arXiv subjects

Robert Davies

Publications and source records attributed to Robert Davies.

7 recordsLinked to original sources

arXiv2026

Divertor topology and vacuum vessel design for stellarators

We present stellarator optimization algorithms for designing the edge magnetic structure in vacuum fields, together with the vacuum vessel. First, we introduce a numerical method that robustly computes periodic-orbit fixed points of any type (elliptic, hyperbolic, or parabolic), which could form the basis of a divertor. To couple divertor and vacuum vessel design, we introduce parametric families of vacuum vessels for which point-to-vessel distances, and their derivatives, can be computed efficiently. The resulting algorithms use signed distance functions to enforce coil-vessel clearance while allowing coils to be placed on or off the vessel. Using these methods, we jointly optimize modular coils and the vacuum vessel to realize a wide range of magnetic topologies for diverting exhaust, including standard X-point divertors, and single- and double-null configurations. For the first time, we show that precise snowflake divertors can be achieved in stellarators. Using this framework, we generate a number of quasi-axisymmetric stellarator designs with compatible vacuum vessels and diverse divertor architectures, which we consider to be candidates for a next-generation STAR Lite prototype.

physics.plasm-ph↗

arXiv2026

Computational studies of giant edge islands and unpaired X-points in HSX and W7-X by manipulating coil currents

We present magnetic configurations in the Helically Symmetric eXperiment (HSX) and Wendelstein 7-X (W7-X), in which the edge magnetic structure is dominated by island chains which are spatially larger than the previously reported configurations. These ``giant" island chains (with rotational transform $\iota=4/3$ or $4/4$ for HSX and $\iota=5/6$, $5/5$ or $5/4$ for W7-X) are obtained by reducing the coil current in main coil 6 for HSX and non-planar coil 5 for W7-X (i.e. the coil nearest the up-down symmetric cross-section $\phi=36^\circ$ for W7-X and $\phi=45^\circ$ for HSX); this appears a sufficient (but not necessary) condition for giant islands. The giant islands create relatively straight X-point legs which transport plasma to the plasma-facing components (PFCs). In the most extreme cases, the island O-points leave the domain of the field line map and the divertor legs of the remaining ``unpaired" X-points do not close around the island. We use the anisotropic heat diffusion code EMC3-Lite to find ``giant island" W7-X configurations which are promising for PFC heat loads. Coil forces analysis (in addition to other effects such as neoclassical transport and magnetohydrodynamic stability) would also be required but are not explored here. It is not known whether giant islands are intrinsically favourable for divertor performance but we demonstrate that such regimes, which are far from the ordinary island divertor, are obtainable and can in principle be studied experimentally. This also reveals the flexibility of existing machines for edge studies beyond their original design space.

physics.plasm-ph↗

arXiv2026

STAR_Lite: A stellarator designed to experimentally validate non-resonant divertors

The non-resonant divertor (NRD) offers a promising exhaust solution for stellarators, combining topological simplicity with resilience to magnetic field perturbations. To experimentally validate the robustness of non-resonant divertors in a quasi-axisymmetric (QA) configuration, we introduce STAR_Lite, a new stellarator experiment at Hampton University. This paper details the design and analysis of the first STAR_Lite coil configuration, STAR_Lite-A. The two field-period configuration manifests an NRD through X-points with zero rotational transform, at the top and bottom of the device. The divertor legs extruding from the X-points are topologically similar to the poloidal divertors of tokamaks. To expand the experimental range, STAR_Lite-A is optimized for experimental flexibility, producing a wide range of distinct QA configurations by only varying the currents in the modular coils. The NRDs not only persist across these configurations, but numerical strike-line simulations confirm that heat exhaust remains resilient to changes in coil currents, with plasma following the divertor legs and creating a toroidal, discontinuous, strike pattern. We further examine the resilience of the NRD to magnetic perturbations caused by manufacturing errors in the modular coils. We find that quasisymmetry and the existence of X-points is well-preserved under these magnetic field changes, but the rotational transform may vary substantially and displacements of the divertor X-points may lead to one X-point having a dominant effect on edge transport. Overall, our analysis indicates a compact, modular design can likely generate a resilient NRD structure while satisfying the practical constraints of a university-scale experiment.

physics.plasm-ph↗

arXiv2025

Characterisation of X- and O-points in Wendelstein 7-X with respect to coil currents

This work analyses vacuum magnetic field topology in Wendelstein 7-X (W7-X) with respect to changes in the current in the superconducting coils. We develop a fast automated scheme to locate fixed points (such as X- and O-points) and calculate the trace of the Jacobian of the field line map for them (Tr(M)), which represents several important properties of the fixed point. We perform two sets of coil current scans: (1) scans where each coil current is varied individually, using the "standard", "high iota" and "low iota" configurations as starting points; (2) a scan of over 200,000 magnetic configurations in which the coil currents are randomly sampled. In both cases we constrain the coil currents to the normal range of W7-X. We verify the principal roles of the non-planar, planar and control coils: the non-planar coils establish island chains with a certain phase; the planar coils modify the location of the island chain by both controlling the iota profile and shifting the configuration "inward" and "outward"; the control coil affects the island size and phase. We also find that |Tr(M)-2| (a quantity closely related to the magnitude of the Greene's residue) tends to increase with the minor radius of the fixed points, and that Tr(M) for X- and O-points can be very differently affected by the control coil current. Finally, we show that |Tr(M)-2| serves as a proxy for island size for internal island chains, which may help identification of suitable experimental candidates.

physics.plasm-ph↗

arXiv2025

Stellarator divertor design by optimizing coils for surfaces with sharp corners

In stellarators, achieving effective divertor configurations is challenging due to the three-dimensional nature of the magnetic fields, which often leads to chaotic field lines and fuzzy separatrices. This work presents a novel approach to directly optimize modular stellarator coils for a sharp X-point divertor topology akin to the Large Helical Device's (LHD) helical divertor using a target plasma surface with sharp corners. By minimizing the normal magnetic field component on this surface, we target a clean separatrix with minimal chaos. Notably, this approach demonstrates the first LHD-like helical divertor design using optimized modular coils instead of helical coils. Separatrices are produced with significantly lower chaos than in LHD, demonstrating that a wide chaotic layer is not intrinsic to the helical divertor. Additional optimization methods are implemented to improve engineering feasibility of the coils and reduce chaos, including weighted quadrature and manifold optimization, a method which does not rely on normal field minimization. The results outline several new strategies for divertor design in stellarators, though it remains to achieve these edge divertor features at the same time as internal field qualities like quasisymmetry.

physics.plasm-ph↗

arXiv2025

Turnstile area as a measure for chaotic transport in magnetic confinement fusion devices

We analyze stochasticity in the magnetic fields of magnetic confinement fusion reactors by calculating the lobe areas of turnstiles - a method developed for characterizing transport into and out of resonance zones in Hamiltonian dynamical systems. We develop an efficient algorithm based on an action principle to calculate this quantity directly from the magnetic field, including stellarator magnetic fields which are sourced by a complicated set of three-dimensional coils. In the analyzed devices, the turnstile area on the inboard (plasma-facing) manifolds is much smaller than the turnstile area on the outboard (wall-facing) manifolds. The application of the turnstile area calculation for the design of future reactors will be discussed.

physics.plasm-ph↗

arXiv2025

The topology of non-resonant stellarator divertors

We apply topological methods to better understand how the magnetic field in the stellarator edge can be diverted away from the confined region. Our primary method is calculating the winding numbers of closed contours, which gives information on the number and nature of fixed points within a bounded region. We first apply this to the non-resonant divertor (NRD) Hamiltonian system, and present a simple explanation for the system's diversion: trajectories are guided away from the confined region by X-points which are "unpaired" i.e. do not have corresponding O-points and therefore do not resemble an island chain. We show how similar phenomena can occur in a similar, axisymmetric Hamiltonian system. Secondly, we find examples of neoclassically optimised stellarators in the QUASR database which divert the magnetic field via unpaired X-points. We present and discuss three examples, each containing novel phenomena which might be desirable for stellarator divertors. These findings broaden the horizons of how magnetic fields can be diverted in realistic stellarators, and may be attractive for future experiments and stellarator reactor design.

physics.plasm-ph↗