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Exploring low-n gyrokinetic simulations

Author: Ronald E. Waltz
Requested Type: Poster Only
Submitted: 2015-01-07 11:30:39

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General Atomics
PO Box 858608
San Diego, CA   92186-5
USA

Abstract Text:
Standard continuum delta-f local gyrokinetic codes like GS2, GYRO, GENE, and GKW are written in a field line following high-n ballooning mode eikonal representation for the amplitudes: δfn(r,θ)exp(inα) where α=φ−q(r)θ is the field line angle, φ the toroidal angle, θ the poloidal angle, n the toroidal mode number, q(r) the safety factor, and r the minor radius flux surface. δfn(r,θ) is assumed to have a slow variation in θ. When operating on δfn(r,θ), the parallel field gradient is (1/Rq)d/dθ with R the major radius , and the radial cross field gradient is d/dr + (inq/r)sθ with s=r(dq/dr)/q the magnetic shear. Most importantly the cross field gradient in the flux surface (inq/r) + (1/r)d/dθ drops the latter slow θ-variation term in the standard high-n gyrokinetic approximation. For nq >> 1 modes, the neglected "low-n" small rho-star term brakes local gyroBohm scaling and is not expected to matter. But what about the n=0 radial (zonal flows and GAM) modes which are known to control the nonlinear saturation of the finite-n drift wave turbulence and transport? The neglected low-n gradient terms for the linear and nonlinear ExB motion have been added to GYRO and novel linear and non-linear low-n gyrokinetic simulations are explored. The n=0 radial modes are now linearly driven (damped?) by the density and temperature gradients and nonlinearly self-coupled. Is the resulting transport higher or lower when the low-n terms are included?

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