GYRO-TWT WITH A HELICAL OPERATING WAVE-GUIDE - NEW POSSIBILITIES TO ENHANCE EFFICIENCY AND FREQUENCY BANDWIDTH

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 eige nwave for a very broad frequency band. The use of such a helical waveg uide as an operating section of a gyrotron traveling wave tube (gyro-T WT) allows significant widening of its bandwidth and an increase in th e efficiency at very large particle velocity spreads. In this paper, t he new concept is confirmed by theoretical analysis and ''cold'' measu rements of the helical waveguide dispersion. Results of a linear and n onlinear theory of the helical gyro-TWT as well as two designs for sub relativistic (80 keV, 20 A) and relativistic (300 keV, 80 A) electron beams are also presented. For both designs, parameters providing a ver y broad frequency band (about 20%) and high efficiency (above 30%) hav e been found, When the transverse velocity spread is increased from ze ro 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 efficie ncy. The theoretical analysis demonstrates important advantages of the helical gyro-TWT over the ''smooth'' one in frequency bandwidth, sens itivity to electron velocity spread, and stability to parasitic self-e xcitation.