Experimental Investigation of a Novel Circuit for MM-Wave TWTs

A single-circuit-section-type W-band FWTWT that utilizes a novel PPM circuit has been built and tested. The novel circuit features voids at the centers of the pole pieces. These voids, which comprise part of the slow wave circuit, occur both at the magnetic field cusps of the large pole pieces and at the magnetic field minima of the small pole pieces. The circuit utilizes thick pole pieces that bring magnetic flux directly to the beam tunnel to support 4-kilowatt beams with lambda _p/L values around 10. For this TWT, a tetrode gun creates ultra-laminar electron beams typically around 0.24 amperes at 18 kV. (See "Experimental Investigation of a Tetrode Gun for MM-Wave TWTs," by A.J.Theiss, R.B.True, and C.J.Meadows, 4th IEEE International Vacuum Electronics Conference, Seoul, Korea, May 28-30, 2003.) When compared to an equivalent beam-stick-type circuit that does not have such voids, transmission for such beams degrades from 99.5% percent down to a maximum of 98%. However this circuit focuses beams as high as 9.25 kW (25 kV, 0.37A) at a 97% transmission level. Outer and inner grid drive levels for 97% transmission of various 0.1-microperveance beams are shown. Output power measurements were made on this W-band FWTWT, which operates around 91 gigahertz on the edge of the passband. Typical small-signal gain data are shown for 21 kV, 0.301 A beams. Observed small-signal gain ripple resulted from coupler matches that degraded both from inductive-post welding and from unexpected alloy flow. Results from MAGIC3D simulations that utilize three-dimensional PPM fields imported from MAXWELL models generally agree with test data. External output-input RF feedback through a frequency filter that eliminates second-passband BWOs provided 10 watts of stable output and 40 watts of regenerative output on again spike