Dielectric and alternative-configuration-metal slow wave structures for W-Band traveling wave amplifiers

Summary form only given. There is a strong need for new types of traveling wave tubes (TWTs) in W-Band with improved power-bandwidth characteristics compared to traditional helix and coupled-cavity configurations. Three classes of alternative slow-wave structure for such TWTs are presently being investigated. The first is a dielectric TWT, which uses an annular dielectric liner surrounding the electron beam. It has been extensively modeled in the large-signal regime using an axisymmetric particle-in-cell code. A design using a 26 kV, 100 mA, 0.36 mm diameter beam inside a moderate dielectric constant liner (90% fill factor, 5.1 cm long) has been completed, and simulations indicate that it is capable of producing 115 W output power at 94 GHz with 10.3 dB large signal gain and a half-power bandwidth of 6 GHz. Extensive parametric studies will be presented. A second class of TWT, based on an all-metal serpentine waveguide geometry, has also been designed and thoroughly simulated with the large-signal particle-in-cell code Neptune. Using a 20 kV, 122 mA beam, an output power of 245 W at 95 GHz is predicted from a two-stage device, with a large signal gain of 30 dB and a half-power bandwidth of 5 GHz. When a variable input power (not exceeding 1 W) is allowed via equalization techniques, greater than 100 W of output power is predicted over the 92-99 GHz frequency range. Based on the highly encouraging simulation results, this serpentine-waveguide TWT is being fabricated for experimental hot tests, using multi-layer ultraviolet lithography and electroforming methods to fabricate the slow-wave structure. A third type of slow wave structure, which resembles a cavity-loaded helix, is presently being investigated with electromagnetic finite-element solvers to determine its dispersion characteristics. The essential idea is to use cavity loading (resonant slots or stubs) along a helix to further slow down the electromagnetic wave, which allows a coarser helix pitch than would - ormally be possible at W-band. This type of structure could potentially enable TWTs with operating characteristics intermediate between those of ordinary helix TWTs and coupled-cavity TWTs. Initial studies indicate a complicated mode spectrum with a variety of slow-wave, fast-wave, and coalesced modes; their variation with respect to geometry will need to be understood to optimize bandwidth and maintain stability.