An electron gun with large compression of a thin annular beam

An electron gun has been designed to produce a beam f01´ generation of high-power microwaves and IR-radiation. It has the following design parameters: energy of electrons 300–500 ke V, beam current > 100 A, longitudinal momentum spread < 1 %, pulse duration 10 µs, repetition rate up to 10Hz. A plane, ring-shaped, thermionic lantanum hexaboride cathode, having a 28-mm average radius and a 6–8 mm width, is used to produce an annula1´ beam. The cathode surface is screened by two focusing electrodes to operate in a space-charge limited regime. Uniformity of temperature distribution across the ring-shaped cathode is provided by a machined-graphite heater. Its current is fed in bifilar geometry. The focusing system must form a laminar beam inside the accelerating gap at rather low magnetic field and transport the beam to an experimental region of about 50-cm length. Inside this region, the beam radius is 2.5 mm and annular thickness less than 1 mm due to a large magnetic field of 15 kGs. Design of the gun and its subsystems, res1llts of computer simulations and experimental tests of the subsystems are described.