Backward-wave oscillators for 70 to 100 kilomegacycles

A pair of backward-wave oscillators employing Karp-type rf structures and having a combined tuning range of 70 to 100 kMc have been developed for the Signal Corps. These tubes, of metal and ceramic construction deliver one to ten milliwatts of cw power while operating at a beam density of 3 amps/cm²and a maximum beam voltage of 3000 volts. A 2000 gauss focusing field for the confined flow electron beam is supplied by an integral permanent magnet. The electrical design considerations of the rf circuit are discussed and the theoretical results are compared with final tube performance. During the devdopment phase of the program, twelve demountable tubes were tested. The design of the demountable tube is unique; has allowed great flexibility to the experimental program and sacrificed nothing with respect to processing schedules or final operating conditions. Constructional features of this demountable tube are described as well as the final tube designs. To obtain suitable cw output in this frequency region it was necessary to protect the wires of the ladder structure from direct bombardment by the use of a beam splitter element. In order to achieve greatest efficiency and maximum output, considerable attention was paid to the reduction of circuit and window losses. Window losses of 2 to 3 db and circuit losses of 6 to 10 db were achieved. Gross power fluctuations over the tuning range were reduced in magnitude by the use of a bulk conductive ceramic in the termination to the rf circuit. This termination is described and its electrical characteristics are presented.