Impurity Flow Research Articles

A Summary on H-Mode Studies in W7-AS

We will give a summary on the status of H-mode studies on W7-AS stellarator. The major H-mode characteristics compare well with those known from the tokamak H-mode. All major characteristics of the H-mode are reproduced: The transition is spontaneous above a power and density threshold; particle and energy confinement improve simultaneously; a transport barrier at the edge develops with steep pressure gradients and ELMs appear; small scale fluctuations are strongly reduced and the development of a radial electric field is indicated by increased perpendicular impurity flow velocity. The temporal development of the transition seems to be distinctively slower than in tokamaks. The H-mode can be initiated by ECRH or NBI, respectively. The power threshold can be smaller than that of tokamaks. With ECRH, the density threshold is found to increase with heating power. The H-mode develops in small windows of the accessible iota range. These operational islands are characterized by a negative electric field already prior to the H-mode and distinct maximum in space potential at separatrix. 10 refs., 6 figs.

Analysis methods for fast impurity ion dynamics data

A high-resolution spectrometer has been developed and used on the MST reversed-field pinch (RFP) to measure passively impurity ion temperatures and flow velocities with 10 μs temporal resolution. Such measurements of MHD-scale fluctuations are particularly relevant in the RFP because the flow velocity fluctuation-induced transport of current (the ‘‘MHD dynamo’’) may produce the magnetic field reversal characteristic of an RFP. This instrument will also be used to measure rapid changes in the equilibrium flow velocity, such as occur during locking and H-mode transition. The precision of measurements made to date is ≤0.6 km/s. We are developing accurate analysis techniques appropriate to the reduction of this fast ion dynamics data. Moment analysis and curve-fitting routines have been evaluated for noise sensitivity and robustness. Also presented is an analysis method which correctly separates the flux-surface average of the correlated fluctuations in u and B from the fluctuations due to rigid shifts of the plasma column.

A fast spectroscopic diagnostic for the measurement of plasma impurity ion dynamics

A high-resolution spectrometer has been developed and used to measure simultaneously impurity ion temperatures and flow velocities in high temperature plasmas with 10 μs temporal resolution (limited by digitization rate). This device is actually a duo-spectrometer: measurements from two different chordal views of the plasma can be made simultaneously via two separate quartz input fiber optic bundles coupled to the entrance slits which are tilted to compensate for line curvature. The dispersed spectra on the exit plane of the duo-spectrometer are coupled via quartz fiber optics to two arrays of 16 photomultiplier tubes each. Measurements made by recording the Doppler broadened and shifted 227.091 nm emission from the C v impurity ions in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) plasma have achieved precisions of <6 eV for temperatures of 150 eV and <0.7 km/s for flow velocities of 6 km/s. Representative results from the MST RFP indicate that the toroidal flow velocity drops and ion temperature increase during sawtooth events in MST.

Extended theory of main ion and impurity rotation and bootstrap current in a shear layer

In this paper, standard neoclassical theory has been extended into the shear layer. Main ion and impurity ion rotation velocity and bootstrap current within shear layer in H-mode are discussed. Inside the H-mode shear layer, standard neoclassical theory is not valid since the ion poloidal gyroradius becomes comparable to pressure gradient and electric field gradient scale length. We present modified expressions of impurity flows and bootstrap current in a shear layer neglecting ion temperature gradient. Comparisons with DIII-D measurements are also discussed.

H-mode of W7-AS stellarator

In W7-AS the H-mode has been observed for the first time in a currentless stellarator plasma. H-modes are achieved with 0.4 MW Electron Cyclotron Resonance Heating with 140 GHz at 2.5 T and high density, with 70 GHz at 1.25 T and lower density and with neutral beam injection. The H-phases display all characteristics known from tokamak H-modes including the development of an edge transport barrier, an increase of the poloidal impurity flow velocity at the edge, the reduction of edge turbulence and ELMs. The power threshold for the H-mode seems to be lower than that in tokamaks and is in agreement with an neBT scaling. Major differences to the divertor H-mode is the small increase in energy content of maximally 30%, the lack of a strong isotope effect both in threshold and in H-mode characteristics and a peculiarly narrow operational range in iota.

The effect of charge exchange on plasma flows

A simple drift-kinetic derivation of the expressions for the poloidal ion and impurity flows in the presence of charge-exchange drag and ion-impurity collisions is presented.

Impurity effects on ηi mode stability and transport

The effects of an impurity ion species on the linear stability of the toroidal ion temperature gradient driven mode (ηi mode) are investigated together with the scaling properties of the corresponding quasi-linear transport coefficients. A stabilizing influence of impurities is found for inwardly peaked or flat impurity density profiles except for small ϵn (ϵn = 2Ln/LB), in which case destabilization is found. However, new impurity driven instabilities are introduced also below the pure ηi mode stability threshold. The ηi mode still accounts for the major part of the quasi-linear ion energy transport, which is reduced accordingly. The direction of the ion particle transport is mainly determined by the shape of the the impurity density profile in relation to the background ion profile. With an inwardly peaked impurity profile, the impurity flow is usually directed towards the edge and the main ion flow is reversed owing to ambipolarity

Multidimensional and Multifluid Plasma Edge Modelling: Status and New Directions

AbstractThe plasma boundary in magnetic confinement devices is observed in experiments and in computations to contain radial and poloidal variations in plasma potential, strong radial electric fields and large poloidal drift flows in addition to the parallel flow toward the plasma sheath boundaries and the particle recycling from the walls. The global power balence is largely influenced by impurity radiation and thus by impurity transport in the edge region. The behavior of the edge region is related to global confinement in ways not yet fully known and motivates further improvements in edge plasma modelling.General multifluid transport equations are reviewed in the context of the current status of plasma edge modelling. It is assumed that the macroscopic transport in the plasma edge region is suitable for a moments description, or fluid model. Progress is well established in computational treatments of the standard paradigm including classical parallel and anomalous cross‐field flux coupled to neutrals in analytic or Monte Carlo steady state computations. Formulation of momentum diffusivities and curvature drift in toroidal coordinates are considered. New directions include defining the dominant influence of drifts and non‐ambipolar flows, improving coupled plasma‐neutral transport calculations, and treating impurity flow in full multifluid models. Calculation of the plasma potential driving the E × B drift in the scrape‐off layer can be extended inside the last closed flux surface by conserving the divergence of the plasma currents. Coupled plasma‐neutral fluids using a neutral diffusion approximation show promise for recycling calculations. Impurity transport based on full multifluid treatment along the field lines has shown good agreement with divertor experiments.Advanced topics for the near future include plasma core‐edge transport coupling and eventually the direct solution of coupled momentum equations rather than the presently assumed cross‐field ‘anomalous’ flux functions. In the edge region, plasma sheath boundary conditions are a significant sink for plasma particles and heat; a small coupling between the momentum components in the plasma edge can be sufficient to drive an ‘anomalously’ large efflux from the core plasma.

Stationary Study of the Boundry Plasma of Large Tokamaks with Divertor

AbstractThe hydrogen plasma and impurity flows parallel to the magnetic field lines in a collissional tokamak scrape‐off layer are numerically investigated. The physical model is developed on the basis of electron‐ion fluid equations for hydrogen plasma and rate equations for impurities. A complex model valid for arbitrary concentration of impurity ions based on 21‐moment Grad approximation is used to describe transport coefficients and thermal forces. Plasma interactions with neutral particles recycling at the divertor plate are taken into account. Steady state spatial profiles of the plasma and impurity (0, C) parameters are computed using the time relaxation method Possibility of helium exhaust is investigated.

Helium Flow Reversal with Neutral Beam Injection and Electron Cyclotron Resonance Heating in TIBER-II

Impurity flow reversal based on coinjected neutral beams and on electron cyclotron resonance heating are considered as possible mechanisms for alpha ash removal from tokamak reactors. Both formalisms are applied to the TIBER-II parameters and preliminary results are reported.

Thermally driven convective cells and tokamak edge turbulence

A unified theory for the dynamics of thermally driven convective cell turbulence is presented. The cells are excited by the combined effects of radiative cooling and resistivity gradient drive. The model also includes impurity dynamics. Parallel thermal and impurity flows enhanced by turbulent radial diffusion regulate and saturate overlapping cells, even in regimes dominated by thermal instability. Transport coefficients and fluctuation levels characteristic of the saturated turbulence are calculated. It is found that the impurity radiation increases transport coefficients for high density plasmas, while the parallel conduction damping, elevated by radial diffusion, in turn quenches the thermal instability. The enhancement due to radiative cooling provides a resolution to the dilemma of explaining the experimental observation that potential fluctuations exceed density fluctuations in the edge plasma (eΦ/Te >n/n0).

Characterization of the Edge Plasma of the CASTOR‐Tokamak

AbstractThe edge plasma of the CASTOR‐tokamak was investigated by Langmuir and collector probes. Te – values derived from both methods agree satisfactorily and are in the range from 15 eV to 45 eV, depending on the minor radius and the magnitude of the discharge current. However, the collector probe measurements indicate an anisotropic plasma and impurity flow in the edge plasma.

An ergodic divertor for tore supra

Three dimensional calculations of energy and particle flows in the ergodic divertor envisioned for the TORE SUPRA experiment are reported. They indicate the possibility of an interesting conduction regime, where the ergodicity caused by the perturbing field is sufficient to establish nearly constant pressure without destroying the temperature gradient in the ergodic layer. Such a regime allows the plasma density to be compressed near the wall and accordingly the temperature to be reduced for a given flux of energy, leading to the same type of situations as in the expanded boundary approach. The transverse diffusion coefficient must be small enough that the particle diffusion flow may be balanced by a flow along the ergodic flux lines. Drag effects on impurity flow along the flux lines may decontaminate the plasma in that regime. A stronger decontamination may be expected at very low temperature allowing deep penetration of neutrals in the layer. However, in such convection regimes, the power extraction is limited to smaller values.

An Investigation of Neutral-Beam-Driven Impurity Flow Reversal as an Impurity Control Method in Future Tokamaks

A theory for the effect of directed neutral beam injection on impurity transport in tokamaks is extended to include temperature gradient effects. The extended theory is compared with experimental data, from PLT, and certain coefficients are adjusted to provide agreement. The adjusted theory is applied to assess the potential of NBI being used to prevent impurities from penetrating to the center of future tokamak plasmas, thereby possibly creating a cold radiating edge. The results indicate the possibility of creating a cold radiating edge in TETR and in future tokamaks represented by FED and STARFIRE.

Modelling of the impurity pumping by a tokamak scrape-off layer

The impurity flow along magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated using a testfluid approach. Transport perpendicular to the magnetic field is approximately included by a local loss time constant τ ⊥. Results for various impurities and hydrogen background plasma parameters show the significance of thermal forces in the high recycling, subsonic flow regime. In this case, impurity flow reversal on “hot” field lines may cause impurity circulation in the scrape-off layer and, as a consequence, a stronger coupling between the main plasma and a separate pumping chamber than previously assumed.

Neutral beam driven impurity flow reversal in tokamaks

A theory is outlined for explaining the observed effects of neutral beam injection on impurities in tokamak plasmas. In the collisional regime, it is shown for a representative model problem that for sufficiently strong co-injection the outward impurity flux component produced by the radial electric field becomes larger than the inward flux components due to the density gradient, rotation and direct beam-impurity momentum exchange, thereby producing an outward impurity flux. In the mixed regime, a preliminary version of the theory provides a reasonable interpretation of the PLT impurity flow reversal experiments when the effective viscous drag coefficient that is observed experimentally is used in the model. This preliminary theory, when adjusted to match the PLT results, predicts significant impurity reversal in TFTR and in a plasma of the size of FED and INTOR.

Modelling of impurity flow in the tokamak scrape-off layer

The impurity flow parallel to the magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated. The rate equations are solved treating each ionization state as a test fluid which interacts with the given hydrogen background plasma via collisions and the ambipolar electric field. Results for typical impurities (O, Fe, He, etc.) show that collisional friction usually forces the impurities to flow nearly at hydrogen speed. Thermal forces, however, can become dominant locally for small Mach number and large temperature gradient (e.g. strong target recycling), causing impurity flow reversal and subsequent accumulation outside the recycling region. The criterion for flow reversal is roughly M <λi/λT where λi is the hydrogen ion mean free path and λT is the temperature gradient length. Self-sputtering at the target plates is calculated, showing the importance of fractional impurity acceleration in addition to the charge-statedependent electrostatic energy gain.

Impurity levels and power loading in the pdx tokamak with high power neutral beam injection

The PDX tokamak provides an experimental facility for the direct comparison of various impurity control techniques under reactor-like conditions. Four neutral beam lines inject > 6 MW for 300 ms. Carbon rail limiter discharges have been used to test the effectiveness of perpendicular injection, but non-disruptive full power operation for > 100 ms is difficult without extensive conditioning. Initial tests of a toroidal bumper limiter indicate reduced power loading and roughly similar impurity levels compared to the carbon rail limiter discharges. Poloidal divertor discharges with up to 5 MW of injected power are cleaner than similar circular discharges, and the power is deposited in a remote divertor chamber. High density divertor operation indicates a reduction of impurity flow velocity in the divertor and enhanced recycling in the divertor region during neutral injection.

Analysis of the impurity flow reversal experiment

Experiments performed on the Impurity Study Experiment (ISX-A) tokamak have shown that poloidally asymmetric injection of hydrogen gas can alter the inward transport of injected impurity atoms. The observed transport times and the magnitude of the flow reversal effect have been compared with a model based on the neoclassical theory of impurity transport; the observations are found to be consistent with theory. The diffusion coefficient is between once and twice neoclassical. Uncertainties in the radial profiles do not, however, permit a truly definitive test of the theory.

Review of the ISX-B experimental program

The ISX-B tokamak is a modestly sized device that is designed to explore questions closely related to the design of future tokamak devices, such as the Engineering Test Facility (ETF). The major program emphasis is on high-β plasma operation using neutral beams to heat plasmas with noncircular cross sections. In addition, substantial efforts are under way or planned in the areas of plasma-wall interactions, coated limiter studies, electron–cyclotron heating, ripple injection, impurity flow reversal, pellet injection, and the application of a bundle divertor. The current status and future plans for these programs are reviewed.