A commercial version of the NASA plasma-contactor hollow-cathode assembly (PC HCA) has been developed. The NASA PC HCA is a highly reliable hollow discharge cathode that is the most thoroughly understood, tested, and mature of all hollow cathodes that have been considered for use in electric spacecraft thrusters. The design of the commercial version preserves all the critical aspects of the technological heritage of the NASA PC HCA necessary for long-term, reliable operation, while incorporating changes that enable mass production at lower cost per unit. Beyond the original intended spacecraft application, the commercial hollow cathodes could prove useful on Earth in plasma research and plasma-based manufacturing processes.

The Commercial Version of the Hollow-Cathode Assembly preserves the technological heritage of the NASA version, but can be mass-produced at lower cost.

Like the NASA PC HCA, the commercial HCA (see figure) is a plasma source that utilizes an Xe or an Xe/Kr mixture as the feed gas and operates in conjunction with several power supplies, which are typically included within a power processing unit. Both the original NASA design and the commercial version call for a barium-impregnated tungsten insert housed within a 0.25-in. (6.35-mm) molybdenum/rhenium tube, and a molybdenum/tantalum keeper assembly, and both designs share an identical geometry and construction throughout a front subassembly wherein all the critical functions of the HCA take place.

There are only two major differences between the NASA and commercial versions. The first is the substitution of an advanced, commercially available heater for a laboratory-type helical swaged heater used in the NASA version. The commercial heater is expected to resist vibration better than the swaged heater does. The heating element in the commercial heater consists of pyrolytic graphite deposited on chemical-vapor-deposited boron nitride ceramic. Heaters of this type are very lightweight and can be operated at extremely high power densities.

A unique characteristic of heaters of this type is that their electrical resistances decrease with temperature up to about 1,000 °C; this characteristic makes it practical to use constant-voltage (instead of constant-current) heater power supplies. Another characteristic of the heater design include low unit-to-unit variability and precise tailorability of resistance. The net benefit afforded by these characteristics is to make it possible to reduce the complexity (and thus the cost) of power-supply circuitry and to increase the reliability of the heater, relative to those of the NASA PC HCA.

The second major difference between the two designs lies in details of the internal construction and mounting configurations in a rear subassembly. Modifications have been made here to enable cost-effective mass production.

This work was done by David Manzella of RMS Aerospace Engineering for Glenn Research Center. Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center, Commercial Tech-nology Office, Attn: Steve Fedor, Mail Stop 4 - 8, 21000 Brookpark Road, Cleveland, Ohio 44135

Refer to LEW-16658.

Motion Control Tech Briefs Magazine

This article first appeared in the February, 2000 issue of Motion Control Tech Briefs Magazine.

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