(TH/P8-46) Global Gyrokinetic Simulations of ρ* and ν* Scalings of Turbulent Transport

Y. Sarazin1), G. Dif-Pradalier1), V. Grandgirard1), P. Angelino1), X. Garbet1), Ph. Ghendrih1), R. Belaouar2), N. Crouseilles2), G. Latu2), E. Sonnendrücker2), S. Jolliet3), B.F. McMillan3), T.M. Tran3), L. Villard3)
1) CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
2) IRMA-Université Louis Pasteur and INRIA Lorraine, 7 rue René Descartes, 67084 Strasbourg Cedex, France
3) CRPP, Association Euratom-Confédération Suisse, EPFL, 1015 Lausanne, Switzerland

Abstract.  Predicting the confinement time in next step controlled fusion devices such as Iter requires to know how the turbulent transport depends on key scaling parameters, more specifically on ρ*, the ratio of the ion Larmor radius to the machine size a, and on the collisionality nustar. Such a scan is performed with the global gyrokinetic code GYSELA, which solves the standard gyrokinetic equation for the full distribution function of the ions with adiabatic electrons. The code achieves more than 80% efficiency on 4 096 processors. Gyrokinetic simulations have predicted the transition from Bohm to gyroBohm scaling when decreasing ρ*, the open issues remaining the precise value of this transition and the underlying physics. Such a scan is performed with GYSELA. While the correlation time is roughly independent of ρ*, the correlation length λc looks consistent with the gyroBohm scaling at small ρ* and well above the linear threshold. Conversely, at large values of ρ* and close to the threshold, λc exhibits a dependence on the system size, consistent with the Bohm scaling. When examining the latter regime, one finds that the gyroBohm scaling is more relevant at small ρ*, suggesting that the transition value between Bohm and gyroBohm scaling is close to ρ* = 0.01 - 0.02 and might depend on the distance to the threshold. Interplay between neoclassical and turbulent transport is addressed with ν* scans. A Fokker-Planck operator has been added to GYSELA, acting in the parallel velocity space only. It is shown analytically that the neoclassical transport coefficients are recovered with such an ion-ion collision operator in the three collisional regimes, banana, plateau and Pfirsch-Schlütter. Below the critical Ion Temperature Gradient for the onset of turbulence, the poloidal equilibrium velocity is found to reverse sign when increasing ν* from the banana to the Pfirsch-Schlütter regime, in good agreement with theoretical predictions. Collisions modify the response of the zonal flows, as well as the turbulent heat flux. The dependence of the turbulent heat flux on ν* is presently being analyzed for these simulations. It is found to combine the reduction of the linear growth rate, the damping effect on zonal flows as well as the spreading into the stable boundary regions and the profile relaxation.

Full paper available (PDF)