Stability analysis of a coaxial-waveguide gyrotron traveling-wave amplifier

Abstract

The gyrotron traveling-wave tube (gyro-TWT) amplifier is known to be highly susceptible to spurious oscillations. This study develops a simulation approach to analyze the stability of a coaxial-waveguide gyro-TWT with distributed wall losses. The interplay among the absolute instabilities, the gyrotron backward-wave oscillations, and the circuit parameters is analyzed. Simulation results reveal that the distributed wall losses effectively stabilize spurious oscillations in the coaxial gyro-TWT. Furthermore, the wall resistivity of the center conductor is shown to be an additional effective mechanism for suppressing oscillations. Under stable operation conditions, the coaxial gyro-TWT with distributed losses is predicted to generate 435kW in the Ka band with 31% efficiency, a saturated gain of 45dB, and a bandwidth of 1.86GHz (≈5.8%) for a 70kV, 20A electron beam with an α(=ν⊥∕νz)=1.0 and an axial velocity spread of Δνz∕νz=5%.

Linear Study of a Large-orbit Coaxial-waveguide Cyclotron Autoresonance Maser Amplifier

“…The four complex roots of k z correspond to four kinds of waves: a backward wave with constant amplitude, a forward wave with constant amplitude, a forward wave with decaying amplitude and a forward wave with growing amplitude. This situation is similar to that in a gyro-TWT amplifier [3][4][5].…”

“…Main points can be drawn from the specific analysis: (1) there is good agreement between the linear theory and the nonlinear simulation in the exponentially growing region of the wave; (2) output power of a large-orbit coaxial CARM amplifier is quite sensitive to the electron-beam energy and average radius; (3) unlike a gyro-TWT amplifier, a large-orbit coaxial CARM amplifier can not operate efficiently in the case of a small beam current; (4) there is the possibility for a large-orbit coaxial CARM amplifier to have megawatt output power in the millimeter wavelength range.…”

“…Previously, the Laplace transform technique was applied to the studies of the cylindrical waveguide gyro-TWT [3,4], the coaxial waveguide gyro-TWT [5], cylindrical waveguide gyro-BTWA [6,7] and cylindrical CARM amplifiers [8]. In the present paper we shall employ this technique to develop a linear theory for the large-orbit coaxial-waveguide CARM amplifier.…”

“…Neglecting the energy spread and velocity spread of the electron beam and following the treatment in Refs. [3,4,[6][7][8], we obtain the dispersion equation…”

A Comparison of the Attenuation of High-Order Mode in Coaxial Waveguide due to Inner and Outer Conductor Losses

“…For a coaxial waveguide, the distributed losses can be implemented on either its inner or outer conductor wall [13,14]. In this paper, based on the analytic formulae of the propagation constants of high-order modes in lossy coaxial waveguide reported in earlier works [15,16], quantitative comparisons of the attenuation of high-order mode in coaxial waveguides resulted from inner and outer conductor wall losses are carried out.…”