type-III Edge Localized Modes Research Articles

First access to ELM-free negative triangularity at low aspect ratio

Abstract A plasma scenario with negative triangularity (NT) shaping is achieved on MAST-U for the first time. While edge localized modes (ELMs) are eventually suppressed as the triangularity is decreased below δ ≲ − 0.06 , an extended period of H-mode operation with Type-III ELMs is sustained at less negative δ even through access to the second stability region for ideal ballooning modes is closed. This documents a qualitative difference from the ELM-free access conditions documented in NT scenarios on conventional aspect ratio machines. The electron temperature at the pedestal top drops across the transition to ELM-free operation, but a steady rise in core temperature as δ is decreased allows for similar normalized β in the ELM-free NT and H-mode positive triangularity shapes.

Edge plasma relaxations due to diamagnetic stabilization

A new mechanism for pressure profile relaxations in an edge tokamak plasma is derived from simulations within the two-fluid three-dimensional turbulence code EMEDGE3D. The relaxation is due to diamagnetic coupling in the resistive ballooning/drift wave dynamics: Unstable modes experience explosive growth at high pressure gradients after a phase in which they are stabilized by the diamagnetic coupling leading to the onset of a transport barrier. The sheared E × B flow does not play any significant role. After relaxation, the transport barrier forms again and it sets the conditions for a new relaxation event, resulting in an oscillatory behavior. We find that energy flux into the scrape-off layer decreases with the increasing oscillation frequency and that the oscillations are tamed by increasing plasma temperature. This behavior is reminiscent of the so-called type-III edge localized modes. A one-dimensional model reproducing the relaxations is also derived.

High resolution density pedestal measurements during edge localized modes by short-pulse reflectometry in the TCV tokamak

This publication presents high spatio-temporal resolution (mm µs−1) density profile measurements of the pedestal top during type I, III, and small edge localized mode (ELM) H-mode plasmas in the tokamak à configuration variable (TCV). These measurements were performed using a novel short-pulse reflectometer. Average inter-ELM density profiles are obtained via conditional averaging using the D trace as ELM indicator. Changes to the pedestal density profile gradients prior to type-III ELMs reveal unique pedestal dynamics leading to the ELM crash which can provide important experimental data for validation of non-linear MHD ELM simulations. The small-ELM scenario is found to feature a ∼25–35 kHz quasi-coherent density fluctuation near the separatrix 0.993–1.05 not observed during a similar type-I ELM discharge. This oscillation is also found in low-field-side magnetic pick-up probes displaying a ballooning character and n = +1 toroidal mode number. This oscillation could help explain the markedly different pedestal dynamics observed in the small-ELM regime.

Formation of radially elongated flow leading to onset of type-III edge localized modes in toroidal plasmas

The dynamic features of triggering of type-III edge localized modes (ELMs) by macro-scale radially elongated flow (REF) in magnetically confined toroidal plasmas are presented. The REF, localized near the mid-plane and with a long radial correlation length at the low field side, was observed prior to the collapse of pedestal in series. It is demonstrated that the REF gets energy from and modulates ambient turbulence via nonlinear three wave interaction. The formation of REF leads to the onset of type-III ELMs, where the collapse of plasma energy occurs in the outer region ~0.3 of the plasma column within a few tens of microseconds without global MHD instabilities. The path of the onset is identified and a clue to the trigger mechanism of violent events in high temperature plasmas is discussed.

Study of core plasma rotation characteristics of RF-heated H-mode discharges on experimental advanced superconducting tokamak

RF-heated H-mode plasmas are readily achieved with lower hybrid current drive and ICRF heating on experimental advanced superconducting tokamak (EAST). Characteristics of H-mode plasma rotation are studied, including the behaviors for non-stationary and stationary H-mode discharges. Experimental results indicate that substantial co-current core rotation increment is observed at L-H transition. For non-stationary discharges with multiple L-H transitions, central plasma rotation varies as the plasma enters and exits the H-mode phase. Rotation increase over L-H transition is linearly correlated with plasma stored energy for both edge localized mode (ELM)-free phases and phases with type-III ELMs. For stationary H-mode discharges with type-III ELMs, core plasma rotation profile is elevated and remains stable during the H-mode phase, although the occurrence of ELMs tends to slow down the core rotation, especially for type-I ELMs where the entire core profiles are affected. Evolution of plasma rotation is fitted with a source-free transport equation and it is found that the momentum transport is dominated by diffusion and explains the flat profile in the core. Based on the Rice scaling and for the same stored energy increase, smaller increase in the core rotation is observed for H-mode discharges with type-III ELMs than for ELM-free discharges. A linear fit indicates that the slope is 75% larger for the ELM-free discharges data.

Parametric Dependence of Type-I and Type-III ELMs and Dynamic Characteristics for ELM Filaments in EAST Tokamak

A simple and effective classification method to distinguish type-I and type-III edge-localized modes (ELMs) in an experimental advanced superconducting tokamak is introduced and some key parameters affecting the type-I and type-III ELMy H-modes are analyzed. The results indicate that the heating power and the edge safety factor q95 have strong correlations with the appearance of type-I and type-III ELMs. By using the high-speed visible-light charge-coupled device camera system, the dynamic characteristics of ELM filaments are studied and the features of type-I and type-III ELM filaments have been compared. The radial outward velocity for type-I ELMy filaments can be up to 1.5-3.5 km/s while for type-III ELMs one is about 0.5-2 km/s. In the process of type-I and type-III ELMy filaments outward propagation, the width of filament decreases gradually, while the velocity of filament increases. In addition, during an ELM event, it can be observed that the earlier the burst of an ELM filament, the higher the radial velocity it will have.

The observation of field-aligned current during type-III ELM in EAST

ABSTRACTMeasurements of current-carrying filaments associated with Type-III edge localized mode (ELM) have recently been made in the Experimental Advanced Superconducting Tokamak by direct probing of the edge plasma using an advanced, fast-moving electrostatic and magnetic probe system. Contrary to expectations, no current filaments were detected near the separatrix. However, a clear signature of current filament has been observed in the far scrape-off layer (SOL) where the difference of the voltage between the divertor plates connecting a filament is sufficiently large, thus strongly suggesting that the current-carrying filaments form in the SOL, rather than being ejected from the plasma inside the separatrix. These findings provide, for the first time, information on the formation and sustainment of current filaments during type-III ELMy H-modes.

Plasma turbulence measured with fast frequency swept reflectometry in JET H-mode plasmas

In this work we present recent achievements to provide precise measurements of turbulence on JET H-mode plasmas using frequency sweeping reflectometry diagnostic. The plasma density fluctuations retrieved from swept reflected signals, first initiated with the Tore Supra reflectometry (Heuraux et al 2003 Rev. Sci. Instrum. 74 1501, Vermare et al 2006 Nucl. Fusion 46 S743, Gerbaud et al 2006 Rev. Sci. Instrum. 77 10E928), provides a radial profile of the density fluctuation level and its spectral structure. Using the complete set of the JET X-mode fast sweeping heterodyne reflectometers we have determined the temporal dynamic of the density fluctuation profile from the edge to the center during an H-mode discharge. At the L-H transition, the turbulence reduction seems to occur, at first, simultaneously from the edge to the center then deepens at the edge at ρ ~ 0.95 and this deepening propagates toward the center with a steepening of the wavenumber spectra. During an edge localized mode (ELM) event, a substantial density fluctuations increase has been observed with a localized turbulent wave front propagating toward the center accompanying a particle transport. We also show that type-III ELMs sustain a steady and high level of plasma turbulence compare to type-I.

Ginzburg‐Landau Model in a Finite Shear‐Layer and Onset of Transport Barrier Nonlinear Oscillations: A Paradigm for TypeIII ELMs

AbstractWe study a Reaction‐Diffusion model describing the nonlinear oscillations of a transport barrier in a finite shear‐layer (width dE ≪ a), where a is the plasma minor radius, based on a 1D reduced model derived to explain nonlinear barrier oscillations in 3D turbulence simulations [P. Beyer, S. Benkadda, G. Fuhr‐Chaudier et al., Phys. Rev. Lett. 94, 105001 (2005)]. We show that this single nonlinear equation encompasses most of the physics of these barrier relaxations. The nonlinear oscillations have common characteristics with type‐III edge localized modes (ELMs), such as a repetition frequency which decreases with increasing power. In addition to the flow shear, the shear‐layer width is also shown to control the nature of the oscillations. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In–out asymmetry of divertor particle flux in H-mode with edge localized modes on EAST

The in–out divertor asymmetry in the Experimental Advanced Superconducting Tokamak (EAST), as manifested by particle fluxes measured by the divertor triple Langmuir probe arrays, is significantly enhanced during type-I edge localized modes (ELMs), favoring the inner divertor in lower single null (LSN) for the normal toroidal field (Bt) direction, i.e. with the ion B × B direction towards the lower X-point, while the in–out asymmetry is reversed when the ion B × B is directed away from the lower X-point. The plasma flow measured by the Mach probe at the outer midplane is in the ion Pfirsch–Schlüter (PS) flow direction, opposite to both B × B and E × B drifts, i.e. towards the inner divertor for normal Bt, and the outer divertor for reverse Bt, consistent with the observed in–out divertor asymmetry in particle fluxes. Since the particle source from an ELM event is predominantly located near the outer midplane, this new finding suggests a critical role of the PS flow in driving the in–out divertor asymmetry. The divertor asymmetry during type-III ELMs exhibits a similar trend to that during type-I ELMs. Strong in–out divertor asymmetry is also present during inter-ELM and ELM-free phases for the normal field direction, i.e. with more particle flux to the lower inner divertor target, but the peak particle flux merely becomes more symmetric, or slightly reversed, for reverse Bt, i.e. reversed B × B drift direction.

Evolution of E × B shear and coherent fluctuations prior to H-L transitions in DIII-D and control strategies for H-L transitions

While operating a magnetic fusion device in H-mode has many advantages, care must be taken to understand and control the release of energy during the H-L back transition, as the extra energy stored within the H-mode transport barrier will have the potential to cause damage to material components of a large future tokamak such as ITER. Examining a scenario where the H-L back transition sequence begins before the E × B shearing layer decays on its own, we identify a long-lived precursor mode that is tied to the events of the H-L sequence and we develop a robust control strategy for ensuring gradual release of energy during the transition sequence. Back transitions in this scenario commonly begin with a rapid relaxation of the pedestal, which was previously shown to be inconsistent with ideal peeling-ballooning instability as the trigger [Eldon et al., Phys. Plasmas 22, 052109 (2015)], despite being otherwise similar to a large type-I Edge Localized Mode (ELM). This so-called transient occurs when the E × B shearing rate ωE×B is significantly larger than the turbulence decorrelation rate ωT, indicating that this is not the result of runaway turbulence recovery. The transient is always synchronous with amplitude and propagation velocity modulations of the precursor mode, which has been dubbed the Modulating Pedestal Mode (MPM). The MPM is a coherent density fluctuation, which, in our scenario at least, reliably appears in the steep gradient region with f≈70 kHz, kθ≈0.3 cm−1, and it exists for ≳100 ms before the onset of back transitions. The transient may be reliably eliminated by reducing toroidal rotation in the co-current direction by the application of torque from counter-injecting neutral beams. The transient in these “soft” H-L transitions is then replaced by a small type-III ELM, which is also always synchronous with the MPM, and MPM shows the same behavior in both hard and soft cases.

Scaling of divertor power footprint width in RF-heated type-III ELMy H-mode on the EAST superconducting tokamak

Dedicated experiments for the scaling of divertor power footprint width have been performed in the ITER-relevant radio-frequency (RF)-heated H-mode scheme under the lower single null, double null and upper single null divertor configurations in the Experimental Advanced Superconducting Tokamak (EAST) under lithium wall coating conditioning. A strong inverse scaling of the edge localized mode (ELM)-averaged power fall-off width with the plasma current (equivalently the poloidal field) has been demonstrated for the attached type-III ELMy H-mode as by various heat flux diagnostics including the divertor Langmuir probes (LPs), infra-red (IR) thermograph and reciprocating LPs on the low-field side. The IR camera and divertor LP measurements show that , in good agreement with the multi-machine scaling trend during the inter-ELM phase between type-I ELMs or ELM-free enhanced Dα (EDA). H-mode. However, the magnitude is nearly doubled, which may be attributed to the different operation scenarios or heating schemes in EAST, i.e., dominated by electron heating. It is also shown that the type-III ELMs only broaden the power fall-off width slightly, and the ELM-averaged width is representative for the inter-ELM period. Furthermore, the inverse Ip (Bp) scaling appears to be independent of the divertor configurations in EAST. The divertor power footprint integral width, fall-off width and dissipation width derived from EAST IR camera measurements follow the relation, λint ≅ λq + 1.64S, yielding . Detailed analysis of these three characteristic widths was carried out to shed more light on their extrapolation to ITER.

Langmuir-magnetic probe measurements of ELMs and dithering cycles in the EAST tokamak

Measurements of the dynamical behavior associated with edge localized modes (ELMs) have been carried out in the Experimental Advanced Superconducting Tokamak (EAST) by direct probing near the separatrix and far scrape-off layer (SOL) using electrostatic as well as magnetic probes. Type-III ELMs and dithering cycles have been investigated near the threshold power for the transition from the low confinement mode (L-mode) to the high confinement mode (H-mode). A precursor is observed prior to type-III ELM events with chirping frequency (130–70 kHz). It is located inside the separatrix and does not lead to considerable particle transport into the SOL. Distinct from type-III ELMs, no precursor modes precede the dithering cycles. It is evident from our measurements that the absence of precursor activity is a good indicator to distinguish the dithering cycles from type-III ELMs. A number of distinct current filaments are identified slightly inside the separatrix, both during type-III ELM events and dithering cycles. The characteristic current topology in these filaments is still ambiguous in our investigations. Furthermore, small ELMs are observed in type-I ELMy-like H-mode discharge regimes on EAST, in which solitary monopolar current filaments are observed to propagate in the SOL.

Effects of edge-localized mode-induced neoclassical toroidal viscosity torque on the toroidal intrinsic rotation in the EAST tokamak

Intrinsic rotation has been observed in lower hybrid current-driven (LHCD) H-mode plasmas with type-III edge-localized modes (ELMs) on Experimental Advanced Superconducting Tokamak (EAST), and it is found that the edge toroidal rotation accelerated before the onset of the ELM burst. Magnetic perturbation analysis shows there is a perturbation amplitude growth below 30 kHz corresponding to the edge rotation acceleration. Using the filament model, the neoclassical toroidal viscosity (NTV) code shows there is a co-current NTV torque at the edge, which may be responsible for the edge rotation acceleration. For maximum displacement ∼1 cm and toroidal mode number n=15, the calculated torque density is ∼0.44 N/m2, comparable with the average edge toroidal angular momentum change rate ∼1.24 N/m2. Here, the 1 cm maximum magnetic surface displacement estimated from the experimental observation corresponds to a maximum magnetic perturbation ∼ 10−3–10−2 T, in accordance with magnetic perturbation measurements during ELMs. By varying n from 10 to 20, the magnitude of the edge NTV torque density is mainly ∼0.1–1 N/m2. This significant co-current torque indicates that the NTV theory may be important in rotation problems during ELMs in H-mode plasmas. To better illuminate the problem, magnetic surface deformation obtained from other codes is desired for a more accurate calculation.

Characterizations of power loads on divertor targets for type-I, compound and small ELMs in the EAST superconducting tokamak

The Experimental Advanced Superconducting Tokamak (EAST) has recently achieved a variety of H-mode regimes with different edge-localized mode (ELM) dynamics, including type-I ELMs, compound ELMs, which are manifested by the onset of a large spike followed by a sequence of small spikes on Dα emissions, usual type-III ELMs, and very small ELMs. This newly observed very small ELMy H-mode appears to be similar to the type-II ELMy H-mode, with higher repetition frequency (∼1 kHz) and lower amplitude than the type-III ELMy H-mode, exhibiting an intermediate confinement level between type-I and type-III ELMy H-modes. The energy loss and divertor power load are systematically characterized for these different ELMy H-modes to provide a physics basis for the next-step high-power long-pulse operations in EAST. Both type-I and compound ELMs exhibit good confinement (H98(y,2) ∼ 1). A significant loss of the plasma stored energy occurs at the onset of type-I ELMs (∼8%) and compound ELMs (∼5%), while no noticeable change in the plasma stored energy is observed for the small ELMs, including both type-III ELMs and very small ELMs. The peak heat flux on divertor targets for type-I ELMs currently achieved in EAST is about 10 MW m−2, as determined from the divertor-embedded triple Langmuir probe system with high time resolution. As expected, type-III ELMs lead to much smaller divertor power loads with a peak heat flux of about 2 MW m−2. Peak power loads for compound ELMs are between those for type-I and type-III ELMs. It is remarkable that the new very small ELMy H-modes exhibit even lower target power deposition than type-III ELMs, with the peak heat flux generally below 1 MW m−2. These very small ELMs are usually accompanied by broadband fluctuations with frequencies ranging from 20 to 50 kHz, which may promote particle and power exhaust throughout the very small ELMy H-mode regime.

Temperature Distribution and Heat Flux on the EAST Divertor Targets in H-Mode

An infrared camera (IR) has been put into operation in the Experimental Advanced Superconducting Tokamak (EAST), which is used to measure the temperature distribution on the surface of lower divertor target plates. With a finite difference method, the heat flux onto the divertor target plates is calculated from the surface temperature profile. The high confinement mode (H-mode) with type-III edge localized modes (ELMs) has been obtained with about 1 MW lower-hybrid wave power on the EAST in the autumn experiment in 2010. The analyzed H-mode discharges were lower single null X-point diverted discharges with a density range of < ne > (1 ∼ 4) × 1019 m−3. The surface temperature of the inner target plate increases with heating power. The peak temperature on the surface of target plates is lower than 200°C with about 2.4 MW heating power. Comparison among the heat flux profiles occurring in different phases in the same discharge has been performed. It indicates that the heat flux profile obviously changes from the ohmic phase to the H-mode phase, and the full width at half maximum (FWHM) of the heat flux profile is the narrowest during the ELM-free H-phase. On the outer target plate, the peak heat flux exceeds 2 MW/m2 during the ELMy H-mode phase, whereas it is only about 0.8 MW/m2 during the ELM-free phase in the same discharge.

First results from H-mode plasmas generated by ICRF heating in the EAST

Deuterium high-confinement (H-mode) plasmas, lasting up to 3.45 s, have been generated in the EAST by ion cyclotron range of frequency (ICRF) heating. H-mode access was achieved by coating the molybdenum-tiled first wall with lithium to reduce the hydrogen recycling from the wall. H-mode plasmas with plasma currents between 0.4 and 0.6 MA and axial toroidal magnetic fields between 1.85 and 1.95 T were generated by 27 MHz ICRF heating of deuterium plasma with hydrogen minority. The ICRF input power required to access the H-mode was 1.6–1.8 MW. The line-averaged density was in the range (1.83–2.3) × 1019 m−3. 200–500 Hz type-III edge localized mode activity was observed during the H-mode phase. The H-mode confinement factor, H98IPB(y, 2), was ∼0.7.

Radiation distribution and energy loss during ELM in EAST

The radiation distribution and radiated energy loss during ELM (Edge Localized Mode) are preliminarily investigated in the Experimental Advanced Superconducting Tokamak (EAST). The radiated power is measured by fast bolometer which is based on compact, multichannel photodiodes array with fast response time of ∼μs. Most of the ELMs are type-III ELMs for the present H-mode discharges. It is found that only a small part of the ELM-induced energy loss is dissipated by radiation. Radiation distribution in divertor region during ELM is compared with that of between ELMs. The radiation profiles are always peaked near the X-point. Power balance is also presented for the ELMy H-mode discharge.

Radiative type-III ELMy H-mode in all-tungsten ASDEX Upgrade

The type-III ELMy H-mode might be the solution for an integrated ITER operation scenario fulfilling the fusion power amplification factor (output fusion power to input heating power) of Q = 10 with simultaneous acceptable steady-state and transient power loads to the plasma-facing components. This highly radiative type-III ELMy H-mode is achieved by nitrogen seeding. Experiments on the tokamak ASDEX Upgrade, in which all plasma-facing components are coated with tungsten, showed favourable confinement conditions (H98(y,2) = 1) at high plasma pressure (βN = 2.4). The power load to the plasma-facing components is as low as ≈3 MW m−2 during the peak heat loads due to edge localized modes (ELMs) at high radiative power fractions of frad ≈ 0.75. In those high-density discharges the central impurity concentration is very low as a result of hollow nitrogen density profiles and minimal erosion of tungsten. The tungsten erosion is not only suppressed in between ELMs, when the divertor is detached, but also during the type-III ELM activity. Such low impurity concentrations in the plasma core might lead to even higher fusion amplification factors in ITER than 10. This is a demonstration of the compatibility of the radiating type-III ELMy H-mode with a tungsten divertor and main chamber wall, with sufficient confinement and favourable power exhaust characteristics. Together with previous results in all-carbon devices this demonstration strengthens the case for a potential application of a Q = 10 scenario on ITER.

Characteristics of edge-localized modes in the experimental advanced superconducting tokamak (EAST)

Edge-localized modes (ELMs) are the focus of tokamak edge physics studies because the large heat loads associated with ELMs have great impact on the divertor design of future reactor-grade tokamaks such as ITER. In the experimental advanced superconducting tokamak (EAST), the first ELMy high confinement modes (H-modes) were obtained with 1 MW lower hybrid wave power in conjunction with wall conditioning by lithium (Li) evaporation and real-time Li powder injection. The ELMs in EAST at this heating power are mostly type-III ELMs. They were observed close to the H-mode threshold power and produced small energy dumps (1–2% of the stored energy). Type-III ELMs produced a time-averaged peak heat flux of about 2 MW m−2 on the target plate, a value which is ∼10 times larger than that of ELM-free phases. A few isolated and large type-I-like ELM events were also observed in EAST with an energy loss of up to 5% of the stored energy. Statistically, the ELM frequencies are several hundred hertz and the frequency appears to decrease with q95, the safety factor at 95% of the flux surface. When an ion cyclotron resonance frequency wave was injected during the H-mode phases, the ELM repetition frequency increased immediately. The frequency and amplitude of type-III ELMs can be effectively influenced by puffing impurity argon gas.