Design of an overmoded W-Band coupled cavity TWT

Abstract

We present the design and cold test validation of a novel, overmoded Traveling Wave Tube (TWT) capable of producing power levels in excess of 100 Watts at frequencies of 100 GHz and above. High power sources at frequencies from the W-Band (70 to 110 GHz) to the THz frequency range are needed for numerous applications including radar, DNP/NMR spectroscopy, and homeland security. The novel TWT design operates in the TM31 mode, of a rectangular cavity, and has transverse dimensions three times larger than a conventional TWT, thus allowing higher power handling capability and less stringent fabrication tolerances. The circuit is also amenable to multiple beam operation which will allow the use of higher beam currents. The concept of dielectric loading in a resonant cavity was utilized to suppress lower order modes and prevent parasitic oscillations. The coupling impedance of the TWT was calculated with the HFSS code and the gain with the MAGIC3D code. The results indicate that with a 0.6 mm diameter electron beam at 50 kV and 0.8 A, over 1 kW of peak output power and a few hundred watts of average output power are achievable at 99 GHz with a linear gain of 32 dB and a -3 dB bandwidth of 0.6 GHz. A cold test structure scaled to a frequency of 15 GHz was designed, built and tested with a vector network analyzer. The results proved that the dielectric loading with strips of Aluminum Nitride works to attenuate the parasitic lower order modes, thus verifying the theoretical analysis. Further cold test measurements showed dispersion and coupling impedance characteristics were accurately modeled by the computer simulations. The novel, overmoded TWT is a very promising approach to achieving high output power at W-Band and is also promising for scaling to frequencies in the 0.2 to 1.0 THz region.