This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support a twenty-four-month research fellowship by Dr. Kyle B. Gustafson to work with Dr. Paolo Ricci at Ecole Polytech Federale in Switzerland.
Plasma turbulence is important in many contexts, from transport of heat and particles in fusion energy research to heating of astrophysical plasmas. An unresolved question of current interest is whether turbulent transport can generally be described using the standard diffusion equation derived from the assumptions of a Brownian random walk. Subtler descriptions of transport in terms of continuous time random walks and fractional diffusion equations have recently been applied to simulations of plasma turbulence. For example, continuous time random walks allow the distribution of step sizes and waiting times for the walkers to be non-Gaussian, possibly with power law dependence. Fractional diffusion equations describe the macroscopic propagation of an ensemble of particles using nonlocal, integro-differential operators in a generalization of the diffusion equation. The results so far have been promising, but comparison with actual experiment and observation has been limited by diagnostic possibilities. This International Research Postdoctoral Fellowship in Lausanne, Switzerland is intended to facilitate a search for anomalous, or non-Brownian, transport indications in a basic plasma experiment and, simultaneously, in simulations of the experiment. This approach will help confirm or deny the relevance of non-Brownian transport in a fundamental experiment, TORPEX, with an unprecedented set of diagnostic tools. It will also verify and validate simulations against highly resolved data from the experiment, while using the simulations to guide the focus of the experimental operation. We use diagnostics including a large Langmuir probe array, fast-ion injection and gas puff imaging with a fast camera. Simulations are based on three-dimensional fluid equations with passive particle tracking in the generated fields. We address the following questions. How do the energy, mass and charge of particles affect the relevance of non-Brownian models for describing transport of those particles? How do the parameters of the device, such as field topology and gradients of temperature and density, affect non-Brownian transport? To what extent do computational results and non-Brownian transport models explain the high-resolution data from TORPEX in various conditions and how can these comparisons be generalized?
The Centre de Recherches en Physique des Plasmas (CRPP) in Lausanne is a major facility for fusion and industrial plasma physics research. The basic toroidal device called TORPEX possesses a completely unique set of diagnostics especially for studying the movement of particles in magnetized turbulence. Simulations of TORPEX have been developed and are ready to be used for particle tracking studies. The CRPP computational facilities are well-equipped for this purpose. This research synergizes with the large and growing plasma physics efforts in Europe, including the landmark ITER experiment. The results of these detailed experiments on plasma microturbulence will be shared with a variety of communities in the U.S. Results from the study of non-Brownian transport will be relevant to transport of turbulent heat and momentum and turbulent heating. In astrophysics, a better understanding of particle energization and heating will be achieved. Fusion energy research depends critically on a proper characterization of particle transport, since an effective machine must have a large energy density in the core. This characterization will be improved by the analysis of the data and simulations of TORPEX in this research.
