Heavy ion beam probe measurements of the equilibrium potential and electrostatic fluctuation profiles on the Madison Symmetric Torus

Measurements of the radial equilibrium potential profiles have been successfully obtained with a Heavy Ion Beam Probe (HIBP) in the core (0.25 < r/a < 0.75) of the Madison Symmetric Torus (MST) Reversed Field Pinch. Typically, φ (r) has a magnitude of 1.0-2.0 kV in standard 380 kA discharges. The core profile of the electrostatic potential fluctuations φ and electron density fluctuations ñ_e(r) have also been measured in MST. The measured φ(r) ranges from 30-50 V_rms and ñ_e(r) ranges from 10-20% for this same standard 380 kA discharge. While most of the data obtained thus far have been for standard discharges at a variety of plasma currents, preliminary measurements have also been obtained for other discharge conditions, including biased discharges and pulsed poloidal current drive (PPCD) discharges. Confinement is significantly improved in PPCD discharges and HIBP measurements obtained thus far show changes in φ (r), φ(r) and ñ_e(r). The general status of HIBP measurements on MST will be presented including representative data from all types of discharges and measurement development issues.