Travelling Wave Tubes (TWTs) are wide band power vacuum electronic amplifiers widely used in microwave communications. A TWT includes an electron gun, a slow wave structure (SWS) usually made of OFHC (Oxygen Free High Conductivity) copper, a collector, a magnetic focusing system and vacuum RF windows to seal the flanges. The electron gun generates an electron beam with given current and voltage, confined by the magnetic system along the SWS. The SWS permits the transfer of energy from the beam to the input RF field, by slowing down the field phase velocity close to the speed of the electrons. This mechanism permits to achieve multi-Watt power at sub-THz making TWTs the only solution for sub-THz high power amplification. D-band TWT can provide 10-12 Watt over 10 GHz band. Due to high cost and technology challenges, TWTs above 70 GHz are not available in the market and only a few prototypes have been built. C. Paoloni invented the double corrugated waveguide (DCW), a mm-wave SWS of easy fabrication and assembly, with low production cost suitable for mm-wave TWTs.