David G. Fearn

An Investigation of Physical Processes in a Hollow Cathode Discharge

The cathodes studied were similar to those used in the SERTII program and in the initial phase of the present work, except that bifilar heaters were employed. Triple carbonate was usually applied to the internal surfaces, although for long periods of operation, various barium dispensers were used. Experiments were conducted in a diode discharge system and mercury vapor was supplied from a conventional vaporizer. There is considerable evidence to suggest that electron emission occurs internally at constant current density J; about 2 to 5 x 10 amp/m. Although these values of J probably cannot be explained by simple thermionic emission despite the use of barium, considerable field-enhancement may occur in the presence of the dense internal plasma. This extends over an active zone having a length determined by the current drawn. Electric fields E in the plasma sheath of about 10 v/m are necessary to account for observed data solely on the basis of this process. Another mechanism capable of giving the required current density and also relying on the presence of an internal plasma covering an active zone is the release of electrons by the impact of metastable atoms. High yields are expected when the excitation energy is close to the work function of the surface, as is the case for mercury impacting on tantalum, and the mechanism does not depend on the presence of an alkali metal. A small cylindrical Langmuir probe near the keeper orifice was used to measure electron temperature Te, electron number density ne and plasma potential Vp in the external plasma. It was also often convenient to use the keeper itself to obtain Te and Vp. Te was in the range 1 to 2.5 x 10K and increased with decrease of flow rate m and increase in /. The former dependence was ascribed to the fall of pressure with m, which resulted in the electrons gaining more energy between collisions. Te was independent of tip temperature T within the range 1000-1400°C. Vp was usually 14-19 v, decreasing as T was raised, and ne was about 10m~. To investigate the internal plasma a cylindrical Langmuir probe was inserted into a cathode. Te, derived from semilogarithmic plots, again decreased with increasing m (Fig. 1). The electron saturation current was used to estimate ne, which

The T6 Hollow Cathode as a Microthruster

*† ‡ A target -based measurement system has been developed and the thrust produced by a T6 ion engine hollow cathode has been evaluated, using argon and krypton as propellants, for discharge current values of 5 -25 A and a wide range of mass flow rates. The calculated values of specific impulse, which are higher for argon , a re generally in excess of those that could be attributed to heating a gas to the rmal equilibrium with the walls. This seem s to suggest , as a first hypothesis, an arcjet -like operation mechanism .

The design and performance of the T6 ion thruster

After many years of development, ion propulsion systems are now being employed increasingly for a wide variety of space missions, in order to take advantage of their high exhaust velocities. These permit the mass of propellant required to conduct a given manoeuvre to be reduced by a factor of 10 or more, with major economic benefits. To date, two ion propulsion systems have been developed in the UK. These systems are described briefly in this paper, then the requirements for future satellites are discussed. It is shown that a family of systems which provide much higher thrust levels than the 10 cm beam diameter UK10 will be needed, certainly in the hundreds of mN range. The process of designing the first of these new thrusters, the 22 cm diameter T6, is described and its performance is evaluated. Constructional details are also covered.

A Study of the Thrust Generated by a T6 Hollow Cathode

The thrust produced by a T6 ion engine hollow cathode has been measured using antarget-based system, with argon, krypton and xenon as propellants, for discharge currentnvalues of 5-25 A and a wide range of mass flow rates. The calculated values of specificnimpulse, which are generally higher for lighter gases, are far in excess of those that could benattributed to heating a gas to thermal equilibrium with the walls. This seems to suggest annarcjet-like operation mechanism.

The promise of electric propulsion for low-cost interplanetary missions

Interplanetary science missions often require a significant propulsive capability. Electric propulsion provides a means of avoiding the large propellant masses associated with chemical systems. Such an approach can lead to the use of small spacecraft, consistent with a low cost mission philosophy. Example applications of electric propulsion to low-cost interplanetary science missions are discussed, including use in cis-Lunar space, Lunar missions, and missions to a range of asteroids.