Advancing the physics basis for quasi-helically symmetric stellarators

Abstract

A new optimized quasi-helically symmetric configuration is described that has the desirable properties of improved energetic particle confinement, reduced turbulent transport by three-dimensional shaping and non-resonant divertor capabilities. The configuration presented in this paper is explicitly optimized for quasi-helical symmetry, energetic particle confinement, neoclassical confinement and stability near the axis. Post optimization, the configuration was evaluated for its performance with regard to energetic particle transport, ideal magnetohydrodynamic stability at various values of plasma pressure and ion temperature gradient instability induced turbulent transport. The effects of discrete coils on various confinement figures of merit, including energetic particle confinement, are determined by generating single-filament coils for the configuration. Preliminary divertor analysis shows that coils can be created that do not interfere with expansion of the vessel volume near the regions of outgoing heat flux, thus demonstrating the possibility of operating a non-resonant divertor.

Magnetic fields with precise quasisymmetry for plasma confinement

“…The low losses in Fig. 6 of Wistell-A, which included another measure of energetic particle confinement besides QH in its optimization [15,34], shows that moderate departures from QH can still be compatible with low test particle losses, perhaps due to the reduced orbit width in QH.…”

“…It has been conjectured (but not proven) that in the absence of axisymmetry, QS may be possible on an isolated surface but it cannot be perfectly realized throughout a volume [11,12]. A number of nominally QS field configurations have been devised numerically [2,7,8,[13][14][15], and one has been realized experimentally [6], but these fields all have substantial deviations from symmetry, discussed below. In these previous studies, QS was only one of several de-sign objectives, which also included e.g.…”

“…Magnetic field strength on the boundary surface of previous magnetic field configurations designed to approximate quasisymmetry, as functions of the Boozer angles. (a) NCSX [7], (b) ARIES-CS [13], (c) a QA developed at New York University [17,18], (d) CFQS [8], (e) a QA developed at the Max Planck Institute for Plasma Physics (IPP) [14], (f) a QH developed at IPP [2], (g) HSX [6], (h) Wistell-A [15].…”

Energetic particle transport in optimized stellarators

“…The quasi-helically symmetric configurations are: the "Wistell-A" configuration which has been described in a previous publication [15]; the "Wistell-B" configuration, a five-field period vacuum configuration optimized with the ROSE code [16] explicitly for quasisymmetry and Γ c ; "Ku4" a four field period configuration from [17] that was optimized for quasisymmetry at high normalized pressure, β. The three quasiaxisymmetric configurations are comprised of the NCSX (specifically "li383") [18,19] and ARIES-CS (specifically: "n3are") [20,21] configurations.…”

“…However, a different metric Γ c [13,14], that seeks explicitly to align contours of J with flux surfaces similar to the omnigeneity constraint, has been shown to correlate better with energetic particle confinement. This metric has been used to optimize quasi-helically symmetric configurations with good energetic particle confinement [7,15].…”