The role of the radial magnetic drift in toroidal ITG turbulence

It is well known that, in toroidal systems, the radial magnetic drift is important for the linear physics of zonal flow (e.g. the Rosenbluth-Hinton residual [1] or geodesic acoustic modes). We here study its role on the nonlinear saturation of zonal flows and ITG turbulence. First, we show how the upshift of the nonlinear ITG critical gradient known as Dimits shift [2], caused by zonal flow that fully suppresses ITG turbulence close to marginality, is strongly dependent on the radial magnetic drift. Secondly, for ITG turbulence far from marginality, we show that the turbulence radial correlation length set by critical balance follows a new scaling set by zonal flow physics, contrary to previous theories assuming perpendicular isotropy [3] or grand critical balance [4]. The anisotropy between the radial and binormal length scales leads to revised scalings, e.g. of the heat flux with the temperature gradient, which is verified using the codes GS2 [5], stella [6], and GX [7,8]. We will show that the radial magnetic drift affects the turbulence through its impact on the zonal flow, and not due to its effect on the non-axisymmetric turbulent fluctuations.

[1] Rosenbluth, M.N. (1998) PRL 80, 724

[2] Dimits, A.M. (2000) PoP 7, 969

[3] Barnes, M. (2011) PRL 107, 115003

[4] Ghim, Y. (2013) PRL 110, 145002

[5] Dorland, W. (2000) PRL 85, 5579

[6] Barnes, M. (2019) JCP 391, 365

[7] Mandell, N.R. (2018) JPP 84, 905840108

[8] Mandell, N.R., et al. (2022) arXiv:2209.06731