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Recent hot test details and inspiring results of the W-band lossy-dielectric-loaded gyrotron traveling-wave tube (gyro-TWT) developed at the University Electronic Science and Technology of China (UESTC) are reported in this letter. Measurement of the gyro-TWT had achieved a pulse output power over 100 kW with a broad bandwidth of 8.5 GHz (92.0~100.5 GHz). For the first time as we know, the W-band gyro-TWT can stably operate with a 10% duty cycle and the average output power is over 10.3 kW in the frequency range. Excellent stability, pure spectrum and mode patterns are all verified in the hot test experiment. [ABSTRACT FROM AUTHOR]

Effects of the output window reflection instability on the performance of our designed ${W}$ -band broadband gyrotron traveling wave tube (gyro-TWT) were studied by the simulation and hot test experiments. Strong oscillations and frequency self-modulation (FSM) in the after-cavity interaction (ACI) region were observed in the zero-driven state with a two-layer window and are verified with the beam-wave particle-in-cell simulation. Oscillations and FSM of 80–81.8 GHz and 91.03–94.68 GHz are, respectively, generated from the TE22 and TE02 modes in the ACI region due to the high window reflection level. By using the “sandwich” and meta-surface windows with low reflection and broad bandwidth, the parasitic ACI oscillations can be effectively suppressed. The maximum saturated output power is improved to 156.8 kW at 94 GHz and is zero-driven stable in our recent hot test experiments. [ABSTRACT FROM AUTHOR]

A circular TE0n mode content analysis technique based on the multi-mode coupler with high mode selectivity is proposed. To improve the mode selectivity, a deformed waveguide, entitled the quad-polar waveguide, which discriminates the azimuthally symmetric TE-mode on the waveguide wall, is introduced. The influence of deformation on the TE01 and TE02 modes’ field pattern, the corresponding transmission loss, and the power capacity in the quad-polar waveguide are analyzed. Based on the analysis, a dual-mode coupler of the circular TE01 and TE02 modes, consisting of an oversized deformed circular waveguide coupler and a pair of circular-to-quad-polar waveguide transitions, is designed for a high-power Ka-band gyrotron traveling-wave tube system. To verify the high mode selectivity of a quad-polar waveguide, a simple coupling with equal-radius and equal-interval holes are used. A prototype is fabricated and measured, and the results show that the coupling coefficient is −40 dB with the directivity of 27 dB for the TE01 mode, suppressing the unwanted mode TE02 mode with 53 dB. For the TE02 mode, the coupling coefficient is −40 dB with the directivity of 25 dB and suppresses the unwanted mode TE01 mode with 75 dB from 30 to 40 GHz. [ABSTRACT FROM AUTHOR]

A broadband dielectric window with high power handling capability for gyrotron traveling wave tubes (gyro-TWTs) had been designed based on the step-type impedance transition structure. The manufactured beryllium oxide (BeO) window can achieve a relative bandwidth of 35.3% with a maximum reflection of −20 dB in the ${X}$ -band, and the corresponding transmission loss is over −0.1 dB. The resonant spikes caused by engineering factors, including assembly tilt and gap, are investigated and analyzed. Thermal analysis shows that the BeO window can achieve a high average/CW power handling capability of over 232 kW within a wide frequency range. The fabricated input window has been successfully applied in our ${X}$ -band broadband and high-power gyro-TWT. [ABSTRACT FROM AUTHOR]

A periodic dielectric meta-surface output window was cold and hot tested to extend the operating bandwidth and suppress the parasitic oscillations for a W-band wideband TE01 mode gyrotron traveling wave tube (gyro-TWT) in this letter. The fabricated beryllium oxide meta-surface dielectric window demonstrates a relative bandwidth of 37.84% with the reflection below −20 dB in the range of 75–110 GHz. The influences of engineering factors and machining errors on the performance are investigated and analyzed. Thermal analysis shows the average power handling capability of the window is over 33 kW within a wide frequency range. Hot test results demonstrate that the meta-surface window is excellent for oscillation suppression and the power handling capability is predicted larger than 20 kW. [ABSTRACT FROM AUTHOR]

Design and measurement of a broadband compact circular TE11 mode input coupler for gyrotron traveling wave tubes (gyro-TWTs) are presented in this article. Different from the T-junction, Y-type, and multiple-hole input couplers, the proposed coupler has a merit of compact dimensions formed by the H-plane of a rectangular cavity. The simulation results show that the port reflection $S_{11}$ is less than −10 dB within 7.9–9.93 GHz. Also, the split transmissions $S_{21}$ and $S_{31}$ are over −3.4 dB in this band. It indicates that the relative bandwidth is 22.8%, and the transmission efficiency is higher than 90%. To further broaden the bandwidth, we propose an improved input coupler with ridges achieving an excellent bandwidth of 28.6%. The designed input coupler has been measured with open boundaries and matched output ports. The simulation and measurement results are in good agreement. The compact TE11 mode input coupler had been applied in our X-band gyro-TWT design. [ABSTRACT FROM AUTHOR]

Gyrotron traveling-wave tubes (gyro-TWTs) have been proved to be an attractive power source in many applications such as high-resolution radars, high-power communication systems, and electronic warfare systems. The main output indexes such as peak and average output power, interaction efficiency, and bandwidth are important for these devices. In addition, a great deal of reliability requirements such as nonstop stable operating time, operation lifetime, and environmental adaptability should be met. In this paper, detailed developing processes of gyro-TWTs in high average power operations are presented, including theoretical analyses, multiphysics simulations of key components, fabrication, and construction of the automatic hot test platform. Finally, the measured results of a Ka-band gyro-TWT for industrial applications are presented. The amplifier exhibits an excellent performance of 100-h nonstop stable operation, a 1-dB fraction bandwidth of 8%, an efficiency of more than 20%, and a peak power and an average power of more 150 and 10 kW, respectively. [ABSTRACT FROM AUTHOR]

A high-power Ku-band TE01 mode gyro-traveling wave tube (gyro-TWT) with a loss loading structure is designed, fabricated, and tested. A high beam quality magnetron injection gun generating 68-kV, 20–30-A electron beam is optimized to provide a high power radiation. A novel lossy ceramic loaded structure with mode selective attenuation ability is proposed to suppress the potential backward wave oscillation. The interaction circuit is analyzed by linear, nonlinear self-consistent theory, and experiment. An improved wideband input structure design is proposed accordingly. In the hot test, the proposed gyro-TWT produces a 420-kW maximum peak power, about 35-dB saturated gain, 23% efficiency, and more than 1.6 GHz (10%)−1-dB bandwidth in a stable operating condition. [ABSTRACT FROM PUBLISHER]

Mode converters for millimeter waves can reduce the transmission loss, generate the desired mode, and facilitate the application of high-power sources. A W-band broadband quasi-optical mode converter which converts a TE0,1 cylindrical waveguide mode into a linearly polarized Gaussian TEM00 mode has been realized and tested in this paper. The converter is composed of a wavefront transforming mirror, a polarization grating, and a phase correcting system. In order to obtain high conversion efficiency over a wide bandwidth and make the structure of the mode converter much simple and compact, the propagation path of the beam is optimized and a wavefront transforming mirror realized by an ellipsoidal mirror rather than a nonquadratic mirror is introduced. The conversion process in each section is displayed and the working principles are discussed in detail. The mode conversion characteristics are investigated by analyzing the measured radiation field pattern. Results of the measurement agree well with the numerical calculations. The proposed W-band mode converter has a conversion bandwidth of 21% when the mode conversion efficiency is over 95% and the cross polarization is below 1.7%. In addition to high conversion efficiency, low cross-polarization, and broad bandwidth, this converter features easy construction and compact structure. [ABSTRACT FROM PUBLISHER]

To accurately measure the performance of dielectric loaded circuits for gyrotron traveling wave tubes, a novel topological structure to generate the TE21○ or TE01○ (superscript represents circular waveguide) mode is proposed and verified in this paper. The topology is constituted by an overmoded TE10□ to TE20□ (superscript represents rectangular waveguide) mode converter with multiple matching steps and a taper from TE20□ to TE21○ or TE01○ mode. This topology allows the TE21○ or TE01○ mode to be excited by changing the taper dimensions. Simulation results reveal that the TE21○ and TE01○ mode converters, respectively, have a 1-dB bandwidth of 12.8 and 9.3 GHz with an excellent performance. Converters for the back-to-back millimeter-wave measurements were fabricated with computerized numerical control and wire cutting techniques. Vector network analyzer measurements are in good agreement with the simulations. [ABSTRACT FROM PUBLISHER]

In order to enhance the efficiency of confocal gyrotron traveling-wave tubes (gyro-TWTs), this paper is intended to present a novel triode-type magnetron injection gun for a 140-GHz 2-kW confocal gyro-TWT. This novel gun, which has a pair of sectorial emitters, is expected to obtain an axial velocity spread of 7.83% and a velocity ratio of 1.01 with an anode voltage of 35 kV and a beam current of 2.4 A. The design procedure has been given in detail. Computer simulations have been carried out using 3-D electromagnetic code computer simulation technology particle tracing. The sensitivity analysis has been performed to study the parametric dependence of modulating anode voltage, accelerating anode voltage, and cathode magnetic field on beam qualities. Results show that this novel gun can be operated stably within a certain parametric scope. [ABSTRACT FROM AUTHOR]

A new curved-profile undepressed collector designed for a planned 35-GHz gyrotron traveling-wave tube (gyro-TWT) is proposed in this paper. This gyro-TWT, which is driven by a 70-kV, 10-A electron beam, is working at the fundamental TE01 mode with an efficiency of 20% and gain of 45 dB. Studies including theoretical analysis, numerical calculation, and computational simulation have been done. Key issues concerning electronic collecting density, dissipated power distribution, transmission performance, secondary electrons, and thermal property are considered. Simulation results show that curved-profile structure collector has a better heat-handling capacity than the conventional line structure collector. [ABSTRACT FROM AUTHOR]

The design of a high-power low-velocity-spread space-charge-limited cusp gun is presented. The Lagrangian formulation is used to analyze the velocity spread and beam ripple (orbit center spread). It shows that the two electron orbit off-center mechanisms, the radial velocity and the nonideal cusp magnetic field, can be adjusted to cancel each other. The characteristics of the cusp gun including the cusp magnet are calculated by cosimulation employing the Maxwell2D, Matlab, Egun, and CST-PS codes. The cusp gun is designed to generate a beam of 8 A at 70 kV with a pitch factor from 1.0 to 1.8 and an axial velocity spread from 3% to 7%. The development of this cusp gun is in progress at the University of Califonia, Davis, for use in the development of harmonic gyrotrons, peniotrons, and gyro-traveling-wave tubes. [ABSTRACT FROM AUTHOR]

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