High-field Side Scrape-off Layer Research Articles

Influence of the magnetic configuration on the high-field side scrape-off layer at ASDEX Upgrade and the role of the secondary separatrix

In tokamaks, radial transport is ballooning, meaning it is enhanced at the low-field side (LFS). This work investigates the effect of the magnetic configuration on the high-field side (HFS) scrape-off layer. Our experiments involved L-mode and H-mode discharges at ASDEX Upgrade, in which we scanned the magnetic configuration from a lower to an upper single-null shape, thus varying the location of the secondary separatrix. We show that the secondary separatrix determines the width of the HFS scrape-off layer, meaning that the density is much lower in the region that is magnetically disconnected from the LFS scrape-off layer, outside the secondary separatrix. Furthermore, we observe that the large density often seen in the HFS divertor drastically decreases as the separation between the primary and secondary separatrices falls below a particular value. This value is different for L-mode and H-mode plasmas and closely matches the power decay length measured at the LFS midplane. We also show how the HFS scrape-off layer density is smaller in an upper single-null than in a lower single-null, when the ionic grad-B drift points down. This difference is likely caused by reversing the E×B drifts in the active divertor when switching the active X-point from the bottom to the top. We further observe that the neutral density in the lower divertor also correlates with the plasma shape and the high-density region in the HFS scrape-off layer. During the shape scans analyzed here, the HFS divertor remained partially detached throughout, with transitory reattachment modulated by ELM activity in H-mode. This work provides novel experimental data that can be leveraged to further the modeling capabilities and understanding of scrape-off layer physics in highly shaped plasmas.

Evolution of the high-field-side radiation belts during the neon seeding plasma discharge in EAST tokamak

Divertor detachment achieved by injecting impurities or increasing density is always accompanied with various local radiation phenomena in the boundary or core plasma. This paper presents the formation and evolution of the high-field-side (HFS) radiation belts during the neon seeding plasma discharge in upper single null configuration with two directions of toroidal magnetic field in EAST tokamak. The neon mixed with deuterium seeding can induce the divertor detachment with strong radiation belts in the HFS scrape-off layer (SOL) region. With the increase of the radiation power, the plasma discharge will transit from H-mode to L-mode, and meanwhile the radiation belts move away from the near X-point to HFS SOL. When the radiation power is high enough, the radiation belts begin to move further to the other X-point along the HFS SOL, and even cause plasma disruption. The results indicate that the behavior of the radiation belts is related to the radiation power, plasma confinement performance and state of divertor detachment, which is useful for developing better feedback control methods to achieve high-performance radiative divertor operation mode.

Influence of the drifts on the double‐peaked emission profile of the visible light in the upper divertor region of EAST

AbstractA novel double‐peaked emission profile has been observed by the visible light imaging system in EAST. In the upper divertor region, two emission peaks in the high‐field side (HFS) scrape‐off layer (SOL) and near the X‐point are identified by tomographic reconstruction. In our previous SOLPS‐ITER simulation work, the emission peaks in the HFS SOL and near the X‐point are attributed to the carbon deposition on the HFS baffle and the dome of the upper tungsten divertor, respectively. The appearance of the emission peak near the X‐point when the outer target is attached was understood by analysing the carbon transport in the private flux region (PFR). However, the simulated intensity of the emission peak in the HFS SOL is lower by one order of magnitude than that in the PFR, while they are at the similar level in the experiment. In this work, the influence of the drifts is further studied by SOLPS‐ITER simulations. Under the favourable toroidal field condition, the HFS SOL plasma transits to the detachment regime. The chemical sputtering of the carbon deposited on the inner baffle increases significantly, as well as the emission intensity of the peak in the HFS SOL. Meanwhile, the C2+ density peak in the PFR decreases due to the E × B drift towards inner target. The emission intensity in the PFR near the X‐point, therefore, decreases moderately. As a result, the intensities of the two emission peaks become comparable, which matches better with the experiment.

Observation of the high-density front at the high-field-side in the J-TEXT tokamak

This article reports the observation of the high-density front at the high field side (HFS) region in the limiter J-TEXT tokamak. In the J-TEXT high density discharges, the high-density front at the HFS scrape-off layer (SOL) region has been clearly identified by the far-infrared laser polarimeter-interferometer. The high-density front forms at the HFS SOL when the plasma density reaches a critical value, it can be up to seven times higher than the density in the low field side region and it stays stable as long as the density remains. In this stable phase, the local radiation is at a low level and it shares common characteristics with the high-field-side high-density (HFSHD) phenomena observed in divertor devices. If density increases continuously, the high-density front expands poloidally and evolves into an unstable ‘moving’ phase while propagating into the main plasma region, which eventually triggers the disruption. The maximum achievable density on J-TEXT seems to be correlated with the evolution of the high-density front, suggesting that the region may play a role in setting the operational limit on J-TEXT. The unstable phase of high-density front seems to share some common characteristics with the well-known multifaceted asymmetric radiation from the edge (MARFE), except that the radiation level in J-TEXT stays at a low level. The density threshold of the high-density front formation and the maximum density of the region itself increase with , consistent with the observation on both HFSHD and MARFE. The MARFE-like behavior of the high-density front in its final stage, suggests that MARFEs might be the result of these fronts forming in or moving into regions where the plasma temperature and impurity concentration are conducive for them to be radiatively unstable.

Investigation of the double peak in the visible-light range of radiative divertor emission profiles on the EAST tokamak

The double-peaked emission profile has been identified by tomographic reconstruction from the signals of the tangentially viewing visible-light imaging system on EAST. In the upper-single-null discharge with neon seeding, two light emission peaks are located near the X-point and in the high-field-side (HFS) scrape-off layer (SOL) on the poloidal cross-section while the outer divertor target is still attached. The double-peaked emission phenomenon has not been reported for other tokamaks. In particular, the existence of the emission peak near the X-point under the attached condition is unexpected because it is usually an indication of X-point multifaceted asymmetric radiation from the edge under the detached condition. Therefore, it is of fundamental importance for the divertor physics to make clear the mechanism of the double-peaked emission phenomenon. The computational simulations with SOLPS-ITER and DIVIMP are performed in this work. The simulated results show the contribution of W impurity to the double-peaked emission profile is negligible with the upper tungsten divertor. Considering the existence of a lower carbon divertor in EAST, the influence of the possible deposition of the carbon on the upper divertor is further investigated. The simulating CIII (465 nm) and CII (514 nm) emissions, which make a significant contribution to the visible-light emission in channel B (blue) and G (green), respectively, are found to be similar tothe experimental results. The emission peak near the X-point is related to the carbon deposited on the dome, while that in the HFS SOL is contributed by the carbon on the inner baffle. The ionization source of the carbon impurity and the net force pointing to the X-point are analyzed to understand the concentration of the carbon impurity near the X-point, which in turn causes the emission peak.

Quiescent regions below the X-point in ASDEX upgrade

Probe measurements of plasma fluctuations in the divertor region of ASDEX Upgrade reveal the existence of two quiescent regions close to the separatrix: one on the low field side scrape-off layer and one on the high field side of the private flux region. The X-point manipulator at ASDEX Upgrade provides the unique possibility of obtaining continuous measurements of the ion-saturation current from the low field side scrape-off layer through the private flux region and into the high field side scrape-off layer.

SOLPS analysis of changes in the main SOL of DIII-D associated with divertor detachment vs attachment and closure vs openness

Edge plasma code SOLPS modeling has been used to assess the effects of divertor detachment vs attachment and closure vs openness on the main scrape-off layer (SOL) plasma in DIII-D. The differences between the inner divertor and outer divertor, and the effects of divertor closure on the outer-midplane SOL plasma, have been analysed. It is found that the divertor shape and plasma state have significant influence on the SOL and pedestal plasma. (i) The inner divertor has a more significant effect on the high-field side (HFS) SOL than the outer divertor does on the low-field side (LFS) SOL, and for increasing main plasma density the ionization region in the HFS moves gradually from the divertor target into the HFS SOL, forming a high-density front in the high density discharge case. Also, particle flux reversal occurs near the separatrix. (ii) It is confirmed that divertor closure has a significant effect on the main SOL and pedestal plasma. Comparing an open and a closed divertor, it is found that for the closed divertor the ne radial profile at the OMP is higher outside the separatrix and lower inside it, due to the combined effects of the D+ and C impurity ion density. Moreover, the open divertor has higher core fueling than the closed divertor, and the closed divertor has better fuel and impurity neutral screening than the open divertor. (iii) The in/out asymmetry is also influenced by the divertor closure, increase of the closure of the outer divertor can substantially reduce the asymmetry. The closed divertor also has much higher HFS SOL density than that of the LFS SOL. These results highlight the strong dependence of neutral/impurity screening and main SOL properties on divertor geometry and plasma regimes, which needs to be taken into account in advanced divertor design for fusion reactors.

Link between divertor conditions and high-field-side/low-field-side midplane density profiles in H-mode plasmas at ASDEX Upgrade

The connection between midplane and divertor conditions is studied on ASDEX Upgrade (AUG) for H-mode scenarios. H-mode discharges with variations in fuelling, heating power and impurity seeding are analysed, enabling to disentangle their impact on the evolution of midplane density profiles. The O-mode reflectometer installed on AUG provides unique insights on the midplane density profiles thanks to the ability to measure simultaneously at the high-field-side (HFS) and the low-field-side (LFS). At the inner divertor, with the onset of detachment, a region of high density is formed (HFS high density front, HFSHD) that expands into the HFS midplane, leading to strong poloidal asymmetries in the main chamber scrape-off layer (SOL) density. A good agreement is observed between the evolution of the density profiles at the midplane and that of the divertor volume density confirming the strong influence of divertor conditions on the midplane density profiles at the HFS. Our results confirm the existence of a relationship between plasma confinement, the shift in the midplane LFS density profile and the presence of the HFSHD with respect to changes in seeding and fuelling. It is established that the separatrix density at the LFS is better correlated with the neutral pressure at the outer target while the HFS SOL density follows the neutral pressure at the inner divertor. A comprehensive characterisation of the edge-localised mode (ELM) evolution at the midplane and divertor is performed demonstrating that also during the ELM cycle the divertor conditions have a strong effect on the SOL density at the midplane. The most striking result is the observation of a HFSHD at the HFS midplane just after the ELM crash associated with strong inner divertor detachment.

The effect of the secondary x-point on the scrape-off layer transport in the TCV snowflake minus divertor

The snowflake (SF) magnetic configuration is investigated as a potential power exhaust solution for a fusion reactor, but also to improve our understanding of divertor physics in general. Unlike a conventional single null (SN), it features an additional nearby x-point, which deeply modifies the magnetic field in the scrape-off layer (SOL) and can thereby affect parallel and cross-field transport of heat and particles. This paper investigates the power exhaust properties of the snowflake minus (SF-) configuration on the TCV tokamak, for Ohmically heated, L-mode, low-density, attached plasmas for a range of x-point separations, magnetic field directions and locations of the secondary x-point (low-field-side (LFS) or high-field-side (HFS)). Due to the relatively large x-point separation in physical space, this study probes x-point transport features in general, rather than reactor relevant aspects of the SF configuration. The target heat fluxes at all strike points are simultaneously monitored with an infrared (IR) thermography system and the kinetic profiles of the SOL at the outer mid-plane with a reciprocating probe (RCP). The placement of the additional x-point is seen to affect the inner-outer divertor power balance. An effective heat flux width for the SOL in the low poloidal field region is inferred from the measured power repartition between the two SOL manifolds created by the secondary x-point. For the LFS SF- configuration (secondary null in the LFS SOL), the is two times larger than that measured by the RCP at the outer mid-plane and by IR at the outer target of a comparable SN, while the outer mid-plane SOL profiles are similar to the SN. This is interpreted as the effect of increased effective cross-field diffusivity in the intra-null region relative to the rest of the SOL. For the HFS SF- configuration (secondary null in the HFS SOL), no such increased transport is observed. The pressure-driven plasma convection expected near the primary null cannot explain the increased , which is instead consistent with interchange ballooning-like turbulence enhanced by the low poloidal field.

Impurity screening behavior of the high-field side scrape-off layer in near-double-null configurations: prospect for mitigating plasma–material interactions on RF actuators and first-wall components

The impurity screening response of the high-field side (HFS) scrape-off layer (SOL) to localized nitrogen injection is investigated on Alcator C-Mod for magnetic equilibria spanning lower-single-null, double-null and upper-single-null configurations under otherwise identical plasma conditions. L-mode, EDA H-mode and I-mode discharges are investigated. HFS impurity screening is found to depend on magnetic flux balance and the direction of B × B relative to the most active divertor. Impurity ‘plume’ emission patterns indicate that both parallel and perpendicular (E × B) flows in the SOL contribute to the ‘flushing’ of impurities towards the active divertor, thereby affecting the overall impurity screening behavior. Despite the fact that the HFS SOL is extremely narrow in near-double-null configurations, this SOL is able to screen locally injected nitrogen at least as effectively as the low-field side (LFS) SOL—up to a factor of 10 more effective, depending on specific plasma conditions and whether the magnetic geometry produces parallel flows that work with or against E × B flows. For situations in which the E × B drift of the impurity ions opposes parallel flow toward the primary divertor, HFS impurity screening is found to be least effective. When E × B drifts assist parallel flow toward the primary divertor, HFS impurity screening is found to be very effective. These data support the idea of placing RF actuators and close-fitting wall components on the HFS of the tokamak. With this configuration, near-double-null magnetic topologies may be used for active control of plasma parameters at the antenna/plasma interface for optimal RF coupling, mitigate the generation of local impurities from plasma–material interactions and, taking advantage of favorable plasma flows and good screening properties of the HFS SOL, further minimize the impact of wall-born impurity sources on the plasma core.

The high field side high density region in SOLPS-modeling of nitrogen-seeded H-modes in ASDEX Upgrade

The understanding of divertor physics and the evolution of divertor detachment is crucial for developing the capability to model power exhaust in current experiments and reliably predict it for future fusion devices. In simulations of ASDEX Upgrade, an experimentally observed region of high density in the high field side scrape-off layer has been recovered. Validated modeling with SOLPS5.0 shows that a detailed match of the high field side scrape-off layer plasma is not only important for local plasma parameters, but can lead to strong changes in global parameters. Drifts play a crucial role in lower-single null discharges with forward toroidal field (∇B→-drift pointing down). Their inclusion changes the spatial extent as well as the radial and poloidal gradients of the high field side high density. Adapted transport coefficients that take into account core fueling by plasma diffusion due to the presence of the high field side density and drift-driven radial fluxes now reconcile the modeled deuterium compression ratio, divertor neutral density, neutral radiation levels and deuterium fueling rates with experimental measurements. The onset of strong volume recombination in the simulations now allows to remove the previously necessary increase of perpendicular transport in the inner divertor from the simulations.

Measurement of N+ flows in the high-field side scrape-off layer of ASDEX upgrade with different degrees of inner divertor detachment

Toroidal and poloidal flows of injected N+ ions were measured in the high-field side (HFS) scrape-off layer (SOL) of ASDEX Upgrade by Doppler spectroscopy with different degrees of HFS divertor detachment. In high-recycling conditions, the results suggest reversed parallel N+ flow away from the inner divertor in the near SOL close to the separatrix, while the flow is towards the inner divertor throughout the SOL in detached conditions. The measured poloidal N+ flows were directed away from the HFS divertor in the near SOL for all density cases. Divertor plasma oscillations, characterized by momentary peaking of the HFS target ion flux and decrease of the HFS SOL density, were observed slightly before the roll-over of the ion saturation current to the HFS target and lead to an increase in the N+ flow towards the HFS divertor. SOLPS and ERO simulations of the experiment predict entrainment below 50% between the velocities of N+ and D+ ions, suggesting that N+ ions are quantitatively a limited proxy for measuring D+ flows. ERO simulations show significantly higher entrainment for higher ionization states, e.g., N2+ and N3+.

The role of the density profile in the ASDEX-Upgrade pedestal structure

Experimental evidence for the impact of a region of high density localised in the high-field side scrape-off layer (the HFSHD) on plasma confinement is shown in various dedicated experiments on ASDEX Upgrade (AUG). Increasing main ion fuelling is shown to increase the separatrix density and shift the density profile outwards. Predictive pedestal modelling of this shift indicates a 25% decrease in the attainable pedestal top pressure, which compares well with experimental observations in the gas scan.Since the HFSHD can be mitigated by applying nitrogen seeding, a combined scan in fuelling rate, heating power, and nitrogen seeding is presented. Significant increases in the achievable pedestal top pressure are observed with seeding, in particular at high heating powers, and are correlated with inward shifted density profiles and a reduction of the HFSHD and separatrix density. Interpretive linear stability analysis also confirms the impact of a radially shifted pressure profile on peeling-ballooning stability, with an inward shift allowing access to higher pressure gradients and pedestal widths.

High-field side scrape-off layer investigation: Plasma profiles and impurity screening behavior in near-double-null configurations

New experiments on Alcator C-Mod reveal that the favorable impurity screening characteristics of the high-field side (HFS) scrape-off layer (SOL), previously reported for single null geometries, is retained in double null configurations, despite the formation of an extremely thin SOL. In balanced double-null, nitrogen injected locally into the HFS SOL is better screened by a factor of 2.5 compared to the same injection into the low field side (LFS) SOL. This result is insensitive to plasma current and Greenwald fraction. Nitrogen injected into the HFS SOL is not as well screened (only a factor of 1.5 improvement over LFS) in unbalanced double-null discharges, when the primary divertor is in the direction of B×∇B. In this configuration, impurity ‘plume’ emission patterns indicate that an opposing E × B drift competes with the parallel impurity flow to the divertor. In balanced double-null plasmas, the dispersal pattern exhibits a dominant E × B motion. Unbalanced discharges with the primary divertor opposite the direction of B×∇B exhibit excellent HFS screening characteristics – a factor of 5 enhancement compared to LFS. These data support the idea that future tokamaks should locate all RF actuators and close-fitting wall structures on the HFS and employ near-double-null magnetic topologies, both to precisely control plasma conditions at the antenna/plasma interface and to maximally mitigate the impact of local impurity sources arising from plasma-material interactions.

A Scoping Study for High-Field-Side Launch of Lower Hybrid Waves on ADX MIT

Launching lower hybrid (LH) waves from the high field side (HFS) of a tokamak offers significant advantages over low-field-side (LFS) launch with respect to both wave physics and plasma material interactions (PM!s). The higher magnetic field opens the window between wave accessibility and the condition for strong electron Landau damping, allowing LH waves from the HFS to penetrate into the core of burning plasma, while waves launched from the LFS are restricted to the periphery of the plasma. The lower parallel refractive index (n||) of the waves launched from the HFS yields a higher current drive efficiency as well. The absence of turbulent heat and particle fluxes on the HFS, particularly in double null configuration, makes it the ideal location to minimize PM! damage to the antenna structure. The quiescent scrape off layer (SOL) also eliminates the need to couple LH waves across a long distance to the separatrix, as the antenna can be located close to plasma without risking damage to the structure. The Advanced Divertor eXperiment (ADX) will include an LH launcher located on the HFS. The LH system for ADX will make use of the existing infrastructure from Alcator C-Mod, including sixteen 250-kW klystrons at 4.6 GHz (total source power of 4 MW), high-voltage power supply, and controls. The ADX vacuum vessel design includes dedicated space for waveguide runs, pressure windows, and vacuum feedthroughs for accessing the HFS wall. Compact antenna designs based on proven technologies (e.g., multijunction and four-way splitter antennas) fit within the available space on the HFS of the ADX. Wave coupling simulations of these launchers with HFS SOL density profiles showing good coupling can be obtained by adjusting the distance between the separatrix and the HFS wall. Guard limiters on each side of the LH antenna protect the structure during ramp-up, ramp-down, and off-normal events.

Measurements of ion cyclotron parametric decay of lower hybrid waves at the high-field side of Alcator C-Mod

Ion cyclotron parametric decay instability (PDI) of lower hybrid (LH) waves is surveyed using edge Langmuir probes on the Alcator C-Mod tokamak. The measurement is carried out simultaneously at the high-field side (HFS) and low-field side (LFS) mid-plane of the tokamak, as well as in the outer divertor region. Different LH spectra are observed depending on the location of the probes and magnetic configuration in L-mode plasmas, with drift direction downward. In lower single null (LSN) plasmas, strong ion cyclotron PDI occurring at the HFS is observed for the first time. This instability is characterized by a frequency separation in sidebands corresponding to the ion cyclotron frequency (ωci) near the HFS scrape-off layer and develops with threshold-like behavior as density increases. In inner wall limited (IWL) plasmas, this HFS instability shows a higher density threshold compared with that in LSN plasmas. The pump width becomes broadened even in the absence of the sidebands. In upper single null plasmas with similar plasma parameters, ion cyclotron PDI sidebands have a frequency separation corresponding to ωci near the LFS and are weaker than those observed in the LSN and IWL plasmas. Correlation between the onset of ion cyclotron PDI and the observed loss of lower hybrid current drive efficiency (Wallace et al 2012 Phys. Plasmas 19 062505) is discussed.

Transport and drift-driven plasma flow components in the Alcator C-Mod boundary plasma

Boundary layer flows in the Alcator C-Mod tokamak are systematically examined as magnetic topology (upper versus lower-null) and plasma density are changed. Utilizing a unique set of scanning Langmuir–Mach probes, including one on the high-field side (HFS) midplane, the poloidal variation of plasma flow components in the parallel, diamagnetic and radial directions are resolved in detail. It is found that the plasma flow pattern can be decomposed into two principal parts: (1) a drift-driven component, which lies within a magnetic flux surface and is divergence-free and (2) a transport-driven component, which gives rise to near-sonic parallel flows on the HFS scrape-off layer (SOL). Toroidal rotation, Pfirsch–Schlüter and transport-driven contributions are unambiguously identified. Transport-driven parallel flows are found to dominate the HFS particle fluxes; the total poloidal-directed flow accounts for ∼1/3 to all of the ion flux arriving on the inner divertor. As a result, heat convection is found to be an important player in this region, consistent with the observation of divertor asymmetries that depend on the direction of B × ∇B relative to the active x-point. In contrast, the poloidal projection of parallel flow in the low-field SOL largely cancels with Er × B flow; toroidal rotation is the dominant plasma motion there. The magnitude of the transport-driven poloidal flow is found to be quantitatively consistent with fluctuation-induced radial particle fluxes on the low-field side (LFS), identifying this as the primary drive mechanism. Fluctuation-induced fluxes on the HFS are found to be essentially zero, excluding turbulent inward transport as the mechanism that closes the circulation loop in this region.

Swinging reciprocating Mach probes for the high field side scrape-off layer in DIII-D

A new pair of in situ reciprocating Mach probes termed swing probes has been deployed on the DIII-D centerpost for the 2012 experimental campaign. When not deployed, the entire assembly is housed in a <5 cm space underneath the centerpost tiles. This design is unique in that the probe swings vertically through the edge plasma, taking measurements along a 180° arc with a 20 cm radius. The motion is powered by actuator coils that interact with the tokamak's magnetic field. Two electrodes maintain a Mach-pair orientation throughout the swing and provide measurements of saturation current, electron temperature, and parallel flow speeds up to the separatrix.

The Scrape-Off Layer in Alcator C-Mod: Transport, Turbulence, and Flows

Research on the scrape-off layer (SOL) plasma of the Alcator C-Mod Tokamak is reviewed. The research has focused on understanding the transport of energy and particles both parallel and perpendicular to the magnetic field. Large differences between the inboard, high-field side SOL and the outboard, low-field side are found. On the outboard side large levels of anomalous cross-field transport of heat and particles exist, with important and far-reaching consequences on recycling, power handling, plasma flows, and possibly core-plasma density limits and rotation. The phenomenon of main chamber recycling is discussed. Parallel and perpendicular transport, together with the heat and particle sources, determine the plasma profiles in the SOL, and these profiles show qualitative differences between near- and far-SOL regions. Particle transport in the near SOL exhibits a strong scaling with collisionality, while transport in the far SOL is clearly convective, with little obvious dependence on collisionality. The anomalously large magnitudes of perpendicular transport are the result of turbulence. Turbulent structures, “blobs,” are largely responsible, and their characteristics have been examined. The turbulent structures are approximately aligned with the field and have k ≪ kperp. Their characteristic size perpendicular to the field is ~1 cm, and their characteristic lifetime is ~1 to 50 μs. The turbulent structures move both radially outward and poloidally at speeds up to ~1 km/s. Evidence that this turbulent transport may play an important role in the core-plasma density limit is presented. Much lower levels of turbulence and no blobs are observed in the high-field-side SOL. For single-null magnetic configurations, plasma in the inboard SOL appears to be almost entirely a result of plasma flow along field lines from the low-field side. Strong parallel flows with sensitivity to magnetic topology are found, along with strong evidence for momentum coupling between these SOL flows and core toroidal rotation.

In–out divertor flow asymmetries during ELMs in ASDEX Upgrade H-mode plasmas

In ASDEX Upgrade, a fast reciprocating probe positioned in the lower divertor, capable of accessing the low-field side (LFS) and high-field side (HFS) scrape-off layer (SOL) just below the x-point, as well as the private flux region, was equipped with a Mach head and used to investigate fast flow fluctuations and in–out divertor flow asymmetries during ELMs. We compare the flow behaviour during ELMs in the three separate regions. Flow enhancement is observed in the HFS SOL, with Mach number values reaching or exceeding M=2, flow reversal in the LFS SOL, and complex fluctuating behaviour in the private flux region (which includes flow reversal). We discuss the possible mechanisms that could drive these observations.