A gyrokinetic model for the plasma periphery of fusion devices

Understanding the turbulent dynamics of the plasma in the periphery of fusion devices - the region extending from the external part of the closed flux surface region to the scrape-off layer - is of crucial importance on the way to fusion energy. Indeed, this region largely controls the plasma heat exhaust, the plasma refueling, the level of fusion ashes and, also, the low-to-high (L-H) mode transition. In the present talk, a gyrokinetic model is presented that can properly consider the main elements at play in the periphery. These include the presence of strong flows, large and small amplitude electromagnetic fluctuations on scale lengths ranging from the electron Larmor radius to the equilibrium perpendicular pressure gradient scale length, and large deviations from thermal equilibrium. Due to the large range of collisionality present across the periphery, the model retains a nonlinear full Coulomb collision operator cast in a form suitable for implementation in a gyrokinetic code. The gyrokinetic equation and the associated equations for the electromagnetic fields are valid for arbitrary deviations from thermal equilibrium and are accurate at arbitrary perpendicular wavenumber values. The formulation of the gyrokinetic model is based on the single gyrocenter dynamics obtained from fully nonlinear second-order accurate electromagnetic gyrokinetic equations of motion, derived from Lieperturbation theory where the fast particle gyromotion is decoupled from the slow drifts. Then, the collective behavior is obtained by the gyrokinetic equation including the collision operator. The gyrokinetic model takes the form of a set of coupled fluid equations referred to as the gyrokinetic moment (gyro-moment) hierarchy equation. To obtain the gyro-moment hierarchy, the gyro-averaged distribution function is expanded onto a velocity-space Hermite-Laguerre polynomial basis. By projecting the gyrokinetic equation, the gyro-moment hierarchy provides the spatial and temporal evolution of the Hermite-Laguerre expansion coefficients, which are velocity moments of the gyro-averaged distribution function referred to as gyro-moments. The Hermite-Laguerre projection of the full nonlinear Coulomb collision operator is performed using a multipole expansion of the Rosenbluth potentials, allowing us to derive a closed form of the Coulomb collision operator in terms of products between gyro-moments. Finally, the self-consistent evolution of the fields is described by a set of gyrokinetic Maxwell’s equations derived from a variational principle. Linear results of the present model are illustrated and show that the Hermite-Laguerre decomposition provides an efficient framework for a numerical implementation with reasonable cost for the study of the turbulent dynamics in the plasma periphery.

Presented at:
18th European Fusion Theory Conference - EFTC 2019, Ghent, Belgium, October 7-10, 2019
Oct 07 2019
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 Record created 2019-11-11, last modified 2020-05-28

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