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Investigation of absolute instability (AI) and convective instability (CI) of the electron cyclotron maser has independently led to the birth of gyrotron oscillators and amplifiers. Here, it is demonstrated that these instabilities can form a cooperative relationship owing to the nonlinear behavior of the stimulated electron beam. The CI can be induced in a zero‐drive system with the assistance of AI, and the ohmic losses of all the excited waves inside the system are greatly reduced, which is called "instability entanglement" here. According to the theoretical and experimental study of a 167/330 GHz gyrotron, when instability entanglement occurs, the ohmic dissipation decreases to one‐ninth of the AI‐only condition, and the output power is enhanced by 20%. This discovery is promising for surpassing the ceiling of output power and frequency of gyrodevices placed by ohmic losses. [ABSTRACT FROM AUTHOR]

In this article, the design of a fourth-harmonic gyrotron with 1-kW-level output power at 1 terahertz (THz) is presented. A unique advantage of this design is that, with a well-optimized magnetic-cusp large-orbit electron gun, the designed gyrotron can operate from first-harmonic to fourth-harmonic in multiple discrete bands by varying the confining magnetic field strength. By carefully balancing the competing modes, the gyrotron can be tuned to operate at six candidate modes, including TE1,2 for fundamental harmonic, TE2,3 and TE2,4 for second-harmonic, TE3,5 and TE3,6 for third-harmonic, and TE4,8 for fourth-harmonic interactions. As the main operating mode, the TE4,8 can generate a peak output power of 1.68 kW, with an output efficiency of about 2.1%, and a magnetically controlled frequency tuning range of about 1.5 GHz around 1 THz. The impact of the longitudinal nonuniformity of the cavity at high-harmonic interaction was also studied. It shows that a radial tolerance of several micrometers will significantly elevate the start-oscillation current and deteriorate output performance. This design is to produce a THz source with ultra-wideband tuning capability ranging from the submillimeter-wave to 1-THz bands. [ABSTRACT FROM AUTHOR]

High-quality axis-encircling electron beam is essential to drive a high-harmonic gyrotron. A magnetic cusp gun that can generate a low-spread axis-encircling electron beam at an extremely high magnetic-compression ratio was developed for a 1-THz fourth-harmonic gyrotron toward continuous-wave operation. The optimized gun could generate an axis-encircling electron beam with 80-kV voltage, 1-A current, and a nearly constant pitch factor of about 1.5 in a wide range of magnetic field strength. The transverse velocity spread was less than 2.1% and the guiding center radius was $26.6 ~\mu \text{m}$. The design was verified by using MAGIC, EGUN, and CST simulations and good agreements were achieved. [ABSTRACT FROM AUTHOR]

Terahertz (THz) vortex beams carrying orbital angular momentum (OAM) with high purity and tunable topological charge (TC) will undoubtedly bring extraordinary capacities for advanced imaging or communication systems. We propose a convenient and efficient method to generate a tunable and broadband vortex beam that is excited by superradiant Smith-Purcell radiation (SSPR) on a helical grating. This scheme fully explores the advantages of natural broadband evanescent wave carried by the electron beam and the Bloch's theorem regulated helical periodic systems. An explicit relation is established between the index of the spatial harmonic wave on the grating and the topological charge of the vortex beam. The electron energy in the SSPR can be customized to ignite the specific spatial harmonics and manipulate the OAM beam, accordingly. The separated radiation region also promises the high purity of the OAM spectrum. The harmonic excitation reduces the communality between the wavelength and the device size and alleviates the difficulty of device fabrication in the THz band. The proposed vortex scheme can not only be handily scaled to microwave and mid-infrared regions, but also bring possibilities to applications based on compact tunable vortex beam sources. [ABSTRACT FROM AUTHOR]

The electron beam of a K-band space traveling-wave tube (TWT) has been investigated in a beam measurement system in this article. The primary beam emitted from a Pierce-type electron gun is scanned by a Faraday cup probe, and the beam current density distribution and envelope are determined. The spent beam is also diagnosed with a novel retarding field energy analyzer (RFEA) by using the monopulse measurement method, and a comparison of velocity distribution between the experimental result and simulation is presented. [ABSTRACT FROM AUTHOR]

A medium-power broadband-tuning terahertz (THz) gyrotron operating at low voltages is especially attractive for applications in biomedicine. This article tends to elaborate on the challenges related to startup oscillation and ohmic loss suppression of an extremely low-voltage THz gyrotron. High interaction efficiency is achievable by applying long transit time of electrons to compensate the unfavorable decline of the beam-wave coupling strength for low-energy electron beams. A cavity structure which favors backward-wave interaction is proposed to extend the frequency tuning range. The theoretical study predicts that the optimal cavity contributes to the robust frequency tunability at low voltages. The results indicate that a continuous tuning range of 10.7 GHz around the 330-GHz band with output power higher than 1 W can be expected by utilizing a 0.3 kV, 0.5 A helical electron beam. [ABSTRACT FROM AUTHOR]

Conventional gyrotron backward-wave oscillators (gyro-BWOs) operate in a low-order mode, e.g., TE $_{0,1}$ mode. As the operating frequency extends to the terahertz (THz) band, the transverse size of low-order mode cavity shrinks, and the power capability is reduced, consequently. A solution to adopt an overmoded interaction cavity with a significantly enlarged index of the operating mode is valid on the condition that the challenging problem of mode competition can be controlled during the broadband frequency tuning. In this paper, a high-order whispering-gallery mode (WGM) THz gyro-BWO with a cathode-end output circuit is investigated. A segment-tapered circuit is applied to suppress the ${Q}$ factors of competing modes and to obtain a the competition-free stable start-oscillation scenario. The theoretical result predicts that the effective frequency tuning range continuously covers between 252.3 and 260 GHz when the ${B}$ -field is changed from 9.41 to 9.96 T. Our studies are beneficial to the development of high-performance sources for THz biomedical and material science applications. [ABSTRACT FROM AUTHOR]

The dispersion relation of relativistic rectilinear electron beam propagating along a guide magnetic field in a dielectric is investigated by cold fluid model. In such a system, due to anomalous Doppler effect, the instability occurs when the electron velocity exceeds the wave phase velocity. The growth rate and spatial growth rate are studied analytically and the nonlinear saturated efficiency is given analytically for the first time. Numerical results show that the saturated efficiency approaches about 10%–30%. The distinctive interaction mechanism is promising for the design of a new kind of compact high-power microwave generation devices. [ABSTRACT FROM AUTHOR]

Gyrotron oscillator, as one kind of efficient terahertz (THz) source, is developed with the capability of frequency and power tuning to meet diverse application demands. Due to system nonlinearity, it is quite difficult to achieve high frequency stabilization (FS) during power tuning without using external auxiliary components. This paper proposes an FS scheme with simultaneous power tuning in gyrotron backward-wave oscillator state. This is realized by compensating the detuning electron cyclotron frequency during the electronic tuning process. The required quasi-linear relationship between the electron-beam pitch factor and the accelerating voltage is numerically verified by using the SLAC EGUN code. The investigation focuses on a Watt-level open-cavity gyrotron oscillator. The tens-of-Watts-level output power can be tuned by over 40% at the FS point, and especially the self-adaptive FS reaches the accuracy of about 10-MHz level. The proposed scheme would be attractive for the high-stability THz-wave heating in material sciences and biomedicines. [ABSTRACT FROM AUTHOR]

Many recent biomedical applications use medium-power frequency-tunable terahertz (THz) sources, such as sensitivity-enhanced nuclear magnetic resonance, THz imaging, and biomedical treatment. As a promising candidate, a low-voltage gyrotron can generate watt-level, continuous THz-wave radiation. In particular, the frequency-pulling effect in a gyrotron, namely, the effect of the electron beam parameters on the oscillation frequency, can be used to tune the operating frequency. Most previous investigations used complicated and time-consuming gyrotron nonlinear theory to study the influence of many beam parameters on the interaction performance. While gyrotron linear theory investigation demonstrates the advantages of rapidly and clearly revealing the physical influence of individual key beam parameters on the overall system performance, this paper demonstrates systematically the use of gyrotron linear theory to study the frequency-pulling effect in a low-voltage gyrotron with either a Gaussian or a sinusoidal axial-field profile. Furthermore, simulations of a gyrotron operating in the first axial mode are carried out in the framework of nonlinear theory as a contrast. Close agreement is achieved between the two theories. Besides, some interesting results are obtained. In a low-current sinusoidal-profile cavity, the ranges of frequency variation for different axial modes are isolated from each other, and the frequency tuning bandwidth for each axial mode increases by increasing either the beam voltage or pitch factor. Lowering the voltage, the total tuning ranges are squeezed and become concentrated. However, the isolated frequency regions of each axial mode cannot be linked up unless the beam current is increased, meaning that higher current operation is the key to achieving a wider and continuous tuning frequency range. The results presented in this paper can provide a reference for designing a broadband low-voltage gyrotron. [ABSTRACT FROM AUTHOR]

A minigyrotron scheme controlled by a compact pulse magnet to excite broadband terahertz (THz) radiation is presented here. In comparison to an open-cavity circuit, the adopted prebunched backward-wave interaction circuit can expand tuning bandwidth tenfold under the control of time-varying magnetic field strength, which also significantly extends the available duration time of the pulse magnet for gyrotron operation. A quasi-optical mode convertor and a Brewster window constitute the output system to transfer the broadband radiation from the circuit into free space. A systematic gyrotron design is also presented. Driven by a low-voltage electron beam, the minigyrotron is predicted to generate radiation with 10-GHz tuning bandwidth around 0.33 THz and a maximum peak power of 2.1 kW with 6-ms pulse duration, using a TE6,2 mode interaction. Such a THz gyrotron with broad tunable bandwidth, kilowatt level power, and with the unique advantage of a compact configuration is the key to high-power THz scientific and industrial applications. [ABSTRACT FROM PUBLISHER]

Spoof surface plasmons (SSP) has become an active research topic in microwave and terahertz (THz) spectrum since its extraordinary optical and physical properties. The strong near field of SSP mode on the corrugated metal surfaces makes it especially attractive for developing a THz electronic source. A THz electronic source based on the efficient generation of SSP modes on the doubly corrugated metallic waveguide is proposed and studied in this paper. The analytical dispersion relations of SSP modes are obtained based on a modal expansion method and the field profiles of SSP modes are also presented by the finite integration method. Besides, the interaction between SSP and injected electron beam is modeled and implemented by particle-in-cell (PIC) simulation based on finite difference time domain algorithm. The gap size between the doubly corrugated metal surfaces can significantly influence the output power and PIC simulation results reveal that output power can be increased from 272 mW to 36.5 W when the gap size decreases from 90 to 40~\mu \textm at the frequency near 1 THz by the 19.55 kV, 1 A injected electron beam within 4.5-mm interaction length. The dependencies of the output performance on electron beam parameters are also investigated and we find that there is an optimized beam voltage for the given operation frequency. Various electron beams of pulse and direct current electron beam are studied and we find that half pulsewidth of periodical electron beam is more preferable than other emissive shape of injected electron beam for the given structure. Our studies on the efficient generation of SSP modes on the doubly corrugated metallic waveguide may provide a new way to develop THz electronic sources. [ABSTRACT FROM AUTHOR]

A terahertz electronic source based on the spoof surface plasmon (SSP) with 2-D subwavelength metallic grating is presented. The SSP dispersion relation of plasmonic grating is derived by a simplified modal expansion method, and the coupling and interaction between the SSP and the electron beam is studied by particle-in-cell simulation. The results reveal that the output performance highly depends on the location of electron beam from grating surface. For an injected electron beam with 19.15 kV and 0.5 A, the SSP output power can reach 22.7 W for the optimized distance of the beam from the grating surface at a frequency near 1 THz for the given structure. Besides, the influence of different electron beam parameters on output power is also investigated and we find that pulse electron beam is preferable than continuous electron beam for good performance. There is an optimized operation frequency for the given beam voltage. Furthermore, output performance can be improved by changing grating structure parameters. By decreasing the grating groove filling factor from 0.8 to 0.2, the SSP output power can be increased from 17.2 to 23.6 W. The SSP power can also be significantly enhanced from 14 to 28.6 W using shallow grating with a groove depth changing from 76 to 56 \mu \textm for the optimized operation frequency with the same electron beam. The present work may provide a new avenue to obtain powerful THz electronic sources. [ABSTRACT FROM AUTHOR]

The requirement of strong magnetic field is one of the major difficulties for terahertz gyrotrons. A plausible solution is to operate at higher cyclotron harmonic denoted as s , in which the magnetic field strength is reduced to 1/ s of the value for the fundamental harmonic operation. This paper presents a systematic theoretical investigation of a fourth-harmonic 400-GHz gyrotron backward-wave oscillator with relatively high efficiency. An axis-encircling electron beam is employed to suppress the mode competition. The operating mode is the TE41 mode. The efficiency and bandwidth are optimized for the magnetic field tuning. Simulations suggest that the fourth-harmonic circuit is capable of achieving highest interaction efficiency $\sim 6.5$ %, and tunable bandwidth 2.8 GHz at 400 GHz. The weak beam-wave coupling and serious Ohm loss on the circuit wall limit the overall performance. [ABSTRACT FROM PUBLISHER]

In this paper, a continuous wave (CW) low-voltage fundamental harmonic W-band gyrotron has been designed and simulated with the help of a particle-in-cell (PIC) software CHIPIC. The output power of 33.4 kW with an electron efficiency 37.1% in the PIC simulation is obtained by the designed conventional gyrotron employing a 30-kV, 3-A electron beam which is produced by a triode magnetron injection gun. In addition, a high-power capacity collector being one of the key components of CW gyrotrons has been evaluated and tested. The peak power density on the collector wall using the spreading electron system is about 390 W/cm^2 in the hot test. The CW output power of 15.8 kW is achieved with the condition of a 29-kV, 1.8-A electron beam in the hot-test experiment of the designed prototype. [ABSTRACT FROM AUTHOR]

A gyrotron capable of both frequency and power tuning is a promising coherent millimeter-THz wave source. A self-consistent nonlinear theory is applied to investigate the electron cyclotron interaction between electron beam and wave modes of axial nonfixed profiles in an extended W-band TE01 mode cylindrical cavity. It is revealed that tuning the magnetic field strength can excite electron cyclotron resonances on forward wave, backward wave, and even simultaneous on both waves, which makes the system operate under distinctive states, namely the gyrotron backward wave oscillation state and the gyromonotron state. In this paper, a W-band prototype gyrotron oscillator based on an extended cylindrical waveguide cavity is built, and the experiment test indicates that the system starts oscillation in a relative wide range of the operation parameters. The measured frequency spectrum reveals the system iteratively switches between the lower order instability axial modes, and it operates under nonstationary oscillation states. The experimental measurement of highest output power \sim8~kW is consistent with the theoretical predictions. An optimized gyrotron circuit with efficiency exceeding 20% and tunable bandwidth over 10 GHz is also presented. The free oscillation behaviors revealed in this paper provide interesting guidance for developing tunable gyrotrons in millimeter-THz wave range. [ABSTRACT FROM AUTHOR]

A double-grating rectangular waveguide system with a round beam channel is proposed as the slow-wave interaction structure of a terahertz (THz) backward-wave oscillator (BWO). Applying the latest microfabrication techniques, the interaction structure is fully realizable and easily assembled. The basic design principle including the beam-wave dispersion relation, interaction impedance, and the system configuration is presented. A detailed sensitivity analysis of this THz BWO circuit is carried out to investigate the structure errors caused by fabrication and brazing. Both upstream and downstream ends are loaded with step tapers to improve the voltage standing wave ratio. The downstream reflection is absorbed by a linear attenuator. The round electron beam is generated by a pierce gun with compression ratio of 15. A 3-D particle-in-cell simulation demonstrates a broad electronic tunable bandwidth, a 3-dB bandwidth exceeding 28 GHz with highest output power 16 W, and interaction efficiency 3.57%. [ABSTRACT FROM PUBLISHER]

An approach to the enhancement of Cherenkov radiation source driven by two electron beams (TEBs) is presented in this paper. The Cherenkov radiation source is composed of a cylindrical grating, and TEBs with a velocity separation are concentric and separated. By a field matched method, the dispersion equation is derived with a linearized fluid theory and solved by numerical calculation. The numerical results show that a considerable enhancement of growth rate is realized at an optimized voltage ratio due to two stream instability. Moreover, with a help of 2-D particle-in-cell simulations, the improvement of output power induced by the two stream instability is demonstrated, and the decreased of output power is observed as the increased distance of two electron beams. [ABSTRACT FROM PUBLISHER]

Using a self-consistent nonlinear code and particle-in-cell simulation, the beam-wave interaction of a W-band four-cavity gyroklystron amplifier has been analyzed. The gyroklystron amplifier operates in the fundamental harmonic TE01 circular electric mode. The dependence of the efficiency and gain on the input power and the electron velocity ratio are studied. The effect of the electron velocity spread on the bandwidth and the electronic efficiency is analyzed. The simulated results show that the designed stable gyroklystron amplifier can produce an output power of over 142 kW, 33.8% electronic efficiency, 37 dB gain, and a 3 dB bandwidth of 1 GHz for a 70 kV, 6 A electron beam. [ABSTRACT FROM AUTHOR]

The design of a ka-band gyrotron traveling wave (gyro-TWT) amplifier is presented. The gyro-TWT amplifier with a severed structure operates in the fundamental harmonic TE01 circular electric mode. The beam-wave interaction is studied by using a particle-in-cell (PIC) code. The simulations predict that the amplifier can produce an output peak power of over 155 kW, 22% efficiency, 23 dB gain, and a 3 dB bandwidth of 2 GHz for a 70 kV, 10 A electron beam with an axial velocity spread Δvz/vz=5%. [ABSTRACT FROM AUTHOR]

A novel broadband millimeter-wave microstrip reflectarray antenna with square-annular patches and combined patches has been presented, aiming to solve the problem of how to obtain a full 360° range of phase shifts for single-layer. The choice of these two-type elements can make the phase shift smoother and increased up to 360°, leading to broader bandwidth. Based on this technique, a single-layer microstrip reflectarray antenna operating at 94 GHz has been designed. The computed results demonstrate the agreement of the main beam steering with the design requirement, and a 1-dB gain bandwidth close to 10% is obtained. The simulation results verify the feasibility of this design. [ABSTRACT FROM AUTHOR]

In a magnetic cusp gun, the canonical-angular- momentum (CAM) spread of the initially emitted electrons is crucial in generating substantial beam velocity spread. A new method called electron beam flow-feature compensation is proposed to build an axis-encircling electron beam with zero velocity spread by optimizing the cross-flow trajectories to compensate for the initial CAM spread. This method provides an effective solution to the velocity-spread problem in terahertz gyrotrons and increases the emission current to a level that is several times higher than the level that can be obtained using current technology. [ABSTRACT FROM AUTHOR]

In this paper, a new compact retarding field energy analyzer (RFEA) is designed for diagnosing electron beams of a K-band space travelling-wave tube (TWT). This analyzer has an aperture plate to sample electron beams and a cylindrical electrode to overcome the defocusing effects. The front end of the analyzer constructed as a multistage depression collector (MDC) structure is intended to shape the field to prevent electrons from being accelerated to escape. The direct-current (DC) beams of the K-band space TWTs with the removing MDC can be investigated on the beam measurement system. The current density distribution of DC beams is determined by the analyzer, while the anode voltage and helix voltage of the TWTs are 7000 V and 6850 V, respectively. The current curve’s slope effect due to the reflection of secondary electrons on the copper collector of the analyzer is discussed. The experimental analysis shows this RFEA has a good energy resolution to satisfy the requirement of beam measurement. [ABSTRACT FROM AUTHOR]

Abstract: A compact Faraday cup reported in this paper is designed to investigate the current distributions of the electron beams of Traveling Wave Tube’s guns. It consists of a 0.06 mm thick molybdenum aperture plate and a copper shield with a graphite collector inside. There are plates with different laser-cut aperture holes that were 100 μm, 50 μm, 20 μm and 10 μm in diameter for measuring electron beams with different sizes. The thermal analysis of the Faraday cup with pulse beam heating was performed and discussed in this paper. The pulse test shows that this device has fast response and small dissipation. A 0.58 beam perveance (μP) electron gun with expected minimum beam radius 1.0 mm was measured with the Faraday cup and the three-dimensional current density distribution and beam envelope were presented. The experiment results show that the design is reasonable for measuring the electron beam with a high resolution. [Copyright &y& Elsevier]