Stellarator magnetic configurations need to be optimized in order to meet all the required properties of a fusion reactor. In this work we introduce the notion of robust optimization via a flat mirror term: a quasi-isodynamic configuration with sufficiently small radial variation of the mirror term can achieve the maximum-J property at low plasma beta. This results in small radial transport of energy and good confinement of bulk and fast ions even if the configuration is not very close to perfect omnigeneity, and for a wide range of plasma scenarios, including low beta and small radial electric field. Reactors based on this concept would be easier to design, as they would be more robust against arror fields, and operate.

In these works, we present KNOSOS (KiNetic Orbit-averaging SOlver for Stellarators), a freely available, open-source code that calculates neoclassical transport in low-collisionality plasmas of three-dimensional magnetic confinement devices by solving the radially local drift-kinetic and quasineutrality equations. The main feature of KNOSOS is that it relies on orbit-averaging to solve the drift-kinetic equation very fast. KNOSOS treats rigorously the effect of the component of the magnetic drift that is tangent to magnetic surfaces, and of the component of the electrostatic potential that varies on the flux surface. In these papers, we show several calculations for the stellarators W7-X, LHD, NCSX and TJ-II that provide benchmark with standard local codes and demonstrate the advantages of this approach.

Energetic ions need to be confined in a fusion-relevant stellarator, so that their energy is transmitted to the plasma and does not damage the plasma-facing components of the device. In this work, we develop a model that, based on the radially-local bounce-averaged drift-kinetic equation, classifies orbits and succeeds in predicting configuration-dependent aspects of the prompt losses of energetic ions in stellarators. Such a model could has later been successfully employed in the optimization stage of the design of new stellarator devices.

A new quasi-isodynamic (QI) stellarator configuration optimized for the confinement of energetic ions at low plasma β is obtained. The new configuration has poloidally closed contours of magnetic field strength, low magnetic shear and a rotational transform profile allowing an island divertor. It shows ideal and ballooning magnetohydrodynamic stability up to β = 5%, reduced effective ripple, below 0.5% in the plasma core. Even at low β, the configuration approximately satisfies the maximum-J property, and the confinement of fast ions is good at β ∼ 1.5% and becomes excellent at reactor values, β ∼ 4%. An evaluation of the D31 neoclassical mono-energetic coefficient supports the expectation of a reduced bootstrap current for plasmas confined in QI configurations. A set of filamentary coils that preserve the good confinement of fast ions in the core is presented.

The stellarator W7-X was able to achieve high-temperature plasma conditions during its first experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.

Achieving impurity and helium ash control is a crucial issue in the path towards fusion-grade magnetic confinement devices, and this is particularly the case of helical reactors, whose low-collisionality ion-root operation scenarios usually display a negative radial electric field which is expected to cause inwards impurity pinch. In this work we discuss, based on experimental measurements and standard predictions of neoclassical theory, how plasmas of very low ion collisionality, similar to those observed in the impurity hole of the Large Helical Device, can be an exception to this general rule, and how a negative radial electric field can coexist with an outward impurity flux.

  • J L Velasco, K McCarthy, N Panadero, S Satake, D López-Bruna, J A Alonso, I Calvo, T Estrada, J M Fontdecaba, J Hernández, R García, F Medina, M A Ochando, I Pastor, S Perfilov, E Sánchez, A Soleto, B Ph Van Milligen, A Zhezhera, and the TJ-II Team. Particle transport after pellet injection in the TJ-II stellarator. Plasma Physics and Controlled Fusion, 58(8):084004, 2016. arXiv / PDF

Core plasma fuelling and density control is a critical issue for developing steady-state scenarios in fusion reactors. In this work we study radial particle transport in stellarator plasmas using cryogenic pellet injection. By means of perturbative experiments, we estimate the experimental particle flux and compare it with neoclassical simulations. Experimental evidence is obtained of the fact that core depletion in helical devices can be slowed-down even by pellets that do not reach the core region. This phenomenon is well captured by neoclassical prediction

A comparative study of energy transport for medium- to high-density discharges in the stellarator-heliotrons TJ-II, W7-AS and LHD is carried out. The chosen discharges exhibit significant ion energy transport, and ion-root conditions, i.e. a small negative radial electric field, were found. Within a core region, the predicted neoclassical energy fluxes comply with experimental findings .

The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows us to simulate density ramp-up experiments and describe from first principles the formation and physics of the radial electric field shear layer. The main features of the transition are perfectly captured by the calculation, and good quantitative agreement is also found. The results presented here, that should be valid for other non-quasisymmetric stellarators, provide a fundamental explanation for a wealth of experimental observations connected to the shear layer emergence in TJ-II.



My most recent published papers can be found in the researcher profile of this section of the site.