Quasi-steady MPD (Magnetoplasmadynamic) arcjet PF-series were developed to understand the cathode jet structures, i.e., to mainly understand pumping acceleration processes of an MPD thruster. The ring anode of PF arcjets is located downstream from the cathode enough to generate a high axial current. Spatial current distribution, electron temperature and electron number density were measured in order to examine the plasma features in the cathode jet region. They were sensitive to the gas species and the anode radius of PF arcjets. Especially, a high-energy region was formed along the center axis of the PF-II arcjet with H2. The current distribution on the cathode cone and the pressure at the cathode tip were also measured with a divided cathode and pressure tap, respectively. Current fractions entering an inner cathode rod at large discharge currents were hardly dependent on gas species and the anode radius of PF arcjets. The pressure at the cathode tip of the PF-II arcjet was good agreement with the JJ
2-dependence curve derived from the electromagnetic acceleration theory.
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