An Overmoded W-Band Coupled-Cavity TWT

A 94-GHz overmoded traveling-wave tube (TWT) has been designed, fabricated, and successfully tested. The TWT operates in the rectangular TM31 mode of the cavity, while lower order modes are suppressed using selectively placed strips of lossy dielectric. The 87-cavity TWT circuit was directly machined from Glidcop, a dispersion-hardened copper. The TWT was tested in a 0.25 T solenoidal magnetic field in 3-mu s pulses. Operating at a voltage of 30.6 kV with 250 mA of collector current, the TWT was zero-drive stable and achieved 21 +/- 2 dB linear device gain with 27-W peak output power. Taking into account 3 dB of loss in both the input and output coupling circuits, the gain of the TWT circuit itself is 27 +/- 2 dB with 55 W of saturated circuit output power. Using the 3-D particle-in-cell code CST Particle Studio, the linear circuit gain was estimated to be 28 dB and the saturated output power 100 W, in good agreement with the experimental results. The measured bandwidth of 30 MHz was significantly smaller than the predicted value of 250 MHz. The overmoded TWT is a promising approach to high-power TWT operation at W-band and to the extension of the TWT to terahertz frequencies.