A water-cooled gas sampling probe capable of steady-state operation at 15 000°K and one atmosphere has been developed. Temperature is measured by a calorimetric technique which can provide high-accuracy calibration under arcjet conditions. The probe also measures velocity and chemical composition and, because of its small size (∼⅛ in. o.d.), is capable of accurate local measurements in the presence of severe gas-stream property gradients. Calibrations in the partially ionized argon-helium environment of an arcjet exhaust indicated an average agreement with mass and energy balance criteria within ½%, and a standard deviation from the mean of less than 3%.

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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. Nomenclature C = most probable particle speed, cm/sec d = Bohm transition region thickness, cm E -electron kinetic energy, ergs E n -electron kinetic energy after n collisions, ergs &E = change in electron kinetic energy, ergs e -electronic charge, esu (also base of natural logarithms) h = Debye length, cm 7 = current density, amp/cm 2 k = Boltzmann constant L = probe diameter, cm m = particle mass, g n = number of collisions in boundary layer N = number density per cm 3 p = pressure, atm Re = Reynolds number T = temperature, °R or °K U = mean flow velocity, cm/sec V = applied potential, v x = axial distance from arcjet nozzle, cm or in. y = distance from probe surface, cm a. = degree of ionization, N € /(NA + N e ) 8 = boundary-layer thickness, cm X = mean free path, cm /z = mobility