Gyro-TWT with a helical operating waveguide: new possibilities to enhance efficiency and frequency bandwidth

Abstract

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

Development of gyrotron traveling-wave tubes at IAP and GYCOM

“…The operating helicalwaveguide eigenmode has sufficiently high and almost constant group velocity at zero axial wavenumber which enables broadband operation of the helical-waveguide gyro-TWT with minimum sensitivity to electron velocity spread [3]. A number of experiments have proved the main theoretical predictions and advantages of gyroTWTs of this type [4][5][6].…”

Theory and simulations of a gyrotron backward wave oscillator using a helical interaction waveguide

“…17 It can also be measured by detecting the polarization angle of a linearly polarized wave when it propagates through the waveguide by using a scalar network analyzer ͑SNA͒. 9,17 In Fig. 1 the measured results using the SNA method are shown and compared with the results simulated from MAGIC using the same operating eigenwave.…”

“…In analogy to gyrotron interactions and Ref. 9, it is possible for one to derive the gyro-BWO beam-wave dispersion:…”

“…9 This allows broadband microwave amplification to be achieved in a gyrotron traveling wave amplifier. 10,11 Such a system is also favorable when used in a gyro-BWO ͑Ref.…”

Analytical Study of the Interaction Structure of Vane-Loaded Gyro-Traveling Wave Tube Amplifier

“…For the disc-loaded gyro-TWT, the controlling parameters for widening the coalescence bandwidth and hence the device bandwidth are the disc radial thickness and the axial periodicity of discs [16,17]. A coaxial helix internally loading a cylindrical waveguide [18][19][20] and a helical groove on the interior wall of a cylindrical waveguide [21,22] can also similarly widen the coalescence bandwidth and hence broadband a gyro-TWT. For optimum performance, the device should be designed such that the desired mode dominates and suppresses all other modes during the transition of the device parameters to the operating point.…”