A theoretical model of neutralized thruster ion beam plasmas has been developed. The basic premise is that the beam forms an electrostatic trap for the neutralizing electrons. A Maxwellian spectrum of electron energies is maintained by collisions between trapped electrons and by collective randomization of velocities of electrons injected from the neutralizer into the surrounding plasma. The theory contains the observed barometric law relationship between electron density and electron temperatures and ion beam spreading in good agreement with measured results.

A preliminary model of ion beam neutralization

Large diameter inductive plasma thrusters

The use of advanced electric propulsion systems is expected to provide cost and performance benefits for future energetic space missions A methodology to predict the characteristics of advanced electric propulsion systems was developed and programmed for computer calculations to allow evaluation of a broad set of technology and mission assumptions The impact on overall thrust system characteristics was assessed for variations of propellant type, total accelerating voltage, thruster area, specific impulse, and power system approach The data may be used both to provide direction to technology emphasis and allow for preliminary estimates of electric propulsion system properties for a wide variety of applications

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Characteristics of primary electric propulsion systems

An investigation of parameters that affect the erosion rates of 30-cm-diameter mercury-ion-thruster components is described. A sputter-erosion model is formulated in terms of the design, operational, and material characteristics of the thruster. The erosion model is applied to the screen electrode, which is assumed to be the life-limiting component of the 30-cm thruster, resulting in a model of wearout lifetime. Results of short-term erosion-rate tests are presented that illustrate the dependence of component wear rates on variables such as discharge voltage, accelerator-grid open-area fraction, ion energy, electrode material, and the partial pressure of facility residual gases such as nitrogen. Test results are compared with wearout rates predicted by the sputter-erosion model.

A model for predicting the wearout lifetime of the LeRC/Hughes 30-cm mercury ion thruster

Ground-based tests of Hg ion thrusters have identified sputter erosion of thruster components as one of the main life limiting phenomena. Subsequent measurements have revealed that sputtering rates can be affected by background gases at pressures as low as 10 to the -10th torr. With the recent interest in thin film technology, sputtering in the presence of reactive gases has been studied in great detail. This paper presents the results of many of those studies and applies them to the sputtering of electric thrusters. A model, which assumes that chemisorption is the dominant mechanism, is applied to the sputtering rate measurements of the screen grid of a 30-cm thruster in the presence of nitrogen. The model utilizes inputs from a variety of experimental and analytical sources. The model of environmental effects on sputtering was applied to thruster conditions of low discharge voltage and a discussion of the comparison of theory and experiment is presented.

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Sputtering in mercury ion thrusters

Experiments are described in which the application of a pulsed magnetic field to a Corbino disk of indium antimonide causes an increase in resistance of about 2 orders of magnitude in a time of 5 μsec. The use of this element as a switch for the transfer of energy from an inductive energy store is discussed.

E. K. Inall

Papers

Application of the Hall effect to the switching of inductive circuits