1. Plasma Dynamics Lab HIBP Abstract Measurements of the radial equilibrium potential profiles have been successfully obtained with a Heavy Ion Beam Probe (HIBP) in the core () of the Madison Symmetric Torus (MST) Reversed Field Pinch. Typically, has a magnitude of up to 1.0-2.0 kV in a standard 380 kA discharge. The core profile of the electrostatic potential fluctuations and electron density fluctuations have also been measured in MST. The measured ranges from 30-50 V rms and 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 very distinct changes in, and. The general status of HIBP measurements on MST will be presented including representative data from all types of discharges and measurement development issues. I. Principle of Heavy Ion Beam Probing I 0 = initial primary current injected into the plasma  ion = ion cross-section for primary to secondary ions l sv = sample volume length n e (r sv ) =electron density at the sample volume k = a multiplying factor between 1~10 due to electrons emitted by the detector plates F p = primary beam attenuation F s = secondary beam attenuation q s = charge of the secondary beam q p = charge of the primary beam High Current Standard Plasmas RESULTS:  ~ 1700 - 2000V E r ~ 1.7 - 2 kV/m PLASMAS: I p ~ 380 kA,  ~ ±1% n e ~.95 x 10 13 cm -3,  ~ ±5% V n=6 ~ 30km/s,  ~±10% F ~ -0.22 T e ~ 300 eV Multiple Shot Analysis Three intervals, 1.5ms duration each ~1.5 ms after crash mid way between crashes ~2.5 ms before crash Potential is Positive Between Sawtooth Crashes After Crash –Phi ~ 1735 V –±125V scatter about trend- line Mid-cycle –Peak phi ~ 2kV –±125V scatter about trend Before Crash –Peak phi ~ 2kV –±150V scatter about trend Scatter may be due to Variations in density Variations in mode velocity Variations in magnetic profiles and thus sample volumes Potential Tracks the Mode Velocity Measured instantaneous potential tracks evolution of the mode velocity m=1, n=6 mode velocity Potential at which v n=6 = 40 km/s does not overlap  at v n=6 = 20 km/s Sample volume ~ r=22cm Data from over 50 shots Differences between  and v n=6 profiles may be due to evolution in B and motion of the sample location Potential and Electric Field Profiles in High I p Discharges Measurement locations determined for data in potential scatter plots Average sample position and potential computed from a 1- 1.6cm range Scatter in the potential and radial location are depicted by the vertical and horizontal ranges Electric field profiles computed from the average potentials and average sample volume spacing After Crash Mid-cycle Before Crash Potential Profile Measurements in Low Current Discharges Electric Field Profiles in Low Current Rotating Plasmas Average E r ~ 1.5kV/m The electric field is outwardly directed Recall, E ~ 2 kV/m in high I p discharges Large error bars on E r (neg. E r ) due to shot to shot scatter in potential and artifacts of data processing Singly charge heavy ions (primaries) are injected into the plasma Some primary ions are further ionized by collisions with plasma electrons The magnetic field separates the secondary ion trajectories from the primary ions. The combined primary and secondary ion trajectories appear as shown in the system figure above The secondary ions are detected by ion collection plates split vertically and horizontally so that four separate currents are monitored This permit measurements of the electric potential, fluctuations of potential and electron density, and magnetic vector potential, localized to the ionization position The secondary beam current I s ( the sum of the four split plate signals) is given by Assuming  ion is a weak function of plasma temperature T e, which is about a few hundred eV in the core of the MST plasma, I s is proportional to the density n e (r sv ) The relative density fluctuation level at the sample volume is then obtained from The currents on the top two plates are summed to produce i upper while the bottom two plates sum to i lower. The energy of the secondary ion leaving the plasma differs from that of the primary ion by the change in potential energy. Thus, to determine potential, we use a Proca and Green type energy analyzer and the following relationship G and F are geometric functions of the analyzer angle V AN is the analyzer voltage and V AC is the accelerator voltage Peak  ~ 1400 V Peak Peak  ~ 500 V lower than in High I p discharges Measurements ~ 2 ms after crash