Pedestal Density Research Articles

H-mode pedestal improvements with neon injection in DIII-D

Improvements in the H-mode pedestal pressure and global energy confinement were observed on DIII-D with both 3 and 6 MW of neutral beam (NBI) heating with neon injection. The pedestal pressure increased, primarily associated with a significant increase in the pedestal density with a relatively smaller decrease in the pedestal temperature. At the same neon injection rate, a rapid decrease in edge localized mode (ELM) frequency and ∼35% increase in plasma stored energy were observed in the 3 MW discharge, while in the 6 MW discharge, the ELM frequency was gradually decreased and the plasma stored energy gradually increased by up to 20%. The measured pedestal widths from both 3 and 6 MW discharges matched the EPED1.0 width scaling, , within 20% deviations before and after neon injection. The peeling-ballooning mode (PBM) stability analysis showed that after neon injection, the ballooning stability boundary was increased, while the peeling stability boundary only dropped once the neon level was relatively high. The ballooning stability boundary increase with neon injection is consistent with increase of the ion diamagnetic stabilization (i.e. including the contribution from carbon, neon as well as deuterium ions) which is used in the criterion to determine when the calculated linear growth rates of PBM are unstable. Consequently, the new ELITE computed PBM boundaries were more consistent with the operating points than when only deuterium was included. This in turn indicates that the impurity may help to improve pedestal pressure through affecting ion diamagnetic frequency in the pedestal region, which could positively lead to the ballooning stability boundary extension.

Impact of E × B flow shear stabilization on particle confinement and density peaking at JET

The impact of the flow shear stabilization on particle transport and density peaking at JET is analyzed in the framework of integrated modelling with the CRONOS code. For that purpose, plasmas from a power scan which show a significant increasing of density peaking with the injected neutral beam injection power have been used as a modeling basis. By means of simulations with the quasi-linear model GLF23 for the heat and particle transport, a strong link between the particle confinement and flow shear stabilization is found. This is particularly important close to the pedestal region where the particle pinch direction becomes strongly inward for high flow shear values. Such impact introduces some non-negligible deviation from the well-known collisonality dependence of the density peaking, whose general trend has been also obtained in the framework of this modelling by performing pedestal density scans.

Identity of the JET M-mode and the ASDEX Upgrade I-phase phenomena

An H-mode plasma state free of edge-localized mode (ELM), close to the L-H transition with clear density and temperature pedestal has been observed both at the Joint European Torus (JET) and at the ASDEX Upgrade (AUG) tokamaks usually identified by a low frequency (LFO, 1–2 kHz), m = 1, n = 0 oscillation of the magnetics and the modulation of pedestal profiles. The regime at JET is referred to as M-mode while at AUG as intermediate phase or I-phase. This contribution aims at a comparative analysis of these phenomena in terms of the density and temperature pedestal properties, the magnetic oscillations and symmetries. Lithium beam emission spectroscopy (Li-BES) and reflectometer measurements at JET and AUG show that the M-mode and the I-phase modulates the plasma edge density. A high frequency oscillation of the magnetics and the density at the pedestal is also associated with both the M-mode and the I-phase, and its power is modulated with the LFO frequency. The power modulation happens simultaneously in every Mirnov coil signal where it can be detected. The bursts of the magnetic signals and the density at the pedestal region are followed by the flattening of the density profile, and by a radially outward propagating density pulse in the scrape-off layer (SOL). The analysis of the helium line ratio spectroscopy (He-BES) signals at AUG revealed that the electron temperature is modulated in phase with the density, thus the I-phase modulates the pressure profile gradient. This analysis gave opportunity to compare Li-BES and He-BES density profiles at different locations suggesting a toroidal and poloidal symmetry of the density modulation. The presented results indicate that the regimes, the AUG I-phase and the JET M-mode, exhibit similar characteristics, which leads to the conclusion that the regimes are likely the same.

Evaluation of tritium burnup fraction for CFETR scenarios with core-edge coupling simulations

A key mission for the next-step fusion tokamak device China Fusion Engineering Test Reactor (CFETR) is demonstrating tritium self-sufficiency, which requires a sufficiently high tritium burnup fraction (fburnup) in order to match a practically achievable tritium breeding ratio (TBR) with the blanket design constraints. Core-edge coupling simulations are performed to investigate the dependence of fburnup on different controlling parameters for CFETR scenarios. Core plasma profiles with a range of pedestal densities are simulated by consistent iterative calculations of equilibrium, transport, auxiliary heating and current drives within the OMFIT framework. The core-SOL integrated COREDIV code is then used to evaluate fburnup with the OMFIT modelled core plasma parameters as input. According to the simulations, fburnup can be effectively increased with a higher pedestal density on account of the fusion power increasing faster than the fueling source required to maintain steady-state. Higher can also increase the fburnup due to increase of fuel recycling. Deeper pellet fueling deposition and lower ratio of particle to thermal diffusivities D/χ can both increase the effective particle confinement time and thus fburnup. However, the effect of helium and other impurities (Ar and W) is shown to reduce fburnup for comparable impurity and main ion transport. Based on our analysis, using present pellet fueling technology, achieving fburnup > 3% for CFETR will be very challenging. This is a lower limit for the required TBR (>1) to match the achievable TBR for tritium self-sufficiency. Furthermore, our study suggests that if fueling can penetrate deeper than r/a < 0.8 under optimistic conditions, the required burnup fraction could be attainable. The modelling results thus provide important suggestions and implications for the optimization of CFETR scenarios and development of advanced fueling systems.

Separating divertor closure effects on divertor detachment and pedestal shape in DIII-D

Comparison between an open divertor and a more-closed divertor in DIII-D demonstrates detachment up to 40% lower pedestal density (ne,ped) in the closed divertor due to a combination of decreased fueling of the pedestal and increased dissipation in the scrape off layer (SOL) in the closed divertor, both resulting from increased neutral trapping in the divertor. Predicting whether the relationship between divertor closure and detachment will hold for an opaque SOL, in which the contribution of ionizing neutrals to fueling the pedestal is lessened, requires separating out different mechanisms contributing to the density difference at detachment. A series of experiments on DIII-D characterizes matched discharges using various divertor configurations to isolate the effects of divertor closure. These experiments show detachment up to 25% lower ne,sep in the closed divertor than in the open divertor, supported by simulations showing increased neutral trapping, and hence, increased dissipation, in the closed divertor. A difference in ne,ped/ne,sep is also seen: for matched ne,sep, the closed divertor has up to 20% lower ne,ped, consistent with modeling showing a smaller ionization fraction inside the separatrix in this case. Understanding how these pieces fit together will help in the development of predictive models of pedestal density and detached divertors compatible with a high performance core.

Estimation of ELM effects on Be and W erosion at JET-ILW

An important W erosion mechanism in JET divertor is the physical sputtering by both, impurity (e.g. Be) and hydrogenic ions hitting the W divertor with energies determined by the pedestal temperature during edge-localized modes (ELMs)—the so-called intra-ELM sputtering of W. The earlier developed analytical approach for the W divertor gross erosion estimation using the Langmuir probe measurements has been improved in this work taking into account the time-resolved pedestal temperature and density drop during the pedestal crash under intra-ELM conditions. The improved model allows reproducing the measured at the divertor tile particle and heat fluxes evolution at the effective magnetic connection length matched with the previous JET- ITER like wall (ILW) studies. The estimates for the tungsten sputtered flux in intra- and inter-ELM conditions for quasi-steady state plasmas executed at the end of the first year of JET-ILW operation (C30C experiment) show that Type I ELMs contribute significantly (∼85%) to the gross tungsten erosion which is in a good agreement with the divertor optical emission spectroscopy (W I 400.9 nm line). ELM filament radial propagation is considered based on the advective-diffusive model and JET-C experiment results. The estimation for the ELM-induced local Be main chamber erosion at JET-ILW reveals the increase of the Be sputtered flux by 30% under intra-ELM conditions.

Impact of divertor material on neutral recycling and discharge fueling in DIII-D

Experiments with the lower divertor of DIII-D during the Metal Rings Campaign (MRC) show that the fraction F of atomic D in the total recycling flux is material-dependent and varies through the ELM cycle, which may affect divertor fueling. Between ELMs, FC ∼ 10% and FW ∼ 40%, consistent with expectations if all atomic recycling is due to reflections. During ELMs, FC increases to 50% and FW to 60%. In contrast, the total D recycling coefficient including atoms and molecules R stays close to unity near the strike point where the surface is saturated with D. During ELMs, R can deviate from unity, increasing during high energy ELM-ion deposition (net D release) and decreasing at the end of the ELM which leads to ability of the target to trap the ELM-deposited D. The increase of R > 1 in response to an increase in ion impact energy Ei has been studied with small divertor target samples using Divertor Materials Evaluation System (DiMES). An electrostatic bias was applied to DiMES to change Ei by 90 eV. On all studied materials including C, Mo, uncoated and W-coated TZM (>99% Mo, Ti, and Zr alloy), W, and W fuzz, an increase of Ei transiently increased the D yield (and R) by ∼10%. On C there was also an increase in the molecular D2 yield, probably due to ion-induced D2 desorption. Despite the measured increase in F on W compared to C, attached H-mode shots with OSP on W during MRC did not demonstrate a higher pedestal density. About 8% increase in the edge density could be seen only in attached L-mode scenarios. The difference can be explained by higher D trapping in the divertor and lower divertor fueling efficiency in H- versus L-mode.

Simulation of supersonic molecular beam injection fueling into H-mode plasmas on EAST using BOUT++

Transport dynamic 2D simulations for supersonic molecular beam injection (SMBI) fueling into H-mode deuterium plasmas on the Experimental Advanced Superconducting Tokamak (EAST) is first simulated using a seven-field two-fluid model in the BOUT++ framework. The SMB is assumed to be injected into plasma from the midplane at the low field side with a fixed width and constant molecular flux. The different densities and injection velocities of SMBI are investigated within the upper single-null geometry of EAST. The simulations indicate that the SMBI has a self-shielding effect on molecules' inward transport into the plasma, and the deposition of SMBI leads to a large increase in plasma density and decrease in plasma temperature. There is a velocity threshold for SMB penetrating the pedestal and depositing at the top of density pedestal. However, the deposition point would be back toward the plasma boundary after SMB arriving at the deepest penetration position. Comparing the different molecular injection velocities and densities, the outcomes demonstrate that the depth of deposition is closely related to the injection velocity rather than the molecular density. The simulated results show good agreement with the EAST experiments by comparing the electron density profiles obtained by simulation and experiment, respectively. These results will be helpful for guiding future experiments, as well as the design of the SMBI system.

The density dependence of edge-localized-mode suppression and pump-out by resonant magnetic perturbations in the DIII-D tokamak

The density dependence of edge-localized-mode (ELM) suppression and density pump-out (density reduction) by n = 2 resonant magnetic perturbations (RMPs) is consistent with the effects of narrow well-separated magnetic islands at the top and bottom of the H-mode pedestal in DIII-D low-collisionality plasmas. Nonlinear two-fluid MHD simulations for DIII-D ITER similar shape discharges show that, at low collisionality (ν*e &amp;lt; 0.5), low pedestal density is required for resonant field penetration at the pedestal top (ne,ped ≈ 2.5 × 1019 m−3 at ψN ≈ 0.93), consistent with the ubiquitous low density requirement for ELM suppression in these DIII-D plasmas. The simulations predict a drop in the pedestal pressure due to parallel transport across these narrow width (ΔψN ≈ 0.02) magnetic islands at the top of the pedestal that is stabilizing to Peeling-Ballooning-Modes and comparable to the pedestal pressure reduction observed in experiment at the onset of ELM suppression. The simulations predict density pump-out at experimentally relevant levels (Δne/ne ≈ −20%) at low pedestal collisionality (ν*e ≈ 0.1) due to very narrow (ΔψN ≈ 0.01–0.02) RMP driven magnetic islands at the pedestal foot at ψN ≈ 0.99. The simulations show decreasing pump-out with increasing density, consistent with experiment, resulting from the inverse dependence of parallel particle transport on collisionality at the foot of the pedestal. The robust screening of resonant fields is predicted between the top and bottom of the pedestal during density pump-out and ELM suppression, consistent with the preservation of strong temperature gradients in the edge transport barrier as seen in experiment.

Localized divertor leakage measurements using isotopic tungsten sources during edge-localized mode-y H-mode discharges on DIII-D

Experiments carried out on DIII-D using a novel setup of isotopic tungsten (W) sources in the outer divertor have characterized how the W leakage from this region depends on both the exact source location and edge-localized mode (ELM) behavior. The sources are toroidally-symmetric and poloidally-localized to two regions: (1) the outer strike point (OSP) with natural abundance of W isotopes; and (2) the far-target with highly-enriched 182W isotopes. With the use of a dual-faced collector probe (CP) in the main scrape-off layer (SOL) near the outside midplane and source-rate spectroscopy, a proxy for divertor impurity leakage is developed. Using this proxy, it is found that for the OSP W location, there is a nearly linear increase of leakage with the power across the separatrix (), which is consistent with the effect of an increased upstream ion temperature parallel gradient force in the near-SOL; trends in the pedestal density and collisionality are also seen. Conversely, it is found that for the far-target W location leakage falls off rapidly as increases and ELM size decreases, which is suggestive that ELM size plays a role in the leakage from this location. Indications for main SOL W contamination is evidenced by the measurement of large deposition asymmetries on the two opposite CP faces. These measurements are coupled with interpretive modeling showing SOL W accumulation near the separatrix furthest from both targets driven by forces parallel to the magnetic field. This experimental setup, together with the target and upstream W measurements, provides information on the transport from different divertor W source locations and leakage. These studies help to elucidate the physics driving divertor impurity source rates and leakage, with and without ELMs, and provide better insight on the link in the chain connecting wall impurity sources to core impurity levels in magnetic fusion devices.

Characterisation of highly radiating neon seeded plasmas in JET-ILW

To study the impact of strong impurity radiation on the energy confinement and the discharge stability at JET-ILW, dedicated high-density, highly heated experiments with neon seeding have been performed. In these experiments an increase of radiation in the core and especially the pedestal region plus a characteristic strong X-point radiator inside the confined region have been observed. The increased radiation inside the separatrix had no impact on the energy confinement. Only at the highest neon puff rates and highest heating powers () a weak H-mode without back-transitions to L-mode (M-mode) could be achieved, while at lower heating powers or seeding levels L-modes and oscillations between H-mode and L-mode could be observed. In H-mode, at highest neon seeding levels the targets are in complete detachment. A degradation of the pedestal density profile was observed when neon was seeded. A correlation between this density pedestal degradation, the appearance of an X-point radiator and the detachment of the targets was found in neon seeded H-mode pulses.In ASTRA-TGLF core transport simulations it could be shown that the increase of the electron temperature in the core with neon seeding is attributed to a partial stabilisation of ion temperature gradient modes.

A low-frequency axisymmetric oscillation in the high-confinement mode pedestal on the EAST tokamak

A low-frequency (~1 kHz) oscillation (LFO) in the pedestal region emerged during the ELM-free phase after the L–H transition on the EAST tokamak. Its characteristics were studied. This oscillation has a magnetic component with a toroidal mode number (n) of 0, indicating an axisymmetric mode, while its poloidal magnetic structure demonstrates an m = 1 standing wave with in–out asymmetry. The density fluctuation shows no phase difference radially, indicating that it is not a radial traveling pulse. Before this axisymmetric oscillation, pedestal density fluctuation was dominated by low-frequency (LF) fluctuation with a frequency of less than 400 kHz. During the phase of axisymmetric oscillation, this LF fluctuation was modulated by this axisymmetric oscillation and thus experienced ‘suppressed’ and ‘recovery’ phases alternately. As a result, both pedestal density and the particle flux onto the divertor were modulated. During the ‘suppressed’ phase of the LF fluctuation, a new high-frequency (HF) fluctuation appeared, with a frequency larger than 500 kHz. The result demonstrates complex turbulence behavior in pedestal region.

Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.

Development of the quasi-optical combiner systems for density profile reflectometers on the EAST tokamak

The reflectometry diagnostics on experimental advanced superconducting tokamak (EAST) are composed of Q-band (32–56 GHz), V-band (48–76 GHz) and W-band (72–110 GHz) microwave reflectometers with X-mode polarization. The three reflectometers with separate frequency bands were launched independently in previous experiments and now are combined together by applying advanced microwave technology. A quasi-optical (QO) combiner/de-combiner by using frequency selective surfaces (FSSs) has been built to combine the three bands so that these waves (32–110 GHz) can be transmitted to the antenna by using one single oversized waveguide. Two double-ridged horns are used for launcher and receiver. The received waves are decoupled by using a QO de-combiner, the same with the combiner. The designed parameters and laboratory test results of the upgraded system have been presented. The density pedestal evolution during the edge localized modes (ELMs) in EAST plasma have been presented as an example for the application of the updated reflectometers.

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

This paper compares the gyrokinetic instabilities and transport in two representative JET pedestals, one (pulse 78697) from the JET configuration with a carbon wall (C) and another (pulse 92432) from after the installation of JET’s ITER-like Wall (ILW). The discharges were selected for a comparison of JET-ILW and JET-C discharges with good confinement at high current (3 MA, corresponding also to low ) and retain the distinguishing features of JET-C and JET-ILW, notably, decreased pedestal top temperature for JET-ILW. A comparison of the profiles and heating power reveals a stark qualitative difference between the discharges: the JET-ILW pulse (92432) requires twice the heating power, at a gas rate of e s−1, to sustain roughly half the temperature gradient of the JET-C pulse (78697), operated at zero gas rate. This points to heat transport as a central component of the dynamics limiting the JET-ILW pedestal and reinforces the following emerging JET-ILW pedestal transport paradigm, which is proposed for further examination by both theory and experiment. ILW conditions modify the density pedestal in ways that decrease the normalized pedestal density gradient a/Ln, often via an outward shift in relation to the temperature pedestal. This is attributable to some combination of direct metal wall effects and the need for increased fueling to mitigate tungsten contamination. The modification to the density profile increases , thereby producing more robust ion temperature gradient (ITG) and electron temperature gradient driven instability. The decreased pedestal gradients for JET-ILW (92432) also result in a strongly reduced shear rate, further enhancing the ion scale turbulence. Collectively, these effects limit the pedestal temperature and demand more heating power to achieve good pedestal performance. Our simulations, consistent with basic theoretical arguments, find higher ITG turbulence, stronger stiffness, and higher pedestal transport in the ILW plasma at lower .

Self-consistent pedestal prediction for JET-ILW in preparation of the DT campaign

The self-consistent core-pedestal prediction model of a combination of EPED1 type pedestal prediction and a simple stiff core transport model is able to predict Type I ELMy (edge localized mode) pedestals of a large JET-ILW (ITER-like wall) database at the similar accuracy as is obtained when the experimental global plasma β is used as input. The neutral penetration model [R. J. Groebner et al., Phys. Plasmas 9, 2134 (2002)] with corrections that take into account variations due to gas fueling and plasma triangularity is able to predict the pedestal density with an average error of 15%. The prediction of the pedestal pressure in hydrogen plasma that has higher core heat diffusivity compared to a deuterium plasma with similar heating and fueling agrees with the experiment when the isotope effect on the stability, the increased diffusivity, and outward radial shift of the pedestal are included in the prediction. However, the neutral penetration model that successfully predicts the deuterium pedestal densities fails to predict the isotope effect on the pedestal density in hydrogen plasmas.

High fusion performance in Super H-mode experiments on Alcator C-Mod and DIII-D

The ‘Super H-Mode’ regime is predicted to enable pedestal height and fusion performance substantially higher than standard H-Mode operation. This regime exists due to a bifurcation of the pedestal pressure, as a function of density, that is predicted by the EPED model to occur in strongly shaped plasmas above a critical pedestal density. Experiments on Alcator C-Mod and DIII-D have achieved access to the Super H-Mode (and Near Super H) regime, and obtained very high pedestal pressure, including the highest achieved on a tokamak (p ped ~ 80 kPa) in C-Mod experiments operating near the ITER magnetic field. DIII-D Super H experiments have demonstrated strong performance, including the highest stored energy in the present configuration of DIII-D (W ~ 2.2–3.2 MJ), while utilizing only about half of the available heating power (Pheat ~ 7–12 MW). These DIII-D experiments have obtained the highest value of peak fusion gain, QDT,equiv ~ 0.5, achieved on a medium scale (R < 2 m) tokamak. Sustained high performance operation (βN ~ 2.9, H98 ~ 1.6) has been achieved utilizing n = 3 magnetic perturbations for density and impurity control. Pedestal and global confinement has been maintained in the presence of deuterium and nitrogen gas puffing, which enables a more radiative divertor condition. A pair of simple performance metrics is developed to assess and compare regimes. Super H-Mode access is predicted for ITER and expected, based on both theoretical prediction and observed normalized performance, to allow ITER to achieve its goals (Q = 10) at Ip < 15 MA, and to potentially enable more compact, cost effective pilot plant and reactor designs.

Role of the pedestal position on the pedestal performance in AUG, JET-ILW and TCV and implications for ITER

The role of the pedestal position on the pedestal performance has been investigated in AUG, JET-ILW and TCV. When the pedestal is peeling-ballooning (PB) limited, the three machines show a similar behaviour. The outward shift of the pedestal density relative to the pedestal temperature can lead to the outward shift of the pedestal pressure which, in turns, reduces the PB stability, degrades the pedestal confinement and reduces the pedestal width. Once the experimental density position is considered, the EPED model is able to correctly predict the pedestal height. An estimate of the impact of the density position on a ITER baseline scenario shows that the maximum reduction in the pedestal height is 10% while the reduction in the fusion power is between 10% and 40% depending on the assumptions for the core transport model used.In other plasmas, where the pedestal density is shifted even more outwards relative to the pedestal temperature, the pedestal does not seem PB limited and a different behaviour is observed. The outward shift of the density is still empirically correlated with the pedestal degradation but no change in the pressure position is observed and the PB model is not able to correctly predict the pedestal height. On the other hand, the outward shift of the density leads to a significant increase of ηe and ηi (where ηe,i is the ratio of density to temperature scale lengths, ) which leads to the increase of the growth rate of microinstabilities (mainly ETG and ITG) by 50%. This suggests that, in these plasmas, the increase in the turbulent transport due to the outward shift of the density might play an important role in the decrease of the pedestal performance.

Pace making of edge localized modes with low-hybrid-wave power pulses in the EAST superconducting tokamak

Pace making of the edge-localized modes (ELMs) by low-hybrid-wave (LHW) power modulation has been observed in the EAST superconducting tokamak when the modulation frequency is sufficiently close to the natural-ELM frequency, i.e. within ±30%fELM. The highest pace-making frequency obtained so far is 120 Hz. The ELM triggering is typically delayed by 1–2 ms relative to the LHW power rising edge, during which the pedestal profiles evolve. The mechanism of the synchronized triggering of ELMs was found due to the LHW-induced pedestal density pumpout. A previous study indicates that LHW generates field-aligned helical current filaments in the scrape-off layer (SOL) (Liang et al 2013 Phys. Rev. Lett. 110 235002), which produces three- dimensional (3D) magnetic topology change at the plasma edge similar to the effect of the resonant magnetic perturbations. The observed pedestal density pumpout, local flattening of the density gradient near the separatrix and steepening near the pedestal top may be induced by the 3D effect. Pedestal linear stability analysis using ELITE code indicates that the pedestal is approaching the peeling-ballooning stability boundary as the profile is getting steep near the pedestal top until the ELM is triggered. The power threshold for the ELM pacing with 2.45 GHz LHW is ∼1 MW.

The effect of divertor closure on detachment onset in DIII-D

Abstract Experiments at DIII-D use different divertor geometries to measure the effect of divertor closure on detachment onset. We compare matched H-mode discharges in the closed (upper) divertor and the open (lower) divertor and use as a detachment onset metric the rollover in peak ion flux to the target, Jsat, as a function of upstream pedestal top density, ne,ped. Measurements of electron temperature in the lower divertor confirm that this metric captures the transition from attached to detached divertor for this data set. Experiments at two different neutral beam injection powers demonstrate an ≈ 20–40% decrease in pedestal density at detachment onset in the closed case compared to the open. High-resolution edge density and temperature measurements and power balance analysis allow us to determine the difference in upstream separatrix density ne,sep at detachment onset, which we find to be ≈ 15–25% lower in the closed divertor case. The effect of closure on detachment seen in separatrix density is hence only about 60–75% the effect seen in pedestal density, with the remainder due to differences in ne,sep/ne,ped. Future devices are expected to have a scrape off layer that is opaque to neutrals, and hence will not have the same pedestal fueling to affect ne,sep/ne,ped; this suggests that the effect of closure on detachment onset seen in future devices will be less than that seen in current experiments, and closer to the magnitude of the effect we see here in the separatrix for a similar degree of divertor closure.