Ivan Calvo

The integration of a breeding blanket (BB) in a stellarator reactor poses a major challenge due to the intricate geometry and the reduced free space between plasma and coils and between neighbouring coils, which also makes maintenance by remote handling techniques difficult. Conversely, in comparison with tokamaks, the BB can benefit significantly from the steady-state operation of stellarators. While previous studies have pointed to the feasibility of a dual coolant lithium lead (DCLL) BB in stellarators such as HELIAS-5B, each device introduces a different set of geometrical constraints and integration requirements that must be addressed through a dedicated design approach. This work presents a sequential study on several aspects that have clear impact on the functionality and efficiency of a DCLL BB integrated into a stellarator design based on the CIEMAT-QI4X plasma configuration. The proposal aims to achieve accurate alignment between the magnetic field and the liquid metal flow to avoid electric insulation of the liquid metal circuits as much as possible. Although it has yet to be confirmed through numerical analysis or experimental testing, the specific alignment strategy proves to be highly effective in reducing pressure drop in the liquid metal circuits. Besides, the neutronic results point to an excellent performance in terms of tritium breeding provided there is an optimal use of the available space between the plasma and the coils, although some arrangements must be done to satisfy also the contemplated shielding requirements. Furthermore, the internal design of the blanket segments and the optimisation of the cooling system allows achieving a valuable performance in terms of balance of plant, while meeting the structural integrity criteria that have been studied at a very preliminary stage.

Good fast-ion confinement is an essential requirement for a fusion reactor. The magnetic configuration of the Wendelstein 7-X (W7-X) stellarator is partially optimized in this regard in a reactor-relevant scenario: it is expected to show improved fast-ion confinement when β is high and the effect of the radial electric field is negligible. The experimental validation of this optimization is difficult since, with the available power, achieving high β under appropriate conditions for the validation is challenging and the effect of the radial electric field is inevitable. In this work, the confinement of fast ions in W7-X has been studied numerically for a variety of scenarios via the ASCOT5 code. The effect of the radial electric field on fast-ion losses is confirmed to be equivalent to the one produced by β , and this is characterized by means of scans on both parameters. Through a preliminary study with experimentally-based profiles, a viable scenario is identified that takes advantage of this effect for the experimental validation of the optimization strategy of W7-X. Finally, the study provides an exhaustive analysis of the entire fast-ion phase space, helping to identify where the fast ions should be generated in such experimental exercise.

Due to their simultaneous optimization for radial and parallel neoclassical transport, quasi-isodynamic (QI) fields have been the main choice of stellarator magnetic configuration for most fusion reactor candidates in recent years. However, achieving a high degree of quasi-isodynamicity often comes at the cost of a strong shaping of the flux surfaces of the stellarator and complex coil geometries. In this work, the concepts of quasi-isodynamicity and piecewise omnigenity (pwO) are combined to form QI-pwO fields. These fields are QI in the low-field region of the magnetic surface, whereas they significantly depart from quasi-isodynamicity in the high-field region without sacrificing the neoclassical transport properties of QI fields. This departure could make it easier to integrate the optimization of neoclassical transport with other physical and technological aspects of a stellarator reactor.

We provide an overview of activities carried out at the TJ-II stellarator aimed at understanding transport from first principles and power balance analysis. These tasks include gyrokinetic simulations with the codes stella and EUTERPE, neoclassical simulations with the code SFINCS, particle deposition calculations with the code HPI2, and heat source estimates with ASCOT5. All these numerical simulation efforts converge, together with transport analyses, to address the transport mechanisms in plasmas with improved confinement through pellet injection—a scenario studied during the past few TJ-II campaigns bearing resemblance to the pellet-fueled high-performance plasma scenarios of observed in W7-X.

A four-field-period quasi-isodynamic stellarator configuration is presented that exhibits small neoclassical and electrostatic turbulent transport, good fast-ion confinement over a wide range of β values, small bootstrap current and an edge island structure compatible with an island divertor. This configuration, called CIEMAT-QI4X, has been obtained by building on the optimization strategy and sophisticating the methods employed in [Sánchez E. et al 2023 Nucl. Fusion 63 066037]. The optimization has been improved by incorporating metrics to control Mercier stability and by enforcing strict constraints on the rotational transform profile to achieve nested toroidal surfaces in the confinement region and a divertor island structure at the plasma edge. Specifically, CIEMAT-QI4X has a 4/4 island chain at the edge that is resilient at least up to β = 4%, even when the bootstrap current is included. A corresponding set of filamentary coils is presented that generates the configuration with enough accuracy to preserve the aforementioned physics properties. In terms of physics performance, CIEMAT-QI4X establishes as a candidate for a stellarator fusion reactor design.

Recent nonlinear gyrokinetic simulations have shown that impurities can strongly modify the turbulent heat flux in stellarator plasmas. Here, the ion-temperature-gradient (ITG) dispersion relation in a plasma containing impurities is analytically solved in certain limits and an expression for the modification of the ITG growth rate by impurities is derived. The analytical expression is the sum of three terms corresponding to three different physical causes (impurity density gradient, impurity temperature gradient and dilution) of the change in the growth rate. The scalings predicted analytically for the modification of the growth rate are shown to be reproduced by linear gyrokinetic simulations. The conditions for reduction or increase of the ITG growth by impurities are also correctly predicted by the analytical solution to the dispersion relation. Finally, a remarkable correlation is found between the analytical expression for the modification of the growth rate and the modification of the turbulent heat flux obtained from nonlinear gyrokinetic simulations.

Until now, quasi-isodynamic magnetic fields have been the only known stellarator configurations that, at low collisionality, give small radial neoclassical transport and zero bootstrap current for arbitrary plasma profiles, the latter facilitating control of the magnetic configuration. The recently introduced notion of piecewise omnigenous fields has enormously broadened the space of stellarator configurations with small radial neoclassical transport. In this Letter, the existence of piecewise omnigenous fields that give zero bootstrap current is proven analytically and confirmed numerically. These results establish piecewise omnigenity as an alternative approach to stellarator reactor design.

Piecewise omnigenous fields are stellarator magnetic fields that are optimized with respect to radial neoclassical transport thanks to a second adiabatic invariant that is piecewisely constant on the flux-surface. They are qualitatively different from omnigenous fields (including quasi-isodynamic or quasisymmetric fields), for which the second adiabatic invariant is a flux-surface constant. Piecewise omnigenous fields thus open an alternative path towards stellarator reactors. In this work, piecewise omnigenous fields are characterized and parametrized in a systematic manner. This is a step towards including piecewise omnigenity as an explicit design criterion in stellarator optimization, and towards a systematic study of the properties of nearly piecewise omnigenous stellarator configurations.

Stellarator magnetic fields that are perfectly optimized for neoclassical transport (with levels of radial neoclassical transport comparable to tokamaks) are called omnigenous. Quasi-isodynamic (QI) 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 QI configurations is to optimize the magnetic field indirectly by minimizing proxies that vanish in an exactly QI 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.