Speaker
Plamen Ivanov
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1098.pdf
Zonal Flow — Drift Wave Interactions in 2D 2-Fluid ITG
P.G. Ivanov1,3 , A.A. Schekochihin1,2 , A.R. Field4
1 Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Rd, Oxford OX1
3NP, UK
2 Merton College, Oxford OX1 4JD, UK
3 St John’s College, Oxford OX1 3JP, UK
4 EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon OX14
3DB, UK
It is believed that most of the heat transport in tokamak plasmas is caused by turbulence.
Numerical simulations have shown that turbulence in tokamaks is regulated by the interaction
of 3 different phenomena — shear of the mean plasma flow, zonal flows (ZF) and drift waves
(DW) [1, 2]. The latter represent wave-like fluctuations in the plasma, driven by gradients in
the background plasma parameters — density, temperature, etc. Zonal flows are Larmor-scale
shear flows in the poloidal direction (around the smaller radius of the torus), which are generated
nonlinearly by the drift-wave turbulence itself.
We aim to find an analytical description of the nonlinear interaction of ZFs and DWs and
the phenomena resulting thereafter — the Dimits shift [1] and the recently discovered state
dominated by coherent structures [3]. Our equations are obtained by taking density and temper-
ature moments of the electrostatic gyrokinetic equation in an appropriate cold ion limit (Ti → 0)
2 ρ 2 . The resulting system of two PDEs
and large collisionality expansion νi ω ∼ ω∗ ∼ νi k⊥ i
is shown to exhibit both a linear ion-temperature-gradient instability, driven by a background
magnetic field and temperature gradients, a nonlinear (secondary) instability towards the de-
velopment of strong zonal flows, as well as the break-up of the ZF-dominated state (tertiary
instability). We present analytical results on both the linear and nonlinear behaviour of the sys-
tem, together with numerical validations.
References
[1] A.M. Dimits et al, Nuclear Fusion 40, 3Y (2000)
[2] B.N. Rogers, W. Dorland, M. Kotschenreuther, Physical Review Letters 85, 25 (2000)
[3] F. van Wyk et al, Journal of Plasma Physics, 82, 6 (2017)
