ACCELERATION OF CESIUM IONS BY MEANS OF A NEGATIVE SPACE-CHARGE SHEATH.

Initial results of an experimental investigation of the electrostatic acceleration of cesium ions in the presence of a transverse magnetic field are reported. No accelerating electrodes are employed. Their function is assumed by a magnetically confined electron sheath which is produced by thermionic emission from a tungsten filament located outside the ion beam. Cesium ions are accelerated in the small gap between the surface of a porous tungsten ionizer and the magnetic field surface intersecting the filament. The magnetic field strength is such that the electron cyclotron radius is small or, at most, comparable to the length of the accelerating gap 1 mm or less, whereas the motion of the cesium ions is essentially unaffected by the magnetic field. Measurements have indicated that a well-collimated ion beam can be obtained. Potential advantages in applying this new concept to electric propulsion may include high perveance and relative freedom from ion sputtering since no accelerating electrodes are used. The main difficulty with this approach, however, is a strong possibility that at high current densities an excessive leakage of electrons to the ionizer could take place through Bohm diffusion. Author