Magnetic interaction between Traveling Wave Tubes and its effect on performance and reliability

Traveling Wave Tubes (TWTs) are used in a number of high power transmitters where there are many tubes operating in close proximity. One example is the ALTAIR radar which features a transmitter equipped with 32 L- 4920 UHF TWTs arranged in 4 groups of 8 tubes. The radar was experiencing out of band non harmonic spurious oscillations in some TWTs and catastrophic TWT failures which resulted in melting the helix at the output end of the tube, on others. The PARCS radar uses the same TWT and there were no similar TWT failures in that transmitter. This paper will describe root cause failure analysis, steps taken to eliminate the cause and the physics of the problem in both empirical and analytical terms.