Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session I16: Sherwood II |
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Sponsoring Units: DPP Chair: Zhi Hong Lin, U.C. Irvine Room: Hyatt Regency Dallas Landmark D |
Sunday, April 23, 2006 10:30AM - 11:00AM |
I16.00001: BREAK
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Sunday, April 23, 2006 11:00AM - 11:30AM |
I16.00002: Renormalization of plasma turbulence leading to fractional differential equations Invited Speaker: Several phenomenological models for plasma turbulent transport have been recently proposed that are based on the use of fractional differential operators. These operators provide with a mathematical framework that may outperform the standard diffusive paradigm when it comes to capturing much of the strange phenomenology encountered in tokamak plasmas: canonical profiles, anomalous confinement scalings, off-axis peaking, etc. The reason is simple: fractional operators are designed to model transport mechanisms that lack characteristic time and/or spatial scales, which appears to be the situation pointed to as relevant by the observed tokamak phenomenology. However, it has remained always unclear how fractional operators (non-local and non-Markovian in nature) can be derived from (and reconciled with) the usual equations for mass, moment and energy turbulent transport. In this contribution we start from the simplest turbulent transport equation (the advection of tracers by a turbulent flow with prescribed statistics) and show how to establish such connection by developing a new renormalization scheme that avoids the limitations of standard quasilinear renormalization theory. This result not only establishes the missing physical link with the phenomenological models but also suggests more efficient avenues for transport modeling in both real experiments and state-of-the-art transport codes. [Preview Abstract] |
Sunday, April 23, 2006 11:30AM - 12:00PM |
I16.00003: Parametric Dependencies of Transport Using Gyrokinetic Simulations Including Kinetic Electrons Invited Speaker: Nonlinear gyrokinetic simulations are used to systematically study the effects of ExB shear, magnetic shear, safety factor q, Ti/Te, collisionality, plasma beta, and shaping on turbulent energy, particle, and momentum transport due to ion temperature gradient (ITG) and trapped electron modes (TEM) using the GYRO code [1]. Previous work has tended to focus on ITG modes with adiabatic electrons for a single reference case. Here, we report on over 300 nonlinear kinetic electron simulations to be used for benchmarking and transport model development. In simulations varying q, the energy transport exhibits a linear q-scaling while the particle diffusivity can be insensitive to q. For shifted circle geometry, the effect of ExB shear on both ITG and TEM transport is well modeled by a simple quench rule. The quench rule needs to be modified, however, for real geometry. The nonlinear results are compared against quasilinear (QL) diffusivity ratios to assess the accuracy of QL theory on a per-mode basis.\newline \newline In collaboration with R.E. Waltz and J. Candy, General Atomics. \newline \newline [1] J. Candy and R.E. Waltz, Phys. Rev. Lett. \textbf{91}, 045001 (2003). [Preview Abstract] |
Sunday, April 23, 2006 12:00PM - 12:30PM |
I16.00004: Long time simulations of microturbulence Invited Speaker: Physics and numerical issues associated with the long time simulations of microturbulence are addressed in the present paper. Firstly, the applications of the Fluctuation-Dissipation Theorem for the noise properties in a nonlinearly saturated system and their subsequent verifications using gyrokinetic particle simulation are discussed. The important finding here is that discrete particle noise, when using insufficient number of particles, will always enhance the transport. Secondly, the use of the global toroidal gyrokinetic particle simulation code (GTC) [1] for investigating the long time behavior of microturbulence is presented. The central issues are: 1) what are the nonlinear physics that are responsible for the steady state turbulence and 2) their numerical convergence and noise properties. The physics focus here is on the ion temperature gradient (ITG) drift turbulence. The effects of the often-neglected velocity space nonlinearity [1] on the production of zonal flow, energy conservation, the evolution of the steady state turbulence and, consequently, the resulting thermal transport are studied. The convergence studies using very large number of particles per cell (from 10 to 1000) have also shown that numerical noise play very insignificant role in the observed steady state thermal transport. This work is supported by the DoE SciDAC GPS Center. \newline \newline [1] Z. Lin et al. SCIENCE 281, 1835 (1998). \newline [2] W. W. Lee, Bull. Am. Phys. Soc. Bull. Am. Phys. Soc. $<$49$>$, No. 8, 135 (2004). [Preview Abstract] |
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