Attempt on Applying Semi-Metallic Supporting Rods to a Wideband Ka-Band Helix TWT

A helix traveling-wave tube (TWT) with semi-metallic rods operating at 26.5-40 GHz is presented. Its promising feature is to provide wide-bandwidth interaction with a compact structure, low gain fluctuation, adequate output power, and electronic efficiency. Such rods may easily transfer the heat out of the helix circuit compared with the conventional helix SWS. Thermal simulation shows that the maximum temperature of this helix structure can be 40 degrees C-56 degrees C cooler than the helix slow-wave structure (SWS) with anisotropic boron nitride (APBN) rods under the same heat loads. Moreover, the deformation of the helix with the semi-metallic rods due to heat conduction is at least 33% smaller than that of the helix with all-APBN rods. The electromagnetic characteristics and the beam-wave interaction based on this SWS are investigated and optimized. The calculation results predict that it potentially could provide saturated output power over 180 W from 26 to 40 GHz when the cathode voltage is 9000 V and beam current is 140 mA, respectively. The corresponding saturated gain and electron efficiency can reach over 43.3 dB and 14.6%, and the gain fluctuation is only about 6-7 dB. Furthermore, the fabrication methods of the semi-metallic rods are explored carefully. The transmission characteristics of the designed helix TWT with semi-metallic rods are experimentally validated by a cold test, showing the voltage standing wave ratio (VSWR) of the high-frequency structure is below 1.98 in the range of 26 to 40 GHz.