New insights into the generalized Rutherford equation for nonlinear neoclassical tearing mode growth from 2D reduced MHD simulations

E. Westerhof, H.J. de Blank and J. Pratt, Nucl. Fusion 56 (2016) 036016 (6pp) http://dx.doi.org/10.1088/0029-5515/56/3/036016.

Two dimensional reduced MHD simulations of neoclassical tearing mode growth and suppression by ECCD are performed. The perturbation of the bootstrap current density and the EC drive current density perturbation are assumed to be functions of the perturbed flux surfaces. In the case of ECCD, this implies that the applied power is flux surface averaged to obtain the EC driven current density distribution. The results are consistent with predictions from the generalized Rutherford equation using common expressions for Δ'bs and Δ'ECCD. These expressions are commonly perceived to describe only the effect on the tearing mode growth of the helical component of the respective current perturbation acting through the modification of Ohm's law. Our results show that they describe in addition the effect of the poloidally averaged current density perturbation which acts through modification of the tearing mode stability index. Except for modulated ECCD, the largest contribution to the mode growth comes from this poloidally averaged current density perturbation.

Redistribution of energetic and impurity ions by sawteeth in ASDEX Upgrade

F. Jaulmes et al, Nucl. Fusion 56 (2016) 112012 (12pp) http://dx.doi.org/10.1088/0029-5515/56/11/112012

In the non-linear phase of the sawtooth, the complete reconnection of field lines around the q = 1  flux surface often occurs resulting in the radial displacement of the plasma core. A complete time-dependent electromagnetic model of this type of reconnection has been developed and implemented in the EBdyna_go code. This contribution aims at studying the behaviour of ions, both impurity and fast particles, in the pattern of reconnecting field lines during sawtoothing plasma experiments in the ASDEX Upgrade tokamak by using the newly developed numerical framework. The results of detailed simulations of the sawtooth crashes are compared with measurements from various diagnostics such as Soft X-Ray (SXR) tomography, Collective Thomson Scattering (CTS) and Fast-Ion D-Alpha (FIDA) spectroscopy.