Targeting measurements of electric fields and turbulence in the Wendelstein 7-X stellarator through optimization of ion beam probe design*

Experiments with a Heavy Ion Beam Probe (HIBP) on Wendelstein 7-X (W7‑X) would advance understanding of core neoclassical and turbulent particle and energy transport. We have carried out a feasibility study for the diagnostic which assumes use of former TEXT-U 2 MeV HIBP equipment. It assumed predictions of electron density (n_e) and temperature profiles with central n_e ≤ 10²⁰ m^-3. We have performed, for the W7-X standard magnetic configuration, detailed simulations of sample volume shapes, sizes, orientations, and locations. They indicate that the radial extent of sample volumes (Δr_eff/a) is typically 0.02, which is advantageous when measuring profiles. They also predict the diagnostic will be sensitive in the plasma region r_eff/a ≥ 0.25 to fluctuations of n_e and electric potential (φ) at levels > 0.5%, widely covering ion-scale turbulence with k_⊥ρ_s < 1. More broadly, simulations for all eight W7-X reference magnetic configurations predict signal levels will be sufficient for study of equilibrium electric fields at all radii and ion-scale fluctuations of n_e and φ for the outer region of the plasma. Relationships between HIBP design parameters, sample volume properties, and the scope of possible physics studies will be discussed.