arXiv subjects

José Luis Velasco

Publications and source records attributed to José Luis Velasco.

7 recordsLinked to original sources

arXiv2026

The Geometry of Flux Surfaces with Quasi-Poloidal Symmetry

Quasi-poloidal (QP) magnetic fields have desirable properties for confining plasma: no radial drift of guiding centres (with positive implications for neoclassical transport), zero Pfirsch-Schlüter current, and a lower level of damping for poloidal flows. Despite their attractive properties, QP fields are not amenable to the near-axis expansion, a major theoretical tool for understanding toroidal fields. In this paper, we provide a novel framework for defining and understanding QP flux surfaces. This framework relies on a simplification that transforms the task of finding a quasi-poloidal flux surface from a 3D problem to a 2D problem. This simplification also applies to asymmetric magnetic mirrors with desirable properties. We sketch how this 2D problem can form the basis of an efficient optimisation problem for finding QP flux surfaces. We leverage this 2D problem for theoretical understanding: for instance, we identify a route to finding QP flux surfaces that are naturally flat mirrors (Velasco et al. 2023). The reduced model is qualitatively checked against numerically optimised QP equilibria. These numerical solutions only satisfy QP approximately, but we predictably find that local discrepancies with the reduced model correspond to significant local QP errors, anomalous parallel currents, and field lines deviating from geodesics.

physics.plasm-ph↗

arXiv2026

Optimizing stellarators with hidden symmetry

Stellarators confine fusion plasmas using three-dimensional magnetic fields composed of nested toroidal magnetic surfaces. In generic stellarators, trapped particles can drift across these surfaces and degrade plasma confinement. Certain topological properties of the magnetic field strength can suppress these drifts. However, conventional stellarator design approaches typically enforce restrictive constraints to realize such properties, thereby segmenting and limiting the accessible configuration space. In this work, we reformulate the conditions for efficient confinement as constraints on a homeomorphic straightening transformation of the field contours. Within this framework, the various families of stellarator magnetic fields optimized for plasma confinement arise naturally as specific realizations of a unified mapping. This new perspective provides a significantly more comprehensive description of viable stellarator configurations, enabling systematic exploration of trade-offs among confinement quality, geometric complexity, and engineering requirements. We illustrate this approach by presenting a highly compact stellarator design that nevertheless achieves plasma performance comparable to that of leading reactor-scale designs with much larger aspect ratios.

physics.plasm-ph↗

arXiv2026

Optimization of stellarator configurations combining omnigenity and piecewise omnigenity

We present a method for optimizing stellarator configurations that combine omnigenity and piecewise omnigenity (pwO). Within the \texttt{OOPS} optimization framework [Liu \textit{et al.}, arXiv:2502.09350 (2025)], we introduce a mapping technique that can ``squeeze'' general omnigenous fields to approximate pwO in the high-field side. Using this approach, we obtain a range of optimized configurations that combine poloidal omnigenity (PO) and pwO, spanning different field periods and aspect ratios. We further show that these configurations are compatible with a magnetic well. The resulting configurations exhibit favorable neoclassical transport and bootstrap current properties while partially relaxing the strict constraints of omnigenity. These results suggest that such configurations are promising candidates for future stellarator reactors.

physics.plasm-ph↗

arXiv2025

Energetic-particle orbits near rational flux surfaces in stellarators: I. Passing particles

Recent simulations have shown that, even when the magnetic field of a stellarator possesses nested toroidal flux surfaces, the orbits of passing energetic particles can exhibit islands. These 'drift islands' arise near rational flux surfaces, where they are likely to enhance alpha-particle transport -- flattening the alpha density profile locally -- unless they can be avoided by suitable design of the stellarator magnetic field. To investigate how this might be achieved, we derive an equation for the drift-island shape in a general stellarator. This result follows from the solution to a more fundamental problem: that of calculating the orbits of passing particles near a rational flux surface. We show that these orbits are determined by conservation of an adiabatic invariant associated with the closed rational-surface field lines. We use this 'transit adiabatic invariant' to prove that there are no drift islands, for all passing particles, if and only if the magnetic field satisfies a weaker version of the Cary-Shasharina condition for omnigeneity; we call such magnetic fields 'cyclometric'. The drift-island width scales as $\sim (ρ_\starδ/s)^{1/2} a$ ($ρ_\star$ is the normalized gyroradius, $δ$ is the deviation from cyclometry, $s$ is the magnetic shear, and $a$ is the minor radius), so large drift islands could arise in low-shear stellarators that are insufficiently cyclometric. To ensure accurate results for very energetic particles, we compute higher-order corrections to the transit invariant. Our calculations agree extremely well with ASCOT5 guiding-centre and full-orbit simulations of alpha particles in reactor-scale equilibria, even at $3.5\text{MeV}$. Finally, we show how our results can also be derived using Hamiltonian perturbation theory, which provides a systematic framework for calculating passing-particle orbits on both rational and irrational surfaces.

physics.plasm-ph↗

arXiv2024

Evaluation of neoclassical transport in nearly quasi-isodynamic stellarator magnetic fields using MONKES

Stellarator magnetic fields that are perfectly optimized for neoclassical transport (with levels of radial neoclassical transport comparable to tokamaks) are called omnigenous. Quasi-isodynamic magnetic fields are a subset of omnigenous magnetic fields in which the isolines of the magnetic field strength close poloidally, which grants them the additional property of producing zero bootstrap current. A frequent strategy in the quest for quasi-isodynamic configurations is to optimize the magnetic field indirectly by minimizing proxies that vanish in an exactly quasi-isodynamic field. The recently developed code MONKES enables fast computations of the neoclassical radial transport and bootstrap current monoenergetic coefficients, and therefore facilitates enormously to assess the efficiency of such indirect approach. By evaluating the large database of intermediate configurations that led to the configuration CIEMAT-QI, the inefficiency of the indirect optimization strategy for minimizing the bootstrap current is illustrated. In addition, MONKES is used to take the first steps in the exploration of a region of the configuration space of piecewise omnigenous fields, a novel family of optimized magnetic fields that has broadened the configuration space of stellarators with low levels of radial neoclassical transport.

physics.plasm-ph↗

arXiv2023

Validating neutral-beam current drive simulations in the TJ-II stellarator

In this paper, we analyze the results of neutral-beam current drive (NBCD) experiments, performed in the TJ-II stellarator, with the aim of validating the theoretical predictions. Both parallel and anti-parallel injection with respect to the magnetic field were explored using co (NBI1) and counter (NBI2) beams at different injected beam power and plasma densities. The fast-ion current driven by both beams was simulated with the Monte Carlo code ASCOT and the electron response to the fast-ion current was calculated analytically using a model valid for an arbitrary magnetic configuration and a low collisionality plasma. The model reproduces with rather good agreement the toroidal current measured in NBI2 plasmas while the current driven by NBI1 is less than half the predicted one. Possible reasons for this discrepancy are discussed.

physics.plasm-ph↗

arXiv2021

Study on impurity hole plasmas by global neoclassical simulation

An impurity hole observed in the Large Helical Device (LHD) is a hollow density profile of an impurity ion species formed in the core plasma where the negative (inward-pointing) ambipolar radial electric field ($E_r$) exists. Although local neoclassical models have predicted the sign of $E_r$ in impurity hole plasmas is negative for the entire minor radius, an experimental measurement of an impurity hole plasma has shown that the $E_r$ changes the sign from negative to positive along the minor radius. In the present work, we investigate neoclassical impurity transport in an impurity hole plasma using a global neoclassical simulation code FORTEC-3D. The variation of electrostatic potential on flux surface ($Φ_1$) is evaluated from the quasi-neutrality condition in multi-ion-species plasma by the global simulation. The ambipolar $E_r$ and neoclassical fluxes are determined in solving a global drift-kinetic equation including the effect of $Φ_1$. By the global simulation, we show that an $E_r$ which changes the sign along the radius is obtained as a solution of the ambipolar condition and with such an $E_r$, impurity carbon flux can be outwardly directed even where $E_r<0$ and the carbon density profile is hollow around the magnetic axis. Furthermore, it is found that the outward carbon flux is only a factor 2-3 from balancing the modeled inward turbulent flux. Our result indicates that we have moved one step closer to reproducing the impurity transport in impurity hole plasmas by kinetic simulation.

physics.plasm-ph↗