Pressure and current effects on the thermal efficiency of an MPD arc used as a plasma source

I several types of laboratory experiments, such as magnetoplasmadynamic power generation, gas dynamic lasers, and basic gaseous plasma studies, a reliable and efficient continuous source of plasma is useful. This paper describes one such source, the magnetoplasmadynamic (MPD) arc operated without an applied magnetic field and presents data on its thermal efficiency, principal heat loss, and arc voltage over a range of argon mass flow rates not previously reported and at several arc currents. A comparison between this data and an existing empirical theory is also made with good agreement. The MPD arc has been studied extensively for possible use as a spacecraft thruster. For this application the arc is operated at low propellant mass flow rates and low-pressure levels. As a plasma source, the MPD arc was found not to require an applied magnetic field; it was also found that the arc can be operated reliably at argon flow rates between 0.08 and 44 g/sec, arc chamber pressures between 26 and 950 torr, and d.c. currents between 200 and 2000 amp. Within these limits the arc produces a steady stream of argon plasma at a thermal efficiency as high as 90%, and with an enthalpy between 4 x 10 and 4 x 10 joule/ kg. Except for the lower current limit (200 amp), these limits on the operating parameters were dictated by the available equipment and, as far as is known, do not represent limits on the MPD arc itself. At currents below 200 amp the arc head was not reliable.