On the measurement of bootstrap and Pfirsch-Schlüter induced magnetic fields in stellarators using a beam based diagnostic *

The magnetic topology of stellarators is altered by bootstrap and Pfirsch-Schlüter currents in ways that can affect neo-classical transport, stability, and divertor performance. Simulation codes, constrained using pressure profiles and external magnetic measurements, are typically used to solve for the equilibrium and infer these currents. Our new technique uses a beam of alkali ions or atoms as test particles to probe the plasma-induced magnetic fields. The beam undergoes collisions with plasma particles, creating a spray of secondary ions that are detected outside of the plasma. The secondary ion momentum depends on the local magnetic vector potential at the beam particle's point of ionization and also along its path.  We are investigating the relative importance of these two contributions and the feasibility of this technique for stellarators by simulating particles passing through the magnetic equilibrium of the Helically Symmetric eXperiment (HSX) and Wendelstein 7-X. We have also developed a prototype detector that's been deployed on HSX to study particle and radiative noise signals that may impact diagnostic measurements.