A helical corrugation of the inner surface of an oversized cylindrical waveguide provides, for certain parameters, an almost constant value of group velocity and close to zero longitudinal wavenumber of an eigenwave for a very broad frequency band. The use of such a helical waveguide as an operating section of a gyrotron traveling wave tube (gyro-TWT) allows significant widening of its bandwidth and an increase in the efficiency at very large particle velocity spreads. In this paper, the new concept is confirmed by theoretical analysis and "cold" measurements of the helical waveguide dispersion. Results of a linear and nonlinear theory of the helical gyro-TWT as well as two designs for subrelativistic (80 keV, 20 A) and relativistic (300 keV, 80 A) electron beams are also presented. For both designs, parameters providing a very broad frequency band (about 20%) and high efficiency (above 30%) have been found, When the transverse velocity spread is increased from zero up to a very high value of 40 %, simulations showed only a 20%-30% narrowing in the frequency band and a 20% decrease in electron efficiency. The theoretical analysis demonstrates important advantages of the helical gyro-TWT over the "smooth" one in frequency bandwidth, sensitivity to electron velocity spread, and stability to parasitic self-excitation

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Abstract-There has been a drive in recent years to produce ultra-high power short microwave pulses f or a range of applications. These hig h power pulses can be produced by microwave pulse com pression. Sweepfrequency based microwave pulse co mpression using smooth bore hollow waveguides is one technique of passive pulse compression, however at very high pow ers this method has some limitation due to i ts operation close to cut-off. A special helical corrugation of a circular waveguide ensures an eigenwave with strongly frequency dependant group velocity far from cut-off, which makes the helically corrugated waveguide attractive for u se as a passive pulse com pressor for very high pow er amplifiers and oscillators. The results of proof-of-principle experiments and calculations of the wave dispersion using a PIC code are presented. In the ex periments a 70ns 1kW pulse from a conventional TWT was compressed in a 2 metre long helical waveguide. The compressed pulse had a peak power of 10.9k W and duration of 3n s. In order to find the optimum pulse compression ratio the w aveguide's dispersion characteristics m ust be w ell known. The dispersion of the helix was calculated using the PIC code MAGIC and verified using an experimental technique. Future work detailing plans to produce short ultra-high power (GW) pulses will be discussed.