Measurement of potential profiles in the MST-RFP using a heavy ion beam probe

The plasma potential and radial electric field have been measured for the first time in the core (r ∼ 16–31 cm) of a hot (T _e ∼300 eV) reversed field pinch (RFP) plasma using a heavy ion beam probe (HIBP) diagnostic. This feat also marks the first ever application of the HIBP on an RFP and includes novel features such as a cross over sweep plate ion injection system and magnetic plasma suppression structures.; Three types of plasmas were investigated in MST: (a) high (380 kA) and low current (275 KA) standard, low-current (b) locked and (c) biased discharges. The plasma potential in MST is measured reliably at times at least 1–1.5 ms away from a sawtooth crash and is found to be positive in all of these cases. Profile measurements are typically made in 100–150 μs. The potential in high current discharges ranges from 1200–2100 V and is strongly dependent on the phase velocity of the n = 6 mode and to a lesser extent on plasma density. In low current discharges, the potential ranges from 900–1200 V and exhibits similar dependence. In locked and biased plasmas, the plasma potential is observed to decrease by 600–800V. The radial electric field in high current discharges is outwardly pointing and typically 2–2.5 kV/m; the value reduces to ∼1.6–2.0 kV/m in low current standard discharges. In low current discharges the radial electric field is observed to co-relate well with plasma flow (as inferred from impurity flow measurements) in the core region. The radial electric field in locked and biased discharges is small or even zero. Potential measurements in PPCD discharges have been preliminarily attempted. Initial observations suggest that plasma potential can drop to negative values due to improved confinement.; The radial electric field measurement utilized a new RFP equilibrium reconstruction program known as MSTFit, which uses experimental data as inputs. The HIBP measurement locations have then been obtained using an ion orbit trajectory solver that utilizes the magnetic field from MSTFit to solve the Lorentz force equation.