Description
Quasi-isodynamic (QI) magnetic fields are a uniquely attractive feature for stellarator power plants due to their low neoclassical transport and small toroidal currents [1], the latter offering robust plasma stability. It is for this reason that several stellarator companies, including Type One Energy [2] and Proxima Fusion [3], have based their power plant designs around QI magnetic fields. Like most properties in stellarators, one must use numerical optimisation techniques to find plasma equilibria that are QI. In this work, we begin by presenting the first method ever developed that is able to find stellarator equilibria with sufficient QI quality for a fusion power plant [4]. Any stellarator reactor must be resilient to magnetohydrodynamic (MHD) and turbulent instabilities, the latter of which limits the performance of Wendelstein 7-X [5], itself a roughly QI stellarator. As such, we next merged our QI optimisation methodology with other novel methods to find a new class of stellarators, “SQuIDs” (Stable Quasi-Isodynamic Designs) [6], which satisfy these conditions alongside excellent QI quality. Findings from Kappel et al. [7] can be included in SQuID designs, yielding configurations that are compatible with relatively simple electromagnetic coils. The final publication in this work therefore presents coil-compatible SQuIDs [8], which additionally feature an island divertor concept, along with other new and interesting physics properties such as an electron root — a region of outward pointing radial electric field — which may result a transport barrier akin to an H-mode placed the plasma radius of the designer’s choosing [9]. Future work on SQuIDs includes introducing an inward particle pinch [10], resulting in further improved fusion power output and plasma confinement.
References
[1] P. Helander and J. N¨uhrenberg. Bootstrap current and neoclassical transport in quasiisodynamic stellarators. Plasma Physics and Controlled Fusion, 51(5):055004, feb 2009.
[2] CC Hegna, DT Anderson, EC Andrew, A Ayilaran, A Bader, TD Bohm, K Camacho Mata, JM Canik, L Carbajal, A Cerfon, et al. The infinity two fusion pilot plant baseline plasma physics design. Journal of Plasma Physics, 91(3):E76, 2025.
[3] J Lion, J-C Angl`es, L Bonauer, A Ba˜n´on Navarro, SA Cadena Ceron, R Davies, M Drevlak, N Foppiani, J Geiger, A Goodman, et al. Stellaris: A high-field quasi-isodynamic stellarator for a prototypical fusion power plant. Fusion Engineering and Design, 214:114868, 2025.
[4] Alan Goodman, Katia Camacho Mata, Sophia A Henneberg, Rogerio Jorge, Matt Landreman, Gabriel Plunk, Hakan Smith, Ralf Mackenbach, and Per Helander. Constructing precisely quasi-isodynamic magnetic fields, 2022.
[5] 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.
[6] Alan G. Goodman, Pavlos Xanthopoulos, Gabriel G. Plunk, H˚akan Smith, Carolin N¨uhrenberg, Craig D. Beidler, Sophia A. Henneberg, Gareth Roberg-Clark, Michael Drevlak, and Per Helander. Quasi-isodynamic stellarators with low turbulence as fusion reactor candidates. PRX Energy, 3:023010, Jun 2024.
[7] 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, 01 2024.
[8] Alan G Goodman, Gabriel G Plunk, Pavlos Xanthopoulos, Michael Drevlak, Joachim Geiger, Robert Davies, H˚akan M Smith, Carolin N¨uhrenberg, Craig D Beidler, Sophia A Henneberg, et al. A quasi-isodynamic stellarator configuration towards a fusion power plant. Submitted Journal Plasma Physics, 2025.
[9] 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.
[10] G. G. Plunk, A. G. Goodman, P. Xanthopoulos, P. Costello, H. M. Smith, K. Aleynikova, C. D. Beidler, M. Drevlak, and P. Helander. Enhanced performance in quasi-isodynamic max-j stellarators with a turbulent particle pinch, 2025.