A particle-in-cell code being developed through a collaboration between the PFC and Keldysh Institute in Moscow is running and able to model the scrape-off layer of diverted tokamaks. Kinetic codes are capable of modelling behavior far from equilibrium where distribution functions are highly non-Maxwellian. Such non-equilibrium behavior occurs near material walls of such divertor target plates and regions of strong spatial variation of plasma parameters. This unique code is the only kinetic code able to model detachment of the edge plasma from the target. The PFC theory group has been recognized for its leadership and is charged by DOE to lead the national Divertor Task Force.
In the MHD area of theoretical studies (Prof. Jeffrey Freidberg and Dr. Jesus Ramos) the quest for a steady state, advanced tokamak operating regime has been advanced through collaboration with numerical specialists from the Keldysh Institute for Applied Mathematics in Moscow who provided several MHD stability codes with the capability of dealing with diverted tokamaks. Collaborations to apply the codes to Alcator, ITER, and TPX-like tokamaks, in search of stabilization of the external kink modes in plasmas with high self-generated ("bootstrap") current fractions are in place. Work is also being carried out to investigate effects of the plasma toroidal rotation on tokamak equilibrium and stability. A collaboration with the Plasma Research Institute in Ahmedabad, India has been initiated in this area.
Theoretical research into the cause of disruptions and its relationship to halo currents has been initiated by Prof. J. Freidberg and graduate students. Avoidance of disruptions would be of great potential benefit to ITER. Prof. Freidberg has also continued to expand his research activities in the area of superconducting magnet design. The problems of interest involve the development of an explanation for the phenomenon of "thermal hydraulic quench back" and basic theoretical studies of the important operating restrictions associated with "ramp rate limits" (with post-doc Ali Shajii and graduate students). A new research activity involving the vitrification of high level nuclear waste by means of an electrodeless melter has also been undertaken. By using the principles of MHD in combination with well established ideas from magnetic fusion, it should be possible to design and construct a high efficiency, high reliability, induction melter even though the molten glass has an electrical conductivity comparable only to sea water. (with Prof. Kevin Wenzel and Dr. A. Shajii).
The RF Theory Group (Prof. Abraham Bers and Dr. Abhay K. Ram) has continued and extended its studies on their original work of high-efficiency mode conversion from fast Alfvén waves (FAW) to ion-Bernstein waves (IBW) for plasma heating and current drive. The (ideal) possibility of achieving 100% mode conversion, as reported last year, was shown to correspond to a critically coupled, internal (to the plasma) resonator containing the mode conversion layer. An extended analysis, including the coupling to an external antenna, has brought into evidence a global resonator system that encompasses the internal one with the mode conversion layer. This new model analysis has been used successfully to understand recent experiments on Tore-Supra (Cadarache, France), that show intense electron heating in the presence of high efficiency mode conversion from FAW to IBW. Our one-dimensional model results also compare favorably with (and explain!) the computational results obtained from the three-dimensional, full-wave, toroidal code ALCYON (France) applied to these experiments.
On the theoretical side, the pioneering work on internal collective modes that can destroy locally the magnetic field confinement configuration have been recognized by the international community. This is a particularly serious issue for the stated objectives of the ITER project. The theoretical transport model that Prof. Coppi proposed in the 1970's, involving the excitation of the so-called ITG modes (toroidal ion temperature gradient driven modes) and trapped electron modes, has found wide acceptance and has been incorporated recently in several transport codes that are used to interpret present experiments. Other theoretical ideas of importance to experimental studies include the so-called "isotopic effect" on plasma confinement (confinement is observed to improve with heavier isotopes of hydrogen). It has also been found to be present in the transport of angular momentum in rotating plasmas. Finally, in the past year significant progress was made in understanding transport in high field devices, such as C-Mod and FTU.
Studies have concentrated on two specific physics areas. The first is the evolution of turbulence during the transition from the low (L) to the high (H) mode of confinement. The transition is found to be accompanied by a reduction in the average amplitude of the line-integrated fluctuations. In the frequency domain, two distinct spectral features were identified: a low-frequency ([[sterling]] 20 kHz) band, which is generally unaffected by the transition, and a broad high-frequency region, which is strongly suppressed in H--mode. In addition, it was determined that the L-mode fluctuations have a nonzero average group velocity in the inward direction. The second area of interest is a periodic H-mode instability known as edge localized mode (ELM), which plays a critical role in the ongoing design of the demonstration reactor ITER. This phenomenon is accompanied by a burst of turbulence whose spatial and spectral structure can be studied by the PCI diagnostic. In particular, the so-called type--III ELM is seen to be characterized by fast coherent modes propagating in the outward direction.
PFC researchers have shown that a comet-shaped tokamak is expected to have improved MHD properties in addition to improved confinement. Furthermore, the oblate shape tends to form an inner x-point, and a divertor design could incorporate a much larger divertor slot length than is possible in the ITER-type geometry. The comet optimizes at low aspect ratio. A reactor based on this concept has a relatively low plasma current and the center post can be shielded. Thus a comet-shaped tokamak may lead to a low aspect ratio reactor, with low plasma current, good confinement and improved reactor features. A three-year experimental program has been developed in detail at the $1.5 - 2.0M per year level. A review of this proposal is in progress.
Finally, Visiting Scientist Dr. Fredrick Seguin is developing a new class of small, radiation resistant x-ray detectors based on photoconductors that could comprise the individual picture elements needed for future x-ray imaging arrays of reactor-based systems.
The group also expanded its work in theoretical physics. For example, two Physical Review Letters were recently published by C. Li and R. Petrasso wherein some of the basic properties of moderately coupled plasmas were delineated (plasmas that occur in the interior of stars and in inertially confined fusion). Such plasmas have relatively high electron densities (n >1023 cm-3) and low electron temperatures (T < 104 eV). Theoretical effort is now directed at enhancing our understanding of the properties of moderately coupled plasmas through improved calculations of characteristic relaxation times and transport coefficients (e.g. the thermal conductivity). This work was also recently featured in an article entitled "Simply Plasma" in Science News (1994). In two review articles in Nature, Richard Petrasso summarized the progress and challenges that confront both laser and magnetic confinement fusion.
In the series of experiments in December 94 and January-February 1995 we obtained information on total plasma radiation from Versator as a function of different impurity injection. These data were analyzed and results indicated, as expected, the strong impact of impurity concentration on total plasma radiation. However the most important data on radiation in the region near 13 nm were not obtained due to the low sensitivity of the detector. Presently we are bench-testing a much more sensitive detector which should allow us to measure time depended radiation in a narrow bandwidth near 13 nm. Our main goal is to obtain data for radiation near 13 nm as a function of impurity concentration and plasma parameters (primarily electron temperature and density). We will use these data for calculating radiation fluxes in a tokamak specially designed as a radiation source. We plan to prepare a proposal for submission to various government agencies.
This year the majority of the Division's work continued to focus on magnetics R&D for the two main Department of Energy, Office of Fusion Energy supported next step tokamak projects, namely the International Thermonuclear Experimental Reactor (ITER), and the U.S. Tokamak Physics Experiment (TPX).
In-house research for both programs concentrates on superconductor development, subscale testing, and magnet design and analysis. Significant results have been obtained in understanding the stability limitations of fast ramping the superconducting coils as well as in a new area of developing fiber optic instrumentation for superconducting coil diagnostics. Fabrication of a moderate sized, new test facility, called the Pulse Test Facility (PTF), was begun for pulse testing of large size superconductors and joints for both the ITER and TPX projects. Prof. Ronald Ballinger's Materials Science and Technology Group has begun a new ITER task for detailed mechanical characterization of the superalloy Incoloy 908 which was initially developed in his laboratory for superconducting magnet applications.
Extensive collaboration with U.S. industries continued under the ITER program for fabrication of the U.S. contribution to the model coil program, including sub-contracts with Lockheed Martin, INCO Alloys International, Teledyne Wah Chang, and Intermagnetics General Corp., among others.
Recent congressional actions on the fusion energy budget for next fiscal year indicate substantial reductions are likely. Although, at this time, the main ITER program funding appears secure, the Technology and Engineering Division has begun actively seeking new programs outside the Department of Energy supported fusion program. New initiatives have resulted in funding through INEL (Idaho National Engineering Laboratory) for a large scale, electromagnetic seismic simulator platform, and from Paramag for design of a superconducting magnetic separation magnet. The Basic Energy Sciences Department of South Korea has expressed interest in MIT assisting them in the development of a new superconducting tokamak called StarX. This is a good match to our relevant experience from the TPX and ITER programs. The Division also has active proposals in several other areas of magnet technology which are likely to result in near level personnel support into the next fiscal year.
PLASMA TECHNOLOGY AND SYSTEMS DIVISION
MIT
Reports to the President 1994-95