Search

To improve the portability of magnets in gyrotron devices, we designed a compact Bitter-type magnet with power consumption optimization theory. This magnet operates at room temperature in a small volume. The theory revises existing electromagnetic theory for non-uniform structural Bitter-type magnets and achieves the lowest energy consumption through iterative optimization. To extend the magnetic field homogeneity region, the ferromagnetic material armature is applied to the Bitter-type system without additional power consumption. Unlike previous manual designs, the proposed Bitter-type magnets can obtain optimal parameters with a significant reduction in computing time. Through the introduction of correction factors, we improve accuracy through multiple verifications of simulations and experiments. On this basis, a room-temperature Bitter-type magnet system for Ka-band fundamental mode gyrotron amplifiers is designed. Its maximum magnetic field strength is 1.1 T, and the length of the homogeneity region is 300 mm. Through optimization, its energy consumption is only 27.5 kW. [ABSTRACT FROM AUTHOR]

Axis coincidence consideration is necessary in the design of gyrotron solenoids; besides, radial field component strongly affects the performance of gyrodevices owing to the transverse particle–wave interaction mechanism in it, especially for high-frequency operation. In this article, the theory of multiple eccentric coils is derived. It is capable of calculating the off-axis field inside the solenoid with specified displacement and tilt. Numerical calculations are validated by 3-D electromagnetic simulation methods. Based on the measured magnet field, the displacement and tilt of a three-coil magnet system are successfully predicted with a global optimization algorithm, and then, the magnet system is well calibrated using a pair of flanges. [ABSTRACT FROM AUTHOR]

This paper developed a method for more accurate permittivity extraction of high-loss materials using the waveguide method to reduce the effect of the air gap. This method improves the Baker-Jarvis algorithm. It makes the errors of S-parameters caused by the air gap compensated by a correction factor β of the form: (S21 + S12) + β(S11 + S22). In our study, the correction results of β for materials on different frequencies and air gap sizes are quantitatively investigated. Furthermore, we propose a method to obtain an optimal frequency-independent β for different materials. A typical high-loss BeO-SiC ceramic commonly used in vacuum electron tubes is analyzed and discussed in detail. It shows that compared to traditionally utilizing only S21 (β = 0), adopting an optimal β can reduce the average errors of the complex permittivity from 17.7% to 7.9% with a 0.10 mm air gap. The proposed method is verified by measurements of unshaved and shaved samples. [ABSTRACT FROM AUTHOR]

In this article, key parameters that characterize field distribution and beam-wave coupling in confocal circuits are studied. The kinetic theory of confocal gyro-TWT considering launching loss is deduced based on it, which is capable of predicting the linear gain rate of confocal circuits with arbitrary mirror widths. Numerical results manifest that the output power/gain versus interaction position agrees well with the nonlinear results in the small-signal regime. In addition, main oscillations in confocal circuits including the near-cutoff oscillation and backward-wave oscillation (BWO) have been examined. Analytical expressions that can predict the near-cutoff oscillation threshold of uniform circuits without considering loss are derived and kz-f plots are introduced for lossy circuits. Furthermore, an algorithm that can automatically search the BWO start-oscillation thresholds of confocal circuits with arbitrary mirror widths has been developed. [ABSTRACT FROM AUTHOR]

In this paper, cold-test experiments to demonstrate the diffractive-wave feedback mechanism in confocal gyrotron travelling-wave amplifiers (gyro-TWAs) are carefully designed and conducted. A broadband HE04 mode exciter with high mode purity and flat transmission is successfully developed firstly, which is then employed to the measurements of various rf circuits. Measured results show good agreement with the simulated ones and the theoretical predictions without considering the feedback. A signal-denoising technology to filter out noises is employed to acquire the net circuit loss. On this basis, the feedback mechanism has been explicitly verified. The experiments provide useful guidance in developing high-performance confocal gyro-devices. [ABSTRACT FROM AUTHOR]

A new topology to excite the HE04 mode in wide bandwidth for confocal gyro-TWAs is presented. The mode exciter consists of a TE04 mode exciter and a TE04–HE04 transition waveguide section formed by two cascaded one-stage tapers. In addition, the closed-to-open confocal waveguide boundaries are studied. Numerical results show that the port reflection of the whole structure is well below −10 dB over 90–110 GHz; the transmission of HE04 mode is better than −1 dB over 94–107 GHz. Even more important, a maximum transmission variation of less than ±0.15 dB over the 1-dB bandwidth and a mode purity of better than 97.2% over the 3-dB bandwidth are achieved. The mode exciter is compact and easy to be fabricated, which can be applied to cold-test experiments of confocal gyro-amplifiers circuits. [ABSTRACT FROM AUTHOR]

The propagation characteristics of a confocal waveguide are related to the radial spheroidal function, which is extremely complicated. A quadratic polynomial fitting (QPF) expression derived from numerical results by interpolation was successfully applied to the development of confocal gyrotrons. However, the QPF expression is valid only for a moderate spheroidal parameter. As a result, it cannot work well for confocal gyrotron travelling-wave tube (gyro-TWT) applications since the Fresnel parameter may be small for stable operations. For the first time, an efficient algorithm is developed to accurately calculate the spheroidal functions. Based on it, the MATLAB code with high accuracy and efficiency is developed to analyze the propagation characteristics of confocal waveguide. The calculated results and the QPF results are compared for different mirror apertures, and then demonstrated by high frequency structure simulator (HFSS). The calculated results show good agreement with the HFSS results for different mirror apertures while the QPF solution agrees with the HFSS results only for large apertures. The code is valid for arbitrary mirror apertures for both the confocal gyrotron and gyro-TWT applications. [ABSTRACT FROM AUTHOR]

Analysis of the performance degradation for W-band lossy ceramic-loaded gyrotron traveling wave tubes (gyro-TWTs) caused by ionization of the released gas molecules is presented in this paper. The gas is released from the lossy ceramics during high average or continuous wave operation and ionized by colliding with the high-energy gyrating electrons. A potential well will be formed by the diffusion and accumulation of the ionization particles after the collision and then degrade the gyro-TWT performance. This process has been simulated based on a simplified model by introducing an equivalent ionization source combining ions and electrons. With a vacuum of 5.1 × 10 - 5 Pa, the theoretically calculated initial energy and equivalent ionization current are 0.01 eV and 0.5 A, respectively. It leads to a deterioration in the output power stability of the gyro-TWT. The performance degradation including spectrum and field pattern caused by gas ionization is also given and analyzed. One of the primary causes is the quality (pitch factor and velocity spread) degradation of the electron beam. According to the simulation and hot test experiments, the vacuum should be maintained below 2.1 × 10 - 6 Pa for stabilizing the amplified operation. [ABSTRACT FROM AUTHOR]

This article presents the development of a dual-state Ku-band gyrotron traveling-wave tube (gyro-TWT) dual-state curved-profile magnetic injection gun (MIG) generating high-quality electron beams, in which the pitch factor is up to 1.2 and 1.35 for the pulse wave (PW) state and the continuous wave (CW) state, separately. A multiobjective genetic algorithm (MOGA) is applied in the profile optimization. Meanwhile, the parametric sensitivity is also discussed in detail. A periodic dielectric-loaded interaction circuit is adopted to suppress oscillations. Furthermore, thermal analyses of the interaction circuit under different duty cycles are also elaborated. The interaction circuit is analyzed in accordance with the proposed nonlinear self-consistent theory and particle-in-cell (PIC) codes. The results of hot experiments show that the dual-state tube obtains the output power of 150/30 kW corresponding to a saturated gain of 40/41 dB and the maximum efficiency of 25%/21% within the bandwidth of 2.0/1.1 GHz for the PW/CW state, respectively. [ABSTRACT FROM AUTHOR]

The phase characteristics of gyrotron traveling-wave tubes (gyro-TWTs) for high-resolution imaging radars have been investigated. Determinant equations of output phase are classified into the cold phase and warm phase in the cases of cold-field and hot-field regions, respectively. The phase–frequency characteristics of cold phase are examined regarding single-mode and multimode operations, respectively. The time stability and the sensitivity to external parameters of warm phase are also investigated. The efficient methods to make the output phase of gyro-TWTs controllable are proposed. The measured output phase of the input coupler and output taper of a Ka-band gyro-TWT is linear with respect to the frequency, without any distortion. Moreover, the warm output phase fluctuation introduced by the RF circuit is less than ±8°. [ABSTRACT FROM AUTHOR]

This paper presents the development of highly efficient gyrotron traveling-wave tube (gyro-TWT) with a nonuniform dielectric-loaded interaction circuit. The high-quality electron beam has been generated by curved-profile magnetron injection gun (MIG). With regard to the profile optimization, simulated annealing algorithm (SAA) and genetic algorithm (GA) are applied. An electron beam with a pitch factor of 1.6 and low-axial velocity spread of 0.9182% have been obtained. The parametric sensitivity is discussed in detail. A nonuniform dielectric-loaded circuit has been employed to suppress both global oscillations and local oscillations. Different thicknesses of dielectric materials have been manufactured and measured, demonstrating the circuit loss of oscillation modes that are enough for stable operation. The interaction circuit is analyzed under the guidance of nonlinear self-consistent theory and particle-in-cell (PIC) codes. The tube achieves a saturated peak output power of 36.24 kW at 93 GHz, corresponding to a saturated gain of 54 dB and a maximum efficiency of 45.3% within a 3-dB saturated bandwidth of 5 GHz. [ABSTRACT FROM AUTHOR]

Backward wave oscillation seriously degrades the stability of gyrotron travelling-wave tubes (gyro-TWTs), especially during high average/continuous wave operation. To solve this problem, a selective mode suppression structure (SMSS) based on the mode coupling principle is proposed and applied in the nonlinear beam–wave interaction region to suppress the parasitic TE11 mode. It is capable of obtaining a high power and improving the tube stability. Simulation results demonstrate that the SMSS can raise the starting current from 10 to 18 A and the starting pitch factor from 1.2 to 1.6. Based on this proposed circuit, a Ka-band TE01 mode gyro-TWT was designed, and the particle-in-cell simulation shows that it can achieve a saturated output power of over 150 kW from 29.7 to 31.7 GHz with a velocity spread of 2.2%. For verification, a SMSS is manufactured and cold tested. The measurement of S-parameters reveals that it can effectively suppress the parasitic TE11 mode. [ABSTRACT FROM AUTHOR]

This paper presents the design and simulation of a broadband Ka-band gyrotron traveling-wave tube operating in the $\text {TE}^{d}_{\text {02}}$ mode at the fundamental cyclotron harmonic with a fully dielectric-loaded circuit. Through loading loss-free dielectric on the entire circuit, the dispersion properties of the circuit are changed to maintain resonance condition at a wide frequency band. A sliced lossy dielectric-loaded structure is employed to provide the required loss for stable operation. The interaction circuit is analyzed under the field matching theory, the nonlinear self-consistent theory, and particle-in-cell codes. A saturated peak output power of 100 kW is achieved at 29 GHz, corresponding to a saturated gain of 44 dB and an efficiency of more than 15% within a 3-dB saturated bandwidth of 6.1 GHz (20% relative bandwidth). [ABSTRACT FROM AUTHOR]

Overmoded-waveguide method (OWM) can effectively alleviate the errors caused by air gaps in waveguide measurements of dielectric properties. In our previous study, both sides of the standard waveguides are enlarged by a same factor (bilateral-OWM). However, as frequency increases to sub-terahertz and terahertz bands, bilateral-OWM will inevitably introduce resonant peaks, which will increase the uncertainty. Therefore, we proposed the unilateral-OWM, which can effectively reduce the occurrence of resonant peaks and improve the measurement accuracy. To verify the feasibility of this method, polytetrafluoroethylene (PTFE) is simulated and measured with bilaterial- and unilateral-OWMs, respectively. Simulation shows that with an overmoded factor of 3.94 and air gap of 0.08 mm, the value of the relative error of εr is basically the same in bilateral- and unilateral-OWMs, but the uncertainty of εr is 0.06% in the bilateral-OWM and 0.01% in the unilateral-OWM. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Design and microwave measurement of a linearly polarized circular TE11 mode coupler are reported in this article. Based on our earlier designed input coupler with smooth wall, we proposed an improved structure with ridges loading resulting in a bandwidth up to 29.2%. Simulation results show the port reflection S11 less than −10 dB is in 7.9–10.6 GHz. And the split transmission S21 and S31are over −3.4 dB in this frequency range. According to the simulation results, the designed input coupler is fabricated and measured with the vector network analyzer. The measurement results show excellent performance, which is in good agreement with the simulation when the conductor loss is taken into consideration. In addition, an input coupler with a cutoff waveguide is designed to improve the efficiency of single-port transmission and the power utilization. Simulation results show the transmission efficiency of the single port achieves 91%. [ABSTRACT FROM AUTHOR]

To set up an automatic hot test platform of gyrotron traveling wave tubes (gyro-TWTs), feedback controls of proportional–integral–derivative (PID) were applied. Simulation of the controlling process of operating voltage on the gyro-TWT cathode had been carried out by MATLAB and then was verified as well as compared by the hot test experiment. The PID control on cathode–anode voltage is typical and similar to the control of beam current achieved by the heater filament. The operating voltage controlling process includes two different stages, i.e., the voltage rise and voltage maintenance phase. By optimizing the operating parameters of the PID algorithm, the simulated voltage rise has a good agreement with the one in the hot test controlling. Furthermore, PID responses after feedback control with sinusoidal and random fluctuations are, respectively, investigated. This paper indicates that the proposed strategy can easily achieve a stable operating voltage. [ABSTRACT FROM AUTHOR]

The theory of multilayer microwave window is investigated and applied to design a broadband high-power output window for a Q-band sheet beam traveling wave tube (SBTWT). The multilayer window consists of a c-axis sapphire disc (the geometry axis parallel to optical axis) in the middle and two identical matching quartz discs tightly attached on both sides. It presents a good multiband wideband and high-power performance from millimeter wave to terahertz range. The thermal property is analyzed and shows a more than 60 kW CW operating capability can be reached. The window is manufactured and cold tested shows a −17 dB reflection bandwidth over 15 GHz (34%). The theory analysis, simulation, and cold test present a good consistency. [ABSTRACT FROM AUTHOR]

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]

A new type of input/output coupler for sheet beam traveling wave tube is designed, analyzed, and measured in this paper. In the design, a special sloped coupling aperture distributed between two paralleled rectangular waveguides in H -plane is employed for compact dimensions, ease of fabrication, attainment of high power capacity, and more convenience for periodic cusped magnet focusing system. A nonlinear-curve profile with Chebyshev-curve distribution is applied to extend the operating frequency bandwidth while suppressing reflections to avoid generating oscillation in the slow-wave structure. Based on the analytical investigation and numerical simulation, an optimized sloping angle of the coupling aperture is obtained to improved performance. Simulations imply that the coupler achieves a broadband bandwidth ( S11< -20 dB) of 23 and 10 GHz in $W$ - and $Q$ -band, respectively, with coupling coefficient above −0.2 dB. Cold test results satisfactorily agree with simulations. [ABSTRACT FROM PUBLISHER]