Kinetic studies of magnetized plasmas in contact with walls

Thermonuclear Fusion, to be achieved in hot plasmas confined by strong magnetic fields, offers the perspective of a safe, practically unlimited and potentially clean source of energy. For both present-day and planned fusion devices, the questions associated with the contact between the plasma and the wall constitute one of the main scientific and technological challenges. This contact is established by the "plasma-wall transition (PWT)" zone, by which we mean the region extending from the "unperturbed" scrape-off layer (SOL) plasma (sufficiently far from the wall) up to the wall. Generally, the PWT zone, which is of central interest in this project, can be subdivided into the presheath, sheath and plasma-wall interaction zones. Central to the solution of the PWT problem is the need to understand and quantitatively predict the detailed physical processes occurring in this zone. Although strong pertinent efforts have been and continue to be going on in a number of plasma physics laboratories, several key aspects of the problem are as yet unsolved, and it is here that the present project purports to contribute. The basic physical processes occurring in the PWT layer are similar to those studied for over 20 years by the project applicant and leader, Prof. S. KUHN, and his co-workers at Innsbruck University`s Department of Theoretical Physics, in the context of Q machines and plasma diodes. These long-term studies have led to substantial know-how in realistic kinetic bounded-plasma modeling, theory and simulation. In addition, the predecessor project P12477-TPH ("Particle-simulation studies of divertor plasmas", Sept. 1998 -Nov. 2000) has enabled this group to more specifically deepen its knowledge in and contribute to the topic of the divertor plasma. The kinetic investigations of the magnetized PWT proposed here will be performed using an appropriate combination of (a) analytic kinetic theory and nonlinear dynamics, (b) particle-in-cell (PIC) simulation, and (c) distribution-function (DF) simulation. The project, whose duration is envisaged to be two years, aims at achieving the following sci-entific goals. (1) A comprehensive, self-consistent kinetic description of the magnetized PWT un-der realistic assumptions will be formulated in great generality. (2) Based thereon, a general framework for obtaining multi-fluid boundary conditions (of the kind needed for multi-fluid simu-lation codes) will be established and applied to several special cases of concrete research interest. In addition, a number of related problems will be tackled, namely (3) Langmuir probes, (4) fast- particle generation in RF fields and related effects, and (5) anomalous impurity diffusion in the presence of electrostatic field fluctuations. For each problem, the conceptual theoretical work re-quired will be accompanied by adequate sets of simulation runs with input parameters relevant to existing and planned tokamaks, thus contributing to the solution of concrete research problems currently of interest to the fusion physics community. Under the coordination of the project leader, four experienced scientists will be working together locally in pursuit of the project goals, namely Dr. Ulrike HOLZMÜLLER-STEINACKER, Dr. Nikolaus SCHUPFER; Dr. David TSKHAKAYA, and Prof. Davy TSKHAKAYA. This research will proceed in close contact with several pertinent research groups both from Austria (Innsbruck, Vienna) and abroad (Berkeley, Cadarache, Garching, Greifswald, Livermore, Prague, St. Petersburg, Varennes), with which collaborations already exist. In addition, the project workers will be open to establish collaborations with other researchers and groups whose expertises and interests are relevant and beneficial to the present project.

This project was primarily devoted to conceptual and detailed studies of the magnetized plasma-wall transition (PWT) and other regions of the tokamak scrape-off layer (SOL). It has produced a large body of new results, to be incorporated next into the framework of Integrated Tokamak Modeling (ITM). Thermonuclear Fusion in hot plasmas confined by strong magnetic fields offers the prospect of a potentially unlimited, safe and clean source of energy. Today`s most promis-ing fusion configuration is the "tokamak", a toroidal vessel containing the hot core plasma surrounded by the cooler SOL, which is bounded by the divertor plates. At present, the main goal of the world-wide fusion effort is the construction of a next-generation experi- mental facility of the tokamak type (ITER), for which the questions of the contact between the plasma and the divertor plates are among the primary scientific and technological chal-lenges. The PWT layer, i.e., the region directly adjacent to the divertor plates, controls the particle and energy fluxes between them and the SOL plasma and, hence, is crucial both for the life expectancy of the expensive divertor plates, for energy exhaust from the toka-mak, and for the overall plasma confinement. It is mainly (but not only) in this context that the present project set out to improve the understanding of the processes occurring in the PWT zone and other parts of the SOL, thus contributing to solving the scientific and technological issues of energy exhaust from the tokamak plasma. The relevance and timeliness of the research was ensured by close collaboration with a number of high-ranking fusion labs, such as IPP Garching and Greif-swald (Germany), JET (Abingdon, U. K.), and CEA (Cadarache, France). The central sci-entific tools used were (i) kinetic plasma theory, and (ii) the properly adapted and improved particle-in-cell (PIC) simulation codes XPDP1 and XPDP2 from Berkeley, USA. The main project results can be summarized as follows: (a) We have investigated a number of unmagnetized and magnetized, time-independent and time-dependent PWT models and (b) derived related boundary conditions for fluid codes simulating the entire SOL plasma. (c) PWT theory and simulation have been applied to calculating detailed Volt-Ampere characteristics of Langmuir probes in collision- dominated plasmas. (d) Our studies of parasitic energy absorption at lower hybrid wave plasma heating have led to new ideas on how to suppress this unwanted effect. (e) We have shown that anomalous diffusion induced by potential fluctuations can gener-ate a radial electric field and, hence, lead to plasma rotation. (f) Kinetic simulations of the SOL with and without edge-localized modes (ELMs) have re-vealed a number of new kinetic effects needing further clarification. In particular, the two-timescale-structure of ELM-induced pulses has been clearly demonstrated.