Abstract Details
Abstracts
Author: Adelle M. Wright
Requested Type: Poster
Submitted: 2019-02-20 22:22:55
Co-authors: S.R. Hudson, R.L. Dewar, M.J. Hole
Contact Info:
Australian National University
Building 145 Science Road
Canberra, ACT 2601
Australia
Abstract Text:
To be physically realisable states, MHD equilibria must exist for some non-trivial timescale, meaning they must be at least be ideally stable and sufficiently stable to the fastest growing resistive instabilities. The existence of at least one `nearby' equilibrium, however, should be a prerequisite for any kind of physically meaningful stability analysis.
Grad [1] argued that 3D MHD equilibria with continuous, non-uniform pressure could exist if pressure gradients were avoided at every rational surface within the plasma volume. The corresponding pressure gradients, however, seemed unnatural.
More recently, Bruno and Laurence [2] proved the existence of 3D MHD equilibria with non-uniform, stepped pressure profiles. The pressure jumps occur at surfaces with highly irrational values of rotational transform and generate singular current sheets. If physically realisable, the dynamical mechanism of formation of these states remains to be understood.
We present a cylindrical equilibrium model with alternating regions of constant and non-uniform pressure, such that pressure gradient is continuous. Our equilibria have continuous total pressure (i.e. no singular current sheets) but discontinuities in the parallel current density.
We examine how the resistive stability characteristics of the model change as we increase the localisation of pressure gradients at fixed radii and approach a discontinuous pressure profile in the zero-width limit. Equilibria with continuous pressure are found to be unstable to moderate/high-m modes and tend towards ideal instability in some cases. We propose that additional geometric degrees of freedom are required to render such states physically realisable, supporting the physical possibility of 3D MHD equilibria with both flux surfaces and stochastic field regions.
[1] Grad, H., Physics of Fluids 10.1 (1967): 137-154
[2] Bruno, O. P., and Laurence, P., Communications on pure and applied mathematics 49.7 (1996): 717-764
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