Gyrotron Traveling-wave Tube Research Articles

Design and experiment of an inverse magnetron injection gun for a Ka-band gyro-TWT with non-superconducting magnet

A triode inverse magnetron injection gun (IMIG) for a Ka-band gyrotron traveling wave tube (gyro-TWT) with non-superconducting magnet is proposed in this paper. Nowadays, the applications of gyro-TWT are intended to expand to moveable and fast-startup systems, so room-temperature magnet and system miniaturization are necessary. For a Ka-band gyro-TWT, to adapt to a room-temperature magnet with 20 mm bore radius, the IMIG form is adopted. Compared with conventional MIG, the gun maximum radius is reduced by 44%. On one hand, a curved emitter and cathode steps are utilized for better beam quality. On the other hand, isolation gaps and a cooling structure are designed to suppress stray electrons. A velocity ratio of 1.14 and a transverse velocity spread of 2.43% are obtained finally. The stray electrons are analyzed, and the IMIG tolerance is also evaluated. Finally, the cathode is fabricated, and its surface morphology is tested. It is then assembled into a gun shell, and the cathode temperatures are measured under both no cooling and cooling conditions. When the temperature of the emitter reaches 1050 °C, the heat powers are 84 and 115 W, respectively. The temperatures of the inner and outer electrodes are low enough to reduce the proportion of stray electrons.

Gyro-TWT Frequency Multiplier

Gyrotron-type amplifiers are being developed for various applications requiring high power in the millimeter wavelength range. Calculations of the optimal designs of gyrotron traveling wave tubes (gyro-TWTs), presented in the article, were carried out using the Gyro-K computer program using a coordinate transformation technique, which made it possible to significantly reduce the calculation time of gyroresonance devices by reducing the three-dimensional problem of excitation of an irregular waveguide to a one-dimensional one. Two options for creating a gyro-TWT using the TE02 wave mode are considered: the first one is operating at the first harmonic of the gyrofrequency, the second is a frequency multiplier. The achievable characteristics of devices in the terahertz frequency range are presented, such as gain bands, efficiency, gain factors and distributions of high-frequency fields in longitudinal and transverse sections. The gyro-TWT frequency multiplier has a gain bandwidth of 7.2 %, an efficiency of 17 %, and a gain of 30 dB.

Performance Degradation Caused by Ionization of the Released Gas Molecules in W-Band Gyrotron Traveling Wave Tube Based on a Simplified Ionization Model

Performance Degradation Caused by Ionization of the Released Gas Molecules in W-Band Gyrotron Traveling Wave Tube Based on a Simplified Ionization Model

Helical Electron Beam Status Online Evaluation for Magnetron Injection Gun

The magnetron injection gun (MIG) is an essential component of the gyrotron traveling wave tube (gyro-TWT). Although the electron beam status influences the performance of the device, it cannot be measured directly in the experiment. An online evaluation module (OEM) for the experiment is developed to calculate the instant beam parameters of MIG. The OEM, by reconstructing the geometry of the MIG and related magnetic field distribution, can obtain the electron beam status under the operating parameters through the online simulation. The beam velocity spread of thermal emission with instant temperature and surface roughness are also considered. The validation is done in a W-band gyro-TWT, and the beam performance is evaluated in the experiment. With a pitch factor of 1.06 electron beam, the velocity spread affected by the electric-magnetic mismatch, thermal emission, and roughness is 1.00%, 0.56%, and 0.43%, respectively. The other beam parameters are also presented in the developed module. The OEM could guide and accelerate the testing process and ensure the safe and stable operation of the device.

Experiment and Power Capacity Investigation of Collector in the 50-kW-Average-Power Q-Band Gyro-TWT

To further increase the average output power of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{Q}$</tex-math> </inline-formula> -band gyrotron traveling wave tube (gyro-TWT), the power capacity and stability of the collector are investigated. In this article, the characteristics of the electron collection, the modeling of the cooling system, and the suppression of stray electrons are analyzed. The dissipated power is associated with the transverse energy of the electron beam. When the gyro-TWT operates at zero drive, the collector has a maximum dissipated power density of 2 kW/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula> for a high average power state. Thermal results indicate a maximum temperature of 381 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C on the collector with an optimized cooling system. By applying a transverse magnetic field, the stray electrons can be completely intercepted by the collector. The hot experiment results indicate that the gyro-TWT (45% duty cycle), driven by a 60-kV–9.82-A electron beam, can withstand a maximum beam power of 265 kW at zero drive. Then, a pulse power of 111 kW, with an average power of 50 kW and an efficiency of 18.8% at 47 GHz, is measured stably. The simulation and experimental results demonstrate the high-power capacity and high stability of the curved collector.

Theoretical Design of a Dual-Band TE01/TE02 Mode Gyrotron Traveling Wave Tube

A dual-band (K/Ka) TE01/TE02 mode gyrotron traveling wave tube is presented in this article. To suppress parasitic oscillations, a lossy-dielectric-loaded interaction circuit is employed. The particle-in-cell simulation results show that when it operates in K-band, the operating mode is the TE01 mode, with a peak output power of 87.1 kW, a saturated gain of 42.74 dB, and a −3 dB bandwidth of 0.7 GHz, and when it operates in Ka-band, the operating mode is the TE02 mode, with a peak output power of 62 kW, a saturated gain of 60.76 dB, and a −3 dB bandwidth of 2 GHz. Moreover, in the operating frequency range of the Ka-band, the overall gain is greater than 57 dB. To meet the requirements of dual-band operating, a dual-state magnetic injection gun is designed, a dual-mode coaxial cavity input coupler is proposed, and a dual-band output system is developed. All of these components showed excellent performance in simulations.

Theoretical designs and experiments on dual-band input coupler for gyrotron traveling wave tube

In this paper, a novel K/Ka dual-band input coupler for the gyrotron traveling wave tube is theoretically and experimentally investigated. This input coupler mainly consists of three modules, a Y-type power division coupler, a coaxial cavity coupler, and a Bragg reflector. When the dual-band coupler operates in K-band, the Y-type coupler couples out the TE11 mode, and simulation results show the −1 dB bandwidth is 3.3 GHz in the frequency range of 17.9–21.2 GHz. In Ka-band, the TE21 mode coupled out by the coaxial cavity coupler, the −1 dB bandwidth is 3.5 GHz within 33.7–37.2 GHz, and the reflection coefficient keeps a pretty low level. The prototype was fabricated and measured to further verify the designed dual-band input coupler's performance. The back-to-back cold test results show that the −3 dB bandwidth of the K-band is about 2.5 GHz, and the −3 dB bandwidth of the Ka-band is 2.44 GHz.

Mode Distortion and Performance Degradation Caused by Electron-Beam Misalignment in W-Band Gyro-TWTs

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.

Long Pulse and High Duty Operation of a W-Band Gyrotron Traveling Wave Tube

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/2.6~2.9 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.

Lengthy Testing of a K-Band Multifrequency Gyro-TWT With Double-Disk External Reflector

We present the results of experimental investigations of multifrequency oscillations in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{K}$</tex-math> </inline-formula> -band gyrotron traveling-wave tube (gyro-TWT) with double-disk external reflector. The system operation in multifrequency regime was tested for a prolonged period of time with temperature monitoring by an infrared camera. Based on the measurement results, the possibility of stable generation for a period of several tens of minutes at the output power level of more than 2 kW is demonstrated.

Simulation and Measurement of an Interaction Structure With Distributed Radiation Coupling Circuit for a High-Power Ku-Band Gyro-TWT

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%.

Experiment and Power Capacity Investigation for a Ku-Band Continuous-Wave Gyro-TWT

In this article, the design and experiment of a high continuous-wave (CW) power Ku-band gyrotron traveling-wave tube (gyro-TWT) are described. The hot experiment results indicate that the TE11 mode gyro-TWT, driven by a 50 kV–4.5 A electron beam, can produce a maximum CW output power of 49 kW, with an efficiency of 21.8% at 16 GHz. The saturated bandwidth is 2 GHz, and more than 30-kW CW output power is achieved from 15 to 17 GHz. However, the outgassing is observed in the low-frequency range, which limits the enhancement of CW output power. Then, the simulation is performed and indicates that the attenuated power distribution and heat dissipation on the dielectric-loaded circuit (DLC) are dependent on the frequency, attenuation of lossy ceramic, and input power. Also, the maximum temperature of lossy ceramic at 15 and 15.5 GHz is too high for stable operation. A nonuniform DLC is utilized to dissipate power dispersedly. The simulation results show that the maximum power loss on a single lossy ceramic is reduced by 77.9% and 48.9% at 15 and 15.5 GHz, respectively. It indicates that the nonuniform DLC could adjust the dissipation power on the lossy ceramics via the attenuation profile varying with frequency and effectively enhance the power capacity of dielectric-loaded gyro-TWT.

Theory and measurement of the single and hybrid-mode excitation and evolution in a lossy-dielectric-loaded waveguide

ABSTRACT This paper investigates the mode excited, evolved, and propagating properties of the lossy dielectric-loaded waveguide (DLW) used in a Ku-band TE11-mode gyrotron traveling wave tube (gyro-TWT). Theoretical calculation and numerical simulation show that different DLW eigenmodes can be excited by the hollow circular waveguide as a single or hybrid-mode. This study clearly reveals the transformation conditions of the single and hybrid-mode, and the mechanism of the resulting specific dispersion, loss, and electromagnetic field distribution. According to the mode excitation principle and its evolution laws, the propagating properties can be accurately controlled by adjusting the dielectric thickness, which can effectively suppress the potential oscillations and improve the electron bunching phase in the gyro-TWT. As a result, the dielectric loading scheme can be optimized by properly arranging the DLWs, thus enhancing the stability and gain of the gyro-TWT. The theory and simulation are verified by the vector network analyzer measurement at Ku-band, and the results show a good agreement.

Study of the Frequency Self-Modulation in Gyro-TWT Based on Two -Band Amplifiers

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.

Time-Domain Simulation of Helical Gyro-TWTs With Coupled Modes Method and 3-D Particle Beam

A new self-consistent time-domain model for the simulation of gyrotron traveling-wave tubes with a helically corrugated interaction space (helical gyro-TWTs) is presented. The new model links classical methods using the approach of slowly varying variables together with an expansion of the electromagnetic field in eigenmodes and advanced full-wave particle-in-cell (PIC) solvers. The aim is to significantly reduce the required calculation time compared to full-wave PIC solvers, while less strict assumptions are introduced as in the classical approaches of slowly varying variables. For the first time, the classical theory of coupled circular waveguide modes for the description of the operating electromagnetic eigenmode in the helical interaction space is combined with a 3-D PIC representation of the electron beam. This allows the simulation of the beam–wave interaction over a broad bandwidth and at arbitrary harmonics of the cyclotron frequency. In addition, arbitrary electron beams (with spreads, offsets of the guiding center from the symmetry axis, and so on) can be investigated. The new approach is compared with the full-wave 3-D PIC code CST Microwave Studio. A good agreement of the simulation results is achieved, while the computing time is significantly reduced.

Design and Measurement of a Novel Overmoded TE01 Mode Converter for a Rectangular Gyro-TWT.

The rectangular gyrotron traveling wave tube (gyro-TWT) with a large aspect ratio (α) has the potential to achieve megawatt-class output power. As an essential component of gyro-TWT, a novel overmoded Ka-band mode converter with an α of 6.19 is designed, analyzed, and cold tested in this paper. Based on the magnetic dipole moment theory, the rectangular overmoded TE01 mode is excited via the rectangular fundamental TE10 mode. The cutoff waveguide is applied to prevent electromagnetic wave transport to the magnetron injection gun (MIG) region and also guarantee higher power electron beam transportation. Simulations predict an operation bandwidth higher than 4 GHz and greater than 99.8% mode purity between 33–37 GHz. To verify this design, the mode converter is manufactured and cold tested. The back-to-back measurement results exhibit a good agreement with the simulation. With similar topologies, the rectangular overmoded TE01 mode can be excited in a different α.

High Average Power Test of a W-Band Broadband Gyrotron Traveling Wave Tube

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.

Design and Cold Test of a G-Band 10-kW-Level Pulse TE01-Mode Gyrotron Traveling-Wave Tube

Overall design and cold test of a G-band gyrotron traveling-wave tube (gyro-TWT) amplifier are presented. This gyro-TWT aims at achieving a goal of 10-kW pulse output power in the range of 210–220 GHz. It is operated in a circular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode at the fundamental cyclotron harmonics, which is driven by a 50-kV, 3-A gyrating electron beam. Low velocity spread diode-type magnetron injection gun (MIG), multichannel TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode input coupler, broadband metasurface output window, and lossy material loaded beam–wave interaction circuit are simulated and partially measured. Particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a saturated output power over 10 kW in the range of 210–228 GHz with a beam velocity spread of 2.29%.

Waveguide Linear-to-Circular Polarization Converter With Cross Polarization Below −40 dB Within 16% Band

A passive waveguide component with circular input and output ports that converts the incident TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,1</sub> mode polarization from circular to linear and vise versa (a polarizer) is under investigation. Compared with a number of previous researches on this topic, including structures with elliptical, rectangular, truncated cross sections, and rectangular waveguides with periodically perturbed walls, the problem of realization of an ultralow cross-polarization factor (below −40 dB) within the maximum bandwidth is theoretically analyzed in detail based on dispersion evaluation. The solution in the form of a rectangular structure with a pair of corrugated walls with precise mechanical adjustment was manufactured and measured yielding the bandwidth of 16%. A polarizer with such characteristics is extremely important when served as an element of a microwave system for input–output through one window of a high-power high-gain gyrotron traveling-wave tube (gyro-TWT) with a helically corrugated waveguide.

Synthesis of Highly Oversized TE₀₁ to TE₁₁ Mode Converter Based on Elliptical Waveguide

Highly oversized mode converters are the critical devices for high power millimeter-wave transmission systems, characterizing high power capacity and low loss. In order to improve the conversion efficiency from TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode, the mode coupling mechanism of oversized elliptical waveguides is explored, and an optimal ellipticity interval to obtain the high conversion efficiency is deduced by analyzing the mode coupling capacity. Considering that ellipticity can affect the power capacity, breakdown thresholds of different operating modes in elliptical waveguides are studied. Based on the analyses, a synthesis method to design the elliptical mode converter integrating the mode conversion efficiency and the power capacity is proposed. A highly oversized mode converter working in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> -band is designed, and the simulation results demonstrate that the mode converter achieves the high conversion efficiency over 95% in a relative bandwidth of 19.6%. In order to validate the design, a prototype of the mode converter is fabricated, and the high purity of outputted TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode is verified by the measurement. The oversized mode converter designed by the proposed synthesis method features high efficiency, broad bandwidth, and high power capacity, enabling its full utilization of the potential of gyrotron traveling-wave tubes (gyro-TWTs).