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Spizzo, G.; White, R.B.; Cappello, S.; Marrelli, L.
Princeton Plasma Physics Lab., Princeton, NJ (United States). Funding organisation: USDOE Office of Science (United States)2009
Princeton Plasma Physics Lab., Princeton, NJ (United States). Funding organisation: USDOE Office of Science (United States)2009
AbstractAbstract
[en] Several heuristic models for nonlocal transport in plasmas have been developed, but they have had a limited possibility of detailed comparison with experimental data. Nonlocal aspects introduced by the existence of a known spectrum of relatively stable saturated tearing modes in a low current reversed field pinch offers a unique possibility for such a study. A numerical modeling of the magnetic structure and associated particle transport is carried out for the reversed-field pinch experiment at the Consorzio RFX, Padova, Italy. A reproduction of the tearing mode spectrum with a guiding center code1 reliably reproduces the observed soft X-ray tomography. Following particle trajectories in the stochastic magnetic field shows the transport across the unperturbed flux surfaces to be due to a spectrum of Levy flights, with the details of the spectrum position dependent. The resulting transport is subdiffusive, and cannot be described by Rechester-Rosenbluth diffusion, which depends on a random phase approximation. If one attempts to fit the local transport phenomenologically, the subdiffusion can be fit with a combination of diffusion and inward pinch. It is found that whereas passing particles explore the stochastic field and hence participate in Levy flights, the trapped particles experience normal neoclassical diffusion. A two fluid nonlocal Montroll equation is used to model this transport, with a Levy flight defined as the motion of an ion during the period that the pitch has one sign. The necessary input to the Montroll equation consists of a time distribution for the Levy flights, given by the pitch angle scattering operator, and a distribution of the flight distances, determined numerically using a guiding center code. Results are compared to experiment. The relation of this formulation to fractional kinetics is also described.
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21 Sep 2009; 30 p; ACO2-09CH11466; Also available from OSTI as DE00965276; PURL: https://www.osti.gov/servlets/purl/965276-GrUtpO/; Plasma Physics Control Fusion (June 2009); doi 10.2172/965276
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