Dormant Cathode Erosion in a Multiple-Cathode Gridded Ion Thruster

Abstract

A rectangular gridded ion thruster discharge chamber is investigated for operation with multiple discharge cathode assemblies. The multiple-cathode approach attempts to increase thruster throughput and lifetime by operating three discharge cathode assemblies sequentially, possibly providing a threefold increase in discharge chamber life. Previous multiple-cathode electric propulsion devices, such as the SPT-100, have shown dormantcathode erosion to be a life-limiting phenomenon. Similar results in a multiple-cathode discharge chamber may decrease the anticipated gain in discharge lifetime. To assess possible dormant-cathode sputtering erosion and to determine the operational configuration that minimizes this erosion, diagnostic cylinders are designed and used to measure plasma properties at the dormant-cathode locations. Each diagnostic cylinder appears similar to the active discharge cathode assembly, but is outfitted with Langmuir probes. Plasma properties are then used in a simple sputtering-erosion model to predict erosion of the dormant cathodes. Results indicate that the device should be operated at the 0 A electromagnet current configuration for minimum dormant-cathode erosion. For this optimum configuration, typical number density, electron temperature, and plasma potential values are 5:0 10 11 cm 3 , 5 eV, and 27 V with respect to cathode common, respectively. The erosion model indicates that the dormant cathodes will suffer preoperation erosion, but the erosion rate is 26 times slower than the active discharge cathode assembly. Compared with a single-discharge-cathode-assembly thruster, the model predicts an increase in lifetime by a factor of 2.9 for a triple-discharge-cathode-assembly device. Nomenclature A = keeper area, m 2 A p = probe area, m 2 E = bombarding ion energy, eV e = electron charge, 1:6 10 19 C I emag = electromagnet current, A I i = ion current, A I si = ion saturation current, A Kn = Knudsen number M i = ion mass, kg m = keeper material mass, kg n e = electron number density, cm 3 n i = ion number density, cm 3 r = probe radius, m S = erosion rate, kg=s T e = electron temperature, eV V = probe voltage, V Y = sputtering yield, atoms/ion i = ion flux, cm 2 s 1 = mean free path, m D = Debye length, m = Laframboise dimensionless current correction

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“…The cathode can be used as a low thrust thruster itself 79 . Cathode erosion also represents a problem reducing the service life 80 .

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