Description
Energetic-ion confinement at finite plasma pressure is a key requirement for optimised stellarators and reactor extrapolation. Wendelstein 7-X (W7-X) has been designed to provide good fast-ion confinement at reactor-relevant β. The recent availability of finite-β W7-X equilibria now enables a systematic assessment of the β-dependence of fast-ion behaviour for realistic beam sources and magnetic configurations.
Fast-ion modelling in W7-X is performed using the ASCOT5 code with finite-β VMEC equilibria including pressure-driven plasma response. The standard and high-mirror magnetic configurations are considered, as both have been shown to provide improved fast-ion confinement. Since finite-β effects can modify orbit topology and fast-ion losses, leading to configuration- and species-dependent wall loads, two representative plasma pressure cases, corresponding to vacuum (β≈0) and finite pressure (β≈2.5%), are analysed. An extra case including magnetic perturbations in the scrape-off layer associated with the island divertor is also considered for the standard configuration to assess their impact on fast-ion confinement.
The fast-ion source from neutral beam injection (NBI) is generated using the BBNBI5 code for hydrogen, deuterium, and helium species, with ionisation coefficients imported from the ADAS database and applied through ASCOT5’s atomic-reactions module [1]. This enables a systematic assessment of mass- and charge-dependent confinement properties. Particular emphasis is placed on helium beam ions as a proxy for fusion-born α-particles. Preliminary results from helium NBI experiments at ASDEX Upgrade [3] using charge-exchange spectroscopy (method similar to [4]) have provided a complementary validation of helium fast-ion modelling, with a good agreement between the model and the measured signal.
Estimated beam-ion losses are used to identify hot spots at plasma facing components, which represents a critical oper- ational constraint. Synthetic signals for a scintillator fast-ion loss detector (sFILD) [2] under construction for W7-X are generated with FILDSIM to support diagnostic design and future validation.
References
[1] P. Ollus et al Plasma Phys. Control. Fusion 64, (2022) 035014
[2] A. van Vuuren et al., Fusion Eng. Des. 204 (2024) 114520
[3] C. Hopf et al Fusion Eng. Des. 123, (2017) 281-284
[4] A. Kappatou et al Nucl. Fusion 61, (2021) 036017