HOLLOW BEAM FORMATION IN HIGHLY CONFINED ELECTRON FLOW.
ARMY ELECTRONICS COMMAND FORT MONMOUTH N J ELECTRONIC COMPONENTS LAB
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The formation of electron beams in low-noise guns for traveling-wave tubes TWTs is investigated. The experimentally observed hollowing-out is explained by a simple model which assumes constant voltage along the beam. The derived relationships are then independent of the particular electric fields which cause the beam spreading. In fact, they only relate initial conditions to quantities further downstream where the beam is again in equilibrium flow. By neglecting an extended accelerating region, the calculation of the radial space charge forces may be accomplished. However, this limits the solutions to cases of negligible emission density at the axis, which is otherwise arbitrary across the cathode. Due to the space-charge cloud in the center of the beam and to the strong radial fields at the edge of the cathode, this condition is usually fulfilled in actual TWT guns. In fact, a number of tubes use hollow cathodes which have no emission at the axis. Author
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