Author: Mark R Cianciosa
Requested Type: Pre-Selected Invited
Submitted: 2017-03-17 13:58:46
Co-authors: A. Wingen, E. A. Unterberg, S. K. Seal, R. S. Wilcox, S. P. Hirshman, P. Piovesan, L. Lao, F. Turco
Oak Ridge National Laboratory
PO BOX 2008 MS6169
Oak Ridge, TN 37831-6
The presence of symmetry breaking 3D structures is changing our understanding of tokamaks as well as enabling ELM mitigation via a non-axisymmetric distortion of the boundary using RMP fields. In steady-state hybrid discharges, high beta and a flat q-profile near unity, can kink an equilibrium into a helical state. Since such hybrid discharges are a proposed operational mode for ITER, a capability to reconstruct weakly 3D equilibria is of vital importance.
Currently, tokamaks rely on axisymmetric reconstructions for feedback control and interpretation of experiment. In the presence of symmetry breaking 3D perturbations, these axisymmetric assumptions are no longer valid. Here we employ V3FIT, a 3D reconstruction code originally developed for stellarators based on Bayesian principles and the highly scalable equilibrium solver PARVMEC. Using gradient based optimization, V3FIT determines the optimal 3D equilibrium and its associated uncertainty. Furthermore, work has begun to couple in the SIESTA code, enabling 3D reconstructions with island topologies.
The challenge of applying these tools to weakly 3D tokamaks, is that the 3D components of signals are difficult to detect. The solution is reconstructing at high model fidelity enabled by high performance computing. In an experiment on DIII-D, SXR and MSE diagnostics measured a toroidal symmetry breaking fluctuation in the core as the equilibrium was rotated. By modeling this time-varying signal as virtual copies of diagnostics displaced in the toroidal direction, 3D information is revealed. Bayesian priors, constrain axisymmetric components to an initial 2D equilibrium. We use this capability to reconstruct and verify, via the rigorous uncertainty analysis within V3FIT, a helical equilibrium in a tokamak. Additionally, progress toward SIESTA based island equilibrium reconstruction will also be presented.
Work supported by US DOE contracts DE-AC05-00OR22725, DE-FG02-04ER54761, and DE-FC02-04ER54698.