Beam buildup in a DEMATRON amplifier

The operation of an RF triggered cold-cathode crossed-field amplifier is briefly described, and the problem is posed of how the beam builds up, since it is necessary for it to remain in an inherently debunching phase region for more than 20 successive impacts on the cathode. A partial answer is provided by numerical calculations showing that the velocity of a repeatedly impacting beam differs from E/B in a way which introduces a new phase-focusing mechanism. It is found that buildup can also occur without exact synchronism over a range of calculated voltages which agree closely with measurement. The principles of two computer programs are described, one covering the buildup region of negligible space charge, and the other the transition and power-generating regions where space charge becomes important. Quantitative conclusions are drawn concerning the velocity and direction of impacts on the cathode, which vary markedly as the space charge density changes.