Simulations of sawtoothing in a current carrying stellarator

Abstract

Numerical MHD simulations are used to gain insight into how sawteeth are affected by three-dimensional shaping from the Compact Toroidal Hybrid (CTH) stellarator field. CTH is a small stellarator-tokamak hybrid and sawteeth are sometimes seen on soft x-ray signals when operating with tokamak like profiles. We use NIMROD to compute numerical solutions having repeated sawtooth relaxations for a sequence of configurations with increasing helical stellarator field strength. The experimentally observed trend of the sawtooth period decreasing as the helical field strength is increased is recovered in the simulations. Careful attention to numerical convergence was required to obtain the results, and these considerations may be relevant to simulations of other phenomena in devices with non-axisymmetric plasmas such as perturbed tokamaks, RFPs, and stellarators.

Power-transfer and fixed-point analysis of sawtooth simulations of a current-carrying stellarator

“…The latter observation is computationally reproduced in Roberds et al. (2016). This work revisits 3D nonlinear NIMROD simulations of sawtooth oscillations in CTH (Roberds et al.…”

“…This work reconsiders CTH sawtooth NIMROD simulations with increasing levels of 3D shaping (Roberds et al. 2016). Our focus is on comparing a tokamak plasma with a stellarator configuration using new analysis tools.…”

Nonlinear MHD simulation of core plasma collapse events in Wendelstein 7-X

“…The 3D nonlinear MHD simulation code is a fundamental tool to understand the nonlinear behavior of stellarator plasmas. Many 3D nonlinear simulations were performed for stellarator plasmas clarifying the importance of the nonlinearity [11][12][13][14][15][16][17][18][19][20][21][22][23]. The 3D nonlinear MHD simulation has been used to understand the CDC [24].…”

Approach to nonlinear magnetohydrodynamic simulations in stellarator geometry

“…Most existing toroidal MHD codes are designed for tokamak applications and hence assume axisymmetric computational domains. While some simple stellarators can be modeled using such codes [3][4][5], most realistic stellarator designs do not permit an axisymmetric surface between the plasma and the coils, and therefore cannot be treated using axisymmetric domains. To our knowledge, the M3D [6] and MIPS [7] codes have developed the capability to allow non-axisymmetric domains, but they have not been used for simulations at transport timescale.…”