Visual display of the spatial distribution of colloidal particle beams.

Abstract

Introduction O the past decade microthrust rockets have been developed that employ charged liquid droplet (colloidal) beams sprayed from a hollow needle held at high potential with respect to an aperture in a ground plane. Although it is not difficult to obtain a stable beam current, the spatial distribution of the beam is usually complex and often asymmetric. Since the beam is produced by intense electric fields at the needle tip, factors such as needle tip shape, extractor hole shape, needle alignment, propellant properties, and mass flow rate influence the beam distribution. In turn, the beam shape fundamentally affects microthrust rocket performance, since beam asymmetry causes unwanted side thrust components and results in the actual thrust being less than that calculated from time-of-flight (TOP) data. These problems indicate the desirability of a rapid, reliable method of checking the beam shape whenever any physical or operating parameters are changed. Several approaches have been used to determine beam distribution. Analysis of TOF data for point by point sampling of the beam is the most complete method. However, it is laborious, time consuming, and relies on maintaining perfect beam stability for the duration of the sampling process. A beam current detector with several concentric rings is easier to use but determines only the average current striking a given section and tells little about beam symmetry or alignment. Observation of beam deposition on a surface is a reliable method, but it represents an average over time. None of these methods can illustrate the beam shape during instabilities or transients. In the following sections two detectors that provide continuous visual display of beam distribution are described: a phosphor screen for instantaneous active display of the beam, and a liquid crystal (LC) screen for averaging the beam over time intervals of seconds to minutes. The visual phenomena are compared with TOF data collected by a beam current detector with five concentric rings.