arXiv subjects

Pedro F. Gil

Publications and source records attributed to Pedro F. Gil.

7 recordsLinked to original sources

arXiv2026

Optical analogy for stellarators: Ridges as caustics and coils as singularities

A common feature of most numerically optimized stellarator geometries is the presence of sharp ridges on outer flux surfaces, irrespective of the rotational transform. Despite their importance, an analytical theory for their existence has been lacking. In this work, we demonstrate that ridges are not artifacts but mathematical necessities. We develop such a theory for devices with quasisymmetry (QS). We demonstrate that QS exhibits close connections with the theory of geometrical optics, following Parker's ``optical analogy" (E.N. Parker, Geophys. Astrophys. Fluid Dyn, 1989). By mapping vacuum QS to the eikonal equation of geometrical optics, we derive the conditions for ridge formation, identified as field line caustics where magnetic field lines focus. Furthermore, we prove a geometric theorem for stellarator coil design: both ridges and filamentary coils must lie on the zero-determinant manifold of the magnetic gradient tensor. This topological constraint unifies the description of plasma ridges and external coils, providing a precise criterion for identifying valid coil locations and explaining the efficacy of the magnetic gradient lengthscale (J. Kappel et al., Plasma Phys. Control. Fusion, 2024) as a coil optimization parameter. We demonstrate that as the device becomes more compact, sharp ridges naturally form on the inboard side in quasiaxisymmetry. We support our analytical theory with extensive numerical evidence.

physics.plasm-ph↗

arXiv2026

A proof-of-concept for automated AI-driven stellarator coil optimization with in-the-loop finite-element calculations

Finding feasible coils for stellarator fusion devices is a critical challenge of realizing this concept for future power plants. Years of research work can be put into the design of even a single reactor-scale stellarator design. To rapidly speed up and automate the workflow of designing stellarator coils, we have designed an end-to-end ``runner'' for performing stellarator coil optimization. The entirety of pre and post-processing steps have been automated; the user specifies only a few basic input parameters, and final coil solutions are updated on an open-source leaderboard. Two policies are available for performing non-stop automated coil optimizations through a genetic algorithm or a context-aware LLM. Lastly, we construct a novel in-the-loop optimization of Von Mises stresses in the coils, opening up important future capabilities for in-the-loop finite-element calculations.

physics.plasm-ph↗

arXiv2026

Stochastic single-stage stellarator optimization using fixed-boundary equilibria

In this paper, single-stage stellarator optimization is combined with stochastic coil optimization to improve the robustness of the stellarator as compared to deterministic methods. The plasma boundary, solved with an MHD solver in fixed-boundary mode, is linked to a set of randomly perturbed coils via the squared flux. The optimizer avoids sharp local minima and can reach improved configurations. Two different configurations obtained with our method, one quasi-axisymmetric and one quasi-helically symmetric, are compared against both the standard stochastic stage II method and the single-stage method. The new configurations shown here yield improved squared flux, quasisymmetry, and particle loss following a posteriori perturbation of the coils.

physics.plasm-ph↗

arXiv2026

Designing A Buildable Optimized Stellarator to Confine Electron-Positron Plasmas

In this paper, the design of the the plasma equilibrium and superconducting coils for the Electrons and Positrons in an Optimized Stellarator EPOS experiment is presented. With newly developed stellarator optimization tools, including single-stage and stochastic optimization, as well as HTS strain, this work demonstrates that it is possible to achieve key metrics for the buildability and confinement properties of the device. In particular, satisfactory quality of quasisymmetry and stellarator robustness is designed, and engineering requirements are met for eight different candidates. A feasibility study is presented that optimizes multiple candidates for different plasma major radii and coil currents, as well as the best EPOS candidate to date.

physics.plasm-ph↗

arXiv2025

Manufacturing Tolerances of Non-Planar Coils for an Optimized Tabletop Stellarator

Stellarator coils are known for their complexity and departure from planarity, along with tight manufacturing tolerances in order to achieve the target magnetic field accuracy. These requirements can lead to increased costs and delays in assembly; failure to meet them can compromise the stellarator's performance. Small-scale experiments offer opportunities to develop and benchmark stellarator coil design and evaluation methods more quickly and at lower budget. In this work, we analyze precise 3D scans of the manufacturing deviations of two 3D-printed coil frames (steel, Ti alloy) and one CNC-machined coil frame (Al alloy), as part of assessing these approaches to fabricating high-temperature superconducting (HTS) coils for a tabletop stellarator. The deviations are measured along the coil length, then modeled using Gaussian processes to extract characteristic length scales. Finally a statistical study of field accuracy is performed using relevant experimental parameters. We conclude that the manufacturing perturbations along the winding path from CNC-machining are almost an order of magnitude lower than those from Additive Manufacturing. Together with high overall fabrication accuracy, this allows for higher magnetic field precision and an improved assembly process.

physics.ins-det↗

arXiv2025

Efficient Computation of Stellarator Coils with an Augmented Lagrangian Optimization Method

Finding feasible coils for stellarator fusion devices is a critical challenge of realizing this concept for future power plants. Current design efforts struggle to navigate the highly nonconvex optimization landscape, spend considerable resources scanning the parameter space, and may produce suboptimal coils. In this work, we present an augmented Lagrangian approach to tackle the ill-posed problem of coil optimization. We illustrate its effectiveness and versatility by generating coils for five stellarators with very different symmetries and magnetic field shaping. In all cases, we find Pareto-optimal coil solutions that in various ways outperform published coil sets.

physics.plasm-ph↗

arXiv2025

A Non-planar ReBCO Test Coil with 3D-printed Aluminum Support Structure for the EPOS Stellarator

We report on the test of a small scale, non-planar coil using non-insulated ReBCO tape wound on a 3D-printed aluminum support structure. A 3D-scan of the winding frame which was printed out of AlSi10Mg using selective laser melting, showed peak manufacturing deviations of 0.3 mm. We tested the coil with 21 turns of 3 mm wide tape cooled both by liquid nitrogen and with a cryocooler. We achieved a central field strength of up to 21 mT which agrees with the prediction suggesting that we manufactured the coil without defects. The peak field was reached at a supply current of 120 A. The current leads showed a contact resistance of $\left(2.25\pm0.13\right)μΩ$. The discharge time $L/R$ was found to be 0.63 s in liquid nitrogen and 2.92 s when cooled by the cryocooler. From this we estimate a winding pack temperature of 41 K.

physics.ins-det↗