Measurement of Turbulent Plasma by the Time-Domain Triple-Probe Method

Abstract

The time-domain triple-probe (TDTP) method has been developed for measurement of turbulent plasma. Rather than implementing the triple-probe states over a three probe array, the TDTP method cycles each of two Langmuir probes through all triple-probe states. Provided the turbulence is bandlimited, fast cycling and large probe impedance allow the establishment of probe-plasma equilibrium for each triple-probe state, and time resolved measurement of floating potential, ion saturation current, and “return potential” at each of the two probes. A total of six measurements are combined with a system of six equations to solve for six plasma variables; electron temperature, electron number density, and plasma potential at each of the two probes. Successful implementation of the TDTP method in the edge plasma and scrape-off layer of TEXT-U measures significant electron temperature fluctuations with poloidal correlation statistics similar to those of floating potential and ion saturation current fluctuations. Electron number density and electron temperature fluctuations are in phase indicating electron pressure fluctuations are important on both sides of the velocity-shear layer.