Description
Good fast-ion confinement is an essential requirement for a fusion reactor. The most restrictive requirement is imposed by the heat load on the reactor walls: alpha particles that are rapidly lost, and thus retain most of their initial energy, could potentially damage the reactor components exposed to the plasma. Moreover, these alpha particles are intended to contribute to heat the plasma. Therefore, their confinement time must be comparable to or larger than the time it takes them to transfer their energy to the plasma and thermalize.
The magnetic configuration of the Wendelstein 7-X (W7-X) stellarator is partially optimized in this regard in a reactor-relevant scenario: it is expected to show improved fast-ion confinement when $\beta$ is high and the effect of the radial electric field is negligible [1]. The experimental validation of this optimization is difficult since, in an experimental device with limited power, achieving high $\beta$ under appropriate conditions for a validation exercise is challenging [2,3] and the effect of the radial electric field is inevitable [4,5]. In fact, previous works [6,7] point to the difficulty, due to the effect of the electric field, of studying the configuration dependence of fast-ion confinement in W7-X.
In this work, fast-ion confinement in W7-X is characterized numerically for a variety of scenarios via the ASCOT5 code [8]. The effect of the radial electric field on fast-ion losses is confirmed to be equivalent to the one produced by $\beta$, and this is characterized by means of scans on both parameters. Through a preliminary study with experimentally-based profiles, a viable scenario is identified that takes advantage of this effect for the experimental validation of the optimization strategy of W7-X regarding fast-ion confinement. This scenario requires that the fast ions are generated in specific regions of the plasma volume and of the phase space, which are identified in this study.
The results presented in this work are relevant for any quasi-isodynamic configuration that is proposed as a reactor candidate, since any intermediate-size device that is designed for the experimental validation of its confinement properties will face the same challenges regarding fast-ion confinement.
[1] M. Drevlak et al., Nuclear Fusion 54(7), 073002 (2014).
[2] R. C. Wolf et al., Nuclear Fusion 57(10), 102020 (2017).
[3] T. Klinger et al., Nuclear Fusion 59(11), 112004 (2019).
[4] Y. I. Kolesnichenko et al., Physics of Plasmas 13(7), 072504 (2006).
[5] E. M. Green et al., Nuclear Fusion, (2026).
[6] J. M. Faustin et al., Nuclear Fusion 56(9), 092006 (2016).
[7] S. A. Lazerson et al., Nuclear Fusion 61(9), 096005 (2021).
[8] E. Hirvijoki et al., Computer Physics Communications 185(4), 1310-1321 (2014).