First measurement of potential in the core region of the madison symmetric torus reversed field pinch

Determination of the plasma potential in the core of the Madison Symmetric Torus (MST) Reversed Field Pinch (RFP) is one of the primary purposes of the MST Heavy Ion Beam Probe (HIBP). In particular, the relationship between the equilibrium electric field and plasma rotation and transport will be investigated. Preliminary measurements inside the core indicate that the plasma potential Φp is positive and in the range of 0.5 kV to 2.5 kV for standard discharges (including the range of observed potentials and possible errors). This is the first such measurement in the core of an RFP with moderate temperature and density. Because an RFP magnetic field is produced by plasma currents, initial experiments have emphasized finding Φp at locations known to be in the core of standard discharges, even though the magnetic field and, thus, the locations themselves are not fully known. The sensitivity of the beam ion orbits to the current profile is being exploited by using the HIBP signals to provide a constraint on the magnetic equilibrium model and thereby help determine the current profile. A simple density scaling experiment has been performed showing a significant decrease in Φp with increasing density. The intriguing, though preliminary results from this and other experiments point to a significant role for Φp measurements in an RFP. (author)