PROBE MEASUREMENTS OF THE DISCHARGE IN AN OPERATING ELECTRON BOMBARDMENT ENGINE

Current-voltage characteristics have been obtained from Langmuirprobes at various positions inside a Kaufman-type ion motor. Data arepresented for the engine in static (no beam, but plasma) and operat-ing (160-ma beam) conditions. The analysis of the probe dataindicates that the electrons in the plasma possess a non-Maxwelliandistribution of energies. To obtain plasma properties from the probecurves, the non-Maxwellian energy distribution is assumed to consistof a primary monoenergetic group of high-energy electrons plus theusual Maxwell distribution. Working from this assumption the probecurves are analyzed for primary and Maxwell energies and densities,and the data for radial positions inside the engine are presented. It isshown from the density curves that the primary electrons are the prin-cipal source of ion production. The beam current as calculated fromprobe data near the exit grids agrees to the correct order of magnitudewith the measured value.I. INTRODUCTION The possibilities of the Kaufman-type electron bom-bardment ion source (Ref. 1) for use as a thrust devicefor deep-space applications is well documented (Ref. 2).Even though the engine in its present state achievesreasonable efflciencies, its operation is not thoroughlyunderstood. It has been studied externally; the variationof beam current, propellant utilization efficiency, etc.,have been measured as the various external operatingparameters such as arc voltage and magnetic field havebeen changed. For a deeper understanding of the sourceoperation, it is necessary to probe the interior of theengine, where the. plasma is being formed. Only in thisway can a theoretieal model be correlated with theactual situation. Then, if a proper model can be found, acalculation of the efficiencies and beam properties canbe made from the operating parameters, and it will bepossible to attempt to improve the engine operation foruse as a thrust devi_e.This paper deals with an experimental program set upto investigate the properties of the plasma inside anoperating Kaufman-type engine. From the results of thisinvestigation it is hoped that a theoretical model can bedeveloped which will sufficiently explain plasma forma-tion so that future engines of this type can be built withpredictable characteristics.Owing to their small size and ability to distinguishplasma properties at different points, Langmuir probes(Ref. 3) were used to probe the inside of the engine.These probes are simple to operate and need a minimumamount of circuitry, advantages not to be overlookedwhen the engine is operated under flight-simulated con-ditions during which the whole device is floating at apotential of about 5 Ice.