Some controversy exists over the role of weak or reversed shear in the formation of internal transport barriers. One theory attributes the formation of internal transport barriers to a gap in global wave structures in the minimum-q region. It is shown that in general no such gap exists, and that energy transport is smooth and increasing across such a minimum-q region.

This is the final report of a panel established as a subcommittee of the U. S. Department of Energy (DOE) Fusion Energy Sciences Committee (FESAC) on Integrated Simulation and Optimization of Magnetic Fusion Systems (ISOFS). The report was requested by the DOE in February 2002 and the approved report was transmitted to the DOE by the FESAC in December 2002. The report addresses the challenge of how to “develop fully integrated capability for predicting the performance of externally-controlled systems including turbulent transport, macroscopic stability, wave-particle physics, and multi-phase interfaces.”

Experimental measurements of electron transport and isochoric heating in 100 J, 1 ps laser irradiation of solid A1 targets are presented. Modeling with a hybrid PIC code is compared with the data and good agreement is obtained using a heuristic model for the electron injection. The relevance for fast ignition is discussed.

The motivations, history, and development of the Monte Carlo algorithm will be described by one of the originators. Early results on equations of state and other applications will be reviewed.

In this paper, we report on an integrated program of experimental, computational, and theoretical studies of sheared zonal flows and radially extended convective cells, with the aim of assessing the results of theory–experiment and theory–simulation comparisons. In particular, simulations are used as test beds for verifying analytical predictions (specifically locality and directionality of energy transfer) of nonlinear dynamics and to investigate the suitability of bispectral analysis for studying nonlinear couplings. Initial comparisons to experimental results are presented, and future experimental studies are motivated. We also present analytic and numerical work investigating the role of structures in transport.

A planar slab of material, for which both the permittivity and permeability have the values of −1, can bring not only the propagating fields associated with a source to a focus, but can also refocus the nonpropagating near fields, thereby achieving resolution beyond the diffraction limit. We study the sensitivity of this subwavelength focus to the slab material properties and periodicity, and note the connection to slab surface plasmon modes. We conclude that significant subwavelength resolution is achievable with a single negative index slab, but only over a restrictive range of parameters.

Several problems in the physics of L→H transition and pedestal formation are examined using a simple and universal sandpile model, which incorporates key features of a confined plasma, namely, diffusion, shear induced bistability of turbulent transport, and a local magnetohydrodynamic (MHD) limit on the gradient. The main focus of this study is the effect of ambient diffusion, representative of neoclassical transport, on hysteresis and edge relaxation phenomena. The transport function of the sandpile bifurcates to a multivalued function with increasing deposition, and, as a consequence, a hysteresis in the L→H→L transition is observed. With pedestal formation, diffusive losses increase at the expense of the turbulent flux. This effect prolongs the time needed to reach the MHD stability boundary, and thus provides a positive feedback on the pedestal. The gradient in the pedestal is more rigid and, due to diffusive smoothing, can reach the critical value at all radii simultaneously. Hence an avalanche, starting at the edge, can span the entire pedestal, thus destroying it. The transport in the core is essentially unaffected by the diffusion.

Two examples of non-perturbative models of intermittency in drift-wave (DW) turbulence are presented. The first is a calculation of the probability distribution function (PDF) of ion heat flux due to structures in ion temperature gradient turbulence. The instanton calculus predicts the PDF to be a stretched exponential. The second is a derivation of a bi-variate Burgers equation for the evolution of the DW population density in the presence of radially extended streamer flows. The PDF of fluctuation intensity avalanches is determined. The relation of this to turbulence spreading, observed in simulations, is discussed.

A set of reduced equations is derived for use in simulations of electromagnetic turbulence in tokamaks, in which the electron transit time scale is removed. The time step is not subject to a Courant condition depending on the electron thermal speed, but instead depends on the Alfvén speed. This allows significantly longer time steps in explicit numerical methods, for relevant values of the ratio of electron pressure to magnetic pressure.

A newly developed continuum gyrokinetic code GYRO has been formulated on a radial grid to operate at finite relative gyroradius in a noncyclic radial annulus with profile variation. The code is used to simulate ion temperature gradient mode turbulence and to demonstrate that gyroBohm scaling can be obtained well above the instability threshold but sufficiently strong profile shear stabilization can break gyroBohm scaling down to Bohm scaling (or worse) near threshold. An adaptive source technique is used to maintain profiles. Clear evidence for nonlocal transport is also found in which the local diffusivity depends on the plasma gradients at some considerable radial distance.

High-pressure gas-jet injection of neon and argon is shown to be a simple and robust method to mitigate the deleterious effects of disruptions on the DIII-D tokamak. The gas jet penetrates to the central plasma at its sonic velocity. The deposited species dissipates >95% of the plasma by radiation and substantially reduces mechanical stresses on the vessel caused by poloidal halo currents. The gas-jet species-charge distribution can include >50% fraction neutral species which inhibits runaway electrons. The favorable scaling of this technique to burning fusion plasmas is discussed.

A sandpile with two stable and two unstable ranges of slopes is presented as a minimal model for the study of H-mode pedestal formation and dynamics. Pedestals are observed to form and expand inward with increasing deposition. Transport bifurcation is not critical to pedestal formation, though the pedestal structure obtained with a second, hard stability boundary is qualitatively different from that found in standard sandpiles. Nonperiodic particle ejection events are observed, but do not initiate pedestal collapse. Pedestal formation alters the spectrum of transport avalanches.

Termination of the runaway electron current generated during plasma disruptions is found in JT-60U during simulated vertical plasma displacement events where the safety factor at the plasma surface qs decreases. For all discharges with runaway electron generation, the runaway current disappears for qs⩾2 with the appearance of spikes in the magnetic fluctuations. The growth rate of the spikes in the magnetic fluctuations decreases by an order of magnitude during the termination of runaway current. Corresponding to the loss of runaway electrons by magnetic fluctuations, heat flux pulses are measured at the inner divertor plates, which indicates interaction of the wall with the runaway electrons. The halo current during runaway termination is small and increases after runaway termination with a dominant toroidal mode of n = 1.

For forty years, plasma turbulence theory has been based on notions of ‘mixing length’, ‘diffusion’, and other ideas from mean field theory. However, there is now a plethora of evidence, from both simulations and experiments, that plasma turbulence is strongly intermittent and that transport is bursty, and composed of a multiplicity of "events" of differing scales. In this case, "predictive modelling via transport coefficients" is an oxymoron. Thus, a fundamentally probabalistic treatment is necessary, i.e. one should aim to calculate probability distribution function (pdf) of flux, rather than an anomalous diffusion coefficient. Of particular interest in this approach is the tail of the pdf, which describes large ‘events’ or avalanches. In this paper, we discuss recent progress in formulating a probabalistic theory of drift wave turbulence and transport. We also report on progress in understanding the more general question of structure formation, particularly in the context of sheared flow.