Experiments using a 25-kw hollow cathode lithium vapor MPD arcjet

Abstract

The performance is reported of a 25-kw, lithium fueled, applied field MPD arcjet which incorporates a unique feed system with an open-ended heat-pipe vaporizer and a hollow cathode. The arc typically operates at currents of 250-500 amp, voltages of 40-60 v, magnetic field strengths between 500 and 3000 gauss, and produces a highly ionized lithium beam which transports 70% of the input electrical power to the beam stop. The ambient tank pressures range as low as 2 X 10 ~ torr. A comparison of hollow cathode and conventional MPD arc performance is made and it is concluded that the hollow cathode arc is superior to the conventional design. Diagnostic experiments performed in the exhaust plume include the determination of axial ion and atom velocities by measurements of Doppler shifts with a scanning Fabry-Perot interferometer and of ion energies by use of electrostatic energy analyzers; the determination of ion temperatures by comparison of experimental and calculated spectral line profiles; the determination of electron temperatures and densities with electrostatic probes; and the determination of current densities by Rogowski loops. These measurements indicate that at a station 25 cm downstream of the arc head the directed ion velocity exceeds 2 X 10 cm/sec. At a station 90 cm downstream the plasma typically has an electron temperature of about 2.3 ev, a density of approximately 1.5 X 10 cm~, and under certain operating conditions more than 40% of the arc current extends 90-cm downstream of the arc head. The efficiency of a process that converts input electrical power into kinetic energy is discussed in terms of a model which sets the plasma into rotation with subsequent expansion in a magnetic field.