In-out Asymmetry Research Articles

SOLPS-ITER drift modeling of neon impurity seeded plasmas in EAST with favorable and unfavorable toroidal magnetic field direction

To better understand divertor detachment and asymmetry in the Experimental Advanced Superconducting Tokamak (EAST), drift modeling via the comprehensive edge plasma code SOLPS-ITER of neon impurity seeded plasmas in favorable/unfavorable toroidal magnetic field (B T) has been performed. Firstly, electrostatic potential/field (/E) distribution has been analyzed, to make sure that and E are correctly described and to better understand drift-driven processes. After that, drift effects on divertor detachment and asymmetry have been focused on. In accordance with the corresponding experimental observations, simulation results demonstrate that in favorable B T the onset of detachment is highly asymmetric between the inner and outer divertors; and reversing B T can significantly decrease the magnitude of in-out asymmetry in the onset of detachment, physics reasons for which have been explored. It is found that, apart from the well-known E × B drift particle flow from one divertor to the other through the private flux region, scrape-off layer (SOL) heat flow, which is much more asymmetrically distributed between the high field side and low field side for favorable B T than that for unfavorable B T, is also a critical parameter affecting divertor detachment and asymmetry. During detachment, upstream pressure (P u) reduction occurs and tends to be more dramatical in the colder side than that in the hotter side. The convective SOL heat flow, emerging due to in-out asymmetry in P u reduction, is found to be critical for understanding divertor detachment and asymmetry observed in EAST. To better understand the calculated drastic power radiation in the core and upstream SOL, drift effects on divertor leakage/retention of neon in EAST with both B T directions have been addressed for the first time, by analyzing profile of poloidal neon velocity and that of neon ionization source from atoms. This work can be a reference for future numeric simulations performed more closely related to experimental regimes.

Modeling of the effects of impurity seeding on plasma detachment and impurity screening of snowflake divertor on HL-2M tokamak by SOLPS-ITER

To address the issues of mitigation and control of the heat loads on the divertor target, a snowflake divertor (SFD) has been proposed on the HL-2M tokamak. In this work, simulations have been performed by using SOLPS-ITER to demonstrate the advantages of SFD on HL-2M on plasma detachment and impurity screening during impurity seeding. Firstly, neon (Ne) and argon (Ar) seeding are chosen for comparison in SFD. It is found that Ar seeding significantly mitigates the in-out asymmetry compared with Ne seeding, mainly in high seeding rate cases. The impurity screening capabilities with Ar seeding are conspicuously better than that of Ne seeding. Subsequently, the SFD and standard divertor (SD) with Ar seeding are compared. The SFD achieves plasma detachment with a seeding rate of more than one order of magnitude lower and has better impurity screening capability than those of the SD. This can be explained by more substantial Ar accumulation in the private flux region near the X-point in SD. Moreover, the simulation shows that D2 puffing near the OMP can drive more Ar ions to the divertor and promote the plasma detachment and impurity screening. Finally, the effects of E × B drift on SFD are studied. It is found that with E × B drift more Ar particles accumulate in the vicinity of both inner and outer targets, especially in the far-SOL region, thus raising the far-SOL power radiation. However, the peak heat flux is mainly located near the separatrix, therefore a higher seeding rate is required to achieve detachment. Moreover, the E × B drift drives more Ar particles away from the core region. In addition, the role of molecules on the plasma momentum loss during detachment is analyzed.

SOLPS-ITER analysis of drift effects on plasma profiles in the EAST scrape-off layer

Drift effects on the plasma profiles of the scrape-off layer (SOL) in the Experimental Advanced Superconducting Tokamak (EAST) have been numerically investigated using the comprehensive 2D edge modeling package, SOLPS-ITER, based on a generic magnetic equilibrium with lower single null configuration. SOL particle diffusivity (D SOL) has been scanned from high (1.0 m2 s−1) to extremely low (0.02 m2 s−1), to gradually highlight the role of drift-based neoclassical mechanisms in radial particle transport. To address the impact of magnetic field direction on drift-driven transport, plasma profiles, flows and currents in the SOL of EAST plasmas, with the toroidal magnetic field (B T) direction favorable and unfavorable for H-mode access, i.e. with the ion B × ∇B drift pointing towards and away from the active X-point, are simulated and analyzed. Results demonstrated that drift-driven transport, considered as the key process in the formation of SOL plasma profiles, is dependent on magnetic field direction and thus SOL flows and currents, as well as SOL widths, can obviously be affected by the direction of drifts. With B T changed from the favorable direction to the unfavorable one, the flattening of the density radial profile as well as the increase in power decay length, in the SOL, can be achieved and can be further enhanced as the weight of turbulent transport (i.e. D SOL) gets reduced, due to the increased contribution of ion parallel viscosity to the radial ion flow. In particular, with D SOL ⩽ 0.05 m2 s−1 in the simulations, the dominant role of drift-based neoclassical mechanisms in the radial particle transport will lead to the formation of the so-called edge density-shelf in plasmas with unfavorable B T. The power scrape-off width in plasmas with unfavorable B T is very insensitive to the turbulent transport level and can remain relatively high even when D SOL has been decreased to an extremely low level. Due to the compressing/widening effect of the drift-driven inward/outward radial particle flow, the simulated power scrape-off width exhibits an in-out asymmetry, which is also dependent on magnetic field direction . This work represents a step towards a deeper understanding of the physics mechanisms determining SOL widths in EAST.

Characterizing the in-out asymmetry in visual crowding.

An object's processing is impaired by the presence of nearby clutter. Several distinct mechanisms, such as masking and visual crowding, are thought to contribute to such flanker-induced interference. It is therefore important to determine which mechanism is operational in any given situation. Previous studies have proposed that the in-out asymmetry (IOA), where a peripheral flanker interferes with the target more than a foveal flanker, is diagnostic of crowding. However, several studies have documented inconsistencies in the occurrence of this asymmetry, particularly at locations beyond the horizontal meridian, casting doubt on its ability to delineate crowding. In this study, to determine if IOA is diagnostic of crowding, we extensively charted its properties. We asked a relatively large set of participants (n = 38) to identify a briefly presented peripheral letter flanked by a single inward or outward letter at one of four locations. We also manipulated target location uncertainty and attentional allocation by blocking, randomizing or pre-cueing the target location. Using multilevel Bayesian regression analysis, we found robust IOA at all locations, although its strength was modulated by target location, location uncertainty, and attentional allocation. Our findings suggest that IOA can be an excellent marker of crowding, to the extent that it is not observed in other flanker-interference mechanisms, such as masking.

SOLPS-ITER drift modelling of ITER burning plasmas with narrow near-SOL heat flux channels

Abstract SOLPS-ITER code simulations with fluid drifts activated are used to examine the consequences for divertor performance, under burning plasma conditions, of a reduction in scrape-off layer (SOL) cross-field transport such that the outboard midplane SOL parallel heat flux width, λq is reduced to the very low values predicted by the experimental scaling in [T. Eich et al., Nucl. Fusion 53 (2013) 093031] for high plasma current operation on ITER. The decrease of λq from the standard value (~3.5 mm) used in the ITER SOLPS modeling database, to λq ~ 1–2 mm expected from the scaling, leads to a strong narrowing of the operational window in terms of divertor heat loading limits. To maintain similar levels of sub-divertor neutral pressure, pn (one of the key divertor operational parameters) as those obtained with standard transport, higher levels of neon impurity seeding are required when transport is reduced, yielding higher Zeff at the upstream separatrix and requiring further integrated modelling to assess the impact on confinement. The simulations also demonstrate that the strong increase in in-out target peak power loading asymmetry at low pn seen for standard transport when drifts are switched on in the code is preserved, and in fact worsens, for low transport. This significantly reduces margins for power handling control at lower pn if heat flux channels will be narrow on ITER.

Exploring SF- in-out asymmetry and detachment bifurcation in HL-2M with E × B by SOLPS

Abstract In this paper, the SF- (with the second X-point located near the outer target surface) in-out asymmetry and detachment bifurcation in HL-2M were explored by the edge plasma code SOLPS5.1 including electric but neglecting magnetic drifts. The modeling results reveal that the radial electric drift component (Epol × B) can play a key role on inducing the SF- in-out asymmetry due to the effective carbon (C) screen capacity and high heat dissipation. The Epol × B drift can enhance the poloidal electric drift component (Er × B), and their synergistic effects can induce a marked in-out asymmetry. Besides, the results showed that the detachment bifurcation in SF- also could occur under given conditions. The presence of four factors (including E × B, C impurity, normal-Bt, and relatively small radial anomalous transport coefficients) forms the necessary conditions for producing the detachment bifurcation. The root reason for the detachment bifurcation formation is due to the strong interaction of Te, E×B, and C impurity radiation loss in the outer divertor region with normal-Bt direction. The detachment bifurcation could be removed or mitigated by weakening their interaction. Finally, the time-dependent detachment bifurcation process is simulated in this work to better understand its physical picture.

Effect of ∇B drift on the H-mode power threshold in upper single null plasmas with ITER-like tungsten divertor on EAST

In this paper, we will experimentally investigate the power threshold (PL-H) in upper single null plasmas with an ITER-like tungsten divertor under different ∇B drift directions on EAST [F. Ding et al., Commissioning and PSI Behavior of the ITER-Like W/Cu Divertor in EAST 22nd PSI, Rome (2016)]. The power threshold for the low (L) to high (H) confinement mode has a clear and positive toroidal magnetic field, BT, dependence when the ∇B drift points toward the primary X-point (B×∇B↑). A factor of 2–3 increase in PL-H is observed for the ∇B drift away from the primary X-point (B×∇B↓). The edge and core impurities quantified by spectroscopy measurements show comparable levels for the transitions for both drift directions. On the other hand, it is found that the divertor Dα emission just prior to the L-H transition is lower for B×∇B↑, compared with that for B×∇B↓. The upper in-out divertor asymmetry, as manifested by particle fluxes measured by the divertor triple Langmuir probe, is most marked for B×∇B↓, and with significantly more particle flux to the outer divertor. The reversing field increases the particle flux into the upper inner and lower outer divertor, reducing the in-out asymmetry. One important distinction between the two field directions has been observed, with respect to the amplitude of the scrape-off layer (SOL) parallel flow. A dedicated experiment under similar target plasma conditions shows a lower SOL density and thus a steeper density gradient slightly inside the separatrix, where a lower PL-H is found for the B×∇B↑, compared to that for B×∇B↓. We, therefore, conclude that the field-dependent SOL plasma conditions play an important role in the transition physics.

Divertor power load studies for attached L-mode single-null plasmas in TCV

This paper investigates the power loads at the inner and outer divertor targets of attached, Ohmic L-mode, deuterium plasmas in the TCV tokamak, in various experimental situations using an Infrared thermography system. The study comprises variations of the outer divertor leg length and target flux expansion, the plasma current and a reversal of the magnetic field direction. The direct impact of the divertor magnetic geometry on scrape-off layer (SOL) transport—parameterised by the SOL power fall-off length , the divertor spreading factor Su and the in-out power asymmetry—is reported for constant core properties. The in-out power asymmetry increases, either with the divertor leg length, or the target flux expansion. The SOL width scales positively with divertor leg length, with a strength that depends on the field direction and differs between the inner and outer divertor. This implies a parametric dependence of that is not explicitly included in current multi-machine scaling laws. The divertor spreading factor at the target , where fx is the target flux expansion, appears unaffected by changes in the divertor geometry and in the plasma current, is independent of the magnetic field direction and is similar between inner and outer divertor. Possible interpretations of these observations using an ad-hoc analytical purely conductive model for the SOL, by ion drifts or by asymmetric turbulent cross-field transport in the divertor are presented. The observed values of are related to existing L-mode and H-mode scaling laws and to similar studies in other tokamaks. Finally, potential implications of these findings for future larger fusion machines are discussed.

Pedestal radial flux measuring method to prevent impurity accumulation

The use of high-z wall materials attempts to shift the fusion challenge from heat handling to impurity removal. We demonstrate that not only the impurity density in-out asymmetry but also the poloidal flow has a major impact on the radial impurity flux direction. This realization provides the first method of measuring the flux from available diagnostics, without the need of a computationally demanding kinetic calculation of the full bulk ion response. Moreover, it affords insight into optimal tokamak operation to avoid impurity accumulation while allowing free fueling.

In-out asymmetry of zonal flow shear and turbulence reduction

In-out asymmetry of ion temperature gradient turbulence in toroidal geometry is studied by performing nonlinear gyrokinetic simulation using the GyroKinetic Plasma Simulation Program code [Kwon et al., Nucl. Fusion 52, 013004 (2012)]. Effects of self-generated zonal flow shear on the in-out asymmetry of radial correlation length and amplitude of turbulence are addressed by varying collisionality. Both quantities exhibit strong in-out asymmetry (longer and higher, respectively, at the low field side) in the absence of zonal flows. When the zonal flow shear (which is higher at the low field side) gets stronger, the radial correlation length decreases with its in-out asymmetry also getting reduced as expected from E×B shear decorrelation theory. On the other hand, in-out asymmetry of turbulence amplitude behaves differently from that of the radial correlation length.

Poloidal asymmetric flow and current relaxation of ballooned transport during I-phase in ASDEX Upgrade

Turbulence driven poloidal asymmetric parallel flow and current perturbations are studied for tokamak plasmas of circular geometry. Whereas zonal flows can lead to in-out asymmetry of parallel flows and currents via the Pfirsch–Schlüter mechanism, ballooned transport can result in an up-down asymmetry due to the Stringer spin-up mechanism. Measurements of up-down asymmetric parallel current fluctuations occurring during the I-phase in ASDEX Upgrade are not responses to the equilibrium by the Pfirsch–Schlüter current, but can be interpreted as a response to strongly ballooned plasma transport coupled with the Stringer spin-up mechanism. A good agreement of the experimental measured limit-cycle frequencies during I-phase with the Stringer spin-up relaxation frequency is found.

Effects of discharge operation regimes and magnetic field geometry on the in-out divertor asymmetry in EAST

This paper aims to investigate the reason why the divertor in-out asymmetry was not obvious as experimentally observed in EAST only considering the classical drifts from previous simulations (Guo et al., J. Nucl. Mater. 438 (2013) 280; Du et al., J. Nucl. Mater. 463 (2015) 485). With consideration of the classical drifts, a series of different typical discharge scenarios in EAST in different magnetic field geometries were simulated by using the SOLPS5.2 code package. The simulated results reveal that the classical drifts make a major contribution to the in-out divertor asymmetry in the high recycling regime (HRR) and partial detachment (one divertor target begins to detach, while the other divertor remains attached) regime. In comparison, in low recycling regime the classical drifts play a much smaller role in the contributions to the in-out divertor asymmetry, which can explain reasonably the reason for it in Guo et al. (J. Nucl. Mater. 438 (2013) 280). In addition, the magnetic field geometry also has a great impact on the classical drifts inducing the asymmetry; it is found that for lower single-null, upper single-null and connected double-null topologies, in HRR the classical drifts play an dominant role in the contribution to the in-out divertor asymmetry, while for a disconnected double null magnetic field configuration, they play a minor role, which is the reason why the in-out asymmetry was unobvious by considering the drifts in Du et al. (J. Nucl. Mater. 463 (2015) 485).

Poloidal asymmetries in edge transport barriers

Measurements of impurities in Alcator C-Mod indicate that in the pedestal region, significant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler et al., Nucl. Fusion 54, 083017 (2014)], a technique based on total pressure conservation to align profiles measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishing [Churchill et al., Nucl. Fusion 53, 122002 (2013)], but remains substantial (nz,H/nz,L∼6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport effects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.

Properties of toroidal Alfvén eigenmode in DIII-D plasma

Linear properties of the toroidal Alfvén eigenmode (TAE) excited by energetic particles (EP) in a DIII-D tokamak experiment have been studied in global gyrokinetic particle simulations treating self-consistently kinetic effects of EP, thermal ions, and electrons. Simulation results of the TAE frequency and mode structure agree very well with the experimental measurements. The non-perturbative EP contribution induces a radial localization of the TAE mode structure, a break-down of mode radial symmetry, as well as a frequency dependence on the toroidal mode number. The simulations further demonstrate the dependence of the growth rate and mode structure on EP pressure gradients. The in-out asymmetry of the mode structure and the experimental identification of the poloidal harmonics have also been clarified.

Effects of heating power on divertor in-out asymmetry and scrape-off layer flow in reversed field on Experimental Advanced Superconducting Tokamak

The dependence of divertor asymmetry and scrape-off layer (SOL) flow on heating power has been investigated in the Experimental Advanced Superconducting Tokamak (EAST). Divertor plasma exhibits an outboard-enhanced in-out asymmetry in heat flux in lower single null configuration for in reversed (ion ∇B drift direction toward the upper X-point) field directions. Upper single null exhibits an inboard-favored asymmetry in low heating power condition, while exhibits an outboard-favored asymmetry when increasing the heating power. Double null has the strongest in-out asymmetry in heat flux, favoring the outer divertor. The in-out asymmetry ratios of qt,out/qt,in and Pout/Ptotal increase with the power across the separatrix Ploss, which is probably induced by the enhanced radial particle transport due to a large pressure gradient. The characteristics of the measured SOL parallel flow under various discharge conditions are consistent with the Pfirsch-Schlüter (PS) flow with the parallel Mach number M∥ decreasing with the line averaged density but increasing with Ploss, in the same direction as the PS flow. The contributions of both poloidal E×B drift and parallel flow on poloidal particle transport in SOL on EAST are also assessed.

Influence of atomic physics on EDGE2D-EIRENE simulations of JET divertor detachment with carbon and beryllium/tungsten plasma-facing components

The EDGE2D-EIRENE code is applied for simulation of divertor detachment during matched density ramp experiments in high triangularity, L-mode plasmas in both JET-Carbon (JET-C) and JET-ITER-Like Wall (JET-ILW). The code runs without drifts and includes either C or Be as impurity, but not W, assuming that the W targets have been coated with Be via main chamber migration. The simulations reproduce reasonably well the observed particle flux detachment as density is raised in both JET-C and JET-ILW experiments and can better match the experimental in-out divertor target power asymmetry if the heat flux entering the outer divertor is artificially set at around 2–3 times that entering the inner divertor. A careful comparison between different sets of atomic physics processes used in EIRENE shows that the detachment modelled by EDGE2D-EIRENE relies only on an increase of the particle sinks and not on a decrease of the ionization source. For the rollover and the beginning of the partially detached phase, the particle losses by perpendicular transport and the molecular activated recombination processes are mainly involved. For a deeper detachment with significant target ion flux reduction, volume recombination appears to be the main contributor. The elastic molecule-ion collisions are also important to provide good neutral confinement in the divertor and thus stabilize the simulations at low electron temperature (Te), when the sink terms are strong. Comparison between EDGE2D-EIRENE and SOLPS4.3 simulations of the density ramp in C shows similar detachment trends, but the importance of the elastic ion-molecule collisions is reduced in SOLPS4.3. Both codes suggest that any process capable of increasing the neutral confinement in the divertor should help to improve the modelling of the detachment. A further outcome of this work has been to demonstrate that key JET divertor diagnostic signals—Langmuir probe Te and bolometric tomographic reconstructions—are running beyond the limit of validity in high recycling and detached conditions and cannot be reliably used for code validation. The simulations do, however, reproduce the trend of the evolution of the line integrated bolometer chord measurements. The comparison between the code results and high-n Balmer line radiation intensity profiles confirms that a strong volume recombination is present during the experimental detachment and may play a role in this process, as suggested by the code.

In-out asymmetry of surface excitations in reflection-electron-energy-loss spectra of polycrystalline Al

We present experimental evidence for differences in surface energy losses between (a) electrons entering a solid from vacuum and (b) electrons leaving the solid into vacuum. Although these so-called in-out asymmetries have been long assumed to exist on theoretical grounds, the present work constitutes a clear experimental observation of the phenomenon. The effect has been exposed by comparing reflection-electron-energy-loss spectra of polycrystalline Al for pairs of conjugate scattering geometries where the directions of the source and the detector were interchanged. Differences of up to 30% in the peak height of surface energy-loss features are observed. The experimentally observed in-out asymmetry has been examined within the semiclassical dielectric formalism using state-of-the-art models for surface scattering of charged projectiles. The theoretical analysis suggests that in-out asymmetry effects are most accentuated for surface-crossing directions close to the surface normal and for high kinetic energies, in good agreement with the observed behavior. The effect is assumed to be present not only for electrons, but in principle for any charged particle.

Predictions of the poloidal asymmetries and transport frequencies in KSTAR

The extended neoclassical rotation theory formulated in Miller flux surface geometry enables unprecedented neoclassical calculations of the poloidal asymmetries in density, rotation velocities, electrostatic potential along the flux surfaces, and of the inertial (Reynolds stress) and gyroviscous transport frequencies, which are strong functions of these asymmetries. This paper presents such calculations of the poloidal asymmetries and non-negligible inertial and gyroviscous transport frequencies in two KSTAR (Korea Superconducting Tokamak Advanced Research) [Kwon et al., Nucl. Fusion 51, 094006 (2011)] Neutral Beam Injection H-mode discharges. The in-out asymmetries in the velocities are an order of magnitude larger than their up-down asymmetries. The magnitudes of the predicted inertial and gyroviscous transport frequencies depend on the magnitudes of the density and velocity asymmetries. The neoclassically predicted density asymmetries are shown to correspond with the reported measurements in tokamaks and the predicted carbon toroidal velocities agree very well with the measurements in KSTAR.

Radiative Divertor Plasma Behavior in L- and H-Mode Discharges with Argon Injection in EAST

Introducing strong radiative impurities into divertor plasmas has been considered as an important way to mitigate the peak heat load at the divertor target plate for ITER, and will be employed in EAST for high power long pulse operations. To this end, radiative divertor experiments were explored under both low (L) and high (H) - mode confinement regimes, for the first time in EAST, with the injection of argon and its mixture (25% Ar in D2). The Ar injection greatly reduced particle and heat fluxes to the divertor in L-mode discharges, achieving nearly complete detached divertor plasma regimes for both single null (SN) and double null (DN) configurations, without increasing the core impurity content. In particular, the peak heat flux was reduced by a factor of ∼6, significantly reducing the intrinsic in-out divertor asymmetry for DN, as seen by both the new infra-red camera and the Langmuir probes at the divertor target. Promising results have also been obtained in the H-modes with argon seeding, demonstrating a significant increase in the frequency and decrease in the amplitude of the edge localized modes (ELMs), thus reducing both particle and heat loads caused by the ELMs. This will be further explored in the next experimental campaign with increasing heating power for long pulse operations.

Divertor asymmetry and scrape-off layer flow in various divertor configurations in Experimental Advanced Superconducting Tokamak

Divertor asymmetry and its dependence on the ion ▿B direction has been investigated in the Experimental Advanced Superconducting Tokamak by changing the divertor configuration from lower single null (LSN), via double null (DN), to upper single null (USN) during one single discharge. Divertor plasmas exhibit the usual in-out asymmetry in particle and heat fluxes in LSN with the ion ▿B direction toward the lower X-point, favoring the outer divertor, especially at high density. The in-out asymmetry is reversed when changing the divertor configuration from LSN to USN, thus clearly demonstrating the effect of classical drifts. DN exhibits an even stronger in-out divertor asymmetry, favoring the outer divertor. A significant top-down asymmetry is also seen for DN, with greater particle and heat fluxes to the bottom divertor. In addition, the parallel plasma flow has been measured by a fast moving Mach probe at the outer midplane, which shows similar magnitude to the Pfirsch-Schlüter flow. Its contribution to the poloidal particle flux is also assessed and comparison is made with that from the poloidal E × B drift.