Initial measurements of plasma potential in the core of the MST reversed field pinch with a heavy ion beam probe

Measurement of the plasma potential in the core of MST marks both the first interior potential measurements in an RFP, as well as the first measuremen ts by a Heavy Ion Beam Probe (HIBP) in an RFP. The HIBP has operated with ( 20-110) keV sodium beams in plasmas with toroidal currents of (200-480) kA over a wide range of densities and magnetic equilibrium conditions. A posit ive plasma potential is measured in the core, consistent with the expectati on of rapid electron transport by magnetic fluctuations and the formation o f an outwardly directed ambipolar radial electric field. Comparison between the radial electric field and plasma flow is underway to determine the ext ent to which equilibrium flow is governed by E x B. Measurements of potenti al and density fluctuations are also in progress. Unlike HIBP applications in tokamak plasmas, the beam trajectories in MST ( RFP) are both three-dimensional and temporally dynamic with magnetic equili brium changes associated with sawteeth. This complication offers new opport unity for magnetic measurements via the Heavy Ion Beam Probe (HIBP). The io n orbit trajectories are included in a Grad-Shafranov toroidal equilibrium reconstruction, helping to measure the internal magnetic field and current profiles, Such reconstructions are essential to identifying the beam sample volume locations, and they are vital in MST's mission to suppress MHD tear ing modes using current profile control techniques. Measurement of the elec tric field may be accomplished by combining single point measurements from multiple discharges, or by varying the injection angle of the beam during s ingle discharges. The application of an HIBP on MST has posed challenges resulting in additio nal diagnostic advances. The requirement to keep ports small to avoid intro ducing magnetic field perturbations has led to the design and successful im plementation of cross-over sweep systems. High levels of ultraviolet radiat ion are driving alternative methods of sweep plate operation. While, substa ntial levels of plasma flux into the HIBP diagnostic chambers has led to th e use of magnetic plasma suppression.