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In this paper, we present a neural network-based framework to compensate for the nonlinear distortion of high-speed continuous variable quantum key distribution (CV-QKD) systems. In practice, the nonlinear impairment of imperfect devices could bring interference to the measured quadrature values and thus compromise the parameter estimation procedure and the secure key rate. Specifically, for a high-speed CV-QKD system, the balanced homodyne detector (BHD) and the following analog-to-digital converter are the primary sources of nonlinear impairment. The nonlinear distortion effect will be more pronounced as the pulse repetition rate increases. We propose an autoencoder-based network to learn the inner patterns of distorted data and then compensate the quadrature measurements in real-time at the receiver’s side. The numerical simulation demonstrates that the excess noise induced by nonlinear impairment is reduced to the $10^{-3}$ level in the shot noise unit. Consequently, such a software approach has the ability to significantly boost the bandwidth efficiency of the BHD and the secure key bit rate. [ABSTRACT FROM AUTHOR]

High control bandwidth is usually restricted in a photoelectric tracking system (PTS) based on a Charge-Couple Device(CCD) with time delay, which hinders a good tracking performance. Generally, a model-based delay-compensation controller called Smith predictor (SP) can help increase the controller gain to promote the bandwidth by separating delay from the control loop. However, the performance promotion is insufficient because the delay still stays in the forward channel which causes errors between output and input. And the increase of the controller gain is still limited due to the effect of model mismatch on stability. In this paper, to solve the problems, a delay-compound-compensation control (DCCC) based on improved SP by trajectory prediction and velocity feedforward is proposed. The additional trajectory prediction is used to further eliminate the effect of delay existing in the forward channel. The additional velocity feedforward is used to further reform the transfer characteristics limited by the controller gain. A Kalman filter-based design method of trajectory prediction is presented and the optimal design principle of feedback and feedforward controllers is given in the face of model mismatch. Experiments demonstrate that the DCCC is valid and could greatly promote the tracking performance in the low frequency. [ABSTRACT FROM AUTHOR]

In this article, a high-isolation power divider with ultrawideband characteristics for simultaneous operation in two adjacent bands is proposed. The power divider is developed based on a four-port coplanar Magic-T. The difference port of the Magic-T consists of a double-ridge gap waveguide (RGW) that supports quasi-TEM mode transmission, which provides the benefit of its broadband characteristics and allows for facilitated assembly. In addition, an inline microstrip line (MSL) containing a probe is inserted into the opposite side of the difference port to form the sum port. This basic configuration has actually achieved high isolation between the output arms and low return loss at each port, and however, its isolation bandwidth is limited by the narrowband matching of the sum port. To overcome this difficulty, a three-port microstrip T-junction is connected in series to the sum port of the Magic-T, resulting in a five-port 3-dB power divider with a wide isolation bandwidth. The process of considering this solution is analyzed and explained in detail in this article and the corresponding simulations and experimental verifications are performed. According to the obtained results, the proposed power divider has excellent overall performance, in particular low loss, ultrawideband, high isolation, and good amplitude/phase consistency. [ABSTRACT FROM AUTHOR]

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]

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 TE01 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 TE01 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%. [ABSTRACT FROM AUTHOR]

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 TE01 to TE11 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 $W$ -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 TE11 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). [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]

Mode converter is one of the most important devices in the high-power microwave (HPM) applications. To conduct an in-depth research for the orthogonal mode converter, a novel 90° compact TM01-TE11 circular waveguide converter working at 9.7 GHz is proposed in this article. This design innovatively integrates the traditional mode converter with 90° bend techniques based on the mode coupling theory, which ensures a compact structure and high-power handling capacity. Calculated and simulated results illustrate that the maximum conversion efficiency of the converter at central frequency reaches 99.4% when the structure length is 535 mm, and the bandwidth reaches 680 MHz when the conversion efficiency exceeds 95%. For demonstration, a prototype is fabricated, and the corresponding experiments are investigated. According to the $S$ -parameter measurement, it has a high conversion efficiency at central frequency. Meanwhile, the high-power performance is conducted by connecting it with a relativistic backward oscillator (RBWO), and the converter works stably under a maximum output power of 4 GW. The good agreement of the theoretical analyses, simulations, and experimental results indicates the feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

This article proposes a method to excite the TM11 mode in circular waveguides. The principle of synthesizing a circular TM mode is deduced based on the electric dipole moment theory first, and the output mode and its purity are analyzed theoretically. Then, a mode conversion scheme to convert the rectangular TE10 into circular TM11 mode via a pair of out-of-phase quasi-TEM mode is proposed. In order to achieve the broadband excitation in a compact size, the ridge gap waveguide is used as a medium between the rectangular and cylindrical waveguide. Simulation results present that the proposed exciter has a propagation bandwidth up to 10 GHz in Ka-band with a mode purity over 98%. A prototype is manufactured and measured, and the presence of TM11 mode and the mode purity are confirmed by a port radiation pattern measurement. The mode exciter features high mode purity, broad bandwidth, and a compact size, which provides a low-power generator for the research of novel gyrobackward-wave devices and devices in the high-power transmission line. [ABSTRACT FROM AUTHOR]

This article introduces a new type of contact-less electromagnetic bandgap (EBG) structure in the form of an embedded bed of nails. Compared with the traditional bed of nails structure, the smooth upper metal plate originally used to provide the perfect electrical conductor (PEC) boundary is replaced by periodic defect grooves, and the metal pins on the base block are embedded in these grooves without any electric contact. One of the advantages of the embedded EBG structure is that it has a robust tolerance for the height of the air gap. Therefore, this structure provides a cost-effective parallel plate mode suppression solution because it allows a low-precision manufacturing process level while maintaining the stability of the bandwidth. On the other hand, the proposed EBG structure can achieve a stopband range of several octaves. This characteristic makes it indeed suitable for wideband gap waveguides (GWs) to provide horizontal open boundaries. To verify the feasibility of the proposed approach, the new ridge gap waveguide (RGW) (with an embedded bed of nails) and the classic RGW (with a traditional bed of nails) with two 90° bends are designed, fabricated, measured, and compared. The experimental and simulation results are quite consistent and indicate that the embedded RGW has prominent advantages in bandwidth and robustness. [ABSTRACT FROM AUTHOR]

A novel fan-out wafer-level packaging (FOWLP) technology with double-sided redistribution layers (RDLs) and tall copper vertical interconnects is described. The design of a dual-polarized magnetoelectric (ME) dipole antenna based on this new packaging technology is presented. It is shown that the antenna with a size of $10\times10$ mm2 achieves good impedance matching and a stable radiation pattern over a wide bandwidth from 25 to 43 GHz, which can cover most of the millimeter-wave (mmWave) bands of the fifth-generation (5G) mobile networks in different countries. The measurement results demonstrate that the antenna is well suited for FOWLP technology and can be used for phased-array modules in 5G wireless communication systems. [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]

An optimum approach for designing a TE01 mode oversized waveguide bend is proposed based on angular-deformed circular waveguide. Due to the deformation of the circular waveguide cross section, the degeneracy of the TE01 mode and the TM11 mode is removed, and the mode conversion inside the waveguide bend is weakened. Characterizations of the angular-deformed circular waveguide with different deformation and its influence on the mode spectrum, the transmission loss, and the power capacity are investigated. Based on the analysis results and the coupling-wave theory, a Ka-band broadband oversized deformed circular waveguide bend with the propagating TE01 mode is designed, fabricated, and measured. A good agreement between the calculation results and the S-parameter measurement of a prototype is achieved. The transmission efficiency is over 95% in the frequency range of 26.5–35.9 GHz (the relative band is over 30%). The near-field test results further demonstrate the high output purity of the TE01 mode in the working frequency band. [ABSTRACT FROM AUTHOR]

To meet the need to transmit increasingly massive volumes of data, a more than 3 kW continuous wave (CW) Ka-band sheet beam traveling-wave tube (SB-TWT) is reported for the first time. Working under unsaturated drive, the maximum gain obtained is 29 dB with a sheet beam of 30.9 kV, 0.82 A, and the efficiency exceeds 24% with a single-stage collector depressed to −15 kV. The 1-dB bandwidth reaches 6 GHz from 26 GHz to 32 GHz and the bandwidth with output power greater than 2 kW can even reach 7GHz from 26 GHz to 33 GHz. The realization of 99.8% static transmission as well as over 97% actual CW transmission of sheet beam is verified successfully, which benefits from adopting periodic cusp magnet (PCM) focusing, and thereby total weight is less than 15 kg. The relevant experimental details are introduced in this letter. [ABSTRACT FROM AUTHOR]

This article proposes a longitudinal output structure in Ka-band sheet beam traveling-wave tube (SBTWT) with an ultrawideband transmission and high breakdown collector voltage. A novel electromagnetic bandgap (EBG) cutoff structure is designed to separate the slow-wave structure (SWS) and the depressed collector. The large stopband property of the EBG structure is applied to achieve not only the high transmission and the low gap leakage but also the long gap distance to provide a high breakdown voltage of the depressed collector. The 3-D simulations of a single longitudinal structure show a full Ka-band (26.5–40 GHz) reflection of less than −20 dB and gap leakage less than −0.02 dB. The entire longitudinal output collector, including two large gaps cutoff structures and the over mode electron collecting parts, is designed and manufactured. The simulations show a bandwidth of 10.4 GHz with a reflection coefficient S11 of lower than −20 dB. The cold test demonstrates the wild bandwidth of ~11.1 GHz with a reflection coefficient S11 of lower than −15 dB. The hot test reveals a stable collector dc voltage of −14.2 kV. [ABSTRACT FROM AUTHOR]

This letter proposes an inline, ultrawideband, and low loss transition from microstrip line (MSL) to ridge gap waveguide (RGW). An efficient conversion between operational mode of RGW and the quasi-TEM mode of MSL is presented by using suspended line patch as a bridge. Furthermore, the effect of loading a capacitive iris on the transition matching performance is studied and a methodology to tune the transition bandwidth is proposed. In order to validate the presented solution, a back-to-back transition prototype is developed and investigated experimentally. The measured results show an insertion and return loss of 0.4 and 15 dB, respectively, over 107% relative bandwidth in the range of 6.2–20.1 GHz. [ABSTRACT FROM AUTHOR]

In this article, the design and experimental verification of a 200-kW Q-band gyrotron traveling wave tube (gyro-TWT) amplifier are presented. A low-velocity spread magnetic injection electron gun (MIG), a new type of multichannel TE01 mode input coupler, and a broadband double-disk output window are adopted in the design to improve the performance of the gyro-TWT. At the same time, the lossy ceramic ring-loaded high-frequency interaction circuit is constructed to provide the required loss for stable operation, and the potential oscillations are effectively suppressed. Saturated peak power of 208 kW and 12.5-kW average output power are measured at 47 GHz by hot test, corresponding to a saturated gain of 50.2 dB, an efficiency of 29.4%, and a −3-dB bandwidth of 5.5 GHz. [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]

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]

This article proposes a synthesis method based on phase modulation to design broadband oversized smooth-walled horns for high-power millimeter-wave applications. The bandwidth of oversized smooth-walled horn is confined by the mismatch of the relative phase between TE11 mode and TM11 mode. By analyzing the mode conversion along horn, it is found that the phase of the generated TM11 mode can be influenced by the TE11 mode. Therefore, the synthesis method uses a slow waveguide taper, in which the generated TM11 mode is in a small fraction and its phase is efficiently influenced by TE11 mode, to modulate the relative phase of TE11 mode and TM11 mode. Using the proposed method, an oversized smooth-walled horn is designed, of which the phase mismatch is ±10° in the range of 32–38 GHz. The simulation results show that the proposed horn has a highly symmetric far-field pattern with a peak gain greater than 28 dBi in the range of 32–38 GHz and the sidelobe level has −19 dB decline compared with the peak gain. A prototype is fabricated and the near-field pattern measurement is carried out. The high correspondence of the simulated near-field patterns with the measured ones proves that the proposed method is an appropriate option to design broadband feed horns for high-power millimeter wave. [ABSTRACT FROM AUTHOR]

In this letter, a 20-kW-average-power Ka-band TE01 mode gyro-traveling wave tube (gyro-TWT) with a cut-off waveguide section is presented and verified by hot test experiment for the first time. To improve the average output power, the new type gyro-TWT adds a cut-off waveguide in high frequency beam-wave interaction circuits. The large signal nonlinear theory calculations and particle in cell (PIC) simulations show that the new type gyro-TWT has broadband and high-power output performances. The hot test experiment shows that the measured maximum average power is 20.3 kW at 34 GHz, corresponding to a saturated peak power of 169.2 kW, a saturated gain of 52 dB, an efficiency of ~26.9%, and 3 dB bandwidth of 3.2 GHz. [ABSTRACT FROM AUTHOR]

An investigation of the single-frequency iteration method for synthesizing mode converter is taken, and the results show that the synthesized structure has an almost constant perturbation period, which limits the bandwidth of the mode converter. Therefore, a multifrequency iteration method based on variable perturbation periods is proposed to design a broadband oversized TE01-to-TE11 mode converter in this article. To avoid the interference among the perturbation periods corresponding to different frequencies, a weight function is introduced to separate these periods from each other. Using the proposed method, a TE01-to-TE11 mode converter is designed with the conversion efficiency over 95% in the range of 32.5–36.2 GHz (the relative bandwidth of 10.77%). To validate the performance of the proposed mode converter, a prototype is manufactured and measured. In the S-parameters measurement, an average conversion efficiency exceeding 93% and an average ohmic loss near 3% are obtained in the range of 32.3–36.2 GHz. The high purity of circular TE11 mode is demonstrated at the output of the fabricated mode converter according to the measured field patterns. [ABSTRACT FROM AUTHOR]

This article proposes a novel method to design a 90° oversized TE01-to-TM11 mode converter for high-power transmission line. In order to obtain a full power conversion at 90° bend angle rather than an acute angle, the mode coupling process inside the oversized circular waveguide bend is analyzed, and a phase rematching method is deduced. Based on the analysis results, a phase rematching structure, consisting of two identical circular-to-elliptical transitions and a uniform circular waveguide which can transform the phase difference from $-\pi $ /2 into $\pi $ /2, is introduced. Two mode converters with the input/output radius of 14 mm are designed at the center frequency of 34 GHz to validate the proposed method. The results show that the mode converter with a limited height has a bandwidth of 4.6 GHz, while the mode converter with a limited width has 4.3 GHz with conversion efficiency over 95%. The mode converter with a limited height is manufactured and measured, and the high-purity outputs are validated by the obtained radiation field patterns at the output port of the mode converter from 32 to 36 GHz. The proposed mode converter provides a new approach for developing a compact, high-efficiency, and broadband high-power millimeter-wave transmission line. [ABSTRACT FROM AUTHOR]

Metamaterial (MTM) operating on zeroth-order resonance (ZOR) is intrinsically suitable for programmable manipulation: It possesses isolated and periodical configuration capable of individually tuning with employed active element PIN diodes; the ZOR is independent of physical length, providing the freedom to utilize an arbitrary unit cell number. Though realizing versatile functionalities, programmable manipulation is not essentially uniform, and even resonance blindness occurs, since the MTM is usually limited to several units for compact size, and more especially, PIN diodes are utilized as an active element with two discrete states 0/1. In this letter, we study the transition zone where the equivalent impedance of PIN diodes is nonlinear to the supplying voltage between 0.6 and 0.7 V. Taking advantages of this nonlinearity, with a compact two-unit-cell MTM, we achieve the programmable functionalities with uniform manipulation without resonance blindness. The proposed programmable ZOR (PZOR) splitting the spectrum into narrow-bandwidth (NB) channels can be applied to NB Internet of Things for 5G in the near future. Moreover, the PZOR with the active element PIN diode needs only less than 1 V supplying voltage, which is significantly lower than other active elements such as microelectromechanical systems (MEMS) or varactors. [ABSTRACT FROM AUTHOR]

The mode converter is a key component in a gyrotron traveling-wave tube (gyro-TWT) application system, which can transform the output circular TE $_{0n}$ modes (in a gyro-TWT loaded with lossy ceramic dielectric or photonic bandgap interaction structure, the output mode is normally a TE $_{0n}$ mode) into a mode conducive to transmission and emission (linearly polarized Gaussian-like HE11 mode). In particular, the bandwidth of the TE01–TE11 mode converter is the most important challenge in the conversion sequence. In this paper, based on the deformation of the transverse cross section and the modified multi-frequency iteration synthesis technique along the longitudinal propagation axis, a novel method for synthesis of a broad bandwidth mode converter is proposed. Then, an application of the method to a wideband serpentine TE01–TE11 mode converter with an elliptical section is given. This novel mode converter including transitional waveguides achieves a 19.18% relative bandwidth (conversion efficiency over 95%) at center frequency 30.5 GHz, which increases nearly 195% compared with the conventional circular design, and its structure length is 1151.65 mm. Overall, this method offers a novel solution to the requirement of broad bandwidth in the high-power microwave transmission system. [ABSTRACT FROM AUTHOR]

In this paper, converters of TE $^{\Box }_{{10}}$ –TE $^{\circ }_{{21}}$ mode and TE $^{\Box }_{{10}}$ –TE $^{\circ }_{{01}}$ mode with similar exciting topologic structures are designed and verified for a cold test of gyro-TWT. Both the proposed converters are composed of three sections, including TE $^{\Box }_{{10}}$ to TE $^{\Box }_{{20}}$ mode converter with ridged twist waveguide, TE $^{\Box }_{{20}}$ to TE+22 mode converter with variable cross-shaped waveguide, and the last section TE+22 to TE $^{\circ }_{{21}}$ or TE $^{\circ }_{{01}}$ mode converter ($^{\Box }$ represents the rectangular waveguide, + represents the cross-shaped waveguide, and ° represents the circular waveguide). The mode converters can be designed step-by-step and exhibit excellent wideband performance and high conversion efficiency. Simulation results reveal that the proposed TE $^{\circ }_{{21}}$ mode and TE $^{\circ }_{{01}}$ mode converters have less than 0.04 and 0.03 dB loss over 13.5- and 12-GHz bandwidths, respectively, while the return loss is below −25 dB. Prototype TE $^{\circ }_{{21}}$ and TE $^{\circ }_{{01}}$ mode converters were fabricated and back-to-back millimeter-wave measurements were performed to verify the design. [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]

A Ka-band broadband TE01-mode overmoded waveguide bend based on an elliptical waveguide structure is proposed for high-power transmission line in this paper. To investigate the mode conversion inside the waveguide bend, the mode coupling coefficients of elliptical waveguide bend are theoretically deduced. The influence of the cross section on the power-handling capacity and the transmission efficiency is studied. Calculation and simulation results present that the proposed overmoded waveguide bend has a propagation bandwidth of 7 GHz (relative bandwidth of 21%) with an efficiency over 95%. A prototype is fabricated and measured. The average transmission efficiency is over 93% in the range of 29–36 GHz according to the S-parameter measurement. The results of near-field test demonstrated that a TE01 mode with high purity is obtained at the output port of waveguide bend. The high-power performance is investigated by connecting it with a Gyrotron traveling wave tube, and the waveguide bend works stable under a maximum output power of 150 kW. The measurement results demonstrate that a novel approach is proposed successfully for the design of high-power waveguide bend. [ABSTRACT FROM AUTHOR]

A high power Ku-band TE01 mode gyrotron traveling wave tube amplifier (gyro-TWT) is designed and experimented in this paper. The emphasis is put on the gyro-TWT with a high average power. In the design process, a low average velocity spread magnetron injection electron gun and a wideband multichannel TE01 mode input coupler are adopted. At the same time, a high average power curved collector is also applied. To suppress the potential gyro-backward wave oscillator interactions, a new type lossy ceramic rings and lossy ceramic pieces hybrid loaded structure is used in the high-frequency interaction circuit of gyro-TWT. In hot test, the developed Ku-band TE01 mode gyro-TWT is driven by a 70-kV 23-A helical electron beam and produce a ~31.5 kW maximum saturated output average power at 16.2 GHz, corresponding to a saturated peak power of ~450 kW (7% duty ratio), a saturated gain of ~40.5 dB, an efficiency of ~28%. The measured 3-dB bandwidth is ~2.1 GHz. The gyro-TWT is found to be zero-drive stable at these operating points. [ABSTRACT FROM AUTHOR]

The electro-optical tracking system with speedability and maneuverability based on stationary platform is confronted with some problems such as external physical disturbance and detector delay. To obtain better stability, which is the key to achieving high tracking precision, a modified Smith predictor method for inertia stabilization loop control is introduced. This method proposes the Smith predictor control scheme based to compensate delay time caused by the signal filtering of fiber gyroscope sensor in the velocity feedback control, and it utilizes disturbance feedforward control to improve the stability of stabilization platform simultaneously. The design of control structure and analysis of the theory are reported. The experiment results indicate convincingly that the proposed method can effectively improve the bandwidth of velocity closed-loop. Moreover, it can drastically enhance the disturbance suppression performance of tracking platform. [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 design and experiment of a high power and broadband Ku-band gyrotron traveling-wave tube (gyro-TWT) amplifier operating in the TE11 mode are presented in this paper. A new-type broadband rotating TE11 mode input coupler is applied to excite operating mode and a broadband three-disk beryllium oxide ceramic output window is used in the designed gyro-TWT. In order to suppress the potential oscillations, expand working bandwidth, and improve the heat dissipation capability of high frequency system, a periodic beryllium oxide and silicon carbide composite lossy ceramic rings loaded high-frequency circuit with a nonlinear small change output taper is adopted in the gyro-TWT. When the beam voltage is 70 kV and the beam current is 12 A, the experiment of this Ku-band gyro-TWT developed shows that the measured maximum output average power is ~20 kW at the frequency 16 GHz, corresponding to a maximum output pulse power of ~200 kW (10% duty ratio), a gain of 43.0 dB and a maximum efficiency of 23.8%, −3-dB saturation gain bandwidth of 2.6 GHz (~16.3%), and −1-dB saturation gain bandwidth of 1.5 GHz (~9.4%). Under the condition of 10-W constant input power driven, the measured −3-dB constant driven bandwidth is 1.4 GHz (~8.8%). [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]

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]

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]

In this paper a methodology to excite a circular high-order TE13-mode via a rectangular TE50 mode was proposed and verified by the millimeter wave measurement. This topology can also be applied to launch other high-order modes (HOMs) such as TE 1n , TE 2n^\prime and TE 0n^\prime (n or n^\prime =1 , 2, 3) modes via different transition rectangular TE _{m0} ( m = 2n-1$ or $m = 2n^\prime $ ) modes, which can be achieved by adjusting the number of coupling apertures and optimizing dimensions of tapered waveguides. A W-band TE13 mode converter was designed and analyzed. Two identical converters connected back to back as a proof-of-principle operating at $Q$ -band were fabricated and measured. Good performance was observed, which was in good agreement with the simulation results. This HOM converter has many potential applications in areas such as gyro-devices, high-power transmission lines, RF multimode systems of accelerators, microwave/plasma, and antenna systems. [ABSTRACT FROM PUBLISHER]

Novel compact input and output couplers for broadband operation of sheet beam traveling wave tubes are analyzed, designed, and cold tested. Multistage stepped waveguide and cascaded Bragg resonators are proposed and applied to improve the performance and extend the operating bandwidth. Optimized curve-Bragg resonators are studied to correct the phase between the reflector and $H$ -bend waveguide, which bring about a further enhancement of the coupler behavior. HFSS simulations predict a −3 dB transmission bandwidth about 38% and 34% are achieved, respectively, in Ku and $Q$ band. Besides, the large beam tunnel property of this kind of coupler benefit the beam transport of sheet beam devices. Different components as the input and output couplers for Ku- and $Q$ -band SB-TWT are manufactured and cold tested. Results of vector network analyzer measurement and simulation present an excellent agreement. [ABSTRACT FROM AUTHOR]

A methodology to generate a circular TE13 mode for the operation of high-order mode (HOM) gyrotron-traveling-wave tubes is presented in this paper. A rectangular TE10 mode is efficiently converted into a circular TE13 mode using Y-type power dividers. Symmetric curved lossy dielectrics and dielectric rings are, respectively, positioned to suppress the potential coupling TE32 and TE71 modes according to their electric and magnetic field distribution properties. An HOM converter without dielectrics loaded was designed, constructed, and cold tested. Back-to-back transmission measurements exhibit good agreement with the results of computer simulations. [ABSTRACT FROM AUTHOR]

Wider bandwidth property of traveling-wave tubes (TWTs) is continuously pursued to satisfy the increased data rates and multiple channels in modern communications. In this paper, the coalesced-mode operation instead of the general single-mode operation for the double staggered grating waveguide (DSGW) slow-wave structure (SWS) is proposed to achieve an ultrawideband property of the TWTs driven by sheet electron beam. Analytical results of the high-frequency properties for the SWS, including the dispersion characteristics, interaction impedance, and transmission property, demonstrate that the coalesced-mode operation for the DSGW is possible. A novel loading approach of the attenuator with good performance, and meanwhile, a compact configuration is proposed to suppress the potential oscillations, thus leading to a steady beam–wave interaction for the coalesced-mode operation. To verify the concept of coalesced-mode operation, particle-in-cell simulation is performed. Simulation results demonstrate that the optimized RF circuit indeed can steadily operate in coalesced mode and produce over 5-kW saturated output power ( $\sim 10$ % efficiency) in a very broad frequency ranging from 37 to 52 GHz, corresponding to a relative bandwidth of 33.7%. [ABSTRACT FROM AUTHOR]

In this paper, a new type input/output (i/o) structure, adopting the multiaperture coupler, for broadband sheet-beam traveling-wave tube was designed and analyzed. This novel coupling structure not only can get a larger electron passway, which will greatly benefit the transmission of the electron beam, without deteriorating electromagnetic transmission characteristics, but also work over an extremely broad bandwidth. The transmission characteristics of this coupling i/o structure are investigated by Ansoft’s high-frequency structure simulator (HFSS). Operating at TE \(_{10}\) mode, the maximum insertion loss and isolation of the i/o structure over 36–52 GHz, which equals to 36.4% bandwidth, are −25 and −20 dB, the transmission coefficient is above −0.3 dB. The results of HFSS were verified by Computer Simulation Technology Microwave Studio. [ABSTRACT FROM PUBLISHER]

Gyrotron traveling wave tubes (gyro-TWTs) are of considerable interest for high-power millimeter and submillimeter radiation sources. The analytical design and experiment of a W-band gyro-TWT loaded with nonuniform periodic lossy dielectric rings is presented in this paper. The gyro-TWT is operating in the circular TE01 mode at the fundamental cyclotron harmonic, and driven by a 60 kV, 8-A gyrating electron beam with velocity ratio 1.2. Hot test results were: 1) maximum peak output power 112 kW; 2) 69.7-dB saturated gain; and 3) 23.3% efficiency at 93.5 GHz. The measured bandwidth with peak power greater than 90 kW is about 4 GHz in the range of 93–97 GHz. The hot test results show excellent agreement with the theoretical prediction. The potential backward wave oscillations involving the spurious modes TE11, TE21, and TE02 are effectively suppressed by the nonuniformly loaded periodic lossy dielectric rings. The W-band gyro-TWT is zero-drive stable at the operating points. It demonstrates that a nonuniform loaded lossy dielectric structure is excellent for suppressing potential spurious oscillations. [ABSTRACT FROM AUTHOR]