FREQUENCY-DOMAIN ELECTROMAGNETIC CHARACTERISTICS OF A 26-KILOWATT AMMONIA ARCJET

Arcjet thrusters employ an are discharge to heat propellant, which exp ands through a nozzle to produce thrust. Spacecraft designers who desi re to exploit the enhanced specific impulse of arcjets have expressed concern about the electromagnetic environment produced by the thruster s. Laboratory tests were performed to investigate the electromagnetic environment produced by a 26-kW ammonia arcjet in the frequency domain from de to 10 GHz using antennas intended to characterize electric an d magnetic fields. Results obtained with a 30-cm monopole antenna expo sed to the arcjet plume correspond to those of similar ground tests at lower power. When the antenna was shielded from electrical contact wi th the plume by a Pyrex(R) cover, signal levels dropped sharply at all frequencies. With a covered 30-cm monopole antenna near the arcjet pl ume, arcjet-on signals exceeded ambient levels over the frequency rang e 10 kHz-5 MHz. The maximum signal level typically exceeded the ambien t level of 40 dB mu V/MHz by approximately 27 dB mu V/MHz near 200 kHz . A likely explanation for the sharp reduction in signal level once th e antenna electrode is covered is that electrons and ions from the plu me are collected on the exposed antenna but not on the covered antenna . Signals on the shielded antenna mag be interpreted as the result of propagating electric fields, but considerable uncertainty remains abou t the held source and the effects of the facility on the measurements. Although the electromagnetic fields produced by an arcjet operating a s part of a satellite are probably lower than preciously expected, the se results raise new concerns about the character and spacecraft compa tibility of the plasma from an arcjet plume.