C. F. Yu

A mode-selective circuit for TE01 gyrotron backward-wave oscillator with wide-tuning range

This study proposes a mode-selective circuit to suppress the competing modes in a TE01 gyrotron backward-wave oscillator (gyro-BWO). The circuit, also functioning as an interaction structure, comprises of several transverse slices. It eliminates the restrictions of the mode-competitions and allows a longer interaction structure to optimize interacting efficiency. Mode-selective effect will be analyzed. Experimental results indicate that the Ka-band TE01 fundamental harmonic gyro-BWO is capable of continuous tuning from 31.4 GHz to 36.4 GHz with a peak efficiency of 23.7% corresponding to 100 kW at Ib = 4.5 A and Vb = 93.6 kV.

W-band TE01 gyrotron backward-wave oscillator with distributed loss

Distributed wall loss is proposed to enhance the stability and tunability of a W-band TE01 gyrotron backward-wave oscillator (gyro-BWO). Simulation results reveal that loss effectively suppresses the unwanted transverse modes as well as the high-order axial modes (HOAMs) without degrading the performance of a gyro-BWO that operates at the fundamental axial mode. Linear and nonlinear codes are used to calculate the interaction properties. The effects of the distributed loss on the starting currents of all of the modes of interest are discussed in depth. The interacting structure is optimized for stability. The calculated peak output power is 102 kW, corresponding to an efficiency of 20%. The 3-dB tuning bandwidth is 1.8 GHz, centered at 94.0 GHz when using 5 A and 100 kV electron beam.

Exciting circular TEmn modes at low terahertz region

This work proposes an approach to generate circular TEmn modes at low terahertz region through sidewall couplings. With proper arrangement of the couplings on the circumference of the waveguide, they then jointly excite the desired mode. A model is developed to calculate the coupling strength and to analyze the mode purity. Accordingly, three mode converters TE21, TE01, and TE41, were designed, built, and tested at W-band. Back-to-back transmission measurements exhibit excellent agreement with the results of simulations. The measured optimal transmissions are 91%, 95%, and 89% with 3 dB bandwidths of 18.3, 24.0, and 20.2 GHz, respectively.

High Performance Circular TE01, Mode Converter

Summary form only given. The development and experimental test of a Ka-band TE01 mode converter are presented. A wave in TE10 rectangular waveguide mode is efficiently converted into the TE01 circular waveguide mode. This converter is composed of a power-dividing section and a mode-converting section. The field pattern and the working principle of each section are analyzed and discussed. A prototype has been built and tested. Back-to-back transmission measurements show excellent agreement with computer simulations. The measured optimum transmissions are 97% with 1-dB bandwidth of 5.8 GHz and 3-dB bandwidth of 7 GHz. High mode purity is predicted in theory and demonstrated in experiment. The field pattern of the circular TE01 mode is directly displayed on a temperature sensitive liquid crystal sheet, where the electric field strength can be discerned from the color spectrum. In addition to three just mentioned advantages, high converting efficiency, high mode purity, and broad bandwidth, this converter also features easy constructions and compact size

A TE21 second-harmonic gyrotron backward-wave oscillator with slotted structure

Second-harmonic gyrotron backward-wave oscillator (gyro-BWO) with a reduced magnetic field strength is a tunable source in the millimeter wave regime, but it has long been impeded by the severe mode competition as a result of low efficiency and narrow bandwidth. This study employs a slotted structure functioning as a mode selective circuit to suppress the lower order transverse modes. In addition, a two-step tapered waveguide is adopted to stabilize the higher-order transverse modes and axial modes. Some important characteristics of the slotted gyro-BWO will be analyzed and discussed. As a calculated result, the interaction efficiency is improved and the stable tuning range is broadened. A stable, Ka-band, slotted second-harmonic gyro-BWO is capable of producing an efficiency of 23% with a 3dB tuning bandwidth of 9% at 5A and 100kV.

Dynamics of Mode Competition in the Gyrotron Backward‐Wave Oscillator

The axial modes of the gyrotron backward-wave oscillator (gyro-BWO) each exhibit a distinctive asymmetry in axial field profile. As a result, particle simulations of the gyro-BWO reveal a radically different pattern of mode competition in which a fast-growing and well-established mode is subsequently suppressed by a later-starting mode with a more favorable field profile. This is verified in a Ka-band experiment and the interaction dynamics are elucidated with a time-frequency analysis.

W-band TE 01 gyrotron backward-wave oscillator with distributed losses

Distributed wall losses are adopted to enhance the stability and tunability of a W-band TE01 gyrotron backward-wave oscillator (gyro-BWO). Simulation results reveal that distributed wall losses can effectively suppress the competing transverse modes and high-order axial modes, but do not significantly degrade the performance of a gyro-BWO operating at the fundamental axial mode. Extensive numerical calculations are conducted. The effects of guiding center radius and waveguide tapering on the start-oscillation currents are examined. Preliminary tuning properties of the gyro-BWO are presented. The gyro-BWO is predicted to yield a peak output power of 100 kW centered at 96 GHz with an efficiency of 20 % and a 3-dB tuning bandwidth of 1.8 GHz at a beam of 100 kV and 5 A.

Generating pure circular TE mn modes

This work presents a methodology of exciting a pure circular TEmn modes using cascaded Y-type power dividers at low terahertz region. The measured transmissions are high and the bandwidths are broad. These Y-type converters are structurally simple but the machining errors are critical.

A mode-selective circuit for TE 01 gyrotron backward-wave oscillator with wide-tuning range

This study proposes a mode-selective circuit to suppress the competing modes in a TE 01 gyrotron backward-wave oscillator (gyro-BWO). The circuit, also functioning as an interaction structure, comprises of several transverse slices. It eliminates the restrictions of the mode-competitions and allows a longer interaction structure to optimize interacting efficiency. Mode-selective effect will be analyzed. Experimental results indicate that the Ka-band TE 01 fundamental harmonic gyro-BWO is capable of continuous tuning from 31.4 GHz to 36.4 GHz with a peak efficiency of 23.7% corresponding to 100 kW at I b = 4.5 A and V b = 93.6 kV.

Development of frequency-tunable, terahertz gyrotron backward-wave oscillator

The gyrotron backward-wave oscillator (gyro-BWO) is a frequency tunable scheme at millimeter/terahertz regime. The continuous frequency tuning can be achieved by varying either the magnetic field or the beam voltage in a non-resonant structure. With a growing interest in terahertz-wave regime, gyro-BWO should be preferably operated with high-order mode due to the power-handling capability. Then the upcoming difficulty would be the terahertz interaction circuit as well as the severe transverse mode competition.