Optical Measurements of Neutral Cesium Mass Flow Rate in Field Emission Thrusters

Field emission electric propulsion thrusters using liquid cesium are currently being actively developed in Europe. For thruster-spacecraft interaction studies and design purposes, it is important to measure the mass flow rate of neutral cesium emitted by the thruster. This measurement requires the characterization of the neutral cesium plume. In this work, this characterization is achieved with a specially developed setup using laser-induced fluorescence velocimetry to probe the three components of the neutral cesium velocity and to measure the absolute number density from a field emission electric propulsion thruster representative of a flight model. The results give mappings of the plume velocity and number density along two planes intercepting the neutral plume for four different thruster operating conditions. Mass flow rates are obtained by integration of the local flow rates. The neutral cesium mass flow rate is in the milligrams/hour range; it increases with thrust and with emitter temperature. Mean neutral velocity can be as high as 800 to 1000ms-11000ms-1. The results show that the neutral mass flow rate is of the same order of magnitude as the ion mass flow rate computed from the thruster current. This work provides the first direct measurement of neutral cesium mass flow rate, thruster efficiency, and, more generally, the first mapping of the neutral plume from this field emission electric propulsion thruster. The technique used could be extended to probe dimeric cesium in the plume. [PUBLISHER ABSTRACT]