Stable, high efficiency gyrotron backward-wave oscillator

Abstract

Stability issues have been a major concern for the realization of broadband tunability of the gyrotron backward-wave oscillator (gyro-BWO). Multimode, time-dependent simulations are employed to examine the stability properties of the gyro-BWO. It is shown that the gyro-BWO is susceptible to both nonstationary oscillations and axial mode competition in the course of frequency tuning. Regions of nonstationary oscillations and axial mode competition are displayed in the form of stability maps over wide-ranging parameter spaces. These maps serve as a guide for the identification and optimization of stable windows for broadband tuning. Results indicate that a shorter interaction length provides greater stability without efficiency degradation. These theoretical predictions have been verified in a Ka-band gyro-BWO experiment using both short and long interaction lengths. In the case of a short interaction length, continuous and smooth tunability, in magnetic field and in beam voltage, was demonstrated with the high interaction efficiency reported so far. A maximum 3-dB tuning range of 1.3GHz with a peak power of 149kW at 29.8% efficiency was achieved. In a comparative experiment with a longer interaction length, the experimental data are characterized by piecewise-stable tuning curves separated by region(s) of nonstationary oscillations, as predicted by theory.

Linear and nonlinear behaviors of gyrotron backward wave oscillators

“…25 Previous studies have revealed some essential properties of the gyro-BWO, and the stable operation regimes were also demonstrated by the theoretical analysis 25 and experiments. 10 However, there are some unresolved issues needed to be further studied for getting a more in-depth understanding of the intriguing electrodynamical system. For example, the effective lengths of the gyro-BWO in the linear and nonlinear regimes, respectively, are drastically different, such that it is difficult to establish a fixed relationship between the start oscillation threshold and the nonstationary oscillation threshold.…”

“…The corresponding wave equation is then given by Dðdx; k m À i@ z ÞAðzÞ ¼ 0; (10) which, noting that @Dðdx;k m Þ @k m ¼ 0, yields the following eigenmode equation for AðzÞ, i.e., h Dðdx; k m Þ À 1 2…”

8-Fold Helically Corrugated Interaction Region for High Power Gyroresonant THz Sources

“…Gyro-devices are fast-wave sources and traditionally are based on a smoothbore waveguide, rather than a slow-wave structure (SWS) that greatly limits the power handling capability of the THz sources. The gyro-backward wave oscillator (gyro-BWO) [5] and gyro-traveling wave amplifier (gyro-TWA) [6] are fast wave devices which can operate using higher order modes where the electrons are bunched in the azimuthal direction and do not rely on a SWS to longitudinally bunch the electrons in the beam.…”

Magnetron injection gun for a broadband gyrotron backward-wave oscillator

“…The gyrotron backward-wave oscillator ͑gyro-BWO͒, which features continuous frequency tunability 3 attracts more and more attention from various applications, such as electron spin resonance, plasma diagnostics, and sensitivity enhancement of nuclear magnetic resonance using dynamic nuclear polarization. [4][5][6][7] Recently, the high efficiency and the broadband tunability of gyro-BWO were achieved at Ka-band 3,8 and X-band, 9,10 respectively. However, little literature has so far been reported at W-band.…”

“…Unfortunately, the ␣ declines sharply with either the increase in the magnetic field B 0 or the decrease in the beam voltage V b , which significantly lowers the efficiency and results in narrow bandwidth. 3,8,19 Based on the above considerations, there is a pressing need to develop a MIG with high and constant ␣ as well as low velocity spread for a W-band or a terahertz gyro-BWO.…”