An experimental investigation of alternative propellants for the helicon double layer thruster

Ion energy distribution functions are measured using a retarding field energy analyser located 7.5 cm downstream of a helicon double layer plasma source, respectively, operating with four molecular gases: nitrogen (N ₂), methane (CH₄), ammonia (NH₃) and nitrous oxide (N₂O). For radiofrequency powers of a few hundred watts, and a magnetic field diverging from about 0.013 T (130 G) in the source to about 0.001 T (10 G) in the exhaust, an ion beam is detected for each propellant over a very similar operating pressure range (∼0.023 Pa (0.17 mTorr) to ∼0.267 Pa (2 mTorr)), as a result of spontaneous electric double layer formation near the exit of the plasma source. The characteristics of the ion beam versus operating pressure closely follow those previously obtained in argon, xenon and hydrogen. The ion beam exhaust velocity in space is found to be in the 17-19 km s^-1 range in N₂, 21-27 km s^-1 range in CH ₄ and NH₃ and 14-16 km s^-1 range in N ₂O.