Cooled electrostatic probe

Abstract

Experiments were performed to evaluate the ability of a water-cooled electrostatic probe to measure local electron temperature, electron density, floating potential, and saturation current ratio in dense plasmas (argon up to 20,000°R at 1 atm). The measurements of electron temperature were calibrated against temperatures obtained from simultaneous local measurements of total plasma enthalpy at different temperatures under conditions of known equilibrium by use of a proven calorimetric probe technique and were found to be in agreement within normal experimental error (3% standard deviation from the mean). Using this electron temperature, the measurements of floating potential and saturation current ratio were found to agree with a first-order theoretical approximation to within the accuracy of the approximation. The cooled calorimetric-electrostatic probe was then used to measure the degree of nonequilibrium in a reduced-pressure argon plasma in terms of the difference between local electron and heavy-particle temperatures. Results were in agreement with analytical predictions of a simple freezing criterion. The probe also provided a semiquantitative measurement of plasma turbulence.