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The Edge Coordinating Committee (ECC) was formed in July 14-15, 2004 when OFES Theory Team invited 14 plasma researchers to a two-day meeting in Germantown, MD to discuss the state of edge-plasma research in the U.S. with a focus on theory and modeling (see http://www.mfescience.org/ecc/ ecc/). At that time, OFES tasked the ECC with providing, in about a six month period, a report on the present status of key issues in this area together with a roadmap of what range of activities should be undertaken in the next five years to resolve these issues. This document is a response to that charge. Future edge-plasma research described here is assumed to fit into a budget constraint of a ''flat budget,'' with some additional activities cited for budget increases of as much as 50%. To obtain some measure of the relative fraction of OFES Theory funding presently devoted to edge plasma research, the OFES Theory Team informally surveyed funded work they support in this area at National Labs, Universities, and industry. John Mandrekas reported to us that approximately 10% of the present budget goes to edge-physics areas at 10 institutions, for a total of {approx}$2.5M each year. While not explicitly estimated, wemore » note that there are also important edge plasma projects in the Plasma Technology program, especially related to plasma/material sputtering interactions, near-surface plasma chemistry and transport, and impurity transport. Likewise, the Confinement Program has important efforts in interpreting edge-plasma data through modeling codes. This initial status/roadmap work of the ECC is complimentary to the Fusion Energy Science Sciences Advisory Committee (FESAC) Panel on Program Priorities (see http://www.mfescience.org/fesac/ fesac/). This Panel is charged with identifying major science and technology issues for all of controlled fusion and also with recommending campaigns to address these issues. The Panel has organized their report along six thematic areas, one of which, termed Plasma Boundary Interfaces, coincides directly with the interests of the ECC. Many of the ECC members have provided input to the FESAC Panel in the edge-plasma area, and an ECC member is on the Panel. The ECC is providing a more detailed examination of edge-plasma issues than is possible in the broad Panel charge and will also work to facilitate ongoing edge-plasma activities beyond the active term on the Panel. While the emphasis of the ECC is initially on theory and model development, there is a strong recognition that comparison and coordination with experiments and experimentalists are essential for meaningful progress; to this end, two members of the ECC are from the experimental community. To gather data for the ECC task of a detailed summary and roadmap, edge-community input has been solicited at two meetings, the first being a workshop at General Atomics on Sept. 27-28, 2004, and the second a community meeting on Nov. 14, 2004 at the APS APS-DPP conference in Savannah, GA. Details of these meetings, topical summaries written by ECC members, and other information on the work of this committee can be found at the ECC website. A consolidation of this material forms the basis of this report. The focus here is on tokamak devices with magnetically diverted scrape-off off-layer (SOL) separating closed and open magnetic field lines. However, there is substantial interest and work on 3D effects that thus begins the expansion of models and tools to include devices with intrinsic 3D variations such as stellarators.« less

1-{1/2}-d BALDUR transport code predictions using recent theoretically-based models for thermal and particle transport are compared to measured profiles of electron plasma density and electron and ion temperatures for TFTR ohmic, L-mode and supershot discharges. The profile consistent drift wave model is found to overestimate ion temperatures at high heating powers, so that a third mode or loss process is needed in addition to drift wave transport (TEM, {eta}{sub i}) and an edge loss model. None of several versions of local multiple mode models, using the 1989 Carreras-Diamond resistive ballooning model, gives T{sub e}, T{sub i} within 20% for all three TFTR regimes studied. 36 refs., 7 figs., 4 tabs.

The computer code DIVERT has been written to trace magnetic field lines in the presence of a divertor. Its purpose is to allow a user to estimate the thickness of the plasma scrapeoff region and to provide a visual mapping of the magnetic field lines near the divertor. Included in the code is the capability to provide auxiliary graphics and compute the field ripple. The code can handle a divertor made up of any arrangement of straight line coil segments and will provide a graph of the field line configuration on output.

The possibility of obtaining ignition in TFTR by means of very centrally peaked density profiles is examined. It is shown that local central alpha heating can be made to exceed local central energy losses (''central ignition'') under global conditions for which Q greater than or equal to 1. Time dependent 1-D transport simulations show that the normal global ignition requirements are substantially relaxed for plasmas with peaked density profiles. 18 refs., 18 figs.

A force balance relation, a representation for the poloidal beta (..beta../sub p/), and expressions for the current densities are derived from the MHD equilibrium relations for an axisymmetric tensor pressure tokamak. Perpendicular and parallel beam pressure components are evaluated from a distribution function that models high energy neutral particle injection. A double adiabatic energy principle is derived from that of Kruskal and Oberman, with correction terms added. The energy principle is then applied to an arbitrary cross-section axisymmetric tokamak to examine ballooning instabilities of large toroidal mode number. The resulting Euler equation is remarkably similar to that of ideal MHD. Although the field-bending term is virtually unaltered, the driving term is modified because the pressures are no longer constant on a flux surface. Either a necessary or a sufficient marginal stability criterion for a guiding center plasma can be derived from this equation whenever an additional stabilizing element unique to the double adiabatic theory is either kept or neglected, respectively.

The nonlinear evolution of ideal MHD internal instabilities is investigated in straight cylindrical geometry by means of a 3-D initial-value computer code. These instabilities are characterized by pairs of velocity vortex cells rolling off each other and helically twisted down the plasma column. The cells persist until the poloidal velocity saturates at a few tenths of the Alfven velocity. The nonlinear phase is characterized by convection around these essentially fixed vortex cells. For example, the initially centrally peaked temperature profile is convected out and around to form an annulus of high temperature surrounding a small region of lower temperature. Weak, centrally localized instabilities do not alter the edge of the plasma. Strong, large-scale instabilities, resulting from a stronger longitudinal equilibrium current, drive the plasma against the wall. After three examples of instability are analyzed in detail, the numerical methods and their verification are discussed.

A reference simulation of a compact ignition tokamak (CIT) limiter discharge is carried out using the 1-1/2-D BALDUR transport code. The parameters for the discharge are as in the CIT-2L design; these parameters are described here, and the results of the simulation are discussed. It is found that plasma ignition can be reached and sustained within the specified constraints.

Transport simulations of the present designs for the INTOR and TIBER ignition devices predict that broad sawtooth oscillations will appear in these experiments. As was noted previously in studies of the Compact Ignition Tokamak, the primary effect of the oscillations is to reduce fusion power production on the average through profile flattening. Due to the disparate time scales for energy and current diffusion between sawtooth crashes, the simulations also produce peaked pressure profiles over a large low shear region inside the q = 1 surface (q is the safety factor). Pressure-driven modes will likely be unstable in this case. 5 figs., 2 tabs.

The successful development of the neutral beam injection method of heating tokamaks has opened up a new range of theoretically predicted phenomena to be explored. This article, intended for the nonspecialist, reviews the existing experimental observations and theoretical understanding of tokamak equilibrium and large scale stability. Then a survey is presented of the new phenomena, such as flux conserving sequences of equilibria and pressure-driven ballooning modes, that are expected to accompany the significantly enhanced plasma pressure to be produced in tokamaks now under construction.

These four papers on high-..beta.. tokamak instabilities, fundamental time scales for flux conserving tokamak heating, intense tokamak heating in the adiabatic approximation, and the Elmo Bumpy Torus were presented at the Workshop on High Beta Tokamaks held at Varenna, Italy on September 1--9, 1977. They will be included in the proceedings of that meeting, to be published by the Department of Energy.

A self-consistent plasma equilibrium model is developed to study the width or ergodic regions and magnetic islands in a periodic cylindrical plasma under the influence of a localized magnetic perturbation, such as that produced by a bundle divertor or ripple coil set. It is found that localized perturbations tend to produce poloidally symmetric annular ergodic regions and poloidally elongated magnetic islands rather than simple magnetic islands. Our plasma model takes into account the flattening of the current profile across each annular ergodic region and the concommitant steepening of the current profile between ergodic regions. Using current profiles inferred from experimental data, saturated tearing mode amplitudes are computed and found to agree with the experimentally observed Mirnov oscillation amplitudes. As the applied magnetic perturbation is turned on and increased, it is observed that the steepened current profile and resulting enhancement of tearing modes produces wider ergodic regions than would be expected from the vacuum magnetic perturbation alone.