Helix TWT's for 10 to 25 KW peak power over octave erequency bands

Abstract

Summary form only given. In designing a unifilar helix traveling wave tube for high peak output power a major constraint has been the necessity for keeping the beam current and voltage below the point at which backward wave oscillations occur. This has made it very difficult to design for more than a few kilowatts peak output power. Two different forms of helix backward wave oscillation suppression have been developed which have made it practical to build stable tubes providing tens of kilowatts of output power over octave frequency bands. Results obtained with both types of BWO suppression are presented and the relative merits of both designs is discussed. One of the designs utilizes a frequency selective and dissipative filter coupled to the helix. This combination has a low pass characteristic which provides a high attenuation at the backward wave oscillation frequencies and minimum loss in the operating band. Backward wave oscillation starting currents have been increased by a factor of 3, in transparent TWTts, with small signal gain greater than 16 db, using the filter. Stable S-band and X-band TWT's have been tested to over 20 kilowatts across octave frequency bands with electron beams of 20 KV and 4 amperes. The size of the filters has restricted their utilization to solenoid focused TWTts. The second method of BWO suppression utilizes a helix velocity step carefully designed so that on one section of the helix the backward wave space harmonic interacts with the fast space charge wave of the beam to effectively cancel the normal BWO interaction which occurs on the other section of the helix. While considerably more complicated to design, this approach has been used successfully to achieve BWO stability on several high peak power helix traveling wave tubes including some PPM focused types. The most noteworthy example has been a high gain experimental tube operating at 31 KV and 10 amperes and delivering 30 to 65 kilowatts of peak output power from 600 to 1200 MHz. Advantages and disadvantages of both designs are discussed and test data obtained on several TWTts in which they are used, are presented. It is predicted that the unifilar helix traveling wave tube now must be considered a viable design for any peak power output up through 25 kilowatts, particularly when a wide frequency band is desired.