Effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster

Abstract

The effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster were investigated in order to improve thrust performance. First, the extracted ion beam current was measured for various levels of strength of the magnetic field. It was found that there is an optimum magnitude of the magnetic field. That this is due to the tradeoff between magnetic mirror confinement and microwave-plasma coupling was confirmed by measurement of the ion saturation current into the antenna of the ion thruster. The ion saturation current was found to decrease with an increase in magnetic field strength, due to the improvement in magnetic mirror confinement. The estimated electron temperature also decreases with an increase in magnetic field strength. This result shows that the increase in magnetic field strength leads to a decrease in microwave-plasma coupling. Next, the ion beam current for three magnetic field shapes was measured by changing the length of the central yoke. The results show that the optimum magnetic field shape depends on the mass flow rate because of the tradeoff between magnetic confinement and ionization probability. For the configurations tested, the 3 mm length central yoke is optimal for low mass flow, whereas 7 mm is the best for high mass flow. Overall, the extracted ion beam current is 21.4 mA, at a xenon mass flow rate of 0.036 mg/s, beam voltage of 1500 V, and incident microwave power of 16 W.