Top Of Pedestal Research Articles

Nonlinear simulation of edge localized mode with pressure profile modified by pellet injection through a BOUT++ three-field MHD model

Abstract A BOUT++ three-field MHD model is extended and applied to study the effect of the pedestal pressure profile modified by the pellet injection from the low field side on the nonlinear behaviors of ELM. The linear simulation shows that the growth rates of all n modes can be enhanced regardless of the deposition location and are more affected by the pellet atoms depositing in the middle of the steep pedestal region. From the nonlinear simulation, an ELM triggering threshold for the rate of the modified pressure over the unperturbed one at the most affected position is demonstrated when the pellet deposits its atoms on the pedestal top, which supports the observation in the non-fuel pellet pacing experiments on EAST. In contrast, when the pellet deposits its atoms within the steep pedestal region, the ELM size is increased at low and high rates but decreased in an intermediate range; compared with the deposition mainly on the pedestal top, fewer perturbed atoms introduced to the steep pedestal region could cause a larger ELM size, which can somehow explain the probabilistic behavior of ELM triggering by small pellet injection on EAST.

Pedestal electron collisionality and toroidal rotation during ELM-crash suppression phase under n = 1 RMP in KSTAR

Excellent reproducibility of KSTAR resonant magnetic perturbation (RMP)-driven, edge-localized mode (ELM)-crash suppression enables us to construct a database reliably for the study of ELM-crash suppression conditions. To establish a high-fidelity database, we have selected one of the frequently used RMP configurations in KSTAR, n = 1, +90° phasing RMP-coil configuration. A series of fitting curves based on edge profile diagnostics data is used for normalized electron collisionality (νe*) and plasma toroidal rotation (Vtor) at pedestal top. Since ITER is expected to employ slowly rotating, low-collisionality, high-density plasmas whose conditions are not readily accessible in the existing devices, the exploration and understanding of these two parameters would be important not only for RMP-driven, ELM-crash-suppression physics but also for the success of ITER. The data points for the ELM-crash suppression phase are in the range of 0.2 < νe,ped*< 1.1 with Zeff = 2 assumption and Vtor,ped> 40 km/s. Suppression thresholds or boundaries in νe,ped* or Vtor,ped are not confirmed in the investigated parameter ranges so far. The KSTAR database still needs additional experimental datasets in ITER-relevant conditions (νe,ped* ∼ 0.1 and low-torque low-rotation) to confirm the boundary of the ELM-crash suppression window in KSTAR and reduce the uncertainties of the RMP ELM-crash control technique in ITER. In both νe,ped* and Vtor,ped parameters, the phase-space distribution of the ELM-crash suppression has no clear distinction from that of the ELM-crash mitigation. Linear discriminant analysis provides a linear combination of parameters relevant to ELM-crash suppression, best separating two data categories. Recursive feature elimination indicates that ne,ped and IRMP, as well as νe,ped* and Vtor,ped, are critical variables in the separation of the data groups.

Experimental identification of edge temperature ring oscillation and alternating turbulence transitions near the pedestal top for sustaining stationary I-mode

A reproducible stationary improved confinement mode (I-mode) has been achieved recently in the Experimental Advanced Superconducting Tokamak (EAST), featuring good confinement without particle transport barrier. The microscopic mechanism of sustaining stationary I-mode, based on the coupling between turbulence transitions and the edge temperature oscillation, has been discovered for the first time. A radially localized edge temperature ring oscillation (ETRO) with azimuthally symmetric structure (n = 0, m = 0) has been identified and it is accompanied by alternating turbulence transitions between an electron diamagnetic drift turbulence (ET) and an ion diamagnetic drift turbulence (IT). The transition is controlled by local electron temperature gradient and strong non-linear couplings between weak coherent mode (WCM) and ET could be identified near the pedestal top, suggesting the unique status of the pedestal top region in sustaining the stationary I-mode confinement on EAST.

The effect of resonant magnetic perturbation with different poloidal mode numbers on peeling–ballooning modes

The effect of resonant magnetic perturbation (RMP) with different poloidal mode numbers on peeling–ballooning (P–B) modes is simulated with the BOUT++ code. In order to investigate the physical mechanism of edge-localized mode mitigated by RMP, a series of RMPs with different poloidal mode numbers are applied into the four-field P–B mode simulation module separately. The results indicate that RMP has a better reducing effect on the energy loss from the pedestal when the resonant position is near the bottom and top of pedestal rather than near the middle. The RMP could influence P–B modes through the following effects: on the one hand, the E × B shearing rate is significantly stronger when the RMP (resonant surface locates at the top of the pedestal) is added, which can suppress the radial propagation of the negative pressure perturbation and reduce energy loss from the pedestal. On the other hand, the coupling of toroidal modes in the nonlinear phase is enhanced when RMPs are added, which motivated the growth of multiple toroidal modes, and the turbulence fluctuation level is weaker with the RMP when the resonant surface is located at the bottom of the pedestal.

Mixed hydrogen-deuterium plasmas on JET ILW

A study of mixed hydrogen-deuterium H-mode plasmas has been carried out in JET-ILW to strengthen the physics basis for extrapolations to JET D-T operation and to support the development of strategies for isotope ratio control in future experiments.Variations of input power, gas fuelling and isotopic mixture were performed in H-mode plasmas of the same magnetic field, plasma current and divertor configuration. The analysis of the energy confinement as a function of isotope mixture reveals that the biggest change is seen in plasmas with small fractions of H or D, in particular when including pure isotope plasmas. To interpret the results correctly, the dependence of the power threshold for access to type-I ELMing H-modes on the isotope mixture must be taken into account. For plasmas with effective mass between 1.2 and 1.8 the plasma thermal stored energy () scales as , which is weaker than that in the ITER physics basis, IPB98 scaling.At fixed stored energy, deuterium-rich plasmas feature higher density pedestals, while the temperature at the pedestal top is lower, showing that at the same gas fuelling rate and power level, the pedestal pressure remains constant with an exchange of density and temperature as the isotope ratio is varied. Isotope control was successfully tested in JET-ILW by changing the isotope ratio throughout a discharge, switching from D to H gas puffing. Several energy confinement times (300 ms) are needed to fully change the isotope ratio during a discharge.

Expanding the parameter space of the wide-pedestal QH-mode towards ITER conditions

Significant parameter space expansion of the wide-pedestal quiescent H-mode (QH-mode) towards more ITER and reactor relevant conditions has been achieved in the DIII-D tokamak. Recent experiments reveal this naturally ELM-free regime can operate over a range of neutral beam injected (NBI) torque spanning from the co- to counter-Ip direction. The accessed torques completely cover the scaled ITER-equivalent applied torque range. Wide-pedestal QH-mode features an edge region where the ExB shear is lower than that of standard QH-mode and H-mode, as well as unique low- to intermediate-k turbulence and a flattening of the pedestal profiles in the low ExB shear region. Wide-pedestal QH-mode has also been extended to lower single null shaping matching ITER average triangularity. A NBI power scan at moderate density and net-zero NBI torque shows no upper power limit to wide-pedestal QH-mode while the confinement quality H98y2 increases with power. At the highest power, H98y2 reaches 1.6 and normalized pressure βN reaches 2.3 simultaneously. The wide-pedestal QH-mode also exhibits improved confinement when (1) transitioning from standard QH-mode, (2) adding core ECH power, and (3) by transitioning from LSN to DN plasma shape. Turbulence suppression by increased ExB shear inboard of the pedestal top plays an important role in all three of these observations. These recent advances make wide-pedestal QH-mode a stronger candidate for an ELM-free, high performance regime for ITER and future reactors.

Coherent modes and turbulences observations with multi-channel Doppler reflectometer on experimental advanced superconducting tokamak

With a newly installed 5-channel W-band Doppler reflectometer (DR) in EAST tokamak, many interesting coherence modes and turbulences have been observed. In this paper, the tearing mode (TM), a novel inter-edge localized mode (ELM) electromagnetic mode, and the eigenmode geodesic acoustic mode (GAM) accompanied by a broadband quasi-coherent mode (QCM) during H-mode stage have been reported. The TM could be directly observed in both density fluctuation and perpendicular velocity fluctuation at the DR channel localized in the magnetic island region; moreover, many high-order TM frequency harmonics, dual turbulence peaks inside and outside the island separatrix, and strong flow shear across the island separatrix could also be observed. A high-frequency (∼230 kHz) electromagnetic mode (toroidal mode number n = 1) could be identified. The mode is generated dozens of milliseconds before the ELM crash and may affect the ELM dynamic process. The eigenmode GAM excited during the H-mode stage is generated near the top of the density pedestal and then propagates radially inward with a radial wavenumber of about 1 cm−1. A broadband QCM with a frequency range of 20–100 kHz is always accompanied by eigenmode GAM and nonlinearly coupled with both GAM and the background turbulence from the bispectrum analysis. All these results convince the validity of the Doppler reflectometer in multi-scale measurement from the mesoscale to microscale.

Stationary high-performance grassy ELM regime in EAST

A stationary, high-performance grassy edge-localized mode (ELM) regime has been successfully accessed on the EAST tokamak since the 2016 campaign. increase via increase of heating power or q95 are both found to facilitate a higher grassy ELM frequency. Edge measurement of absolute extreme ultraviolet radiation indicates that the affected area by grassy ELMs is localized at the pedestal region and the perturbations there induced by grassy ELMs can be 90% smaller than that by type-I ELMs. Parameter scan demonstrates that the grassy ELM regime has good density control capacity, and the access to the grassy ELM regime is independent of the toroidal field direction and low-hybrid-wave power. Statistical analysis indicates that the grassy ELM regime is highly reproducible in a wide parameter space when edge safety factor and poloidal beta are simultaneously high enough ( and ). High triangularity contributes to the increase of grassy ELM frequency while the requirement on high internal inductance in the JET tokamak for grassy ELMs cannot be found in EAST. The operational space in pedestal top collisionality for the EAST grassy ELMs is in the range of . The exploration towards lower is mainly limited by the available heating power. Evolution of a coherent mode at pedestal top suggests that grassy ELMs most likely generate in the steep-gradient region of the pedestal, rather than on the pedestal top or near the separatrix. The grassy ELM regime offers a highly promising approach for further exploration of long-pulse high-performance H-mode operations in EAST, with potential application to the Chinese Fusion Engineering Test Reactor (CFETR) as the baseline scenario and a primary solution for the control of ELMs.

Real-time pedestal optimization and ELM control with 3D fields and gas flows on DIII-D

The capabilities of the DIII-D tokamak’s plasma control system (PCS) were expanded to allow for pedestal optimization and edge localized mode (ELM) control. Three proof of principle control schemes are presented that were successfully implemented and tested. These use multiple inputs from real-time (RT) diagnostics like a based ELM monitor and edge profile measurements from Thomson scattering (TS) as well as 3D, i.e. non-axisymmetric, magnetic fields and gas puffs as actuators to regulate the density pedestal.The first scheme targets to optimize the access of ELM suppression induced by non-axisymmetric magnetic perturbations (MPs). The conducted set of experiments identifies a path dependence of plasma confinement on the applied MP amplitude. The controller aims to transition into ELM suppression at the minimum 3D field amplitude and reduces it further afterwards, allowing for partial confinement recovery. Another pedestal control scheme is deployed to compensate the density ‘pump-out’ in MP ELM suppression by regulating the gas puff. This uses RT TS diagnostic data, extracting the pedestal height from the electron density () profiles and enables studies of the transition into and out of MP ELM suppression at constant density. A limit cycle behavior of edge rotation and MP amplitude persists under these conditions. The third control scheme combines MPs and gas puffs as actuators to perform pedestal density trajectory control to access Super high confinement mode (H-mode) and furthermore, allowing the integration of a radiative divertor in this regime. While MPs mainly impact the pedestal top density, the control scheme allows to loosen the tight coupling of pedestal top and separatrix density evolution.With respect to ITER, the achieved results emphasize the need for an advanced control system to keep MP amplitude close to but above the ELM suppression threshold at all times, enabling high confinement and, respectively, at high fusion energy gain factor (Q). Furthermore, pedestal control enables detailed physics studies in present-day tokamaks and allows the exploration of core-edge integrated plasma scenarios.

The role of edge resonant magnetic perturbations in edge-localized-mode suppression and density pump-out in low-collisionality DIII-D plasmas

Two-fluid nonlinear MHD simulations using the TM1 code demonstrate that the formation of magnetic islands at the top and bottom of the H-mode pedestal, together with the strong screening of resonant fields in the gradient region of the pedestal, can account for ELM suppression and density pump-out by n = 2 resonant magnetic perturbations (RMPs) in low-collisionality DIII-D ITER similar shape (ISS) plasmas. Using experimentally relevant transport coefficients, neoclassical resistivity, electron collisionality, and RMP amplitudes, nonlinear MHD simulations reproduce the observed level of density reduction (density pump-out) in DIII-D due to the formation of narrow magnetic islands in the resistive foot of pedestal. For large amplitude RMPs (Br/Bt > 1×10−4) simulations predict resonant field penetration and resulting in pressure reduction at the top of the pedestal, consistent with experimental observations at the onset of ELM suppression. The predicted reduction in the height and width of the pedestal by magnetic island enhanced collisional transport provides a quantitative mechanism for the stabilization of the peeling-ballooning mode (PBM). Importantly, these simulations predict strong screening of resonant fields in the steep gradient region of the pedestal due to strong E×B rotation and diamagnetic flows. However, if the plasma resistivity is made artificially larger (∼ 10X) than neoclassical, the simulations predict magnetic stochasticity throughout the plasma edge and the collapse of the pedestal due to the reduction in the penetration threshold with increasing resistivity. A scaling relation for resonant field penetration at the pedestal top, using several hundred nonlinear simulations, reproduces the density and E×B rotation dependence of the ELM suppression threshold observed in DIII-D. Simulations using ITER model equilibria suggest that the penetration threshold at the top of the ITER pedestal will be lower than in DIII-D due to the anticipated lower perpendicular flow velocities in ITER, resulting in the weaker screening of resonant fields.

The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas

Exhaust power components due to ELMs, radiation and heat transport across the edge transport barrier (ETB) between ELMs are quantifed for H-mode plasmas in JET-C and JET-ILW for comparison with simulations of pedestal heat transport. In low-current, JET-ILW pulses with a low rate of gas fuelling, the pedestal heat transport is found not to be stiff, i.e. the effective, mean heat diffusivity does not increase with the electron temperature gradient across the pedestal and the parameter increases with the conducted loss power across the pedestal, with the latter saturating at mean values . This increase in pedestal temperature gradient is partly due to a relative reduction of the ion neo-classical heat transport (which is more significant at low plasma current) with decreasing collisionality at higher power. In JET-ILW pulses, significantly more power is required at a high gas puffing rate to achieve a similar pedestal pressure and normalised confinement to that in otherwise similar JET-C pulses without gas-puffing. The increased heat transport across the JET-ILW pedestals is caused by changes to the pedestal structure induced by the gas puffing, which is required to mitigate contamination by W impurities sputtered from the target plates. In high-power JET-ILW pulses, the radiated power is dominated by that from W, which exhibits a highly asymmetric poloidal distribution due to toroidal rotation. During the ELMy H-mode phase, the W is concentrated in the outer ‘mantle’ region () inside the pedestal top by a favourable alignment of profile gradients, where it can be effectively flushed by ELMs. Transport analysis reveals that the strong mantle radiation cools the outer region of the plasma, causing more of the heat to be lost through the electron channel. However, direct cooling by W radiation from the ETB region is shown to be insignificant compared to the power conducted through the pedestal.

Achieving a robust grassy-ELM operation regime in CFETR

We have identified a robust grassy-edge localized mode (ELM) operation regime for future tokamak reactors. The regime exists within a pedestal top electron collisionality (ν*) window at high global poloidal beta (βp). The existence of an upper ν* limit for grassy-ELMs is consistent with results previously reported in experiments (Oyama et al 2010 Nucl. Fusion 50 064014), while the existence of a lower ν* limit has not been reported previously. Using EPED and BOUT + +, a theoretical model that quantitatively explains the physics of the grassy-ELMs within the window, which distinguishes them from the small mixed-ELMs at lower ν*, is presented for the first time. A peeling-ballooning stability boundary is obtained by scanning the operating density space. The change in density corresponds to a change in ν* that affects the pedestal bootstrap current. High βp leads to a strong Shafranov shift, which affects the flux surface averaged pressure drive. The two effects combine to create a peeling-dominated window in intermediate ν* buffered by ballooning-dominated regimes. Only the peeling-dominated regime shows a cyclic behavior in the perturbed pressure during the nonlinear simulation of an ELM crash, reminiscent of grassy-ELM dynamics. Similarly, the energy released across the separatrix is demonstrated to be significantly smaller. The quick recovery of the ELM crash is explainable by the rapid rise of a low n kink-peeling instability when the pedestal current Iped exceeds a threshold at high βp. It minimizes the excursion beyond marginal stability and is absent in the ballooning-dominated regime. Comparison with recent experiments over a range of βp and ν* strongly supports the physical picture proposed by the modeling.

Setting the H-mode pedestal structure: variations of particle source location using gas puff and pellet fueling

Experiments in DIII-D show that a particle source location inside the top of the H-mode pedestal (pellets) maintains a higher pedestal pressure than an edge source (gas fueling) through a widening of the electron temperature pedestal with reduction of the temperature gradient. The effect of these two fueling schemes on the H-mode pedestal structure was examined in DIII-D by comparing controlled pellet-fueled and gas-fueled discharges across a fueling scan up to 40 torr l s−1. High resolution electron profiles reveal that gas fueling lowers the pedestal pressure as the density profile shifts radially outwards and the separatrix density increases, while pellet fueling maintains a constant pedestal pressure. The neutral source locations from pellets and gas are determined with the PELLET and UEDGE codes, respectively, and quantify the particle source localization. Pellets provide significant ionization inside the pedestal top while gas puffing localizes ionization in the scrape-off-layer and pedestal foot, broadly consistent with the density profile structure influenced by the source. ELMs are observed to increase in frequency and reduce impurity content as fueling is increased. Stability analysis with ELITE shows that both conditions are near the type-I ELM corner of the peeling–ballooning stability diagram, which is altered significantly by the introduction of pellets. Since transport mechanisms are not observed to change substantially with particle source location, wider pedestals allow the pellet-fueled discharges to retain higher pedestal temperatures at similar pedestal densities. EPED1 is tested to capture the pedestal pressure, under-predicting the height with pellets and over-predicting the height with gas by ∼15%. These results have important implications for future reactors where pellet fueling will be the primary particle source due to an opaque scrape-off-layer by showing that the neutral source location plays a role in setting the structure of the H-mode pedestal.

Dynamics of the pedestal transport during edge localized mode cycles at ASDEX Upgrade

The dynamic behaviour of the ion and electron energy, particle and momentum transport measured during type-I edge localized mode (ELM) cycles at ASDEX Upgrade is presented. Fast measurements of the ion and electron temperature profiles revelead that the ion and electron energy transport recover on different timescales, with the electrons recovering on a slower timescale (Cavedon et al 2017 Plasma Phys. Control. Fusion 59 105007). The dominant mechanism for the additional energy transport in the electron channel that could cause the delay in the electron temperature gradient () recovery is attributed to the depletion of energy caused by the ELM. The local sources and sinks for the electron channel in the steep gradient region are much smaller compared to the energy flux arriving from the pedestal top, indicating that the core plasma may dictate the local dynamics of the recovery during the ELM cycle. A model for the edge momentum transport based on toroidal torque balance that takes into account the existence of poloidal impurity asymmetries has been developed. The analysis of the profile evolution during the ELM cycle shows that the model captures the dynamics of the rotation both before the ELM crash and during the recovery phase.

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 < 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.

A new criterion for controlling edge localized modes based on a multi-mode plasma response

A new technique for multi-mode plasma response extraction is presented, as well as a new criterion to identify modes associated with edge localized mode (ELM) control. This technique extracts modes by separating the full poloidal cross-section structure from the phase difference between upper and lower resonant magnetic perturbation (RMP) coils . The dependence of extracted modes are compared to the effect of ELM control using both n = 2 and n = 3 RMP in wide q95 operating regimes. It shows that the mode with highest resonant plasma response near the pedestal top is correlated with ELM mitigation or suppression, even if the mode is not the dominant one in plasma response.

Unified modeling of both resonant and non-resonant neoclassical transport under non-axisymmetric magnetic perturbations in tokamaks

A numerical model for neoclassical transport under nonaxisymmetric magnetic perturbations in low collisionality plasmas in tokamaks is developed. To take into account bounce-drift resonances and magnetic drift effects, a Fourier decomposition of the drift kinetic equation in new coordinates, rather than bounce average of it, is employed. A pitch angle scattering collisional operator is used to keep accuracy in the nonresonant regimes or resonant plateau regimes with resonant pitch near pitch space boundaries. Full toroidal geometry effects are also included to increase the accuracy in the modeling of neoclassical transport in the resonant regimes. Neoclassical transport in the most important collisionless regimes, including resonant super-banana plateau and bounce-drift resonances regimes, nonresonant 1/ν and ν−ν regimes, and the transitions between them, can be modeled simultaneously in this model by numerically solving the drift kinetic equation. By application to the neoclassical toroidal viscosity modeling in one discharge in the EAST tokamak, it is found that the bounce-drift resonances dominate the contributions near the plasma core where the plasma E→×B→ drift frequency is close to the bounce frequency, while the precessional resonance dominates the contribution near the edge pedestal top where the E→×B→ drift frequency is close to zero.

Promising High-Confinement Regime for Steady-State Fusion.

A reproducible stationary high-confinement regime with small "edge-localized modes" (ELMs) has been achieved recently in the Experimental Advanced Superconducting Tokamak, which has a metal wall and low plasma rotation as projected for a fusion reactor. We have uncovered that this small ELM regime is enabled by a wide edge transport barrier (pedestal) with a low density gradient and a high density ratio between the pedestal foot and top. Nonlinear simulations reveal, for the first time, that the underlying mechanism for the observed small ELM crashes is the upper movement of the peeling boundary induced by an initial radially localized collapse in the pedestal, which stops the growth of instabilities and further collapse of the pedestal, thus providing a physics basis for mitigating ELMs in future steady-state fusion reactors.

High frequency edge coherent modes studied with the ultra-fast swept reflectometer on ASDEX Upgrade

The results on high frequency coherent modes developing at the plasma edge after L–H transitions are presented. These edge coherent modes (ECMs) observed in the I-phase, during ELM-free and ELMy H-mode phases have common characteristics, such as their frequency of 50–200 kHz, radial position and scaling of the frequency with the pressure gradient. The ultra-fast swept reflectometer with the sweep time of 1 μs has provided measurements of the mode dynamics and localisation. The ECMs propagate in the electron diamagnetic direction in the laboratory frame and are localized in the region between the pedestal top and the steepest pressure gradient.