Helical core formation and evolution in high-field tokamaks and its extrapolation to ITER

Large, spontaneous m/n = 1/1 helical cores are predicted in tokamaks such as ITER with extended regions of low- or reversed- magnetic shear profiles and q near 1 in the core. The helical core is a saturated internal kink mode; its onset threshold is determined by (dp/dρ)/B_t² = const. along the threshold. Helical cores occur frequently in Alcator C-Mod [1] during ramp-up when slow current penetration results in a reversed shear q-profile. Using a technique for 3D equilibrium reconstruction, the onset and early development of a helical core in C-Mod was reconstructed in a time series. It is found that a reverse shear q-profile as well as a hollow pressure profile reduce the onset threshold, enabling helical core formation. In C-Mod the pressure profile becomes hollow due to impurity radiation in the plasma core, which is also expected to occur in ITER. Beneficial effects can include sawteeth stabilization; helical cores flatten the q-profile, driving it toward q = 1. Combined with plasma rotation this suggests current redistribution, so called flux-pumping, which has been observed in DIII-D helical cores [2] and modeled theoretically [3]. On the other hand fast ion confinement is predicted to degrade [4]. A cross-machine comparison of the helical core onset threshold for discharges from C-Mod, DIII-D and ITER confirms that while DIII-D is marginally stable, C-Mod and especially ITER are highly susceptible to helical core formation. Predictions for ITER show a large helical core with a size of 50% of minor radius in the 15 MA standard H-Mode scenario.

[1] L. Delgado-Aparicio et al., PRL 110, 065006 (2013)

[2] P. Piovesan et al., Nucl. Fusion 57, 076014 (2017)

[3] S.C. Jardin et al., PRL 115, 215001 (2015)

[4] D. Pfefferlé et al., Nucl. Fusion 54, 064020 (2014)