Abstract Details
Abstracts
Author: Kissman H. Franco
Requested Type: Poster
Submitted: 2026-03-20 10:22:58
Co-authors: C. R. Sovinec
Contact Info:
University of Wisconsin-Madison
1500 Engineering Drive
Madison, Wisconsin 53706
US
Abstract Text:
ULq plasmas, defined by 0<q(a)<1, occupy an intermediate regime between tokamaks and reversed-field pinches and provide a useful platform for studying self-organization, relaxation, and transport in low-shear plasmas. In the Madison Symmetric Torus, this regime can be sustained without disruptive kink activity because of the close-fitting conducting shell and feedback-controlled programmable power supply. However, the balance of drives that sustain the relaxed configuration at different q(a) values is not well understood.
Here, we apply nonlinear visco-resistive MHD modeling in toroidal geometry with the NIMROD code to simulate ULq dynamics in detail. Starting from vacuum magnetic field, plasma current is driven with a proportional-differential loop-voltage controller that models the experiment. Ohmic heating and anisotropic thermal transport are included to capture the evolution of current and pressure self-consistently with transport and fluctuations. Current scans reproduce key features of low-q tokamak and ULq operation, including sawtooth-like 1/1 dynamics for q(a)>1 and self-organized ULq states whose dominant toroidal mode changes with the evolving safety-factor profile and the location of low-order rational surfaces.
To understand the drives that sustain the relaxed states, time-averaged profiles extracted from the nonlinear results are used as equilibria for linear computations. An energy-integral relation is applied to quantify the relative contributions of pressure-gradient and parallel-current drive based on the resulting eigenmodes, enabling a physics-based interpretation of unstable mode. For selected ULq equilibria, the linear computations identify both tearing- and interchange-parity eigenmodes, with the dominant structure depending on the relaxed profiles obtained nonlinearly. These results connect nonlinear self-organization to linear stability and clarify how different mode families govern relaxation in this underexplored regime.
Characterization: 1.0
Comments: