Diagnostics - New Segmented Probe for Low Disturbing Plasma Measurements in Hall Thrusters

Electrostatic probes are widely used to measure spatial plasma parameters of the quasi-neutral plasma in Hall thrusters and similar E x B electric discharge devices. Significant perturbations of the plasma, induced by such probes, can mask the actual physics involved in operation of these devices. In Hall thrusters, probe induced perturbations can affect 100% changes of the discharge current and plasma parameters relative to their steady state values. A possible cause of these perturbations is secondary electron emission induced by energetic electrons of the plasma bulk from the insulator ceramic tubes in which the probe wire is inserted. In fact, a probe design (shown below), in which a low secondary electron emission material, such as metal, shields the probe ceramic tube, significantly reduce these perturbations. A segmentation of this shield can be useful in order to prevent probe-induced perturbations, which might be caused by shortening of the plasma through the conductive shield especially for the thruster operating regimes with high discharge voltages. In a set of experiments with a segmented shield probe, the thruster was operated in the input power range of 500-2.5 kW and discharge voltages of 200-500 V, while the probe induced perturbations of the discharge current were about 3% of its steady state values.