Beam buildup in a DEMATRON amplifier

In a nonreentrant, cold cathode, self-modulated cross-field amplifier (DEMATRON), the beam is built up by secondary emission from successive impacts on the cathode. The buildup process poses a problem because acceleration of electrons into paths which intersect the cathode only occurs in an inherently phase-defocusing phase range, while complete buildup requires that an electron bunch remain in this phase range for at least 20 successive impacts. Then, when buildup is complete, the bunch must be shifted in phase by 180° before it can deliver useful power to the rf wave. In doing so it has to pass over an "electron trap" -- a phrase region in which the impact velocity on the cathode is so low that the secondary emission ratio is much less than unity, and the bunch tends to be reabsorbed.