Design of phase velocity tapering of W-band folded waveguide travelling wave tubes for efficiency enhancement

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Summary form only given. Folded waveguide slow wave structure is broadly used in W-band TWTs due to some advantages of easy-fabrication compared to helix and coupled cavity circuit, all metal structure to stand with high dissipation power, relatively wide bandwidth characteristic and etc. Due to the low coupling impedance and high attenuation factor in W-band folded waveguide circuit, the efficiency of actual TWTs is very low. The preliminary model of our 10W level CW TWT has only 1.5% electron efficiency <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> . Both the pulsed folded waveguide TWTs manufactured by BVERI <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and L3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> have less than 3% electron efficiency among its working bandwidth. There is a great essential to increase the efficiency of W-band folded waveguide TWTs.In this work, we explored the efficiency enhancement method of phase velocity tapering which is broadly used in helix and coupled cavity TWTs. The tapered circuit design is based on a W-band CW TWT. This baseline TWT is driven by a 75mA electron beam of 16kV in 140mm length circuit. Simulation by code MTSS predicts 50W output power with electron efficiency of 4%. Two prototype tubes have been manufactured and tested. About 40W output power has been achieved near the center frequency. For this tapered folded waveguide TWT, we used negative ph ase velocity tapering approach. The input section which was used to establish the growing wave has a constant pitch of 0.5 5 mm. Before the negative step tapering, an increased phase v elocity section was used to gather the electrons into bunches as effectively as possible. At the end of the circuit, an appropr iate reduced phase velocity section with pitch of 0.52mm was used to extract the energy from the electron beams. We use MTSS to optimize the negative tapering pitch profile which predicts that the output power could achieve above 70 W with 10 GHz bandwidth. The highest power at center frequency reached about 75 W. The electron efficiency was improved from 4% to about 6% by applying this very effective negative phase velocity tapering method.