Production of Axially Focused Colloid Beams

Abstract

Production of Axially Focused Colloid Beams T application of colloid sources to micro thrust rockets has been extensively discussed in the literature.' These colloid sources, which have usually been operated at voltages of 5 kV to 9 kV, have exhibited two problems. Firstly, the colloid beams produced, when analyzed by time of flight (TOF) techniques, have been shown to contain a broad distribution of charge to mass ratios. As a result, the beam thrust efficiency, defined as r — 2 / , where u is beam particle velocity and denotes average over particles in the beam,' has been found to be as low as O.7. Secondly, the spatial distribution of these colloid beams has usually been divergent and asymmmetric, leading to asymmetrically distributed mass flow rate, charge to mass ratio and thrust. These effects, which were found by point to point TOF measurements, degrade rocket performance by giving side thrust components. They also introduce uncertainties into the calculation of axial thrust from simple TOF measurements. Clearly, better micro thrust rocket performance could be obtained by finding means for producing axially focused colloid beams with narrow distributions of charge to mass ratio. This note describes the results of a series of experiments using a conventional colloid source operating with a relatively lowly doped Nal—glycerol fluid. It is indicated that, with capillary voltages greater than about 13 kV, axially focused beams can be reliably produced, with average charge to mass ratios variable between 10 coul/kg and 100 coul/kg. The beams exhibit a narrow distribution of charge to mass ratios, with thrust efficiencies exceeding 0.90 in general. The work formed part of a study of means for providing high specific impulse beams by post-acceleration of beams with low charge to mass ratios. The source used sprays Nal doped glycerol from a single capillary tube positioned flush with the outer surface of a flat extractor plate. The extractor hole diameter was 4.75 mm. Capillary tube sizes ranged from 0.10 mm i.d. by 0.20 mm o.d. to 0.25 mm Id. by 0.45 mm o.d. In each case the capillary tip had a funnel shaped internal bevel with a total included angle of 120°. Both stainless steel and Pt—Ir capillaries were used. Fluid doping levels of between 2.5 g and 7.5 g NaI/100 ml glycerol were used. Fluid pressures during source operation ranged from 2 cm to 20 cm Hg, while capillary voltages ranged from 3 kV to 20 kV. Spatial distributions of the beams were observed using a phosphor screen and a segmented electrical detector. The phosphor screen, which has been described elsewhere, gives an instantaneous visual display of the beam that may be photographed. The segmented electrical detector, in conjunction with a thyratron switching circuit to control the capillary voltage was used for TOF measurements. This detector has five concentric rings, electrically insulated from each other, whose outer perimeters subtend angles of 5°, 15°, 25°, 35°, and 45° at the capillary tip. Initial attempts to produce an axially focused beam were unsuccessful. Operation of the source at voltages between 5 kV and 9 kV produced beams which were off axis and asymmetric