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Acceleration of propellant in arcjet devices by thermal and self-magnetic forces is investigated theoretically and experimentally. An axially symmetric direct current arc is operated within a rapidly expanding supersonic plasma flow, which is achieved in a free-expansion plume as well as in various nozzle configurations with wide opening angles. According to low mass flow rates, pressures of about 20-100 mm Hg are built up in the vicinity of the tungsten cathode. This leads to cathode operation that, showing a large spotless arc attachment, is different from that known at high pressures. The anode is situated downstream in the lowpressure region, typical values of ambient pressure being 0.1-3 mm Hg. High-speed motion pictures and oscillographic measurements seem to indicate that the anode attachment is of a diffuse type. The experiments cover a current range from 200-2000 amp with argon and 200-1000 amp with helium and hydrogen as working gases. In a nozzle configuration, the Mach number of the exhaust stream is observed to be increased by self-magnetic forces. Thermal acceleration, however, still contributes substantially to thrust production in the devices discussed here.Nomenclature A = area B = magnetic field strength c -velocity of sound F = thrust h = enthalpy I = arc current J sp = specific impulse j = current density M = Mach number ra = mass flow rate n = polytropic exponent p = pressure q = heat added to flow per unit mass r = radius v = velocity 7 = ratio of specific heats r] = efficiency IJLQ = permeability TT = pressure ratio p = density T = control volume