Tsun-Hsu Chang

Theory and experiment of a 94 GHz gyrotron traveling-wave amplifier

Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because o...

Frequency tunable gyrotron using backward-wave components

We present a frequency tunable scheme for the gyrotron at millimeter/submillimeter regime. Unlike step-tunable type where oscillation jumps discretely in different transverse modes, this scheme allows a smooth tuning within a single transverse mode using its backward-wave component. The characteristics of backward-wave interaction will be shown. A proof of principle experiment was conducted with a sealed cavity. The result shows the oscillation frequency smoothly transitions over a wide range of 6 GHz from 134 to 140 GHz. The proposed mechanism is capable of producing medium output power with broad frequency tunability up to sub-terahertz region.

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.

An ultra high gain gyrotron traveling wave amplifier

Physics and technology issues of importance to the high gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the suggestion of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. An experimental Ka-band gyro-TWT stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz.

Theoretical investigation of a high efficiency and broadband subterahertz gyrotron

This study investigates the electron dynamics of gyrotron interaction in a tapered waveguide and proposes an interaction structure for a 0.2-THz TE02 reflective gyrotron backward-wave oscillator (gyro-BWO). The design shows the advantages of high power, continuous and broadband frequency tuning, mode converter free, and inherently possesses mode-selective ability to suppress the potentially competing TE22 mode. Calculated results indicate that the reflective TE02 gyro-BWO is capable of generating a peak power of 3.3 kW corresponding to 26.3% interacting efficiency with a broad 3dB-tuning-bandwidth of 6.9 GHz at beam current Ib = 0.5 A and beam voltage Vb = 25 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.

Magnetron injection gun for a broadband gyrotron backward-wave oscillator

The magnetron injection gun is capable of generating relativistic electron beam with high velocity ratio and low velocity spread for a gyrotron backward-wave oscillator (gyro-BWO). However, the velocity ratio (α) varies drastically against both the magnetic field and the beam voltage, which significantly limits the tuning bandwidth of a gyro-BWO. This study remedies this drawback by adding a variable trim field to adjust the magnetic compression ratio when changing the operating conditions. Theoretical results obtained by employing a two-dimensional electron gun code (EGUN) demonstrate a constant velocity ratio of 1.5 with a low axial velocity spread of 6% from 3.4–4.8 Tesla. These results are compared with a three-dimensional particle-tracing code (computer simulation technology, CST). The underlying physics for constant α will be discussed in depth.

Dual-function circular polarization converter for microwave/plasma processing systems

Generation of a uniform density plasma has been a key consideration in the design of microwave/plasma processing systems. A circularly polarized wave can generate a plasma with good azimuthal uniformity as well as provide strong resonant interaction with the plasma electrons. In this article, we report the development of a three-port polarization converter which efficiently converts the TE10 wave of a standard rectangular waveguide into a circularly polarized TE11 wave of a cylindrical waveguide. Employing the same principle for mode conversion, the converter is also made to function as a protective device of the high power microwave source by providing a separate port for the return and damping of the reflected wave. Such a converter has been analyzed, constructed, and tested. At the operating frequency of 2.45 GHz, test results indicate a 25 dB return loss, 97% end-to-end polarization conversion efficiency, and better than 20 dB rejection of the reflected wave. The structural simplicity of the device al...

Polarization-controllable TE21 mode converter

We report the concept and development of a Ka-band mode and polarization converter that efficiently converts a TE10 rectangular waveguide mode into either a linearly or a circularly polarized TE21 cylindrical waveguide mode. The converter is composed of a power-dividing section, a mode-converting section, and a polarization-transitioning section. The converting process in each section is displayed and the working principles are discussed. A prototype has been built and tested. The measured results agree well with the numerical calculations for both linear and circular polarizations. The measured optimum back-to-back transmission is 94% with a 1-dB bandwidth of 4.1 GHz for the linear polarization. As for the circular polarization, the measured optimum transmission is 97%, but the corresponding bandwidth is indistinct due to some resonant dips. The reasons and impact for the dips are discussed. A bandwidth of 3.9 GHz is obtained for a single circular converter; meanwhile, an approach to eliminating these un...