Modelling of (2,1) NTM dynamics with flow in JET advanced scenarios

Abstract

Experimental observations show that the βN threshold for (2,1) NTM excitation is increased by flow shear, but the physical explanation for this trend is still unclear. In this work, we investigate this issue by performing numerical experiments addressing the dependence of the critical island width on toroidal plasma rotation with the full MHD toroidal code XTOR (Lütjens and Luciani 2010 J. Comput. Phys. 229 8130–43), on the basis of a typical JET advanced tokamak case. We find that for situations where the Lundquist number is increased towards the experimental value, the (2,1) NTM is weakly destabilized by flow shear at low magnetic Prandtl number Prm, while the threshold remains nearly insensitive to the flow at high Prm. This weak effect of rotation shear also holds close to the linear regime, where an equivalent of the Δ′ concept adapted to nonlinear simulations does not indicate any significant variation with flow shear. The experimental trend is therefore not recovered, and possible explanations for this disagreement are discussed. A simple model of anisotropic viscous tensor shows that the high toroidal viscosity does not influence the value of the threshold, but comparison with experimental measurements suggests that the effective Prm seen by the mode is, however, larger than its small collisional value. Finally, the scaling of dimensionless parameters to ITER range is discussed.

Bifurcation of magnetic island saturation controlled by plasma viscosity

“…The saturation can be compared with two important characteristic widths: the viscoresistive width and the curvature width. The visco resistive width 1/6 1/3 Prm δ η ∝ ην [6] is evaluated from the computed fields as the distance between inflexion points of the perturbed poloidal magnetic field, as described in [20]. At S 10 0 7…”

Control of neo-classical double tearing modes by differential poloidal rotation in reversed magnetic shear tokamak plasmas

“…Chandra et al investigated the influence of the toroidal flow on the tearing mode instability in toroidal geometry, indicating that the toroidal flow has a stabilizing effect due to the modification in the q-profile arising from a flow induced 'Shafranov' like shift [24,25]. More recently, the attention has shifted from the classical tearing mode to the neo-classical tearing mode [26][27][28][29][30]. Experimental observations in the spherical torus NSTX showed that the toroidal rotation shear can increase the neoclassical drive required for NTM onset for various trigger types [31].…”

Neoclassical viscous stress tensor for non-linear MHD simulations with XTOR-2F

“…Non-linear MHD modelling has addressed several aspects of the NTM issue, such as the non-linear threshold (i.e. critical island width), the saturation size or the coupling between NTMs [4][5][6][7][8][9][10][11][12][13]. In these works, the MHD model describes the dynamics of a single fluid, with an ad hoc bootstrap current in Ohm's law.…”