle fluxes measured by using the ELPA at the divertor target are mapped to the outboard mid-plane, and the fluxes are quite well matched to the radial profile of the particle flux from the FRLPA measurement. Finally, the peak heat flux at the divertor target during edge localized modes (ELMs) can be estimated to be up to 1.0 MWm−2 for the neutral beam power of ~ 2.7 MW by using T e at the separatrix and λ Te from the FRLPA measurements. In this work, the experimental results from the probe measurements in the KSTAR are presented.

Progress of the Keda Torus eXperiment Project in China: design and mission

The Keda Torus eXperiment (KTX) is a medium-sized reversed field pinch (RFP) device under construction at the University of Science and Technology of China. The KTX has a major radius of 1.4 m and a minor radius of 0.4 m with an Ohmic discharge current up to 1 MA. The expected electron density and temperature are, respectively, 2 × 1019 m−3 and 800 eV. A combination of a stainless steel vacuum chamber and a thin copper shell (with a penetration time of 20 ms) surrounding the plasma provides an opportunity for studying resistive wall mode instabilities. The unique double-C design of the KTX vacuum vessel allows access to the interior of the KTX for easy first-wall modifications and investigations of power and particle handling, a largely unexplored territory in RFP research leading to demonstration of the fusion potential of the RFP concept. An active feedback mode control system is designed and will be implemented in the second phase of the KTX program. The recent progress of this program will be presented, including the design of the vacuum vessel, magnet systems and power supplies.

Onion skin model (OSM) analysis of EAST SOL plasmas

Two-dimensional maps of the Experimental Advanced Superconducting Tokamak (EAST) scrape-off layer (SOL) plasma conditions for ohmic, L-mode and H-mode discharges are reconstructed using an onion skin model (OSM) coupled in DIVIMP together with the Monte Carlo neutral transport code, EIRENE. The boundary conditions for OSM calculation are taken from the measurements of the Langmuir probe built into the divertor targets. The OSM-calculated values of the outboard mid-plane electron density, ne, and temperature, Te, are compared with the mid-plane measurements of ne and Te from a fast reciprocating probe. Some other characteristics of these SOL plasmas are also derived from the OSM solution, reflecting that the upstream plasma conditions are governed by the SOL collisionality to a large degree. Values of at the low-field side and the high-field side mid-plane are derived separately as a function of the distance to the separatrix for ohmic, L- and H-mode discharges, showing that increases with the distance to the separatrix at both sides and that the values of at the low-field side tends to be higher than that at the high-field side. is found to be larger than by a factor of 2–3 for all the discharges considered here. In addition, before the use of the OSM method of extracting and for EAST discharges, the reliability of this method is assessed by taking SOLPS-generated target n, T profiles as boundary conditions and by comparing the OSM-extracted cross-field transport coefficients with those input in the SOLPS modelling.

Analysis of the Gas Puffing Performance for Improving the Repeatability of Ohmic Discharges in the SUNIST Spherical Tokamak

The gas puffing performance plays a key role in repeatable discharges in the Sino-UNIted Spherical Tokamak (SUNIST) experiments. In this paper, temporal evolution of the gas pressure in the vacuum vessel and the dependence of the repeatability of plasma discharges on different timing arrangements between the gas puffing pulse and the Ohmic field have been experimentally investigated. The results show that, after a fast rising phase, the gas pressure becomes quasi-stationary. In the regime of the discharges being started up when the gas pressure has already reached the quasi-stationary state for about 37 ms, an improved repeatability of the plasma discharges is achieved.

Development of internal magnetic probe for current density profile measurement in Versatile Experiment Spherical Torus.

An internal magnetic probe using Hall sensors to measure a current density profile directly with perturbation of less than 10% to the plasma current is successfully operated for the first time in Versatile Experiment Spherical Torus (VEST). An appropriate Hall sensor is chosen to produce sufficient signals for VEST magnetic field while maintaining the small size of 10 mm in outer diameter. Temperature around the Hall sensor in a typical VEST plasma is regulated by blown air of 2 bars. First measurement of 60 kA VEST ohmic discharge shows a reasonable agreement with the total plasma current measured by Rogowski coil in VEST.

Calculation and Characteristics Analysis of Lithium Ion Batteries’ Internal Resistance Using HPPC Test

Internal resistance of battery can really reflect its own characteristics, which including health status of the battery, inconsistency, state of charge, thermal runaway. Method to calculate the internal resistance of the battery using HPPC (Hybrid Pulse Power Characterization) test which proposed by the FreedomCAR is introduced. Ohmic resistance, polarization resistance, discharge resistance and regen resistance are calculated based on MATLAB programming using HPPC test. The differences between the four kinds of internal resistance of lithium ion batteries are compared. Characteristics of the four kinds of internal resistance in charged-state and discharged-state are analyzed.

Real-time control for long ohmic alternate current discharges

The ISTTOK tokamak has a long tradition on alternate plasma current (AC) discharges, but the old control system was limiting and lacked full system integration. In order to improve the AC discharges performance the ISTTOK fast control system was updated. This control system developed on site based on the Advanced Telecommunications Computing Architecture (ATCA) standard now integrates the information gathered by all the tokamak real-time diagnostics to produce an accurate observation of the plasma parameters. The real-time actuators were also integrated, allowing a Multiple Input Multiple Output (MIMO) control environment with several synchronization strategies available.The control system software was developed in C++ on top of a Linux system with the Multi-threaded Application Real-Time executor (MARTe) Framework to synchronize the real-time code execution under a 100μs control cycle. In addition, to simplify the discharge programming, a visual Human–Machine Interface (HMI) was also developed using the BaseLib2 libraries included in the MARTe Framework.This paper presents the ISTTOK control system and the optimizations that extended the AC current discharges duration to more than 1s, corresponding to 40 semi-cycles without apparent degradation of the plasma parameters. This upgrade allows ISTTOK to be used as a low-cost material testing facility with long time exposures to nuclear fusion relevant plasmas, comparable (in duration) with medium size tokamaks.

Experimental Advanced Superconducting Tokamak/material and plasma evaluation system material migration experiment

A test assembly of carbon coated molybdenum tiles was exposed to a sequence of well-diagnosed ohmic helium discharges in the Experimental Advanced Superconducting Tokamak (EAST) using the material and plasma evaluation system. The test assembly was configured to approximate, on a reduced scale, the profile of a first-wall panel in ITER. Net erosion and deposition of carbon was determined from measurements of the thickness of a carbon surface layer by ion-beam analysis before and after the plasma exposure. The highest erosion of 800 nm, or about one third of the initial thickness, occurred near the EAST midplane on the side facing the ion-drift direction. Erosion decreased in toroidal and poloidal directions with increasing distance from the plasma. Net deposition was not observed anywhere above the limit of detection. This experiment provides data needed to benchmark codes being used to predict erosion/deposition in ITER.

Surface erosion and modification of toughened, fine-grained, recrystallized tungsten exposed to TEXTOR edge plasma

In order to evaluate the applicability of toughened, fine-grained, recrystallized (TFGR)-W to tokamak edge plasma environment, two TFGR-W specimens (TFGR-W 1.1wt%TiC and TFGR-W 3.3wt%TaC) were exposed to 31 identical ohmic discharges in the TEXTOR tokamak by means of a limiter lock system. The highest surface temperature reached was about 1300 °C. Under these temperature conditions, the bulk microstructure and dispersoids distribution of both TFGR-W remained intact, suggesting that these TFGR-W tungsten materials have sufficient stability under these plasma loading conditions. The erosion of TiC dispersoids on the surface was enhanced by plasma exposure above 1150 °C, while such enhanced erosion was not observed for TaC dispersoids probably due to the higher melting temperature of Ta than Ti.

Core intrinsic rotation behaviour in ASDEX Upgrade ohmic L-mode plasmas

The ASDEX Upgrade intrinsic rotation database has been expanded to include a large number of measurements from ohmic L-mode discharges covering a wide range of plasma densities, currents and magnetic fields. This database was then used to study the rotation behaviour across the transition from linear to saturated ohmic confinement (LOC/SOC). At low collisionality the plasma is in the LOC regime and the toroidal rotation profile is in the co-current direction. As the collisionality is increased and the plasma transitions from LOC to SOC, the core rotation decreases resulting in a hollow, counter-current profile. Even deeper in the SOC regime, however, a second reversal back towards the co-current direction occurs. Linear gyrokinetic calculations indicate that these reversals cannot be explained by a transition from a trapped electron mode (TEM) to an ion temperature gradient (ITG) dominated regime. Rather, the analysis indicates that the intrinsic normalized rotation gradient, u′, depends strongly on local plasma parameters, in particular on . Taken together with turbulent particle transport theory, these results suggest that the co- to counter-current directed rotation reversal occurs in the TEM regime due to profile changes and not due to transition from TEM to ITG. The second reversal back to the co-current direction is explained by the reduction of observed in ITG dominated plasmas. Lastly, a simple linear model for the residual stress assuming a fixed poloidal tilt angle of the turbulent eddies was applied to the database. This model is able to capture the main parameter dependencies observed in the data and demonstrates that at least two processes contributing to the poloidal tilt of the turbulent eddies are needed to reproduce the experimentally observed u′ values: one proportional to the sign of the turbulence propagation and the second independent of ωr.

Growth and decay of runaway electrons above the critical electric field under quiescent conditions

Extremely low density operation free of error field penetration supports the excitation of trace-level quiescent runaway electron (RE) populations during the flat-top of DIII-D Ohmic discharges. Operation in the quiescent regime allows accurate measurement of all key parameters important to RE excitation, including the internal broadband magnetic fluctuation level. RE onset is characterized and found to be consistent with primary (Dreicer) generation rates. Impurity-free collisional suppression of the RE population is investigated by stepping the late-time main-ion density, until RE decay is observed. The transition from growth to decay is found to occur 3–5 times above the theoretical critical electric field for avalanche growth and is thus indicative of anomalous RE loss. This suggests that suppression of tokamak RE avalanches can be achieved at lower density than previously expected, though extrapolation requires predictive understanding of the RE loss mechanism and magnitude.

Plasma Response to Supersonic Molecular Beam Injection in J-TEXT

Recently, hydrogen fueling experiments with supersonic molecular beam injection (SMBI) system have been performed in the J-TEXT tokamak. To evaluate the effects of the injection amount of SMBI on plasma behaviors, moderate and intensive SMBs have been separately injected and compared with each other in Ohmic discharges. With moderate SMBs, electron density increases about twice as before, the size of magnetic island slightly decreases, and the edge toroidal rotation speed in a counter-current direction, measured by a high resolution spectrometer (Carbon V ion, 227.09 nm, r/a = 0.7–0.8), is accelerated from 8 km/s to 12 km/s. The state of higher electron density with moderate SMBI can be maintained for a long period, which indicates that plasma confinement is improved. However, with intensive SMBs, the accompanied magnetohydrodynamic (MHD) activities are triggered, and the electron density increases moderately. The edge toroidal velocity is decreased, in certain cases even reversed in the co-current direction. The statistical result of experimental data for moderate and intensive SMBs suggests a preferred fueling amount (less than 3.2×1019) to improve the SMBI fueling efficiency in experiments.