Breaking the symmetry of tokamaks in just the right way

Nik Logan
Columbia University
Tuesday, March 5, 2024
11:00 am
FRH 4135

Abstract: Tokamaks are designed and optimized as axisymmetric confinement
devices, but always contain some level of symmetry breaking. Levels of
“3D” non-axisymmetric fields on the order of one part in ten thousand
(δB/B ~ 1e-4) can be sufficient to induce tearing modes, stop the
stabilizing plasma rotation and cause sudden losses of confinement
known as disruptions. On the other hand, similar or even larger breaks
in the asymmetry can stabilize edge localized modes (ELMs) that cause
problematically high heat fluxes in the typical high confinement mode
(H-mode) reactor scenarios. This talk reviews how purpose built,
non-axisymmetric coils have been used to correct the unwanted
asymmetries that cause disruptions while intentionally breaking the
tokamak symmetry in ways that benefit reactors. Experience on existing
machines is combined with Generalized Perturbed Equilibrium Code’s
(GPEC’s) modeling of the multimodal, non-axisymmetric plasma response
to optimize the geometric coupling between coil arrays and the physics
of interest in the plasma core or edge. This forms the physics basis
for several coil designs being built now on existing and new machines.

Laszlo Bardoczi