Laser-induced fluorescence velocity measurements of a diverging cusped-field thruster

Measurements are presented of the most probable time-averaged ion velocities within the acceleration channel and in the plume of a diverging cusped-field thruster operating on xenon. Xenon ion velocities for the thruster are derived from laser-induced fluorescence measurements of the 5d[4](7/2)-6p[3](5/2) xenon ion excited state transition centred at lambda = 834.72 nm. The thruster is operated in both a high-current mode, where the anode discharge current is shown to oscillate periodically, and a low-current mode where operation is relatively quiescent. In the low-current mode, ion emission is predominantly in the form of a conical jet, whereas in the high-current mode, the emission is still divergent but more diffuse throughout the cone angle. These time-average measurements provide insight into the structure of the acceleration region. However, discerning the mechanism for the diffuse ion emission in the strongly oscillating high-current mode will require ion velocity measurements capable of resolving the time-dependent behaviour of the discharge.