Pedestal Stored Energy Research Articles

Impact of pedestal density gradient and collisionality on ELM dynamics

BOUT++ turbulence simulations are conducted to capture the underlying physics of small ELM characteristics achieved by increasing separatrix density via controlling strike points from vertical to horizontal divertor plates for three EAST discharges. BOUT++ linear simulations show that the most unstable modes change from high-n ideal ballooning modes to intermediate-n peeling–ballooning modes and eventually to peeling–ballooning stable plasmas in the pedestal. Nonlinear simulations show that the fluctuation is saturated at a high level for the lowest separatrix density. The ELM size decreases with increasing separatrix density, until the fraction of this energy lost during the ELM crash becomes less than 1% of the pedestal stored energy, leading to small ELMs. Simulations indicate that small ELMs can be triggered either by the marginally peeling–ballooning instability near the peak pressure gradient position inside the pedestal or by a local instability in the pedestal foot with a larger separatrix density gradient. The pedestal collisionality scan for type-I ELMs with steep pedestal density gradient shows that both linear growth rate and ELM size decrease with increasing collisionality. On the contrary, the pedestal collisionality and pedestal density width scan with a weak pedestal density gradient indicate small ELMs can either be triggered by a high-n ballooning mode or by a low-n peeling mode in a low collisionality region 0.04–0.1. The simulations indicate the weaker the linear unstable modes near marginal stability with small linear growth rate, the lower nonlinearly saturated fluctuation intensity and the smaller turbulence spreading from the linear unstable zone to stable zone in the nonlinear saturation phase, leading to small ELMs.

Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

The EUROfusion JET-ILW pedestal database is described, with emphasis on three main issues. First, the technical aspects are introduced, including a description of the data selection, the datasets, the diagnostics used, the experimental and theoretical methods implemented and the main definitions. Second, the JET-ILW pedestal structure and stability are described. In particular, the work describes the links between the engineering parameters (power, gas and divertor configuration) and the disagreement with the peeling-ballooning (PB) model implemented with ideal magnetohydrodynamics equations. Specifically, the work clarifies why the JET-ILW pedestal tends to be far from the PB boundary at high gas and high power, showing that a universal threshold in power and gas cannot be found but that the relative shift (the distance between the position of the pedestal density and of the pedestal temperature) plays a key role. These links are then used to achieve an empirical explanation of the behavior of the JET-ILW pedestal pressure with gas, power and divertor configuration. Third, the pedestal database is used to revise the scaling law of the pedestal stored energy. The work shows a reasonable agreement with the earlier Cordey scaling in terms of plasma current and triangularity dependence, but highlights some differences in terms of power and isotope mass dependence.

Nonlinear ELM simulations based on a nonideal peeling–ballooning model using the BOUT++ code

A minimum set of equations based on the peeling–ballooning (P–B) model with nonideal physics effects (diamagnetic drift, E × B drift, resistivity and anomalous electron viscosity) is found to simulate pedestal collapse when using the BOUT++ simulation code, developed in part from the original fluid edge code BOUT. Linear simulations of P–B modes find good agreement in growth rate and mode structure with ELITE calculations. The influence of the E × B drift, diamagnetic drift, resistivity, anomalous electron viscosity, ion viscosity and parallel thermal diffusivity on P–B modes is being studied; we find that (1) the diamagnetic drift and E × B drift stabilize the P–B mode in a manner consistent with theoretical expectations; (2) resistivity destabilizes the P–B mode, leading to resistive P–B mode; (3) anomalous electron and parallel ion viscosities destabilize the P–B mode, leading to a viscous P–B mode; (4) perpendicular ion viscosity and parallel thermal diffusivity stabilize the P–B mode. With addition of the anomalous electron viscosity under the assumption that the anomalous kinematic electron viscosity is comparable to the anomalous electron perpendicular thermal diffusivity, or the Prandtl number is close to unity, it is found from nonlinear simulations using a realistic high Lundquist number that the pedestal collapse is limited to the edge region and the ELM size is about 5–10% of the pedestal stored energy. This is consistent with many observations of large ELMs. The estimated island size is consistent with the size of fast pedestal pressure collapse. In the stable α-zones of ideal P–B modes, nonlinear simulations of viscous ballooning modes or current-diffusive ballooning mode (CDBM) for ITER H-mode scenarios are presented.

Characteristics and control of the type I edge localized mode in JT-60U

The detailed characteristics of the precursor of the type I edge localized mode (ELM) have been studied in JT-60U using diagnostics with high temporal and spatial resolution such as a microwave reflectometer, electron cyclotron emission (ECE) heterodyne radiometer and grating polychromator. Coherent density and temperature precursors have been observed before the collapse phase of type I ELM. The growth rate of the precursor is evaluated to be γ/ωA ∼ 10−3 for several edge pedestal conditions. From the phase delay between ECE signals measured at two toroidal locations and the frequency of the precursor, the toroidal mode number is experimentally evaluated as n = 8–10 or 14–16 assuming that the precursor rotates toroidally with the same toroidal rotation speed of carbon impurity. It is found that the dominant n varies with each ELM under the same plasma condition. The ratio of the pressure gradient inside the pedestal (∇pin) to the pressure gradient within the pedestal (∇pped) has been confirmed as an important parameter in determining the ELM energy loss (ΔWELM) normalized to the pedestal stored energy (Wped), ΔWELM/Wped. From the comparison of the reduction rate in the ion temperature profile due to ELMs, a larger reduction rate within the pedestal and a wider ELM affected area are observed in the plasma with larger ∇pin/∇pped. When the plasma near the top of the pedestal on the high-field side is heated by an electron cyclotron wave (ECW) power of 1.57 MW, the ΔWELM/Wped is reduced by ∼35%, together with an increase in the ELM frequency. The increasing rate of the ELM frequency with the heating power is about four times larger in the ECW injection case than the natural power dependence observed in the neutral beam injection case.

Nonlinear Simulations of Peeling-Ballooning Modes with Anomalous Electron Viscosity and their Role in Edge Localized Mode Crashes

A minimum set of equations based on the peeling-ballooning (P-B) model with nonideal physics effects (diamagnetic drift, E×B drift, resistivity, and anomalous electron viscosity) is found to simulate pedestal collapse when using the new BOUT++ simulation code, developed in part from the original fluid edge code BOUT. Nonlinear simulations of P-B modes demonstrate that the P-B modes trigger magnetic reconnection, which leads to the pedestal collapse. With the addition of a model of the anomalous electron viscosity under the assumption that the electron viscosity is comparable to the anomalous electron thermal diffusivity, it is found from simulations using a realistic high-Lundquist number that the pedestal collapse is limited to the edge region and the edge localized mode (ELM) size is about 5%-10% of the pedestal stored energy. This is consistent with many observations of large ELMs.

Pedestal and scrape-off layer dynamics in ELMy H-mode plasmas in JET

Pedestal and scrape-off layer (SOL) dynamics due to edge localized modes (ELMs) have been studied on JET with improved diagnostic capability. The new high resolution Thomson scattering system enables detailed measurement of the space and time evolution of the Te and ne pedestal profiles. The pedestal and SOL dynamics for type I ELMy H-mode plasmas have been studied for a wide range of plasma conditions. During a short period of <200 µs after the ELM event radial profiles of filaments in the SOL electron density and temperature have been observed. After that period the SOL density is increased and remains high for several milliseconds. During the same period the electron temperature shows no increase compared with the pre-ELM values. This SOL dynamics has been observed for a wide range of plasma parameters and is independent of plasma pedestal collisionality. For the first time on JET the convective and conductive ELM energy losses have been quantified using the new kinetic profile measurements. The findings provide detailed confirmation of earlier observations based on different measurements and analysis. The pedestal region perturbed by the ELM is the same for both density and temperature and the ELM effect extends up to about 20% of minor radius. The convective energy losses do not vary significantly and are ∼5% of the pedestal stored energy (Wped) over a large range of pedestal collisionality from below to above whereas the conductive losses strongly decrease from ∼20% of Wped to 5% of Wped with increasing . The experimental observations are compared with a simple model based on losses being driven by parallel transport.

Balance of the stored energies sustained by the internal and edge transport barriers and effects of ELMs and L–H transitions in JT-60U

To understand key physics processes determining radial profiles of the kinetic plasma parameters in the advanced tokamak operation scenarios, correlations between the edge transport barrier (ETB) and the internal transport barrier (ITB) have been studied in the JT-60U tokamak device. It has been found that the edge pedestal poloidal beta, βp-ped, increases almost linearly with the total poloidal beta, βp-tot, over a wide range of the plasma current for type I ELMing H-mode plasmas, and this dependence becomes stronger with increasing triangularity. This dependence is not due to the profile stiffness, since the dependence is the same regardless of the existence of ITB. As the stored energy inside the ITB-foot radius (WITB) increases, the total thermal stored energy (Wth) increases and then the pedestal stored energy (Wped) increases. On the other hand, as Wped increases, the ELM penetration expands more inwards and finally reaches the ITB-foot radius. At this situation, the ITB-foot radius cannot move outwards because of the erosion by ELMs. Then the fractions of WITB/Wth and Wped/Wth become almost constant. It has also been found that the type I ELM expels/decreases the edge toroidal momentum larger than the edge ion thermal energy. The ELM penetration for the toroidal rotation tends to be deeper than that for the ion temperature and can exceed the ITB-foot radius. The ELM penetration is deeper for CO-rotating plasmas than CTR rotating plasmas. In both cases, the ELM penetration is deeper in the order of the toroidal rotation (Vt), the ion temperature (Ti) and then the electron temperature (Te). The L–H transition also changes the Vt profile more significantly than the Ti profile. At the L–H transition, the pedestal Vt shifts into the CTR-direction deeply and suddenly without a change in Ti, and then the pedestal Vt grows further together with a growth of the pedestal Ti in a slower timescale. Such changes in Vt by ELMs and L–H transitions may affect degradation/evolution of ITBs.

Characteristics of the H-mode pedestal in improved confinement scenarios in ASDEX Upgrade, DIII-D, JET and JT-60U

Pedestal and global plasma parameters are compared in conventional ELMy H-modes and improved confinement discharges from ASDEX Upgrade (AUG), DIII-D, JET and JT-60U with varying net input power. Both electron and ion pedestal pressures are studied. The pedestal top pressure pPED increases moderately with power in all tokamaks, in broad agreement with the power dependence of the IPB98(y, 2) scaling. Higher pedestal pressures are observed in AUG improved H-modes and in JT-60U high βpol discharges at q95 ∼ 6.5 and high triangularity. For all machines and all scenarios a robust correlation between the total and the pedestal thermal stored energy is observed, with the ratio of the two varying between ∼0.3 and 0.5. However the relative importance of pedestal and core confinement varies from regime to regime. In AUG the confinement improvement with respect to the IPB98(y, 2) scaling is due to improved pedestal confinement in improved H-modes with early heating and to both improved pedestal and core confinement in improved H-modes with late heating. In DIII-D hybrid discharges the increase in confinement factor compared with conventional H-modes is due to improved confinement in the plasma core. JT-60U reversed shear H-modes have strong internal transport barriers and thus improved core performance. In all four tokamaks improved edge stability is correlated with increasing total βpol and H98(y,2) increases with pedestal βpol. The analysed multimachine data set supports a scaling expression for the pedestal stored energy derived under the assumption that the dominant loss term for the pedestal is by thermal conduction in the edge transport barrier region.

ELM frequency dependence on toroidal rotation in the grassy ELM regime in JT-60U

A systematic study of the effect of the level of toroidal plasma rotation at the top of the ion temperature pedestal () on the edge localised mode (ELM) characteristics in JT-60U has been performed. The level of toroidal plasma rotation was varied by using different combinations of tangential and perpendicular neutral beam injection (NBI). In the grassy ELM regime at high triangularity (δ) and high safety factor (q), the ELM frequency clearly increased up to ∼1400 Hz, when counter (ctr) plasma rotation was increased. The response of the ELM frequency was independent of poloidal beta (βp) in the range 0.84 < βp < 1.88 at δ > 0.53. Even in non-rotating plasma with balanced-NBIs, a high ELM frequency of ∼400 Hz was observed without a large energy loss. When the frequency of the plasma rotation in the co-direction of the plasma current became higher than ∼1 kHz, type I ELMs with a frequency of ∼20 Hz was observed. The achieved pedestal pressure and plasma confinement were similar both in plasmas with type I ELMs and in plasmas with grassy ELMs. The energy loss due to grassy ELMs was evaluated from the reduction in the electron temperature, and the ratio of the energy loss to the pedestal stored energy was less than 1%.

Roles of Toroidal Rotation at the Plasma Edge, Toroidal Field Ripple and Configuration on ELMs in the JT-60U Tokamak

Recent experimental studies are presented regarding ELMy H-mode plasma having Type-I Edge Localized Modes (ELMs) in the JT-60U. Toroidal rotation scan experiments in inward-shifted, small-volume plasma show that the ELM energy loss decreases as the toroidal rotation at the plasma edge increases in the direction counter to the plasma current. Specifically for a large volume plasma having a toroidal field ripple of ∼2% at the plasma edge, small Type-I ELMs are observed whose ELM energy loss normalized by the pedestal stored energy is smaller than that of an acceptable ELM size in the ITER. However, the pedestal pressure tends to decrease when plasma volume increases. No remarkable effect of reduced toridal field ripple due to the installation of Ferritic Steel Tile (FSTs) inside the vacuum vessel on the JT-60U on ELMs for large volume plasma is seen in the ELM energy loss normalized by the pedestal stored energy. These new findings suggest that the toroidal field ripple itself may not directly affect the normalized ELM energy loss, and that toroidal rotation at the plasma edge as well as plasma configuration might play important roles in the prediction of ELM size in future devices.

Impact of the edge pedestal characteristics on the integrated performance in advanced tokamak operation modes in JT-60U

Characteristics of the edge pedestal has been studied for the high βp ELMy H mode and the reversed shear ELMy H mode in JT-60U. The contribution of the pedestal is around 20% for the confinement improvement factor, the normalized beta, and the bootstrap fraction. The upper boundary of the fusion performance measure increases with the pedestal poloidal beta, βp-ped. At high triangularity, δ, βp-ped increases with the total βp (βp − tot) almost linearly for positive shear type I ELMy discharges. This dependence is not due to the profile stiffness, since the dependence is the same for the discharges both with and without an internal transport barrier (ITB). In the reversed shear ELMy H mode, βp-ped increases with βp-tot on the same line as the positive shear cases except at high q95 = 8–9.3. In the high plasma current regime, the final structure with both an ITB and edge transport barrier seems to be determined by the balance between the expanding ITB-foot radius and deepening ELM penetration: the ELM penetration radius deepens with increasing pedestal stored energy and then reaches the ITB-foot radius. The ITB-foot seems to behave as a barrier against the ELM crash penetration, and shrinks after a few ELM attacks, and the ELM penetration follows the shrinking ITB-foot.

Characterization of Type-I ELMs in tangential co-, balanced- and counter- plus perpendicular NBI heated plasmas on JT-60U

Effects of plasma rotation and losses of fast ions on Type-I ELM characteristics have been systematically studied in the JT-60U tokamak, scanning combinations of NBI (tangential co-, balanced- and counter-NBI plus perpendicular NBI) in the three types of plasma configurations (corresponding toroidal field ripple at the plasma edge, δr ∼ 0.4, 1.0 and 2.0%). New findings on the Type-I ELM characteristics are as follows: smaller ELM energy loss normalized by pedestal stored energy, ΔWELM/Wped, and faster ELM frequency, fELM, are confirmed in the counter-NBI in comparison with the co-NBI discharges. Nevertheless, the power loss due to ELM, PELM (= ΔWELM × fELM), normalized by heating power crossing the separatrix, PSEP, is constant regardless of the direction of the momentum injection at each plasma configuration. In contrast, the PELM/PSEP decreases with increasing δr. The relationship between ELMs, rotation and the losses of fast ions are discussed.

Observations of the collapse and recovery of the temperature pedestal using diagnostics with the fast temporal resolution in JT-60U

We investigated the relative change in ion temperature (ΔTi/Ti), electron temperature (ΔTe/Te), pedestal stored energy (ΔWELM/Wped), and toroidal rotation velocity (Vt) due to type-I ELMs using diagnostics with fast temporal resolution in JT-60U. The increase in Vt was found in the direction of the plasma current right after the ELM. The recovery of Vt was found to be faster than that of Wdia, Te and Ti. It is also found that ΔTi/Ti can be larger than ΔTe/Te.

Pedestal conditions for small ELM regimes in tokamaks

Several small/no ELM regimes such as EDA, grassy ELM, HRS, QH-mode, type II and V ELMs with good confinement properties have been obtained in Alcator C-Mod, ASDEX-Upgrade, DIII-D, JET, JFT-2M, JT-60U and NSTX. All these regimes show considerable reduction of instantaneous ELM heat load onto divertor target plates in contrast to conventional type I ELM, and ELM energy losses are evaluated as less than 5% of the pedestal stored energy. These small/no ELM regimes are summarized and widely categorized by their pedestal conditions in terms of the operational space in non-dimensional pedestal parameters and requirement of plasma shape/configuration. The characteristics of edge fluctuations and activities of ideal MHD stability leading to small/no ELMs are also summarized.

Energy loss for grassy ELMs and effects of plasma rotation on the ELM characteristics in JT-60U

The energy loss for grassy edge localized modes (ELMs) has been studied to investigate the applicability of the grassy ELM regime to ITER. The grassy ELM regime is characterized by high frequency periodic collapses of 800–1500 Hz, which is ∼15 times faster than that for type I ELMs. The divertor peak heat flux due to grassy ELMs is less than 10% of that for type I ELMs. This smaller heat flux is caused by a narrower radial extent of the collapse of the temperature pedestal. The different radial extent between type I ELMs and grassy ELMs agrees qualitatively with the different radial distribution of the eigenfunctions as determined from ideal MHD stability analysis. The dominant ELM energy loss for grassy ELMs appears to be caused by temperature reduction, and its ratio to the pedestal stored energy was 0.4–1%. This ratio is lower by a factor of about 10 than that for type I ELMs, which typically have between 2–10% fractional loss of the pedestal energy. A systematic study of the effects of counter (CTR) plasma rotation on the ELM characteristics has been performed using a combination of tangential and perpendicular neutral beam injections (NBIs) in JT-60U. In the high plasma triangularity (δ) regime, ELM characteristics (e.g. amplitude, frequency and type) can be changed from type I ELMs to high frequency grassy ELMs as the CTR plasma rotation is increased. On the other hand, in the low δ regime, complete ELM suppression (QH-mode) can be sustained for long periods up to 3.4 s (∼18τE or energy confinement times), when the plasma position in terms of the clearance between the first wall and the plasma separatrix is optimized during the application of CTR-NBIs. In JT-60U, a transient QH phase was also observed during the CO-NBI phase with almost no net toroidal rotation at the plasma edge.

Fast dynamics of type I ELMs and transport of the ELM pulse in JT-60U

Simultaneous fast ELM measurements using a reflectometer, interferometers, Dα intensity and a magnetic probe reveal the detailed characteristics of type I ELMs. From the phase signal of the reflectometer indicating the radial movement of the cut-off layer, four different phases in the ELM event, i.e. a precursor phase, collapse phase, recovery phase and a relaxation phase, were observed. In the collapse phase, the radial extent of the collapse of the density pedestal reached up to twice the pedestal width. A fast drop of the horizontal interferometer signal was observed, while the vertical interferometer on the high-field side edge plasma only exhibited a small and delayed reduction. These features of the plasma response due to ELMs might indicate a non-uniform collapse of the density pedestal in the poloidal direction, localized near the low-field side (LFS) midplane. Expelled particles from the LFS midplane were measured by using scrape-off layer Mach probes, and the heat flux to the divertor target was measured using a fast infrared TV camera. The time constant of the peak heat load was comparable to that of the enhancement of the plasma flow towards the divertor target. The normalized ELM energy loss by pedestal stored energy in JT-60U was found to be less than 10% of the pedestal stored energy. It shows weak dependence on the pedestal parameters such as electron collisionality, parallel ion loss time and the ratio of the pedestal density to the Greenwald density limit.