Effect of Exhaust Magnetic Field in a Helicon Double-Layer Thruster Operating in Xenon

A xenon ion beam is spatially characterized by using a retarding-field energy analyzer positioned 7 cm downstream of a helicon double-layer thruster (HDLT) operating at 500-W radio-frequency power, 0.07-mtorr (9.33 x 10(-3) Pa) gas pressure, and with an exhaust magnetic field diverging from a maximum of about 142 G (0.0142 T) inside the thruster to about 26 G (0.026 T) at the probe location. The beam is formed by acceleration through the potential drop of a double layer (DL). It is found that, for constant operating pressure, increasing the maximum exhaust magnetic field from about 60-236 G (0.006-0.0236 T) induces an increase of both the ion-beam energy and the ion-beam-to-down-stream-plasma-flux ratio, both indicators of an increased thruster efficiency. Hence, the specific impulse can be controlled by using the exhaust magnetic field in the HDLT.