Effect of magnetic and physical nozzles on plasma thruster performance

Plasma cross-field diffusion in a magnetic nozzle is inhibited by increasing the magnetic field strength in a helicon plasma thruster attached to a pendulum thrust balance, while maintaining constant plasma density and electron temperature in the source tube, i.e. a constant plasma injection into the magnetic nozzle, where the field strength near the radio frequency (rf) antenna is less than 210G and the operating argon pressure in the vacuum chamber is 0.8mTorr. Inhibition of the cross-field diffusion yields a higher electron pressure in the magnetic nozzle and a resultant larger thrust. The thrust component arising from the magnetic nozzle approaches the theoretical limit derived from an ideal magnetic nozzle approximation where no plasma is lost from the nozzle and there is an azimuthal plasma current originating from the electron diamagnetic drift. It is also shown that the momentum of the plasma lost from the magnetic nozzle is captured by a physical nozzle attached at the source exit resulting in a larger thrust. Two physical nozzles of different sizes (nozzle 1: 10.5 cm in length with a maximum diameter of 20 cm, nozzle 2: 26 cm in length with a maximum diameter of 36 cm) are tested. The maximum thrust of 20 +/- 1mN is obtained for 25 sccm argon propellant and 2 kW rf power with a reflection power less than 5W, which gives a specific impulse of 2750 +/- 165 s and a thrust efficiency of 13.5 +/- 1.5%.