Resonances of plasma in stellarators contribute to energetic particle loss

Over the decades, stellarators have shown particular promise for future fusion reactors by confining plasma into a corkscrewing loop. Fusion requires energetic particle confinement, but the capability of stellarator confinement with regards to energetic particles, such as alpha particles, is still debated. Such behavior is essential for assessing the relevance of the stellarator configuration for a fusion reactor.

White et al. examined the resonances of high-energy particles in several of today’s stellarators to describe how significant particle loss occurs in such devices. The group modeled the Wendelstein 7-X, the Helically Symmetric Experiment, the Large Helical Device, and quasi axisymmetric and quasi helically symmetric designs to find these resonance characteristics.

“Most of the work in stellarator research involves the construction of the magnetic field structure,” said author Roscoe White. “I am interested in the particle orbits in these fields, because only at low energy do particles follow the magnetic field. At higher energy, particles drift across field lines, so the field does not describe everything about particle orbits.”

The group found interaction with the toroidal variation of the equilibrium of particles at high energy led resonances in some stellarators to take on large widths even in the absence of perturbations due to instabilities.

“A resonance in a stellarator can produce a standing wave of particle orbits matching the toroidal dependence of the field, with a large island,” White said. “The islands short circuit radial transport, providing paths for loss.”

White hopes the work will inspire others to explore the effect of resonance on particle loss.

Source: “Poor confinement in stellarators at high energy,” by Roscoe White, Andreas Bierwage, and Stephane Ethier, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0094458 .