A magnetic nozzle radiofrequency (rf) plasma thruster having a stepped-diameter source cavity is operated with upstream and downstream gas injection cases; the imparted thrust, the plasma density, and the rf magnetic field are measured. The plasma produced by a 13.56 MHz rf generator is sustained only with low magnetic field strength at the solenoid center of about 30-300 Gauss for the driving frequency of 13.56 MHz and the downstream gas injection; the larger thrust than the upstream gas injection case is obtained. The magnetic field strength giving the maximum thrust increases when changing the operating frequency to 40.68 MHz, implying the efficient plasma production by a low field helicon mode. The axial density measurements show that the maximum plasma density is located near the thruster exit for the downstream gas injection, while the maximum density location is observed at the upstream side of the source for the upstream gas injection. Large amplitude rf magnetic field near the thruster exit and in the magnetic nozzle is simultaneously observed for the low-magnetic field with the downstream gas injection, implying the propagation of the helicon wave. The measured axial wavenumber is qualitatively understood by a simple dispersion relation assuming a radial wavenumber determined by the plasma-vacuum boundary. It is shown that the low-field helicon mode contributes to an increase in a specific impulse since the high density plasma can be sustained for the low gas flow rate, i.e. in the low pressure condition.
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