Spatially Mapping the Ion Energy Distributions in Hollow Cathode Discharges

Distributions of ion energy-per-charge were collected with an electrostatic energy analyzer directed along various lines-of-sight in the near-plume region of a hollow cathode operating in a laboratory discharge chamber. While the analyzer was stationary, the cathode and anode assembly were mounted on a rotary and a linear stage, allowing measurements along the cathode axis and at off-axis angles up to 90°; the linear stage enabled translational motion which allowed for measurements parallel to the cathode axis. At angles below 45°, three distinct ion populations were identified: two low-energy narrow peaks and a broad high-energy “tail” with energies-per-charge in excess of 5 times the discharge voltage. Detection of low-energy ions above 45° was not possible since they were confined by the anode voltage. At large angles, the intensity of the high-energy ion signal decreased to become indistinguishable from noise in the data system; however, based on previous results, the failure to detect high-energy ions at large angles has been attributed to the large distance between the ESA line-of-sight and the cathode axis increased from 0° to 16°, the location of the low-energy ion peaks shift but there is no apparent difference in the high-energy ion tail. Increasing the separation angle further yields a decrease in intensity at all energies. For low energies-per-charge, the shape and intensity of the unidirectional ion-energy distribution does not change when the cathode is translated up to 2.5cm; however, as the distance from the cathode centerline increases, the signal intensity of the high-energy ions decreases in a manner similar to the decrease in high-energy ions seen with increasing angles above 30°. These results indicate that magneto-gas dynamic forced did not accelerate the ions observed in this experiment; additional experiments will elucidate the mass and energy dependence of the mechanisms responsible for high-energy ions.