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Experiments performed in DIII-D demonstrate that higher plasma current and heating power combined with impurity seeding facilitate the achievement of divertor detachment with a higher pedestal pressure and higher plasma performance in H-mode plasmas with a baffled closed divertor compared with an open divertor. Dedicated experiments were carried out to study the impact of power, plasma current and impurity seeding on divertor detachment with ion B × ∇ B directed into the divertor favorable for the L–H transition. With a factor of three variation in heating power and with only D2 puffing, no significant difference in the separatrix density at detachment onset was found. The higher heating power leads to higher impurity concentration and wider scrape-off layer (SOL) width, and reduces the detachment onset density to one similar to that in lower-power plasmas. Higher current requires higher pedestal and line-averaged densities to achieve divertor detachment; however, the increase in separatrix density at increasing plasma current is found to be less pronounced. Initial calculations found that both power scan and plasma current scan datasets are qualitatively consistent with theory after considering the change in impurity concentration and heat flux width. This also motivates the future extensive study of transport and divertor impurity behavior in order to have a quantitative comparison between experiment and theory. Compared with an open divertor, a closed divertor facilitates detachment onset at ∼40% lower line-averaged plasma density. Additional N2 seeding facilitates the achievement of detachment at a lower separatrix density and thus a higher pedestal temperature, which is beneficial for advanced tokamak scenarios. Higher heating power requires a higher N2 puffing rate to achieve the same degree of detachment, while a higher N2 puffing rate leads to lower detachment onset line-averaged density, both of which agree with theory. In contrast to the narrower pedestal in an open divertor approaching detachment, the pedestal density width in a closed divertor increases with density. The density gradient increases with line-averaged density at higher plasma current, but remains nearly unchanged at lower plasma current. In particular, compared with discharges with low power, at high heating power the pedestal density gradient is much weaker, while the SOL density is significantly higher and wider. At the same plasma current, both pedestal pressure gradient and temperature gradient decrease linearly with the line-averaged density but remain similar across different heating powers. Even with different plasma current and heating power, the normalized pressure gradient remains identical. As a result, achievement of divertor detachment with a higher pedestal pressure and higher plasma performance is shown in a closed divertor, which is important for improving core–edge integration as one of the critical issues for future tokamak fusion reactors. [ABSTRACT FROM AUTHOR]

Spontaneous mitigation of giant ELMs and appearance of grassy ELMs have been observed repeatedly at low pedestal collisionality () in the high-power (Pinj > 13 MW) hybrid scenario in DIII-D. Higher βp and higher q95 appear to be beneficial to achieving the grassy ELM regime. The grassy ELM H-mode plasma shows high energy confinement performance (H98y2 up to over 1.5) under the conditions of high neutral beam torque and high core rotation. The pedestal width appears to exceed the EPED1.0 model prediction by more than 50%. Pedestal stability analysis performed with the ELITE code indicates that the stability against low-n kink/peeling modes is improved with increased auxiliary heating power and the operational point in the grassy ELM regime is located near the ballooning boundary. The pedestal stability characteristics during the grassy ELM crashes have been investigated in comparison with the giant ELM crashes based on plasma profiles experimentally measured with high resolution and accuracy. It has been found that the underlying mechanism for the observed small-amplitude ELM crashes is mainly the expansion of the ballooning stability boundary induced by an initial radially localized collapse in the pedestal, which helps to stop the growth of instabilities and further collapse of the pedestal. The effect of electron density pedestal on mitigating edge localized instabilities has been analyzed by numerical simulation, suggesting that the electron density pedestal characterized by high ne,sep/ne,ped and low density gradient helps to stabilize peeling-ballooning modes because of a low pressure pedestal gradient and to lower the ballooning boundary mainly because of a low ion diamagnetic frequency in the pedestal region, thus triggering ballooning instabilities and producing the intrinsic grassy ELMs. Numerical simulation of the Chinese fusion engineering test reactor (CFETR) with the SOLPS code indicates that the separatrix density might be insensitive to the electron diffusivity in the pedestal region and increase with the power flowing from the core region to the edge region. Pedestal stability analysis suggests that the flat density pedestal with high separatrix density obtained in the high-power plasma in CFETR would make the operational point close to the ballooning boundary, which is considered to help destabilize ballooning instabilities and facilitate the access to the grassy ELM regime. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

MHD stability at edge pedestal in a QH-mode plasma in DIII-D was analyzed by taking into account plasma rotation and ion diamagnetic drift effects. We have found that the coupled rotation and ion diamagnetic drift effects can stabilize a kink/peeling mode in the QH-mode plasma rotating in the direction counter to the plasma current, although it has been recognized the rotation alone destabilizes the mode regardless of its direction. The physics mechanism responsible for the stabilization was identified as reduction of the destabilizing effect by dynamic pressure through the coupling between the rotation and the ion diamagnetic drift. The coupling effect can be harnessed to both stabilize and destabilize the kink/peeling mode by switching the rotation direction, the trend which could be the reason that the QH-mode plasmas in DIII-D favor toroidal rotation counter to the plasma current direction. [ABSTRACT FROM AUTHOR]

Recent experiments on DIII-D have shown it is possible to create and sustain wide pedestal quiescent H-mode (QH-mode) plasmas with zero net torque from neutral beam injection (NBI) for the full discharge duration. Wide pedestal QH-mode has many of the features of the previously investigated QH-mode while having the advantage of increased edge pedestal pressure and excellent energy confinement time. Both QH-mode variants operate without edge localized modes. Accordingly, these new discharges demonstrate that significant input torque is not essential to the exploitation of wide pedestal QH-mode in future devices that are expected to have small or non-existent NBI torque. Developing operating conditions that allowed net zero torque access to wide pedestal QH-mode required implementing several techniques to avoid locked modes including minimizing intrinsic error fields, avoiding large sawteeth, and driving toroidal rotation via neoclassical toroidal viscosity. [ABSTRACT FROM AUTHOR]

In the wall-stabilized high-β plasmas in JT-60U and DIII-D, interactions between energetic particle (EP) driven modes (EPdMs) and edge localized modes (ELMs) have been observed. The interaction between the EPdM and ELM are reproducibly observed. Many EP diagnostics indicate a strong correlation between the distorted waveform of the EPdM and the EP transport to the edge. The waveform distortion is composed of higher harmonics (n ⩾ 2) and looks like a density snake near the plasma edge. According to statistical analyses, ELM triggering by the EPdMs requires a finite level of waveform distortion and pedestal recovery. ELM pacing by the EPdMs occurs when the repetition frequency of the EPdMs is higher than the natural ELM frequency. EPs transported by EPdMs are thought to contribute to change the edge stability. [ABSTRACT FROM AUTHOR]

Understanding the physics of the L–H transition power threshold scaling dependencies on toroidal field and density is critical to operating and optimizing the performance of ITER. Measurements of long-wavelength (k⊥ρI < 1) turbulent eddy dynamics, characteristics, flows, and flow shear in the near edge region of DIIID plasmas have been obtained during an ion gyroradius scan (varying toroidal field and current) and density scan in a favourable geometry (ion ∇B drifts towards the X-point), in order to determine the underlying mechanisms that influence the macroscopic L–H power threshold scaling relations. It is found that the normalized integrated long wavelength density fluctuation amplitudes (ñ/n) in the pedestal increases with ρ* approaching the L–H transition. The turbulence poloidal flow spectrum evolves from geodesic acoustic mode dominant at lower power to low-frequency zonal flow (LFZF) dominant near the L–H transition, and the effective shearing rate correspondingly increases. An inferred Reynolds stress, , from BES velocimetry (inferring velocity field from imaging) measurements is found to significantly increase near the L–H transition. At lower electron density, a clear increase of the LFZF is observed prior to the L–H transition, which is not evident at higher density. Taken together, these results are qualitatively consistent with the electron density and toroidal field scaling of the L–H transition power threshold. [ABSTRACT FROM AUTHOR]

The pedestal structure in NSTX is strongly affected by lithium coatings applied to the PFCs. In discharges with lithium, the density pedestal widens, and the electron temperature (Te) gradient increases inside a radius of ψN ∼ 0.95, but is unchanged for ψN > 0.95. The inferred effective electron thermal and particle profiles reflect the profile changes: is slightly increased in the near-separatrix region, and is reduced in the region ψN < 0.95 in the with-lithium case. The profile shows a broadening of the region with low diffusivity with lithium, while the minimum value within the steep-gradient region is comparable in the two cases. The linear microstability properties of the edge plasma without and with lithium have been analysed. At the pedestal top microtearing modes are unstable without lithium. These are stabilized by the stronger density gradient with lithium, becoming TEM-like with growth rates reduced and comparable to E × B shearing rates. In the region ψN > 0.95, both the pre- and with-lithium cases are calculated to be unstable to ETG modes, with higher growth rates with lithium. Both cases are also found to lie near the onset for kinetic ballooning modes, but in the second-stable region where growth rates decrease with increasing pressure gradient. [ABSTRACT FROM AUTHOR]

Based on semianalytic modeling of experimental H-mode density profiles, a pedestal density limit is computed for a prototypical next generation burning plasma. For a range of input assumptions, the projected limit lies near or above Greenwald density limit [M. Greenwald, J. L. Terry, S. M. Wolfe, S. Ejima, M. G. Bell, S. M. Kaye, and G. H. Neilson, Nucl. Fusion 28, 2199 (1988)]. Two separate models are developed and utilized: the first model determines the edge density profile from consideration of cross-field transport in the steep gradient region and the distribution of fueling sources. It is shown that the observed shape of the density profile in H-mode can be explained with constant particle diffusivity within the separatrix. The second model determines the separatrix density above which the conduction limited scrape-off layer (SOL) transport is no longer sustainable. These results imply that further increases in pedestal density (while maintaining separatrix density below the power balance limit) are achievable through control of the fueling profile, e.g., with shallow pellets from the high-field side and divertor pumping. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the correlation among geodesic curvature of magnetic field lines, plasma rotation, and improved confinement regimes in tokamaks. Toroidal magnetic confinement configurations of all the charged particles; Minimization of the neoclassical transport; Reduction of the geodesic curvature.

Examines the structure and scaling of the High-mode pedestal for discharges in the DIII-D tokamak. Indication of ion measurement and C[sup 6+]; Prediction on pressure gradient magnitude in the barrier; Description of scalings based on the poloidal ion gyroradius.