Characterisation of electrospray properties in high vacuum with a view to application in colloid thruster technology
The operational environment of colloid thrusters is high vacuum (10-3 _ 10-6 mbar) however, much of the experimental data collected to date to identify parameter relationships in cone jet mode electrosprays (ES), such as current-volumetric flow rate scaling laws, has been conducted in atmospheric conditions. This highlights a need for electrospray data under high vacuum conditions. Electrospray experimental data was collected using medium conductivity solutions (0.0025 -0.0160 S/m) of TEG doped with sodium iodide in high vacuum. These sprays were obtained from a stainless steel capillary and a disk counter electrode with central aperture. An online flow measurement system is described, which has been developed during this research to measure the fluid volumetric flow rate, concurrently with applied voltage and spray properties such as spray current and cone, jet and spray geometry. This automated flow measurement system was used to measure flow rates as low as InUs with an absolute accuracy of 0.3nUs and a resolution of 0.03nus. It is identified that this system may be easily adapted for lower flow rates and higher resolutions. The ES data collected demonstrates, for the first time, the detailed dependence of volumetric flow rate upon the applied voltage. The sensitivity of nominal flow rate to applied voltage was found to be higher for lower nominal flow rates. For a volumetric flow rate -4nLIs a 25% a change in flow rate per kV was recorded over a cone-jet mode stability range spanning -1.5kV. This volumetric flow rate voltage sensitivity holds particular significance for colloid thruster systems, which operate at or near minimum flow rate conditions. The current was found to have a power law dependence on flow rate similar to the current scaling laws of F. de la Mora and Gahan-Calvo however the exponent of this power law differs significantly from these scaling laws. A study considering the effect of charge carrier mobility in simple 1: 1 electrolytes shows that the exponent of the power law current-flow rate scaling increased with increasing charge carrier mobility. Contrary to the various scaling laws the spray current was found to be dependent on electrostatic conditions. The sensitivity of the emitted current to the applied voltage was also found to increase with increasing nominal volumetric flow rate. The geometrical parameters of cone angle, spray angle and jet length were measured for varying TEG/Nal solution conductivity. Cone geometry was found to be relatively independent of conductivity in the range tested. Jet length was found to have an inverse relationship with solution conductivity.