Simulations of the Compact Toroidal Hybrid Using the Finite Element Extended MHD Code NIMROD

Abstract

Magnetohydrodynamic (MHD) modeling of magnetic confinement devices is an important tool in the understanding of plasma processes. With the nearly axisymmetric tokamak being the leading candidate for a plasma fusion reactor, much of the work in plasma modeling has been focussed on two-dimensional modeling. Recently, the importance of three- dimensional effects has become apparent in the control of disruptive modes such as the Edge Localized Mode (ELM) and the resistive wall mode. Thus modeling of three-dimensional plasma configurations is a critical step forward in the process of designing a viable plasma fusion reactor.

This dissertation describes modeling of the Compact Toroidal Hybrid (CTH) stellarator/tokamak hybrid at Auburn University using the extended MHD code NIMROD. The CTH has a high degree of toroidal shaping and a circular boundary and is thus a good candidate for modeling with NIMROD. Building on initial work by Schlutt et al., ohmic heating, temperature dependent thermal diffusivity, initial temperature and density profiles, temperature dependent resistivity, and time dependent loop voltages were added to the model to increase physical fidelity. This updated model is used to investigate current hesitations in the current rise of CTH discharges.

Initial modeling shows large, symmetry-breaking islands growing with an m = 2, n = 1 structure. These islands have a large effect on plasma current and temperature profiles and the growth rate of the associated magnetic mode scales similarly to the resistive interchange mode (γ ~ η^1/3). The growth of large islands for lower rotational transforms is investigated using the driving loop voltage as a feedback mechanism to keep the total plasma current constant. Doing this, symmetry-breaking islands formed for ι- = 1/3, but the growth time- scale was much longer than the time-scale of hesitations in the experiment. To sharpen edge gradients, the perpendicular thermal diffusivity was lowered. This change greatly affected the current drive, causing symmetry-breaking islands to grow for rotational transforms as low as ι- = 1/10.