Stable quasi-isodynamic stellarators with low turbulence as fusion reactor candidates*

Quasi-isodynamic (QI) stellarators are uniquely attractive fusion reactor candidates due to their low neoclassical transport, excellent confinement of fusion-born alpha particles, and vanishingly small net toroidal currents [1]. To be a viable fusion reactor, a stellarator design must be MHD stable, have low enough turbulent heat losses to alleviate the temperature clamping present in the Wendelstein 7-X experiment [2], and include set of buildable electromagnetic coils. Combining the work in [3] with the findings from [4], we present a new approach to finding MHD stable QI stellarator geometries with superb fast-ion confinement and considerably lower turbulence than W7-X, which can be realized by a set of relatively simple electromagnetic coils. Their small net toroidal currents, along with their optimised rotational transform profiles, make these configurations compatible with an edge island divertor. We have additionally found that these configurations can be designed to have an electron root — a region of outward-pointing radial electric field — which provides an elegant solution to the problem of impurity accumulation [5]. As such, these designs are alluring candidates for future stellarator experiments and reactors.

References

[1] P. Helander and J. Nuhrenberg. Bootstrap current and neoclassical transport in quasi-isodynamic stellarators. Plasma Physics and Controlled Fusion, 51(5):055004, Feb 2009.

[2] M. N. A. Beurskens and et al. Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas. Nuclear Fusion, 61(11):116072, Oct 2021.

[3] A. G. Goodman, et al. Quasi-isodynamic stellarators with low turbulence as fusion reactor candidates. PRX Energy 3, 023010, Jun 2024.

[4] J. Kappel, M. Landreman, and D. Malholtra. The magnetic gradient scale length explains why certain plasmas require close external magnetic coils. Plasma Physics and Controlled Fusion, 66, Jan 2024.

[5] C. D. Beidler, M. Drevlak, J. Geiger, P. Helander, H. M. Smith, and Y. Turkin. Reduction of neoclassical bulk-ion transport to avoid helium ash retention in stellarator reactor. Submitted to Nuclear Fusion, Jan 2024.