GB1094738A - Charged particle sources - Google Patents

Abstract

1,094,738. Thermionic cathodes; electron guns; ion sources. CSF COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. March 1, 1966 [March 1, 1965], No. 8847/66. Heading H1D. A source of charged particles, i.e. electrons or positive ions, comprises a parallel walled cavity 4 having an internal electron emissive coating and within which a plasma of an ionizable substance, e.g. caesium supplied from a reservoir 10, is formed by contact ionization; the two parallel walls 1, 3 are maintained at two different temperatures whereby a voltage difference is developed between them so that with a suitably small spacing between the walls, e.g. 0À2-2 mm., an additional energization of the electrons within the plasma is achieved and an arc discharge is established across the cavity. Under these conditions the escape of unionized neutral atoms is avoided. The upper or lower wall may be maintained at the higher temperature and the walls may be of the same or different materials. The following examples are given: (1) tungsten walls at 1900‹ K and 1200‹ K producing 1À2 volts; (2) tantalum at 1900‹ K and tungsten at 1200‹ K producing 1À5 volts, and (3) rhenium at 2500‹ K and molybdenum at 1900‹ K producing 1À5 volts. In cases (2) and (3) the wall 3 at the higher temperature is formed of the material of higher work function. The caesium is brought to a temperature of 600‹ K and a pressure of the order of 4 Torr. The lower wall may be provided with a re-entrant lip (15, Fig. 3, not shown), opposite the charged particle exit orifice 2 and the caesium inlet pipe 8 may enter the cavity through the side wall (4, Fig. 4, not shown). Electron or ion streams, e.g. for ionic propulsion, may be produced according to the signs of the focusing and accelerating electrodes 5, 6.