Global gyrokinetic ion temperature gradient turbulence simulations of ITER

• Published in: Plasma physics and controlled fusion 2013-07, Vol.55 (7), p.74017-8
• Main Authors: Villard, L, Angelino, P, Bottino, A, Brunner, S, Jolliet, S, McMillan, B F, Tran, T M, Vernay, T
• Format: Article
• Language: English
• Subjects: Circularity; Computational fluid dynamics; Computer simulation; Elongation; Fluid flow; Ion temperature; Turbulence; Turbulent flow
• ISSN: 0741-3335; 1361-6587
• DOI: 10.1088/0741-3335/55/7/074017

Global gyrokinetic simulations of ion temperature gradient (ITG) driven turbulence in an ideal MHD ITER equilibrium plasma are performed with the ORB5 code. The noise control and field-aligned Fourier filtering procedures implemented in ORB5 are essential in obtaining numerically healthy results with a reasonable amount of computational effort: typical simulations require 109 grid points, 109 particles and, despite a particle per cell ratio of unity, achieve a signal to noise ratio larger than 50. As compared with a circular concentric configuration with otherwise similar parameters (same ρ* = 1/720), the effective heat diffusivity is considerably reduced for the ITER MHD equilibrium. A self-organized radial structure appears, with long-lived zonal flows (ZF), modulating turbulence heat transport and resulting in a corrugated temperature gradient profile. The ratio of long-lived ZF to the fluctuating ZF is markedly higher for the ITER MHD equilibrium as compared with circular configurations, thereby producing a more effective ITG turbulence suppression, in spite of a higher linear growth rate. As a result, the nonlinear critical temperature gradient, R/LTcrit,NL, is about twice the linear critical temperature gradient, R/LTcrit,lin. Moreover, the heat transport stiffness above the nonlinear threshold is considerably reduced as compared with circular cases. Plasma elongation is probably one of the essential causes of this behaviour: indeed, undamped ZF residual levels and geodesic acoustic mode damping are both increasing with elongation. Other possible causes of the difference, such as magnetic shear profile effects, are also investigated.