Modeling and measurement of energetic particle slowing down in Wendelstein 7-X

The energetic particle slowing down model in the BEAMS3D stellarator neutral beam code is compared to analytic models and experimental data from the Wendelstein 7-X experiment (W7-X). Recently, the first neutral beam experiments were performed in W7-X, providing validation of neutral beam deposition codes (Lazerson S.A. et al 2020 Nucl. Fusion 60 076020). This work builds upon that work, and follows the gyro-center orbits of the neutral-beam-generated fast ions to the plasma boundary. Slowing down times based on measurements of diamagnetic energy changes are compared to simulation data. A discharge solely heated by neutral beam injection is used to compare neoclassical heat flux estimates to neutral beam fueling, heating, and current drive. Experimental estimates of electron heat diffusivity suggest that electron turbulence is destabilized by density peaking in the discharge. Neutral beam current drive dominates over bootstrap current, resulting in a reversal of the toroidal current, as seen experimentally. Particle losses and heat flux through the equilibrium boundary are described. The effects of the magnetic configuration and plasma density on such parameters are also assessed. Benchmarking based on analytic estimates and other energetic particle codes is presented.

Automated summary

The study compares an energetic particle slowing down model in the BEAMS3D stellarator neutral beam code with analytic models and experimental data from the Wendelstein 7-X experiment. Validation of neutral beam deposition codes was achieved through recent neutral beam experiments in W7-X. The research demonstrates that neutral beam current drive dominates over bootstrap current, resulting in a reversal of the toroidal current, and density peaking can destabilize electron turbulence in the discharge.