Session BI1: Pedestal, SOL and Divertor

Critical gradients and plasma flows in the edge plasma of Alcator C-Mod

Recent experiments in both L- and H-mode plasmas on Alcator C-Mod have lead to a fundamental shift in our views of edge transport physics: transport in the `near' scrape-off-layer (SOL) region may be more appropriately described in terms of a critical gradient phenomena rather than a diffusive and/or convective transport paradigm. L-mode pressure gradients, normalized by the square of the poloidal magnetic field strength (i.e., $\alpha _{MHD})$ appear invariant in plasmas with the same normalized collisionality, despite vastly different currents and magnetic fields. These data suggest that local gradients are pinned to a `critical gradient' condition, which is sensitive to local collisionality -- a behavior that connects with first-principles electromagnetic fluid drift turbulence simulations [1]. H-mode pedestal gradients are found to follow a nearly identical scaling [2]. Thus, the near SOL, which forms the base of the H-mode pedestal, may play a key role in its creation. Prior to an L-H transition, strong SOL plasma flows are found to set a flow boundary condition for the confined plasma [3]. With favorable \textit{Bx}$\nabla B$ direction (i.e., \textit{Bx}$\nabla B$ pointed toward active x-point) these flows tend to spin the plasma in the co-current direction, perhaps reducing the L-H threshold power. Indeed, we find the edge profiles of the L-mode target plasmas to be fundamentally different, depending on the x-point topology: higher values of $\alpha _{MHD}$ are observed for favorable \textit{Bx}$\nabla B$ direction, independent of the direction of $B$ -- supporting evidence that SOL flows play a role in affecting the observed `critical gradient' value. \newline [1] LaBombard, B., \textit{et al.}, Nucl. Fusion \textbf{45} (2005) 1658 \newline [2] Hughes, J.W., \textit{et al.}, Phys. Plasmas \textbf{13} (2006) 056103 \newline [3] LaBombard, B., \textit{et al.}, Nucl. Fusion \textbf{44 }(2004) 1047.   

Effect of Island Overlap on ELM Suppression by Resonant Magnetic Perturbations in DIII-D

Recent DIII-D experiments show that the degree of magnetic island overlap in the plasma edge is a good predictor for suppression of edge-localized modes (ELMs), consistent with theoretical expectations. For fixed resonant magnetic perturbation (RMP) strength, ELM suppression is obtained over a finite window in the edge safety factor ($q_{95} )$ indicating a resonant effect. In H-mode plasmas, ELM suppression is obtained over an increasing range of $q_{95} $ by either increasing the RMP strength that produces the islands, or by adding $n=1$ perturbations to ``fill in'' islands across the edge plasma. Large Type-I ELMs are completely suppressed by applying $n=3$ RMPs in the presence of $n=1$ error-field correction and small $n=2$ and 3 field-error components in plasmas with electron pedestal collisionality of $\sim $0.1 and shape similar to ITER. In these experiments, the region of island overlap is changed by varying either: 1) the strength of the applied $n=3$ RMP, 2) the edge q-profile, 3) the combination of $n=3$ and $n=1$ perturbations, or 4) the up-down parity of the applied $n=3$ RMP. Each case agrees with theoretical expectations that the island overlap region width (vacuum fields) needs to be at least several times the width of the pedestal to completely eliminate ELMs. Theory predicts that the plasma response in rotating plasmas reduces the RMP amplitude from the vacuum level (RMP screening). Experiments to validate this theory have examined the detailed dependence of ELM suppression on the width of the island overlap region for two different values of edge toroidal rotation. Experimental validation of theoretical models for ELM suppression represents an important scientific advance that will provide the foundation for designing ELM control systems in future devices.   

Influence of Beta, Shape, and Rotation on the H-mode Pedestal Height in DIII-D

Recent experiments on DIII-D aimed at improving our understanding of the H-mode pedestal have shown that the observed pedestal gradient and pedestal width are sensitive to variations in plasma shape and global $\beta $ but relatively insensitive to plasma rotation. These dependencies are critical to the extrapolation of present results to ITER due to the sensitivity of fusion performance on pedestal height. Using single parameter scans to isolate these effects, the pedestal pressure was observed to increase as either plasma shaping or global $\beta $ was increased due to an increase in the width and the gradient of the pedestal pressure. In both cases, stability analysis indicated that the increased pressure gradient is consistent with peeling/ballooning theory. Stronger shaping increases the edge stability limit, allowing the pedestal pressure gradient to increase. At the same time, the pedestal width grows with the increasing pressure gradient until the MHD stability limit is reached and an ELM occurs. In the same manner, increased $\beta $ improves the edge stability limit through increased Shafranov shift, with the pedestal gradient and width increasing. The increase in pedestal width at higher total pedestal pressure is correlated with a larger ion gyroradius as suggested by a number of theories. Gyroradius dependence will be examined with respect to previous results and scaling of the pedestal height to ITER. Increased toroidal rotation was observed to have minimal impact on the pedestal height even though core energy confinement improved. Increased toroidal rotation does not significantly change the stability limit, resulting in the pedestal width and gradient remaining unchanged. These results suggest that the improvement in core confinement as rotation increases is independent of pedestal performance.   

Progress in ITER-relevant exhaust physics at JET

Plasma boundary research during the 2006-2007 campaigns at JET has made advances in several ITER-relevant exhaust physics issues. On the critical question of fuel retention, dedicated gas balance experiments provide unequivocal evidence for long term retention of 10-20{\%} in both L-mode and ELMing H-mode, in fair agreement with that obtained from campaign averaged post-mortem analysis. In-situ quartz microbalance (QMB) techniques coupled with visible spectroscopy and modelling show that long range migration of carbon to remote areas and is a major contributor to this retention. The QMB data also register an ELM induced erosion rate increasing exponentially with ELM energy. Large Type I ELMs (up to $\sim $1MJ), comparable to the smallest expected in ITER, provoke divertor and X-point radiation in the range 50-100{\%} of the ELM energy with an in-out asymmetry favouring the inner divertor. This is consistent with the observed trend for the ELM to deposit more energy on the inboard target for normal field direction and the presence of redeposited layers, which have lower threshold for ablation under high power fluxes. The magnitude and time variation of these power fluxes can be reproduced quantitatively by new PIC simulations of kinetic transport parallel to the field in the SOL. The spatial patterns and magnitudes of energy deposition during ELMs have been measured for the first time on JET main chamber wall surfaces using IR thermography, confirming the presence of field aligned filaments carrying a small fraction of the ELM energy. Meanwhile, the small, convective ELMs previously found in JET at ITER relevant pedestal collisionalities, have been recovered in the new MarkIIHD divertor configuration and a clear threshold in q$_{95}$ identified for their occurrence.   

Divertor Heat Flux Reduction and Detachment in the National Spherical Torus eXperiment.

Steady-state handling of the heat flux is a critical divertor issue for both the International Thermonuclear Experimental Reactor and spherical torus (ST) devices. Because of an inherently compact divertor, it was thought that ST-based devices might not be able to fully utilize radiative and dissipative divertor techniques based on induced power and momentum loss. However, initial experiments conducted in the National Spherical Torus Experiment in an open geometry horizontal carbon plate divertor using 0.8 MA 2-6 MW NBI-heated lower single null H-mode plasmas at the lower end of elongations $\kappa $=1.8-2.4 and triangularities $\delta $=0.45-0.75 demonstrated that high divertor peak heat fluxes, up to 6-10 MW/ m$^{2}$, could be reduced by 50-75{\%} using a high-recycling radiative divertor regime with D$_{2}$ injection. Furthermore, similar reduction was obtained with a partially detached divertor (PDD) at high D$_{2}$ injection rates, however, it was accompanied by an X-point MARFE that quickly led to confinement degradation. Another approach takes advantage of the ST relation between strong shaping and high performance, and utilizes the poloidal magnetic flux expansion in the divertor region. Up to 60 {\%} reduction in divertor peak heat flux was achieved at similar levels of scrape-off layer power by varying plasma shaping and thereby increasing the outer strike point (OSP) poloidal flux expansion from 4-6 to 18-22. In recent experiments conducted in highly-shaped 1.0-1.2 MA 6 MW NBI heated H-mode plasmas with divertor D$_{2}$ injection at rates up to 10$^{22}$ s$^{-1}$, a PDD regime with OSP peak heat flux 0.5-1.5 MW/m$^{2}$ was obtained without noticeable confinement degradation. Calculations based on a two point scrape-off layer model with parameterized power and momentum losses show that the short parallel connection length at the OSP sets the upper limit on the radiative exhaust channel, and both the impurity radiation and large momentum sink achievable only at high divertor neutral pressures are required for detachment.   

Dynamics and generation mechanisms of meso-scale structures in tokamak edge plasmas.

In recent years, it became clear that intermittent convective-like transport associated with meso-scale coherent structures extended along the magnetic field lines is often dominant in the cross-field transport of tokamaks, stellarators, and linear devices. Such structures can propagate in a ballistic way toward the wall for the distance of tens of centimeters and can strongly enhance plasma energy and particle transport and plasma-wall interactions. The apparent examples of such meso-scale structures in the edge and the Scrape-off Layer (SOL) plasmas are Edge Localized Modes (ELMs) and blobs, and pellet clouds in the core of fusion devices. Significant amount of theoretical and computational work on the physics of coherent structures has been done to date. It was found that in many cases, an effective gravity (e.g. due to curvature effects in a tokamak) plays a very important role in the dynamics of the evolution of the structures. It turns out that reduced 2D models with different closures accounting for the parallel plasma dynamics give tractable and useful approach to understand the main features of meso-scale structures. In particular, the dynamics of the propagation of ELMs and blobs in the SOL plasma is understood rather well. However, while the origin of ELMs is widely assumed to be due to peeling-ballooning instabilities, the generation mechanism (-s) of blobs are still dim. Here we review the models of blobs developed so far and present new both analytic and modeling results describing the mechanisms of the blob generation triggered by sub-critical phenomena related to the ballooning drive.