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This letter presents the design and experimental results of a 152-kW W-band gyrotron travelling-wave amplifier (gyro-TWA) that boasts an enhanced 3-dB instantaneous bandwidth ranging from 8.8 GHz to 12.5 GHz, and 1-dB bandwidth from 7.0 GHz to 11.5 GHz. This was achieved by powering the gyro-TWA with a 60 kV, 11 A electron beam. The amplifier demonstrates an output power exceeding 120 kW and an interaction efficiency above 19.1% within 89.8-101.3 GHz. This performance improvement was made possible by using a curved-profile circuit that matches with the gyrotron solenoid across a wide frequency band. The theoretical predictions, particle-in-cell simulations and experimental results are compared and found to be in good agreement.

Insufficient attenuation and poor heat dissipation in the beam-wave interaction section are critical limitations in developing the megawatt-level gyrotron traveling wave tube (gyro-TWT). To solve these problems, we propose a water-based lossy waveguide with high attenuation and heat dissipation in this letter. It comprises a vacuum region, a lossless BeO ceramic, and flowing water. Measured results indicate that the proposed structure can achieve an attenuation higher than 10dB/cm at 15.5-17.0GHz. The maximum attenuation at 16.3GHz can even exceed 29dB/cm. It is sufficient to suppress oscillations at large currents. In addition, the flowing water has a higher power capacity than conventional lossy dielectric. The proposed structure paves the way for the development of megawatt gyro-TWTs.

This letter presents an approach to generate the sub-nanosecond microwave pulse with a high power compression factor using a compact reflection structure. In the experiment, an input chirped pulse with the frequency nonlinearly sweeping from 12.03 to 15.53 GHz is compressed, and the generated pulse has a 3 dB pulse width of 0.257 ns and a power compression factor of 43.3. The experimental result is in good agreement with our simulation. This method can potentially enhance the power level of broadband vacuum electron amplifiers.

To satisfy the demand for high-power microwave sources in the terahertz band, a G-band dielectric-load gyro-TWT is fabricated and experimental demonstrated for the first time in this letter. The high-precision assembly and concentricity adjustment technology are proposed to minimize test deviations. The hot test result indicates that the gyro-TWT, driven by a 48.3 kV-3.05 A electron beam with a duty cycle of 10%, achieves a maximum pulse power of 34.1 kW, a gain of 59.8 dB, and an efficiency of 23.2% at 216 GHz. More than 10 kW output power is produced between 213 to 221 GHz. This result represents the highest output power in the terahertz band dielectric-load gyro-TWT currently.

The fluctuation of current can destroy the performance of the gyrotron traveling-wave tube (Gyro-TWT). We found that the fluctuation of the current is related to the generation of trapped electrons in the interaction circuit. Through experimental and simulation verification, it has been found that the oscillation characteristics of the current are consistent with the motion characteristics of the trapped electrons in the interaction circuit. This indicates that trapped electrons have a significant impact on the current. To suppress the trapped electrons, a novel magnetron injection gun (MIG) with the rear electrode (MSRE) has been designed. The absorbed power of the interaction circuit has decreased by 50%. When a W-band Gyro-TWT with MSRE is operated at 60.5 kV/10.15 A, zero drive with a pulse duration of $200~\mu $ s, and a duty cycle of 15%, there is no current fluctuation. The tube achieves a peak power of 136 kW, a gain of 71 dB, and an average power of 20.54 kW at 94 GHz. The hot test shows that MSRE can suppress the generation of trapped electrons. This greatly enhances the stability of the tube.

In the hot test of our G-band (0.21–0.23 THz) TE01-mode gyrotron traveling wave tube (gyro-TWT), the beam current was increased from 3.0 to 3.9 A to obtain a higher gain and output power. However, a strong TE21-mode backward-wave oscillation occurred in the nonlinear interaction region. To solve this problem, a novel mode-suppression circuit (MSC) is proposed, designed, and measured in this article. The MSC contains six transverse slot-coupled loops surrounded by an external lossy dielectric ring, which can provide a sufficient coupling loss for the parasitic TE21 mode without affecting the transmission of the operating TE01 mode. Based on the MSC, a simulated loss of 8.09 dB/cm is provided for the TE21 mode near the oscillation frequency. Considering the engineering realization, the beam voltage and pitch factor are set to 50 kV and 1.0, respectively. Particle-in-cell simulation shows the MSC-based gyro-TWT has excellent zero-driven stability and can achieve a saturated output power over 20 kW in the range of 210–228 GHz with a beam velocity spread of 3.01%. For verification, an MSC was manufactured and cold tested. The cold-tested results agree well with our analysis.

In this article, we propose and measure a high-power Ku-band gyrotron traveling wave tube (gyro-TWT) with external coupling feedback to generate wide-band multifrequency signals. With varying the beam current, pitch factor, coupling coefficient, and feedback loop length, strong oscillations from single-frequency to multifrequency regimes of the self-excitation TE11 mode are observed in the zero-driven state. Particle-in-cell (PIC) simulation shows the gyro-TWT can achieve a maximum output power of 192.86 kW and 56.67 MHz interval between main spectral components in the range of 14.85–17.46 GHz. It was confirmed by a proof-of-principle experiment based on a Ku-band gyro-TWT with an external feedback loop, which yielded 2.6 GHz multifrequency signals with an interval of 54 MHz and maximum output power of 174.25 kW at a duty cycle of 1%. The generated multifrequency radiation signals may have potential applications in microwave wireless communication, jamming of radar systems, and microwave frequency combs.

In this article, the design and power capacity investigation of the gyrotron traveling wave tube (gyro-TWT) with more than 100 kW continuous-wave (CW) in ${Q}$ -band are described. An input coupler is designed with a photonic bandgap isolator to ensure a wide bandwidth and low microwave leakage. A nonuniform dielectric-loaded circuit (DLC) is proposed to address the severe microwave loss in lossy ceramics at high power levels. The curved depressed collector with a multigap output isolator provides an effective approach to further enhance the overall efficiency and power capacity of high-frequency gyro-TWTs. An output window, equipped with boundary heat transfer channels, is employed to improve mechanical strength. Simulation results indicate that the ${Q}$ -band TE01 mode gyro-TWT, driven by a 55 kV–9.1 A electron beam (3.2% transverse velocity spread), can produce a maximum output power of 145 kW, with a gain of 74 dB and an overall efficiency of 40% at 48.5 GHz. The power output exceeding 100 kW is achieved in the 47–50 GHz range, with an overall efficiency of over 30%. With the efficient thermal management system, the maximum temperature on the DLC, collector, and output window are 190 °C, 241 °C, and 134 °C, respectively. The simulation results demonstrate the feasibility of the gyro-TWT for more than 100 kW CW stabilized operation.

The large-signal theory for curved-profile circuit and magnetic field of gyro-TWT is presented. According to it, an optimization method is proposed to achieve effective wave-particle interaction over a wide frequency band through a curved-profile circuit with cutoff sections and gyrotron solenoid magnetic field. It avoids strict limit of grazing magnetic field along the circuit position to improve the design freedom. On this basis, a Ka-band gyro-TWT operating at TE01 mode is theoretically designed and well validated by particle-in-cell (PIC) codes and hot-test experiments. The developed amplifier achieves an output power of above 134 kW range from 27.7 to 31.7 GHz, with an interaction efficiency of above 22.3% over the entire band.

In response to the urgent requirement for a high-power millimeter-wave system with small size, low cost, and excellent maneuverability in W-band, a compact high average power sheet beam extended interaction oscillator (SB-EIO) based on periodic cusp magnet (PCM) is designed, fabricated, hot tested and reliability verified. The maximum output power is 10.2 kW at a working duty of 1%. When the duty is increased to 27%, an average power of 2.2 kW is achieved, corresponding to a $400 \mu \text{s}$ pulse and 675 Hz repetition frequency. The device can be repeatedly turned on and off quickly during a 6-minute operating time and the single longest running time exceeds 60 min. The 96.0% transmission rate of the sheet beam (45.5 kV, 2.49 A) is verified. The overall weight of SB-EIO including the magnet and external package is less than 10 kg, which is ideal for equipping movable platforms.

Electron beam transportation stability is important to the gyro-traveling-wave tube (gyro-TWT). For conventional ${G}$ -band TE01 mode magnetron injection gun (MIG), the transportation performance is unsatisfactory because of the small interaction tunnel. For the transportation problem, a MIG with a slotted cathode for ${G}$ -band TE13 mode gyro-TWT is proposed in this article. The MIG is designed and optimized, and a velocity ratio of 1.15 and a transverse velocity spread of 0.58% are obtained. Furthermore, the cathode has been produced and fabricated. A thermal test, a surface morphology test, and a computed tomography (CT) scan have been carried out. Then, beam quality and transportation stability are analyzed when thermal expansion, morphology deviation, stray electron, and magnetic field misalignment exist, respectively. The beam transportation performance is good under all conditions, and the total simulated velocity spread of the main electron is 3.3%.

In this letter, we reported a gyrotron traveling wave tube (gyro-TWT) featured broadband and high power at the University Electronic Science and Technology of China (UESTC), which had achieved a long pulse width up to 2 ms and a maximum duty cycle of up to 40%. Two typical operation states, i.e. high voltage/current (60.5 kV/9.0 A) and low voltage/current (46 kV/2.6 $\sim $ 2.9 A), are respectively hot tested for high pulse and average power. With a duty cycle of 40%, the measured maximum average power is up to 20.1 kW.

Mode distortion and performance degradation caused by an OFF-axis gyrating electron beam in our W-band gyrotron traveling wave tube (gyro-TWT) were studied by the particle-in-cell (PIC) simulation and verified with the hot test experiments. The simulated electron beam misalignment may lead to a mixed pattern, mainly composed of TE01, TE02, TE21, and TE22 modes. They will further deteriorate the performance by disturbing the bunched electron beam. When the misalignment is over 0.1 mm, the operating mode shows a significant distortion, and the output power is reduced to 66% of that without misalignment at 93 GHz. For the prototype gyro-TWT with an estimated beam misalignment of about 0.1 mm, the measured saturated output power drops by more than 57% below 93.5 GHz compared with the normal one, whose estimated misalignment is less than 0.05 mm. The downward trend proves that the beam misalignment is the main factor causing the performance degradation at the W-band and above.

In this article, a planar quasi-optical waveguide (PQOW) circuit is proposed for vacuum electron devices (VEDs) for the first time. The circuit is made of a series of metamaterial pillars attached to a pair of parallel plates for phase compensation. It can propagate quasi-optical modes as a pair of concave mirrors does, while the lower order HE $_{{0}{n}}$ modes experience much heavier circuit loss compared to that of convention confocal waveguide circuits, which can improve the backward oscillation (BWO) stability of gyro-amplifiers. As an example, a W-band gyro-amplifier with PQOW circuits to replace conventional confocal circuits is designed. The 3-D particle-in-cell simulation demonstrated that the circuit is capable of producing stable high-power radiation within 96–108 GHz, with an electron efficiency of about 15% over the entire bandwidth.

To realize a high-gain wideband millimeter-wave amplification, we present and simulate a ${W}$ -band gyrotron traveling wave tube (gyro-TWT) with cascaded coaxial- and circular-waveguide amplifiers. The amplifiers are operated in a coaxial and circular TE01 mode, respectively, and driven by two 60-kV gyrating electron beams with a pitch factor of 1.2 but different beam currents of 1 and 10 A. They are designed to possess relatively low individual gains below all oscillation thresholds to maintain high stability and achieve a high total cascaded gain for the whole gyro-TWT. A coupling circuit is adopted to cascade these two sections, converting the coaxial TE01 mode to the circular one with a −3-dB bandwidth of 6.8 GHz (94.1–100.9 GHz). particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a maximum saturated output power of 239.8 kW at 100 GHz, with a gain greater than 60 dB over a −3-dB bandwidth of 8.0 GHz (94.5–102.5 GHz).

The large-signal theory, including the BEAMLETS model and averaged coupling impedance formalism, of confocal gyrotron traveling-wave amplifiers (gyro-TWAs) with diffractive loss has been derived. As an example, a ${W}$ -band gyro-TWA with uniform confocal circuits and curved-profile magnetic field is theoretically studied and then well validated by three-dimensional particle-in-cell (3-D-PIC) simulation in detail. The results predicted by our code achieve excellent agreement with the PIC simulation over 92–104 GHz regarding the electron efficiency and radio frequency (RF) wave efficiency profile along the circuit position and saturation amplification performance, with a saturation input power difference of less than 20%.

Design and measurement of a distributed radiation coupling (DRC) circuit for the Ku -band TE11-mode gyrotron traveling wave tube (gyro-TWT) amplifier are presented. Different from the dielectric-loaded (DL) waveguide, the proposed DRC circuit can suppress the parasitic modes by achieving sufficient radiation coupling losses. It has the merit of lowering the microwave power loss density, which can improve the tube stability and average power-handling capability. Based on this proposed DRC circuit, a Ku -band TE11-mode gyro-TWT was designed and the particle-in-cell (PIC) simulation shows it can obtain a saturated output power of over 150 kW from 15.3 to 17.2 GHz for an electron beam with a velocity spread of 4.35%. As a comparison, the maximum average microwave power loss density is only 20% of the DL circuit under the same operating parameters. For verification, a DRC circuit was manufactured and cold tested. The maximum deviation between the measured results and our analysis is within 10%.

Periodical radiation spectrum with a frequency difference of ~160 MHz is observed in hot-test experiments of Ku-band gyrotron traveling-wave tubes (gyro-TWTs). By the means of eigenmode analyses and particle-in-cell (PIC) simulation considering the whole output transition section from the interaction circuit end to the output window, we found that the circuit oscillation is pronounced because of frequency self-modulation (FSM). As a comparison, the cutoff taper contributing to the FSM is removed and a new amplifier has been developed; hot-test results indicate that the periodical property has been greatly alleviated as expected and the stability of the amplifier is well improved.

In this article, a collaborative design method for gyrotron magnetron injection gun (MIG) and solenoid is proposed for the first time. On this basis, the shape of the electrodes and solenoid sizes is optimized by the means of global optimization techniques. Simplified theory and numerical methods of MIG have been developed, and the results agree well with the PIC simulation. By using the new method, the calculation time is reduced and the numerical stability has been greatly improved compared with that directly solving the Lorentz equation. As an example, a helical electron beam with a velocity pitch ratio of 1.20 and a transverse velocity spread of 1.65% is obtained for G-band gyro-TWT.