Edge Localized Mode Activity Research Articles

Negative triangularity scenarios: from TCV and AUG experiments to DTT predictions

Abstract Experiments, gyrokinetic simulations and transport predictions were performed to investigate if a negative triangularity (NT) L-mode option for the Divertor Tokamak Test (DTT) full-power scenario would perform similarly to the positive triangularity (PT) H-mode reference scenario, avoiding the harmful edge localized modes (ELMs). The simulations show that a beneficial effect of NT coming from the edge/scrape-off layer (SOL) region ρ tor > 0.9 is needed to allow the actual NT L-mode option to perform like the PT H-mode. Dedicated experiments at TCV and AUG, with DTT-like shapes, show an optimistic picture. In TCV, experiments indicate that even with the relatively small triangularity of the DTT NT scenario, a large beneficial effect of NT comes from the plasma edge and SOL, allowing NT L-modes to outperform PT L-modes with the same power input, reaching the same central pressures as PT H-modes with twice as much applied heating power. For AUG, NT plasmas go into H-mode more easily than for TCV, but always present much smaller pedestals compared with PT plasmas with the same input power, showing a much weaker or absent ELM activity. However, NT has a smaller beneficial effect for AUG than for TCV, with NT pulses outperforming PT pulses with the same input power only for an ECRH-only case with relatively low input power. For the considered AUG cases, PT pulses perform better than NT ones at higher ECRH power or with mixed NBI and ECRH power. Based on this analysis, the NT option is a viable alternative for the DTT full power scenario, providing high performance plasmas with reduced or absent ELMs.

Density modification in the scrape-off-layer during LHW coupling at EAST

The results presented in this paper are an extension of our recent studies (Kong et al 2013 Plasma Phys. Control. Fusion 55 065007, 065008) on lower hybrid wave (LHW) coupling. The effects of LHW power, edge recycling, edge-localized-mode (ELM) activity and gas puffing on the scrape-off-layer (SOL) density modifications will be further studied in this paper. The SOL density profile usually depends on the competition between an increment by ionization of neutrals and a depletion by the ponderomotive force (PMF) in L-mode plasma. Experimental results indicate that the SOL density with a lower edge recycling intensity could be reduced by a higher LHW power because ionization of a few neutrals cannot provide sufficient electrons to increase the SOL density and thus degrade the LHW coupling. In addition, in H-mode plasma, it is observed that the significant reduction of RC is associated with higher LHW power compared with the oscillation of RC during ELM activity, implying the priority of neutral gas ionization by LHW power. Studies also indicate that gas puffing in the GIM_e side could be more efficient in improving LHW coupling due to local ionization, inferred by the luminosities on the surface of the poloidal limiter (PL) and confirmed by SOL density measurement. The evolution of edge density with various puffing rates exhibits increments of far SOL density focused on the narrower layer several centimeters in front of the PL, possibly suggesting that the ionization of neutral gas mainly occurs in this region.

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.

Studies of edge localized mode mitigation with new active in-vessel saddle coils in ASDEX Upgrade

The ASDEX Upgrade tokamak is currently being enhanced with a set of in-vessel saddle coils for non-axisymmetric perturbations aiming at mitigation or suppression of edge localized modes (ELMs). Results obtained during the first experimental campaign are reported. With n = 2 magnetic perturbations, it is observed that type-I ELMs can be replaced by benign small ELM activity with strongly reduced energy loss from the confined plasma and power load to the divertor. During these phases with ELM mitigation, no density reduction (density ‘pump-out’) is observed. ELM mitigation has, so far, been observed in plasmas with different shape and different heating mixes and, therefore, different momentum input. The ELM mitigation regime can be accessed with resonant and non-resonant perturbation field configurations. The main threshold requirement appears to be a critical minimum plasma edge density which depends on plasma current. So far it is not possible to distinguish whether this is an edge collisionality threshold or a critical fraction of the Greenwald density limit.

Characteristics of Edge MHD Modes and ELM Activity Observed in Large Helical Device Plasmas

AbstractIn the Large Helical Device (LHD), nonlinear evolution of strongly destabilized edge MHD modes, which are thought to be resistive interchange modes, induce repetitive bursts of magnetic fluctuations, soft X‐ray (SX) fluctuations, and extreme ultra‐violet (XUV) fluctuations, and generates a train of sharp spikes in Hα emission signals. These Hα spikes exhibit the character of edge localized mode (ELM). The ELM activities are also observed in high beta or high density L‐mode plasmas having steep pressure gradient at plasma edge as well as H‐mode plasmas with the L‐H transition. The responsible instabilities for ELMs in LHD plasmas are thought to be resistive interchange modes, and are clearly different from those in a tokamak where peeling‐ballooning modes are the candidates. The repetition frequency of ELMs (fELM) increases with the increase of external heating power, of which the character is similar to that of type I ELMs in tokamaks. The product of fELM and the relative ELM amplitude (δHα/Hα) is roughly in proportion to the input heating power, where the ELM induced loss energy (ΔWp) increases with δHα/Hα (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of steady-state scenarios compatible with ITER-like wall conditions

A key issue for steady-state tokamak operation is to determine the edge conditions that are compatible both with good core confinement and with the power handling and plasma exhaust capabilities of the plasma facing components (PFCs) and divertor systems. A quantitative response to this open question will provide a robust scientific basis for reliable extrapolation of present regimes to an ITER compatible steady-state scenario. In this context, the JET programme addressing steady-state operation is focused on the development of non-inductive, high confinement plasmas with the constraints imposed by the PFCs. A new beryllium main chamber wall and tungsten divertor together with an upgrade of the heating/fuelling capability are currently in preparation at JET. Operation at higher power with this ITER-like wall will impose new constraints on non-inductive scenarios. Recent experiments have focused on the preparation for this new phase of JET operation. In this paper, progress in the development of advanced tokamak (AT) scenarios at JET is reviewed keeping this long-term objective in mind. The approach has consisted of addressing various critical issues separately during the 2006–2007 campaigns with a view to full scenario integration when the JET upgrades are complete. Regimes with internal transport barriers (ITBs) have been developed at q95 ∼ 5 and high triangularity, δ (relevant to the ITER steady-state demonstration) by applying more than 30 MW of additional heating power reaching βN ∼ 2 at Bo ∼ 3.1 T. Operating at higher δ has allowed the edge pedestal and core densities to be increased pushing the ion temperature closer to that of the electrons. Although not yet fully integrated into a performance enhancing ITB scenario, Neon seeding has been successfully explored to increase the radiated power fraction (up to 60%), providing significant reduction of target tile power fluxes (and hence temperatures) and mitigation of edge localized mode (ELM) activity. At reduced toroidal magnetic field strength, high βN regimes have been achieved and q-profile optimization investigated for use in steady-state scenarios. Values of βN above the ‘no-wall magnetohydrodynamic limit’ (βN ∼ 3.0) have been sustained for a resistive current diffusion time in high-δ configurations (at 1.2 MA/1.8 T). In this scenario, ELM activity has been mitigated by applying magnetic perturbations using error field correction coils to provide ergodization of the magnetic field at the plasma edge. In a highly shaped, quasi-double null X-point configuration, ITBs have been generated on the ion heat transport channel and combined with ‘grassy’ ELMs with ∼30 MW of applied heating power (at 1.2 MA/2.7 T, q95 ∼ 7). Advanced algorithms and system identification procedures have been developed with a view to developing simultaneously temperature and q-profile control in real-time. These techniques have so far been applied to the control of the q-profile evolution in JET AT scenarios.

The role of edge current-driven modes in ELM activity

We propose a model for edge localized mode (ELM) evolution which goes beyond linear stability arguments by hypothesizing that peeling modes initiate a Taylor relaxation (a constrained minimization of the magnetic energy) of an outer annular plasma region. The relaxation has two effects on peeling mode stability: (a) As the relaxation process proceeds radially inwards it leaves in its wake a Taylor state, which for conventional tokamak ordering is simply a flattened equilibrium toroidal current density. This effect acting in isolation would provide a destabilizing effect (for conventional current profiles the edge current density would increase); (b) The formation of a (negative for conventional current profiles) skin current at the plasma–vacuum interface which has a counteracting stabilizing effect on peeling modes. For a finite relaxed annulus, these two opposing effects can balance and give a configuration that is stable to all possible peeling instabilities. The radial extent of the relaxed region required for stability can be calculated using this balance. This then leads to model predictions for ELM characteristics such as the widths and mode numbers, the magnitude of the attendant energy losses and the natural (deterministic) scatter in these quantities. We compare these model predictions with a number of experimentally observed ELM properties. Further, expanding the governing equations gives analytic expressions for ELM widths in terms of localized edge parameters. Peeling modes can occur even when the critical pressure gradient for the onset of ballooning modes has not been reached. For this reason ‘type III’ ELMs, which typically occur just above the threshold for L–H transitions, may be best described by this model.

Characteristics of ELM activity and fueling efficiency of pellet injection from different locations on DIII-D

Pellet injection from several different locations has been used on the DIII-D tokamak to study core fueling and transport in H-mode and L-mode plasmas. These experiments have provided a variety of conditions in which to examine the fueling efficiency and edge localized mode (ELM) interaction with pellets injected into DIII-D plasmas. The fueling efficiency, defined as the total increase in number of plasma electrons divided by the number of pellet fuel atoms, is determined by measurements of density profiles before and just after pellet injection. New injection ports on the DIII-D inner wall enable high field side (HFS) pellet injection from both the midplane and 30 cm above the midplane. These ports, in addition to the previously existing top vertical ports and outside midplane port, enable a comparison of the effect of injection location on fueling efficiency and ELM activity. We find that the ELMs triggered from HFS injected pellets and vertical HFS injected pellets are similar to the nominal background ELMs in ELMing H-mode plasmas. In contrast, the low field side (LFS) injected pellets trigger large-magnitude ELMs that lead to a strong reduction in fueling efficiency. The fueling efficiency of the HFS injected pellets is found to be significantly higher than with the LFS injected pellets and remains high even with significant heating power.

Traveling-Wave Antenna for Fast-Wave Heating and Current Drive in Tokamaks

The traveling-wave antenna for heating and current drive in the ion cyclotron range of frequencies is shown theoretically to have loading and wavenumber spectra that are largely independent of plasma conditions. These characteristics have been demonstrated in low-power experiments on the DIII-D tokamak, in which a standard four-strap antenna was converted to a traveling-wave antenna through use of external coupling elements. The experiments indicate that the array maintains good impedance matching without dynamic tuning during abrupt changes in the plasma, such as during L- to H-mode transitions, edge-localized mode activity, and disruptions. An analytic model was developed that exhibits the features observed in the experiments. Guidelines for the design of traveling-wave antennas are derived from the validated model.

Overview of the recent experimental results in JT-60 and JFT-2M

The recent experimental results on H-mode in JT-60U and JFT-2M are reported. The enhanced confinement mode with the edge transport barrier ( H-mode) in JT-60U is characterized by the high edge ion temperature. The confinement improvement is well correlated with the edge ion temperature. The H-mode at low magnetic field ( approximately 2 T) in JT-60U has similar features to the H-mode in JFT-2M: rapid reduction of Dalpha emission. However, in the H-mode at high magnetic field ( approximately 4 T) and at low density, no clear transition is observed in JT-60U, although the confinement is still improved with the edge transport barrier. Collisionality and the width of the transport barrier are also investigated in both tokamaks, and are compared with Shaing's theory (1989). Improvement of the high-pp mode discharge with the H-transition was demonstrated in JT-60U. Owing to this improvement, the record value of the fusion triple product (1.1*1021 m-3 s keV) has been achieved. Quasi-steady H-mode operation with H-factor of approximately 1.8 has been also demonstrated in JT-60U by controlling the repetition rate of edge localized modes (ELMs). In JFT-2M, ELM activity has been successfully induced by a new and active method using helical magnetic field perturbation, and as a result, a steady H-mode has been achieved.

H-modes under steady-state conditions in JET

Two H-mode regimes have been identified in JET in which edge localized modes (ELMs) maintain steady-state conditions. In the first regime, strong gas puffing was used in combined (ICRF plus NBI) heating experiments at powers of up to 20MW. Rapid ELM activity occurred and at moderate powers ( approximately 8MW) steady-state H-modes with durations of up to 18s and energy confinement times of up to 95% of the JET/D-IIID scaling were established. At high toroidal beta ( beta N>or=1.5) H-mode plasmas were also found to exhibit regular ELM behaviour which resulted in steady-state H-modes with confinement enhancement of approximately 90% of the JET/D-IIID scaling. This paper examines the plasma properties of these regimes and assesses their implications for steady-state H-mode operation in ignited plasmas.

Edge localized mode activity as a limit cycle in tokamak plasmas.

A model of edge localized modes in tokamak plasmas is presented. A limit-cycle solution is found in the transport equation (time-dependent Ginzburg-Landau type), which has a hysteresis curve for the gradient versus the flux. A periodic oscillation of the particle outflux and an L-H intermediate state are predicted near the L-H transition boundary. A mesophase in spatial structure appears near the edge.