Plume Divergence and Discharge Oscillations of an Accessible Low-Power Hall Effect Thruster

Hall effect thrusters (HETs) are an increasingly utilized proportion of electric propulsion devices due to their high thrust-to-power ratio. To enable an accessible research thruster, our team used inexpensive materials and simplified structures to fabricate the 44-mm-diameter Western Hall Thruster (WHT44). Anode flow, discharge voltage, magnet current, and cathode flow fraction (CFF) were independently swept while keeping all other parameters constant. Simultaneously, a Faraday probe was used to test plume properties at a variety of polar coordinate distances, and an oscilloscope was used to capture discharge oscillation behavior. Plasma plume divergence angle at a fixed probe distance of 4.5 thruster diameters increased with increasing anode flow, varying from 36.7° to 37.4°. Moreover, divergence angle decreased with increasing discharge voltage, magnet current, and CFF, by 0.3°, 0.2°, and 8°, respectively, over the span of the swept parameters. Generally, the thruster exhibited a strong oscillation near 90 kHz, which is higher than a similarly sized HET (20–60 kHz). The WHT44 noise frequency spectra became more broadband and the amplitude increased at a CFF of less than 1.5% and greater than 26%. Only the low flow and low voltage operating conditions showed a quiescent sinusoidal discharge current; otherwise, the discharge current probability distribution was Gaussian. This work demonstrates that the WHT44 thruster, designed for simplicity of fabrication, is a viable tool for research and academic purposes.

An analytical model for top width of jet footprint in abrasive waterjet milling: a case study on SiC ceramics

In two-axis/five-axis macro/micro milling with abrasive waterjets (AWJs), the ‘top width of the jet footprint' is one of the critical measures that determine the dimensional accuracy of the part to be manufactured. In two-axis milling, the top width of the jet footprint is influenced by operating parameters such as the diameter of the focusing nozzle, the jet plume divergence in air, the standoff distance, and the jet feed rate, and in complex shape machining using five-axis milling, it is also influenced by the jet impingement angle, i.e. the angle between the jet axis and the horizontal workpiece plane. Hence, in this work, an analytical (i.e. geometrical) model for the top width of the jet footprint was proposed and tested on silicon carbide (SiC) workpeices by considering the diameter of the focusing nozzle, the jet plume divergence in air, the standoff distance, and jet impingement angle. The influence of jet feed rate was included in the model by the use of a parameter defined as the ‘effective jet plume divergence'. SiC workpieces were considered in this study, since they offer a reduction in the ‘noise' created by plastic deformations thus creating an appropriate environment for the calibration of the proposed geometrical model. Moreover, with its low machinability and chipping/fracture tendency, a SiC workpiece represents a good case study on niche applications of AWJs for manufacturing high-value-added products made of advanced materials. Assessment of the proposed model was carried out by comparing the top width of the jet footprint predicted by the proposed model with the experimental values obtained at various jet impingement angles in the range 40—90° at both high and low jet feed rates. From the results, it was observed that the top width of the jet footprint decreases gradually with an increase in jet impingement angle. Although the actual jet plume divergence does not change with the jet feed rate, the effective jet plume divergence, i.e. the divergence that can initiate erosion, decreases at higher jet feed rates. Furthermore, it was found that the prediction error slightly increases at shallower jet impingement angles owing to the instability in the leading portion of the jet structure and this is significant at higher jet feed rates.