Development and Implementation of a Four-Tip 3D Mach Probe Arranged in a Tetrahedral Geometry on MST device*

To examine the momentum redistribution processes and study general helicities during plasma relaxation in the RFP, a new prototype 3D Mach probe has been designed and tested to measure 3D plasma flow. The probe consists of four biased molybdenum electrodes to measure the 3D ion velocity. The geometric addition of the logarithms of ion saturation currents allows 3D vectors to be calculated from the tetrahedral geometry. Additionally, the probe has three orthogonal magnetic coils to measure the equilibrium and fluctuating magnetic field. Plasma perturbation should be minimized, so the conducting shell of MST and return electrodes at various distances from the tips are all being tested to determine the optimum location for the return current. A multi-channel power supply is used to bias the Mach tip electrodes and measure the collected current of each tip and the return electrode with voltage dividers. All five currents are connected to a 100 kHz isolation amplifier. The magnetic signals are integrated with a 250 kHz bandwidth. All signals are digitized at 1 MHz. Preliminary results, calibrations, drawings and photos will be presented.