Optimisation of stellarator reactor design to enable electron-root operation

Heavy impurity transport, helium ash exhaust and turbulence are important issues for future stellarator reactor designs as they can greatly influence reactor performance. A positive radial electric field in the plasma core of stellarators is expected to be beneficial in expelling heavy impurities from the core [1], improving the exhaust of helium ash [2], and possibly reducing turbulence. Therefore, scenarios which allow for such a solution of ambipolarity, often called electron root, are interesting for plasma operation in future stellarator reactors. In this work [3], we show that the optimisation of stellarator designs for such a root is possible in quasi-isodynamic stellarators at reactor-relevant magnetic configurations and plasma parameters. The space of plasma parameters at which the electron root can exist is maximised by targeting the ratio of electron to ion thermal radial transport coefficients, using the optimisation framework SIMSOPT [4] and the drift-kinetic solver SFINCS [5]. The optimised configurations have an electron root, favourable tungsten screening, helium ash exhaust and reduced thermal ion neoclassical transport.

[1] K. Fujita et al., 2021, Nuclear Fusion 61, 086025

[2] C. D. Beidler at al., 2024, in submission process

[3] E. Lascas Neto et al., 2024, submitted to JPP, arXiv:2405.12058

[4] M. Landreman et al., 2021, Journal of Open Source Software 6 (65), 3525

[5] M. Landreman et al., 2014, Physics of Plasmas 21 (4), 042503