A large dielectric vacuum facility.
I the experimental study and developmental testing of plasma propulsion devices of various types, interference of the vacuum chamber walls with the exhaust plume has proved to be a major limitation on simulation of space operation. This interference may take the form of an artificial constriction of the exhaust plume dimensions, of a recirculation of a portion of the ambient gas in the chamber through the plume, of a participation of the walls in the arc current conduction pattern, or of remote electromagnetic interactions between currents in the plume and induced currents in the walls. The first difficulty clearly relates to chamber size; the second relates to chamber size and to the ambient pressure that can be sustained during thruster operation. The remaining interactions are fostered by the electrical conductivity of the metallic walls of the conventional stainless steel or aluminum vacuum chambers. The desirable facility would thus seem to be a large, dielectric-walled vacuum vessel, capable of low ultimate pressure. Glass or quartz has the most desirable vacuum properties but is essentially unmanageable in the desired sizes. Plastic materials are structurally more convenient, but considerable uncertainty has surrounded their vacuum behavior for the large surface exposures involved in this application. In connection with a program of pulsed plasma acceleration studies described in detail elsewhere," design, construction, and testing of a 50-ft plastic vacuum tank was undertaken, as much to determine its actual vacuum capabilities as to provide a more satisfactory environment for experiments on the ejected plasma. Specifically, this is a vessel of l-in.-thick Plexiglas G~l in the form of a cylinder 3 ft in diameter, 6 ft long, with a hemispherical fixed end and a dish-shaped removable end. Ports are provided for the plasma accelerator at the removable end and for exhaust sampling diagnostics at the fixed end. Each side of the tank also has four large access ports for pumping equipment, gages, diagnostic probes, and photographic flats. Figure 1 shows a sketch of this vessel. The tank was fabricated at a nearby industrial shop§ in the following way. The main body of the vessel was formed by heating a l-in.-thick plane sheet of Plexiglas to about 350°F and bending it over a half-cylinder mandrel, whereon it was