In-out Asymmetry Research Articles

Active control of divertor asymmetry on EAST by localized D2 and Ar puffing

The divertor asymmetry in particle and power fluxes has been investigated on the EAST superconducting tokamak [S. Wu and EAST Team, Fusion Eng. Des. 82, 463 (2007)] for both single null (SN) and double null (DN) divertor configurations. D2 and Ar puffing at various divertor locations has also been explored as an active means to reduce peak target heat load and control divertor asymmetry. For SN, peak heat load on the outer divertor target is 2–3 times that on the inner divertor target under typical ohmic plasma conditions. DN operation leads to a stronger in-out asymmetry favoring the outer divertor. D2 and Ar puffing promotes partial detachment near the strike points, greatly reducing peak target heat load (over 50%), while the far-SOL divertor plasma remains attached. What is remarkable is that the particle flux is even increased away from the strike points when the B×∇B drift is directed toward the divertor target, thus facilitating particle removal.

Recent progress on divertor operations in EAST

The first systematic assessment of divertor performance was made in the EAST superconducting tokamak for both single null (SN) and double-null (DN) divertor configurations under Ohmic and L-mode plasma conditions. Particle and heat fluxes are higher at outboard divertor targets than those at inboard divertor targets for both SN and DN. However, the DN divertor configuration exhibits a much stronger in-out asymmetry favoring outer divertors. DN operation also leads to an up-down asymmetry with higher particle fluxes to the divertors with their ∇B drift toward the X-point. Reversing toroidal field direction shows a strong influence on the divertor asymmetries. The plasma exhibits a large amplitude broadband turbulence in the outer divertors for both SN and DN, which is significantly reduced in the inner divertors, especially for DN. This coincides with a broad divertor SOL profile on the outboard side. Initial comparisons are also made with SOLPS-B2/EIRENE modeling.

Divertor Plasma Properties for a Lower Single-Null Discharge in EAST

Divertor plasma properties for lower single-null (LSN) configuration in EAST, under a water-cooling graphite wall condition, are presented. A typical shot is selected for analyzing electron temperature, density and power flux at targets. Both higher electron density and lower electron temperature are found at the inboard target, compared to those at the outboard target. A higher power flux is also found to be at the outboard target because of in-out asymmetries for LSN-shaped discharge.

Effect on the divertor and scrape-off layer plasma properties of the distribution of power and particle influxes from the core

Calculations of the profiles along the field lines within the divertor and scrape-off layer (SOL) of differences in the plasma ion density, temperature, parallel current, parallel flow velocity, and electrostatic potential, which result from using different poloidal distributions of the particle and heat influxes crossing the separatrix from the core plasma into the SOL, are presented and discussed vis-à-vis experimental observations. The calculations show that the stronger outboard than inboard particle and heat fluxes into the SOL caused by the geometric compression/expansion of flux surfaces predicted by magnetohydrodynamic equilibrium calculations lead to a prediction of higher plasma temperature at the outboard divertor than at the inboard divertor, a result that is consistent with experimental observation and that confirms a previous prediction (made without accounting for drifts) of a possible cause of the observed in-out divertor power asymmetry. The calculations also illustrate the effect of the poloidal distribution of particle and power influx into the SOL on the flow velocity, parallel current, and electrostatic potential distributions in the SOL and divertor.

Scrape-off Layer Plasma Flow in L- and H-Mode Plasmas on JT-60U

Significant progress has been made in understanding the scrape-off layer (SOL) mass transport along magnetic field lines — the SOL flow. Understanding the driving mechanisms of the SOL flow was summarized based on experiments in the JT-60U tokamak plasmas. Fast SOL flow with parallel Mach numbers of 0.2-1 was generated from the low magnetic field side (LFS) SOL to the high magnetic field side (HFS) divertor for the ion ∇B drift direction toward the divertor. The SOL flow pattern was formed mainly by the LFS enhanced in-out asymmetry in diffusion and by classical drifts in the torus. Detachment of the divertor plasma affected enhancement of the SOL flow at the HFS SOL. Dynamics of the SOL flow were measured during the transient event of edge localized modes (ELM), and the flow pattern of the plasma filaments was clarified at both SOLs. The radial movement of the ELM filaments at the LFS SOL was sometimes faster than the parallel convective transport to the divertor target, which caused the heat loading to the first wall. Filament structures with temporal peaks and flow velocities comparable to the ion sonic level were also determined in the HFS SOL, but they appeared only near the separatrix. The delay after start of the ELM was shorter than the parallel convection time from the LFS midplane, suggesting that part of the ELM filaments was ejected into the HFS SOL.

A Survey of Thermal Instabilities in Tokamak Plasmas: Theory, Comparison with Experiment, and Predictions for Future Devices

The strong temperature dependence, over certain temperature ranges, of the radiation cooling rate of low-Z impurities, of the atomic physics cooling and particle source rates associated with recycling and fueling neutrals, of the ion-electron recombination particle loss rate, of the turbulent transport loss rate, and of the fusion alpha-particle heating rate have all been identified as “drivers” of thermal instabilities in the coupled plasma particle, momentum, and energy balances. This paper surveys the experimental observations of a number of abrupt transition phenomena in plasma operating conditions - i.e., density-limit disruptions, multifaceted asymmetric radiations from the edge (MARFEs), divertor MARFEs, detachment, in-out divertor heat flux asymmetries, H-L and L-H transitions, confinement, and pedestal deterioration - or anticipated in future reactors - i.e., power excursions - their theoretical interpretations in terms of thermal instabilities driven by the temperature dependence of various radiative and atomic physics cooling mechanisms, and a comparison of theoretical prediction with experimental observations. Also surveyed are theoretical predictions of thermal instabilities in the power balance driven by the strong positive temperature dependence of the fusion heating rate.

Up-Down and In-Out Asymmetry Monitoring Based on Broadband Radiation Detectors

Understanding the origin of asymmetries, either in plasma flows reaching the divertor region or in plasma emissivity from the confined volume, is a question of great interest in magnetic confined plasma research.In the TJ-II stellarator, a set of bolometer systems distributed around the vacuum chamber has been installed to monitor up-down and in-out radiation asymmetries that are observed in transitions to different confinement regimes. Although plasma emissivity is a complex magnitude, a comparison of signals from twin detectors placed at selected positions may be of help to distinguish between the different causes of asymmetry. Moreover, this is a nonperturbing method that is especially appropriate when plasma wall clearance is small.

In-Out Asymmetry of Divertor Plasma Flows in Heliotron/Torsatron Devices

Understanding the reason for the divertor flow asymmetry in heliotron/torsatron–type fusion devices is important for the safety of the in-vessel components, such as divertor plates subject to the direct impact of the plasma or the electrical probes and thermocouples for measurements of the particle and energy fluxes to the divertor plates. In previous work, the divertor flow distributions were studied mainly with focusing on the up-down asymmetry in a heliotron-type fusion device, Heliotron E. This paper analyzes the in-out asymmetry of divertor flows and discusses the effects on this asymmetry of the magnetic axis position (the horizontal shift due to variation of the vertical magnetic field or plasma pressure) and power of neutral beams.

Tokamak edge model validation and improvement

Because of the complexity of the physics in the edge of divertor tokamaks, extrapolation from present-day machines to future reactor or reactor-type devices is done using two-dimensional-edge codes. Amongst the issues which areimportant for future machines, are those of inboard/outboarddivertor power asymmetries and the compression of hydrogen and helium.The issues driving the in-out power asymmetry are explored, and areasonable match is found to existing experiments, as well as an`adjustable knob' that could be used to further improve the match.In this work, one further effect that is known to play a role - thatof E×B and diamagnetic drifts - is identified but not analysed,because the codes have not yet been able to include the drift terms incombination with the impurities and kinetic neutrals. It is alsoshown that the codes reproduce the fact the only small differenceswere seen on the ASDEX Upgrade experiment in the compression of heliumwhen the DivII divertor was replaced by the DivIIb configuration.

Study of particle pumping characteristics for different pumping geometries in JT-60U and DIII-D divertors

Particle pumping characteristics were compared between pumping from the inner side private flux region (IPP) and pumping from both sides of the private flux region (BPP) in the JT-60U W shaped divertor, and between JT-60U IPP and pumping in the DIII-D lower baffled divertor. The pumping flux for BPP is smaller than that for IPP by about a factor of 2 with weak in-out asymmetry of recycling neutral flux and by a factor of 3.5-6.5 with strong in-out asymmetry. The reduction of the pumping flux for BPP is consistent with Monte Carlo simulations, where backflow at the outer pumping slot is observed due to in-out recycling asymmetry. The pumping flux in DIII-D at Ip = 0.8 MA and BT = 1.6 T is comparable to or smaller than that for JT-60U IPP at Ip = 1.0 MA, BT = 3.8 T and Ip = 1.5 MA, BT = 3.5 T in the same density regime. In the DIII-D divertor with pumping from the private flux region, the pumping flux decreases with increasing in-out asymmetry. The pumping flux normalized by the integrated Dα emission over the whole plasma exhibits a similar dependence on the distance between the pumping slot and the strike point in JT-60U IPP and the DIII-D lower divertor with pumping through the outer divertor plasma region.

In-out asymmetry of divertor temperatures in tokamaks

A 2-D numerical simulation of the edge plasma inASDEX Upgrade is used to show that above a critical density a bifurcation occurs, leading to strong poloidal in-out asymmetry of the electrontemperature in the divertor. At densities well below and above thecritical one, the asymmetry is less pronounced. Although drift termsare not included in the model, these findings can explain experimentalobservations from Alcator C-Mod and ASDEX Upgrade: (a) the clamping ofthe temperature in the inner divertor after the bifurcation and (b) the large temperature asymmetry immediately above the criticaldensity. The position of the colder divertor leg is determined by asmall asymmetry which is amplified by the bifurcation. In thesimulation the basic asymmetry is due to different lengths of the fluxtubes in the plasma edge. In the experiment the asymmetry effect ofdifferent field line lengths is combined with the action of E × B drifts. Comparison of X point top and bottom measurements thenproduces evidence that the E × B drift effects are dominant. Theeffect of the difference in field line length is still present in theexperiment but E × B produces a stronger effect.

Contribution of the ion diamagnetic flow to poloidal rotation and pressure asymmetries in the SOL

Under typical conditions of high recycling divertor plasmas, the ion diamagnetic flow makes the largest contribution to the ion poloidal rotation in the SOL. Owing to toroidal effects, the net poloidal rotation in the SOL gives rise to ion parallel Pfirsch-Schlüter flows, which are sustained by an up-down pressure asymmetry which reverses with toroidal field (Bϕ) reversal. The up-down pressure asymmetry in turn gives rise to net, surface averaged, radial currents j̄r arising from the surface averaging of local diamagnetic currents jr = (1/rB)(∂p/∂θ) due to toroidal effects. The radial divergence of radial currents j̄r can exist in the SOL because of electrical contact with the target. The resulting net toroidal j̄r Bθ force in the direction of the main plasma current creates an in-out pressure asymmetry in favour of the ion drift side (which is the inner side in normal Bϕ and the outer side in reversed Bϕ plasmas). This pressure asymmetry should result in higher density and low temperature plasma in the inner divertor leg in normal Bϕ discharges and should also make the distribution of density and temperature between the targets more equal in reversed Bϕ discharges, consistent with the main trends observed in experiments with toroidal field reversal. The proposed mechanism can therefore provide an alternative explanation (to the effect of the E × B drifts analysed earlier) for the observed changes in target asymmetries associated with Bϕ reversal.

Comparison of basis functions in soft x-ray tomography and observation of poloidal asymmetries in impurity density

Natural basis functions (NBF), also known as natural pixels in the literature, have been applied in tomographic reconstructions of soft x-ray (SXR) measurements in the JET (Joint European Torus) tokamak. The results are compared with those obtained with local basis functions (LBF), and those obtained with a conventional constrained-optimization tomography method. Truncated singular value decomposition is used as the inversion method. Reconstructions without a priori information, such as the NBF reconstructions, are, as can be expected, less good than reconstructions in which a priori information is used, as in the conventional method, but the reconstructions are shown to be reliable by means of phantom simulations. Reconstructions with the same number of LBFs as NBFs without a priori information are comparable to NBF reconstructions, although the latter seem to be somewhat better. No significant changes in results, apart from smaller reconstruction errors, are obtained if the measuring system has more regular or complete coverage than the JET SXR system. The various tomography methods are used to assess whether a newly observed in-out asymmetry in the SXR emission during the injection of nickel into an RF-heated plasma, with the peak on the inboard side, is real. A possible explanation for the asymmetry in emissivity is an increased nickel density on the inboard side as a result of an RF-induced increase of the hydrogen-minority density on the outboard side.

Recent results from divertor and scrape-off layer studies at JET

Recent progress in the study of divertor and scrape-off layer plasma phenomena inJET is reviewed. Three pumped divertors (Mark I, Mark IIA/AP and Mark IIGB) have beeninstalled and exploited under reactor relevant conditions. With increased divertor closure,the particle exhaust rate has increased and neutral compression factors of >100 have beenobtained with the Mark IIGB divertor. Helium enrichment factors of >0.2 have been measuredunder a wide range of conditions and satisfy the minimum requirements for ITER. Excellentpower handling has been demonstrated in all divertor configurations. Fast infrared camerameasurements exhibited broad deposition profiles during type I ELMs and energy densities of∼0.12 MJ/m2, which extrapolate to unacceptable levels for ITER. During the recentDT experiments, the co-deposition of tritium on cold shadowed surfaces in the inner divertorwas identified as an important form of long term tritium retention. This has serious implicationsfor the divertor design and tritium inventory in a next step tokamak. Core plasma purity hasnot improved with enhanced divertor closure or decreased main chamber neutral pressure.Studies of the chemical sputtering yield showed a dependence on surface temperature andhydrogen isotope. This accounts for the observation of increased carbon impurity productionin Mark II (at 500 K) relative to Mark I (at 300 K). The variation of the disruptive (L mode)density limit appears to be more strongly dependent on the local impurity production ratethan on the divertor geometry. Significant progress has been made in the study of divertordetachment, and volume recombination has been spectroscopically identified. With increasingisotope mass, detachment and the disruptive density limit occur at lower main plasma density,as predicted by the EDGE2D/NIMBUS codes. Using differential gas fuelling in the Mark IIGBdivertor, it was possible to modify the in-out asymmetry of the divertor plasma parametersand energy deposition for the first time.

Finite element modelling of TdeV edge and divertor with E*B drifts

Simulation results are presented from a finite element calculation of transport in the edge and divertor of TdeV. In addition to the usual transport processes, this model also includes a calculation of the electrostatic potential and the associated E*B drifts. The inclusion of E*B drifts is shown to be essential in the calculation of the parallel and toroidal flow velocities. With the inclusion of drifts, the effect of toroidal magnetic field reversal is studied, particularly with regard to in-out asymmetries in the Halpha radiation and power deposited on the plates. Relatively weak asymmetries are calculated on the basis of these drifts alone, and it is concluded that the much stronger asymmetries observed experimentally must be associated with other physical processes not yet included in the model. The transport equations are based on fluid conservation equations for particles, parallel (to the magnetic field) momentum, electron and ion energy. For simplicity, a single ion species is considered and, when modelling recycling, neutral transport is treated in the diffusion approximation. All transport equations are discretized with finite elements on a triangular unstructured mesh that is locally aligned with the flux surfaces. The use of such a mesh allows an accurate representation of the simulation geometry, with boundaries of arbitrary shapes and complexity. In particular, it is possible to account for divertor plates that are not orthogonal to the flux surfaces without any penalty for the validity of the transport equations. The analysis presented here concerns a low density attached ohmic TdeV discharge in a single null geometry, with the X point above the midplane

Dynamics of Transition to Enhanced Confinement in Reversed Magnetic Shear Discharges

A simple model of the transition to enhanced confinement in reversed shear discharges is presented. The proposed transition mechanism relies on a synergism between electric field shear suppression of turbulence aided by reduced curvature drive due to magnetic shear reversal or reduction. Profile structure and transport barrier propagation dynamics are predicted. A novel analytical theory for the time evolution of the barrier foot-point location is presented. The model predicts that the transition threshold has favorable dependence on pretransition temperature ratio $({T}_{i}{/T}_{e})$, in-out asymmetry in the $E\ifmmode\times\else\texttimes\fi{}B$ shearing rate (i.e., lower for larger Shafranov shift), density profile peakedness, and unfavorable scaling with density. Optimal confinement occurs in discharges where deposition is peaked within the magnetic shear reversal radius.

Field reversal effects on divertor plasmas under radiative and detached conditions in JT-60U

Reversal effects of the toroidal field Bt on the principal divertor plasma parameters were investigated under radiative and detached divertor conditions in L mode discharges. The ion flux to the inboard separatrix strike point decreased before a MARFE occurred, irrespective of the ion Del B drift direction. The local electron temperature, Te, div, decreased to around 10 eV. The maximum fraction of power radiated in the divertor was comparable between the two directions of Bt. With the power flowing into the two divertor fans being slightly larger on the outboard than on the inboard, a nearly symmetric in-out heat load was observed for the ion Del B drift away from the target. This was due to the outboard enhanced asymmetries in the particle flux and radiation loss distributions. From the viewpoint of in-out symmetry in the target heat load and Te, div, operation with the ion Del B drift away from the target plate is desirable as long as the attached divertor condition is maintained. On the contrary, during the MARFE, although deterioration of the energy confinement as well as the increase in the fuelling efficiency were comparable, for the ion Del B drift towards the target the plasma did not detach completely, and the strong in-out asymmetry in the particle recycling was relaxed to a relatively symmetric distribution. From the viewpoint of particle exhaust to the divertor, operation with the ion Del B drift towards the target is favourable

Characteristics of heat flux and particle flux to the divertor in H-mode of JT-60U

Heat flux and particle flux behavior in H-mode is studied in a comparative manner. It was confirmed that the multiple peak structure of heat flux during ELM activity has a role in reducing the average value of a peak heat flux at the divertor. In order to characterize heat and particle flux during ELM activity, the ELM part and the steady state part of heat flux and particle flux were determined and statistically analyzed. A large in-out asymmetry of peak ELM heat flux density was found. The asymmetry is almost unaffected by the ion grad- B drift direction. In-out asymmetry of both ELM and steady-state parts of the particle flux were found to be similar.

Density Fluctuations in the Advanced Toroidal Facility (ATF) Torsatron

Density fluctuations in low-collisionality, low-beta ({beta} nearly equals 0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron have been studied using a 2-mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher {kappa}{sub PRP}{rho}{sub s} (nearly 1) than is typically seen in tokamaks. The in-out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii {rho} from 0.5 to 0.85) increases from n/n >= 1% when the density profile flat to n/n >= 3% when the density profile is steepened. These observations are in qualitative agreement with theoretical expectations for helical dissipative trapped electron modes (DTEMs), which are drift-wave instabilities associated with particle trapping in the helical stellaratormore » field. 5 refs., 5 figs.« less

Analysis of cold divertor concepts for ITER

Two different ITER geometries (a V-shaped and a gas-bag like divertor) are analysed with respect to their pure plasma behaviour, ELMs and impurity transport. The V-shaped divertor shows a local balance within each divertor with a rather symmetric behaviour for pure deuterium plasmas. Being relatively closed, the deep V is able to control the detachment on inner and outer leg separately better. It has easier access to high recycling at lower midplane separatrix densities. It is able to buffer grassy ELMs of some kJ going to the divertors. The gas-bag is dominated by a more global divertor gas balance due to its open configuration. The interaction between inner and outer divertor is complicated, appearing also in the ELM simulation, where the capability to buffer a small ELM is not as good as for the V-shaped configuration. In both configurations for multifluid calculations with beryllium produced by sputtering and neon prescribed by the density of its highest charge state at the innermost flux surface strong radiation close to the X-point develops, however for the V-shaped configuration a much more symmetric situation is obtained, whereas for the gas-bag a strong in-out asymmetry is seen, that means the inner divertor is cooled much more than the outer one.