STARTING ENERGY AND CURRENT FOR A LARGE-DIAMETER FINITE-LENGTH BACKWARD-WAVE OSCILLATOR OPERATED AT THE FUNDAMENTAL MODE

We study the starting conditions for a large diameter (diameter/wavele ngth = 4.8) finite length backward wave oscillator designed for 24-GHz operation at the fundamental TM(01) mode. This geometry is very promi sing for high power handling capability. We analyze two separate thres hold conditions. First, finite length effects give rise to a threshold in electron beam energy below which oscillations cannot be sustained at any beam current. The second is the more familiar current threshold known as a start current. It is also found that the growth rate for t he fundamental mode can be much larger than those of other higher orde r modes thus leading to coherent operation of large diameter sources f ree from mode competition.