Abstract Details
Abstracts
Author: Richard Nies
Requested Type: Poster
Submitted: 2025-03-13 17:38:22
Co-authors: F.I. Parra
Contact Info:
Princeton University / PPPL
Nassau St
Princeton, 08540
USA
Abstract Text:
For small magnetic shear values, ion-scale microinstabilities can become extended along the magnetic field due to the weakened stabilisation by finite Larmor radius effects. In such cases, the electrons cannot be assumed to respond adiabatically despite their fast parallel propagation speed. Extended modes have been observed in numerous gyrokinetic simulations with kinetic electrons [Hallatschek & Dorland ’00, Dominski et al. ’15, Volčokas et al. ’23, ’24] and have recently been suggested to be an important electromagnetic saturation mechanism through nonlinear flattening of the safety factor profile [Volčokas et al. ’25]. These modes could prove crucial to understand the turbulence physics in stellarators, see e.g. the precise quasisymmetric stellarators of [Landreman & Paul ’22], and in tokamak reversed shear scenarios.
We present an analytical theory of extended modes using a multi-scale expansion of the linear gyrokinetic equation at small magnetic shear (see [Hardman et al. ’22] for the large magnetic shear case). We derive an integral dispersion relation whose solutions replicate results from gyrokinetic simulations using stella [Barnes et al. ’19]. To validate the theory and elucidate the physics of the modes, we perform extensive scans in magnetic shear, safety factor, ion to electron mass ratio, binormal wavenumber, density gradient, ion temperature gradient, and electron temperature gradient. We show the modes can be explained by a balance between the parallel electron dynamics, the polarisation of ion gyrocentres, and the compression induced by toroidicity.
Characterization: 1.0
Comments: