HSX as an example of a resilient non-resonant divertor

Abstract

This paper describes an initial description of the resilient divertor properties of quasi-symmetric (QS) stellarators using the HSX (Helically Symmetric eXperiment) configuration as a test-case. Divertors in high-performance QS stellarators will need to be resilient to changes in plasma configuration that arise due to evolution of plasma pressure profiles and bootstrap currents for divertor design. Resiliency is tested by examining the changes in strike point patterns from the field line following, which arise due to configurational changes. A low strike point variation with high configuration changes corresponds to high resiliency. The HSX edge displays resilient properties with configuration changes arising from the (1) wall position, (2) plasma current, and (3) external coils. The resilient behavior is lost if large edge islands intersect the wall structure. The resilient edge properties are corroborated by heat flux calculations from the fully 3-D plasma simulations using EMC3-EIRENE. Additionally, the strike point patterns are found to correspond to high curvature regions of magnetic flux surfaces.

Advancing the physics basis for quasi-helically symmetric stellarators

“…This paper discusses results from optimizations to produce quasi-helically symmetric (QHS) equilibria that simultaneously demonstrate multiple desirable properties for advanced stellarators. These properties include excellent neoclassical and energetic particle confinement, a reduction in turbulent transport and a functional non-resonant divertor (Bader et al 2017;Boozer & Punjabi 2018). The baseline targets for the configuration also avoided low-order rational surfaces, and included a vacuum magnetic well to avoid interchange instabilities.…”

Minimum magnetic curvature for resilient divertors using Compact Toroidal Hybrid geometry

“…The first two conditions were shown in a previous publication [9]. The third one was not explicitly stated previously, but is a natural outcome of the result that strike lines tend to exit through regions of maximum curvature, as shown in figure 4.…”

“…Turning to the resilient properties of CTH, there is not a strong difference between the configurations despite changes in the plasma current, rotational transform, and LCFS shape. Like the QHS equilibrium studied previously [9], the CTH equilibrium when operated in stellarator mode has a divertor that is resilient to the effects of plasma current.…”

“…In the calculations presented here the step size is 0.1 mm and the field line is moved every 2 radians. This method was described in more detail in [9,10,18], where it was often referred to as a "spiraling out" method. As before, field lines are followed in both forwards and backwards directions for both cases.…”

“…Rather, nonresonant divertors rely on the fact that stellarators, optimized for reduced neo-classical transport, tend to have outer flux surfaces shaped with distinct toroidally localized "ridges" [7,8] Flux is preferentially expelled near these ridges in a manner that is largely independent of the plasma profiles and changes to the equilibria. These features were explored in previous work [9,10] which examined the ability of a quasi-helically symmetric (QHS) equilibrium to remain resilient to changes in wall position, plasma current, and perturbations by external coils. It was found that the QHS equilibrium in question, that of the Helically Symmetric eXperiment (HSX), was resilient to changes in configurations provided that the last closed flux surface (LCFS) or large magnetic islands did not intersect the target surfaces.…”