Piecewise omnigenous magnetohydrodynamic equilibria as fusion reactor candidates

TL;DR

Piecewise omnigenity generalizes traditional omnigenity to allow distinct constant-J regions on a flux surface, enabling collisionless confinement with greater design flexibility. The authors develop a DESC-based optimization targeting a pwO field $B_{pwO}( heta,eta)$ to minimize $\delta B$ on the flux-surface $s_0=0.5$ while enforcing reactor-relevant constraints. The resulting configuration CIEMAT-pw1 exhibits $|\delta B|/B_{pwO}<1\%$ at the target surface, reduced neoclassical transport and bootstrap current comparable to QS/QI cases, robust MHD stability across beta, and favorable alpha-particle confinement with modest turbulence, all compatible with an island divertor. These findings expand the set of viable stellarator reactor candidates and outline next steps toward coil feasibility and full reactor integration.

Abstract

In piecewise omnigenous magnetic fields, charged particles remain perfectly confined in the abscence of collisions and turbulence. This concept extends the traditional notion of omnigenity, the theoretical principle upon which most of existing magnetic fusion reactor designs, including tokamaks, are based. While piecewise omnigenity broadens the range of potentially viable stellarator reactor candidates, it is achieved by relaxing the requirement of continuity in the magnetic field strength, which could appear to pose significant challenges for the design of magnetohydrodynamic equilibria. In this work, a stellarator magnetic configuration is presented that satisfies the ideal magnetohydrodynamic equilibrium equation and that achieves unprecedented levels of piecewise omnigenity. As a result, it exhibits favorable transport characteristics, including reduced bulk radial (neoclassical and turbulent transport), bootstrap current and fast ion losses. In addition, the configuration displays robust MHD stability across a range of \b{eta} values and possesses a rotational transform profile compatible with an island divertor. Collectively, these features satisfy the standard set of physics criteria required for a viable reactor candidate which, until now, were believed to be attainable only by certain types of omnigenous stellarators.

Figures (14)

• Figure 1: Flux surface of a stellarator magnetic configuration (red/blue colours correspond to a larger/smaller $B$) with the guiding-center trajectories of a passing particle (black), an unconfined trapped particle (red), a confined trapped particle (green), and a transitioning particle (pink). Gray arrows represent the bounce-averaged drifts and the dashed circle highlights one of the transitions.
• Figure 2: $B$ on a flux surface of an axisymmetric tokamak or an exactly quasi-axisymmetric stellarator (top left), of an exactly QS field with helical symmetry (center left), of an exactly QI field (bottom left); $B$ on the boundary of NCSX (top right), HSX (center right) and W7-X high mirror (bottom right). Dashed gray lines represent magnetic-field lines.
• Figure 3: Diagram depicting the different families of optimized stellarator fields.
• Figure 4: $B$ on a flux surface of an exactly pwO field close to quasi-isodynamicity (left) and an exactly pwO field far from omnigenity (right).
• Figure 5: Top and side views (top and bottom figures, respectively) of the magnetic field strength on the $s=1$ flux surface of CIEMAT-pw1, with the shape of several toroidal cuts of the flux surfaces.