Formation of wide-aperture electron beams in electron accelerators with explosive emission cathodes

Abstract

The problems concerning the formation of electron beams of microsecond duration, electron energy 500-600 keV, and current densi~ up to 20-30 A/cm e with a rectangular cross section of area 0.1-1 m e in high-current electron accelerators with explosive emission cathodes are considered. The designs of vacuum diodes capable of producing such beams to be used in high-power lasers and for ionization of gas in large volumes are presented.For pumping high-power excimer lasers and ionization of gas in large volumes in radiation chemistry and technology, electron beams of microsecond duration with an electron energy of up to 500-600 keV, a cross-sectional area of 0.1-1 m 2, and a relatively high current density of -10-20 A/cm 2 are needed. The only practical method to produce such electron beams is to use electron accelerators based on high-power pulse generators and high-current vacuum diodes with explosive emission cathodes. This paper considers some problems concerning the formation of such beams and presents some designs of highcurrent electron diodes for their production.There are two factors that most strongly affect the parameters of microsecond electron beams of large cross section. The first one is the generation and expansion of the cathode plasma and the second one is the magnetic field induced in the diode by the flowing electron beam current. To attain a uniform distribution of emission centers over the cathode surface, the emitters are made of materials which provide generation of plasma within the risetime of the voltage pulse with the average electric field in the electrode gap being -20-30 kV/cm. Graphite is the most common cathode material, although other materials are used as well [ 1 ]. To reduce the time spread in plasma formation, the electric field at the emitters is increased, and in this connection cathodes with pointed or blade-shaped emitters as well as cathodes with the emitting surface made of fibrous graphite (e.g., carbotextim PU) are used.