Particle sources, profiles and transport in neutral beam heated plasmas at Wendelstein 7-X

A promising path towards the generation of net energy gain from deuterium tritium fusion reactions is magnetic confinement fusion. To negate the detrimental effect of particle drifts on the confinement properties a rotational transform, which means a twist of the magnetic field lines, is needed. In a stellarator this twist is generated by external field coils. As a consequence, it can be run in steady-state and is not prone to disruptions caused by current driven instabilities, making it an attractive candidate for a fusion reactor. On the other hand, in a general stellarator geometry the broken rotational symmetry leads to increased neoclassical heat and particle losses. Wendelstein 7-X (W7-X) is a stellarator of the HELIAS (helical advanced stellarator) line which was successfully optimized to significantly reduce these losses and is now dominated by turbulent transport. A well-founded understanding of the particle transport is essential for further optimization and to comprehend deuterium-tritium fueling as well as helium exhaust. Additionally, it has been shown that gradients of the plasma density can strongly influence the energy confinement in toroidal devices and thereby their performance, particularly at W7-X. Theoretical and experimental knowledge of the particle transport is a necessity in order to predict and manipulate density profiles. The presented work focuses on neutral beam heated and fueled discharges at W7-X. It touches on the topic of diagnosing fast-ion confinement using neutral beams as fast-ion sources. An improved neutral beam injection model and a new formation model are implemented in the Bayesian inference framework Minerva and verified against a computationally more costly Monte-Carlo code. It is shown that electron density profiles, from the plasma core to the last closed flux surface, can be inferred from beam and halo emission data, measured by a spectroscopy system at W7-X. Additionally, the main ion temperature profile is inferred from the thermally Doppler broadened halo emission. With accurately inferred beam injection parameters to correctly model the beam fueling profile and verified density and temperature profiles, a detailed particle balance study is conducted. A clear transition of the anomalous main ion and electron particle transport is identified in neutral beam heated plasmas during a strong density peaking phase.