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Electron-optic systems (EOSs) with high compression are required for THz-band vacuum-tube electron devices to reduce the cathode load, which is necessary to increase lifetime and enable operation in a continuous-wave (CW) mode. However, it is difficult to achieve high compression of multiple electron beams. In this article, we present the design and simulation of EOS with triple elliptic electron beam for a 0.2-THz traveling-wave tube. The triode electron gun with planar electrodes provides $3\times 0.06$ -A electron beam with compression factor of 16. At the cathode, the current density is 26.31 A/cm2 while in the beam tunnel it is higher than 400 A/cm2. The magnetic focusing system with 1.34-T axial magnetic field consisting of Nd–Fe–B permanent magnets and pole pieces is also designed. The numerical simulation predicts stable beam transmission at 25-mm distance.

The electron optical systems (EOS) play a vital role in vacuum electron devices (VED). This letter reported the design and experiments of the sheet EOS for the 1.03 THz traveling wave tube (TWT). A high-current-density sheet electron beam (SEB) electron gun generating up to 20.3-mA at 22.4-kV is designed, based on the requirements of this TWT. Due to the very high operating frequency, more than 1 THz, the miniature tunnel and beam waist with a dimension of 0.05 mm * 0.148 mm and 0.025 mm * 0.114 mm, respectively, will lead to a very high beam current density (906.9 A/cm2). Therefore, a 1-Telsa uniform magnetic system is designed, to ensure such a high-current-density SEB passes through the 27-mm tunnel. Finally, the primary experiments indicate that 7.56-mA current reaches the collector. [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]

Characteristics of annular electron flow formed in electron gun with a multitip field emitter were determined experimentally and by numerical simulations, and the possibility to achieve electron beam current up to 75 mA was demonstrated. The gun performance was stable in technical vacuum conditions—at the pressure of ~10−7 torr. Electron velocity distributions in a flow on transverse and longitudinal components were determined. It was shown that the relative root-mean-square transverse velocity spread varied with radial coordinate within the annular beam wall from ca. 6% to 30%, while the total spread in the electron flow was about 60%. [ABSTRACT FROM AUTHOR]

The electron beam quality of an electron gun is the key factor for the performance of vacuum electron devices. It is reported here that a convex carbon nanotube (CNT) emitter cathode is realized, improving the focus and laminar flow of the electron beam for a cold cathode gun. A mechanical polishing treatment was used to fabricate a convex shaped emitter of the cold cathode from vertically aligned CNTs, and it showed that the convex shaped emitter could effectively overcome the edge emission effect and improve the distribution uniformity. Also, the convex CNT cold cathode was utilized to fabricate a coaxial electron gun, and it presented that the beam spot of the gun is regular and uniform. Compared with the planar CNT emitter cathode, the convex emitter cathode demonstrated better uniformity and compatibility on the quality control of the electron beam. The present results indicate a viable route for improving the electron beam quality of nano-materials’ cold cathode gun. [ABSTRACT FROM AUTHOR]

In this article, the design and experiment of a hollow electron beam (HEB) electron optics system (EOS) for Ka-band extended interaction klystrons (EIKs) are presented. To realize the fast switch of the current emission at a low cutoff voltage, an electron gun with two focus electrodes (FEs) is chosen. The emitting surface of cathode is a ringed spherical surface. The first focus electrode (FE1) is around the cathode, and the second focus electrode (FE2) is set in the central hole of the cathode. The current emitted from the cathode surface is about 2A, when the operating voltage is 25 kV. Three simulation codes, Egun, Superfish, and CST, are used for designing the electron gun and the permanent magnet focusing system (PMFS). Some sensitivity analyses with respect to the PMFS’s critical parameters are presented. The simulation shows the dc beam transmission is 100%. The electron beam, confined by 6000 Gauss uniform field, propagates through the drift tube of 37 mm with good laminarity and small ripple. The emission current can be suppressed when the voltage of the FEs is −3.4 kV. According to the results of simulation, the Ka-band EIK had been constructed and the hot-test experiments had been fulfilled in succession. The experimental results exhibit that the dc beam transmission is about 99% and the beam can sustain 98% transmission at high frequency (HF) operation. In addition, the output power is higher than 8 kW in the bandwidth of over 100 MHz. [ABSTRACT FROM AUTHOR]

In the present article, a multi-cavity W-band gyro-twystron amplifier operating in fundamental TE01 mode has been designed, investigated for its beam-wave interaction behavior, and its performance has been enhanced by optimizing a diode type electron gun with a nominal velocity spread and stagger-tuning technique. The particle-in-cell simulation of the present gyro-twystron has predicted a peak RF output power of ~84 kW with a saturated gain of ~37 dB. The use of staggered RF cavities improved the bandwidth up to 1.5 GHz, which was found to be 2.5 times more than the synchronously tuned W-band gyro-twystron. The power conversion efficiency of the present stagger-tuned amplifier has been calculated as ~22%. A single anode magnetron injection gun has been designed and modeled to form the gyrating electron beam with 65 kV, 6 A, and 4% velocity spread by using 2-D electron optics code. A collector with an effective collecting area and an output window have been designed and studied to extract the remaining energy of spent electrons and collect the RF power from the vacuum environment, respectively. [ABSTRACT FROM AUTHOR]

Gyrotrons operating in high-order modes often suffer from severe mode competition. For the selective excitation of the desired mode, triode-type electron guns are often used. In the case of triode-type guns, where the beam voltage and the mod-anode voltage can be varied independently, the mode excitation during the startup process is studied in this article by using a generalized approach. The term “generalized approach” means that the obtained results can be valid for gyrotrons operating at arbitrary voltages and in any mode. The conditions for excitation of the modes are analyzed for different types of startup scenarios. The goal of the study is to find such relations between the beam and mod-anode voltages during the gyrotron startup that only the desired mode will be excited. It is shown that by using a specific triode-type startup scenario with a proper timing for the rise of the mod-anode voltage with respect to the beam voltage, the initial excitation of the desired mode can be realized in the cases of practically any mode density. The mode interaction in such a case is studied; also, the dependences of the results on the timing relation between the two voltages and on the voltage rise speed are considered. This article also contains the numerical analysis of a specific megawatt (MW) gyrotron with a triode-type gun. [ABSTRACT FROM AUTHOR]

Experiments have demonstrated high-power amplification (100 MW class) from a traveling wave amplifier driven by a modulated electron beam. The modulated electron beam is generated by applying a modulated voltage pulse from a nonlinear transmission line (NLTL) to the cathode of the electron gun. The system showed that over the designed operating range of the disk-on-rod (DoR) TWT amplifier (1.2–1.4 GHz), the peak frequency of the amplifier output tracked the peak frequency of the NLTL to within ±2%; however, the output was not phase stable. Measured RF energy gain for this frequency regime ranged from 1.5 at 1.2 GHz up to 5.0 at 1.4 GHz). [ABSTRACT FROM AUTHOR]

A new approach to realization of a high cyclotron harmonic gyro-oscillator based on the frequency multiplication effect is proposed and numerically studied. This approach utilizes axially asymmetric electron beam optics, which nevertheless may be as simple as the one of the conventional gyrotrons. Numerical simulations show that the proposed method can provide a watt-level output power in THz and sub-THz frequency ranges using existing electron guns. [ABSTRACT FROM AUTHOR]

To improve the performance of a pencil beam electron gun for terahertz traveling wave tubes (TWT), a support vector regression (SVR) method combined with a goal attainment algorithm, which is used to optimize its structural parameters considering the periodic permanent magnetic (PPM) focusing system, is proposed in this article. Moreover, a parameter correction method to reduce the impact of the thermal expansion on the electron gun is proposed. An electron gun for 0.22-THz TWT gun is developed, in which a 30-mA/23.8-kV electron beam emitted by a curved barium–tungsten cathode with a beam loading of 4 A/cm2 is achieved with the optimized parameters. To the high current density for THz beam–wave interaction, the area compression ratio of electron beam is used in beam tunnel and reaches 144. The experimental results are very similar to the simulation results, which indicate that the methods would be beneficial to the development of TWT. [ABSTRACT FROM AUTHOR]

A novel electron-optical system (EOS) for ${G}$ -band traveling-wave tubes (TWT) is investigated by simulation and verified by experiment. The EOS includes a sheet beam electron gun, a tunable periodic cusped magnetic focusing (TPCMF), and a collector. An elliptical focus electrode (FE) featuring a simple geometry and easy fabrication is utilized to compress the pencil beam into the sheet beam with a cross section of 0.4 mm $\times0.1$ mm. The electron gun with a cylindrical cathode of 0.8 mm diameter generates the sheet beam with voltage of 22 kV and current of 115 mA. Based on the sheet beam electron gun, the TPCMF with easy assembly and compact size is then proposed. The experimental results show that the beam transmission is 92.1% within a beam tunnel (its length 21.2 mm and cross section 0.8 mm $\times0.15$ mm). The EOS is suitable for ${G}$ -band TWT. [ABSTRACT FROM AUTHOR]

The need for enhanced performance of high-power RF vacuum electron devices has led to the investigation of multiple-beam, sheet beam, and annular beam configurations. A key issue with such devices is the magnetic field shaping required producing high-power, laminar beams. Field shaping is difficult when Pierce-type gun geometries are employed. The development of high current density cathodes makes the necessary beam power achievable without compression. Such cathodes can operate within a uniform magnetic field yielding advantages for both single and distributed-beam RF devices. However, the quality of the resulting beams presents problems. A project to optimize beam quality in zero-convergence electron guns was undertaken by Calabazas Creek Research, Inc. (CCR) and North Carolina State University (NCSU). The surprising result was that high-quality electron beams can be generated in uniform magnetic fields using convex (dome)-shaped cathodes. The underlying physics involves perturbation of the beam cyclotron motion by a nonadiabatic radial electric field impulse. This article examines this physical mechanism and extends the initial result to additional diode and beam geometries. [ABSTRACT FROM AUTHOR]

Ubiquitous wireless distribution of multigigabit per second data rate for enabling new 5G and 6G paradigms can be only achieved by exploiting the wide frequency bands available in the sub-THz spectrum (90–305 GHz). The high total attenuation at sub-THz, in particular because of rain and humidity, poses a substantial challenge to achieve long links, not yet resolved because of the lack of amplifiers with adequate transmission power. Sub-THz traveling wave tubes (TWTs) are emerging as key components to ensure high signal-to-noise ratio over a large coverage area or for long distance. This article will describe the design and fabrication of a novel TWT for enabling point to multipoint wireless distribution at ${D}$ -band (141–148.5 GHz). To be suitable for the wireless market, TWTs need to be low cost and easily manufacturable for large-scale production. The proposed ${D}$ -band TWT uses a double corrugated waveguide as slow wave structure and a new electron gun, both designed for easy assembly and low fabrication cost. This article describes the design process, the development of the parts of the TWT, and the first prototype assembly. [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]

In this article, a trigger unit has been implemented to generate a high-current-density electron beam from a pseudospark (PS) discharge-based electron gun. A trigger pulse power supply unit has been designed and developed to provide the seed electrons from the ferroelectric cathode (FEC). The FEC has been used inside the hollow cathode cavity of a single-gap PS discharge-based electron gun. The electron gun has been operated in the self-breakdown as well as the trigger-based breakdown mode. A comparative analysis has also been performed between self-breakdown and trigger breakdown modes with the developed trigger unit’s help for different applied gap voltages ranging between 0 and 16 kV. The designed trigger unit is based on a high-voltage-gain isolated dc–dc converter and a pulse-forming network. It is compact and capable of generating a negative pulse of ~–5 kV with a pulsewidth of ~500 ns. It provides better control over PS discharge to produce a high-current-density electron beam. [ABSTRACT FROM AUTHOR]

Nanosecond pulse output at megahertz level is wide interest because of its potential application in the experiment of cell function electric field regulation and accelerator electron gun injector. In this article, a pulse generator based on the combination of the Blumlein pulse forming line and the boost method is proposed. Different from the traditional Blumlein line, in this module, the RLC circuit is used to improve the pulse voltage amplitude and energy conversion efficiency. According to the different inductance values, the generator has two working modes: discontinuous and continuous with different output characteristics. RF-Si-mosfet is used as the main switch and used dual-switch timing logic control to adjust the output waveform parameters. It can obtain the output pulse frequency two times the switching frequency, effectively reducing the single switching loss. By constructing a prototype, the pulsewidth of 5 ns, maximum repetition of 10 MHz and 2.5-kW peak power is realized. In addition, a dc equivalent pulse method is proposed, which is suitable for the thermal simulation of switching at high frequency and gives the heat dissipation measures. The effectiveness is verified by simulation and experiment. [ABSTRACT FROM AUTHOR]

We present experimental characterization of a 20-kV, 130-mA thermionic electron gun, which is a component of a W-band traveling-wave tube (TWT) power amplifier. The electron beam is strongly focused by a 6.6-kG permanent magnet solenoid, producing a peak current density up to 800 A/cm2. The cathode temperature, focus electrode voltage, and modulating anode voltage are varied to characterize the emitted cathode current and beam transport through the beam tunnel of the TWT RF circuit. We observe the variation of the cathode current from 107 to 155 mA and peak beam transmission of 95% measured at the collector. Experimental results are compared to 2-D simulations with the electron gun and collector design code MICHELLE. [ABSTRACT FROM AUTHOR]

The 220-GHz continuous wave (CW) folded waveguide (FWG) traveling wave tube (TWT) is presented as the high-power amplifier for the terahertz (THz) communication system in this article. This tube is designed and fabricated including FWG circuit, electron gun, periodical permanent magnet system, coupling window, and thermal management structure. The test results show that with the pencil beam of 20.5 kV and 52.4 mA, the output power is 16 W and the corresponding bandwidth is 7 GHz from 213 to 220 GHz, and the maximum power and the corresponding gain are 30 W and 31.2 dB at the frequency of 217 GHz. [ABSTRACT FROM AUTHOR]

The SCAPE (SuperConducting Arbitrarily Polarizing Emitter) undulator is under development at the Advanced Photon Source (APS). This new undulator requires a cryostat that will be designed based on expected heat loads. For instance, the expected heating of the beam chamber by electron beam is estimated to be at a level of 182 W – much higher than in planar SCUs. This and other challenges require careful thermal analysis of the LHe-based SCAPE cryostat. A detailed thermal model of the LHe-based SCAPE cryostat has been created in ANSYS. This paper presents calculated cooling capacity and temperatures of the SCAPE cryostat for the static and dynamic heat loads. [ABSTRACT FROM AUTHOR]

Carbon nanotube (CNT) cold-cathodes hold much promise in a variety of millimeter-wave and terahertz vacuum electronic radiation devices due to their inherent near instantaneous temporal turn-on and near-ideal ideal field electron emission performance. Here we report on the development of a CNT cold-cathode Ka-band backward-wave oscillator (BWO). Using a novel beam compression stage, theoretical studies, simulation results, and empirical findings collectively demonstrate that this device affords an unprecedentedly high output power of 230 W at a technologically important operating frequency of 33.65 GHz. The developed magnetic injection electron gun achieves a high emission current of 265.5 mA (emission current density of 188.3 mA/cm2) and a high focused beam current density of 18.5 A/cm2, which our studies suggest, is essential to the BWOs high output power. [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]

This article presents the results of a design study of an electron-optic system (EOS) for a sheet-beam G-band traveling-wave tube. An electron gun providing a 0.1-A, 20-kV sheet beam emitted by a 0.8 mm × 0.8 mm curved cylindrical cathode is designed. The beam dimensions at waist position are 0.05 mm × 0.8 mm with a compression ratio of 16 in the vertical direction. A magnetic system providing 1.1 T magnetic field, which is nearly seven times higher than the Brillouin field, is designed, and beam transmission in the 0.1 mm × 0.85 mm beam tunnel is studied by 3-D particle-in-cell (PIC) simulation. It is demonstrated that the beam has a complex structure of particle distribution, i.e., a high-density central part (core) and a peripheral low-density part (halo). The effect of assembly tolerance on beam transmission is studied. The prototype gun is fabricated, which compresses the beam to 0.1 mm by electrostatic field only. The 135-mA current with 95% transmission through the 0.2-mm anode aperture is measured. [ABSTRACT FROM AUTHOR]

A sheet beam electron gun with high compression ratio and long transmission distance performance for W-band traveling-wave tube (TWT) is presented. A beam focusing electrode (BFE) with two pairs of unequal length outstretched structures is designed to provide equal positions of beam waist in both horizontal and vertical planes, which is beneficial to obtain large compression ratio and keep the beam cross section shape over a long distance. The operational voltage is set as 30 kV. The beam current is 0.505 A, and the corresponding cathode current density is about 5.59 A/cm2. The simulation results show that the electron gun can provide a stable sheet beam under a 0.85-T magnet field. The compression ratio of the obtained sheet beam can be over 35, and the sheet beam can be transported through a 126-mm beam tunnel without interception with a 0.6 fill factor. The parameter sensitivities of the gun are analyzed and discussed as well. [ABSTRACT FROM AUTHOR]

This article presents the design and analysis of a staggered double-vane (SDV) slow-wave structure (SWS) for $W$ -band amplifier, with 20-dB gain and a very high bandwidth (~25%). The use of dual Bragg reflector at either end of the interaction structure increases the impedance matching and the radio frequency (RF) coupling efficiency at the input and output ports, thereby reducing the RF leakage at the electron gun and collector ends, from 15% to 25% to less than 0.6%. The attenuator section is simple to fabricate and optimally designed in order to provide an effective isolation (>20 dB) between the input RF signal in the input section and the RF signal reflected from the output section. The dispersion analysis, the transmission analysis of each section, and the beam–wave interactions were simulated using the Dassault system’s computer simulation technology (CST) eigenmode solver, time-domain solver, and the particle-in-cell (PIC) solver, respectively. The proposed design of the SDV SWS conclusively provides an enormous bandwidth of ~25 GHz with 20-dB gain for $W$ -band amplifier, when compared to its solid-state counterparts and earlier reported work as per the author’s knowledge. [ABSTRACT FROM AUTHOR]

In this article, the design approach for a ${V}$ -band miniaturized oscillator with a pseudospark-based high-current-density sheet electron beam source is presented. The hollow cathode electron gun has been designed and developed. In the experiment, the ~10-A current in the hollow-cathode phase has been measured. The high-frequency structure comprised of a truncated sine waveguide slow wave structure (TSW SWS) with wideband-matched output couplers has been designed, optimized, and simulated. The simulations show good transmission characteristics of the structure. The operation of a backward-wave oscillator with the designed TSW SWS driven by a 10-A, 40-kV sheet electron beam has been investigated by using 3-D particle-in-cell simulations. The maximal output power of 70 kW has been attained in the simulation. Frequency tuning within the 10-GHz frequency band has been demonstrated. [ABSTRACT FROM AUTHOR]

In this article, RF behavior studies on a $W$ -band continuous wave (CW) gyrotron for industrial, scientific, and medical applications are presented. Two different designs are carried out for the output power level of 50 and 100 kW. The mode competition with the first and second harmonic neighboring modes is carefully investigated, and TE6,2 and TE10,4 are chosen as the operating modes for these designs. The cold cavity design and self-consistent computations for power and efficiency are done to determine the geometrical and beam parameters of the desired modes using the in-house code, Gyrotron Design Suite Second Harmonic Version 2018 (GDS2H-V. 2018). The desired output power is obtained for two different designs at the chosen cavity modes. Additionally, electron guns along with magnetic guidance systems are designed to generate an electron beam with the required beam parameters for the two designs. [ABSTRACT FROM AUTHOR]

The design methodology of a sheet beam electron gun has been discussed and, subsequently, used for the design of a gun intended for use in a 0.3 THz traveling wave tube amplifier. The initial gun design parameters have been calculated analytically using a theoretical approach. Then, starting with these calculated parameters, the designed gun is then simulated using a commercial 3-D simulation code “CST Particle Studio” and optimized to get the target beam parameters. The circular-shaped cathode with an elliptically shaped beam-focusing electrode and elliptically shaped anode has been used to produce the good laminar elliptical sheet beam. The effect of varying the several parameters, such as height, width, and distance of focusing electrode, cathode–anode distance, and anode height have been studied to investigate their sensitivity on the target beam parameters. The simulation results predict that the designed sheet beam electron gun produces an elliptical beam with a desired beam cross section of 0.33 mm $\times0.066$ mm at a beam-waist position of 6.61 mm from the curved cathode emission surface corresponding to the beam parameters of 12 kV beam voltage and 55 mA beam current. [ABSTRACT FROM AUTHOR]

The key subsystems of a high efficiency material processing gyrotron are designed at an operating frequency of 27.885 GHz. A triode-type electron gun is designed to launch hollow spiraling electron beam. The outer diameter of the gun is kept at 50 mm to lower inside diameter of the magnet. Start oscillation current for TE02 operating as well as competing modes is plotted. The output power and efficiency are predicted to be 18.4 kW and 46%, respectively, based on single mode time-dependent analytical equation. Beam–wave interaction is studied by employing particle-in-cell (PIC) simulation. Electronic efficiency and output power are increased by operating the tube in hard excitation region. An axially slotted cavity is adopted to abridge mode competitions. The output power and electronic efficiency are obtained to be 20.08 kW and 50.2%, respectively, without considering any velocity spread of the electron beam. However, the efficiency is reduced to 45%, when a moderate transverse velocity spread of 5% is considered. The power efficiency is enhanced to 67.07% by introducing a 3-staged depressed collector. [ABSTRACT FROM AUTHOR]

The Ka-band extended interaction klystrons (EIK) have been developed at the Institute of Electronics, Chinese Academy of Sciences (IECAS), to satisfy the requirement for the high-power millimeter-wave sources in scientific researches. In this work, two kinds of high compression electron gun with the solid or hollow beams, the derivative electron optics systems using the compact permanent magnet uniform focusing and the shared efficient interaction circuit based on the multigap ladder cavity, have been studied in detail and well designed through a mass of calculations. After the mechanical design, finely manufacturing, and a series of technical processes, the Ka-band EIK prototype tubes have been successfully built and tested. For the solid beam scheme, the maximum output power achieves 20 kW in the bandwidth of 60 MHz, and, correspondingly, the gain of 53 dB and the efficiency of 24% are observed. The measured −1 and −3 dB bandwidths are about 220 and 350 MHz, respectively. Meanwhile, the beam transmission is excellent, that is over 97% in dc state and over 90% in RF operation. For the hollow beam case, an over 91% dc transmission test has finished. [ABSTRACT FROM AUTHOR]

In this article, a novel approach for optimizing a practical 270° bent electron gun has been proposed by using JUPYTER NOTEBOOK, an open-source web application utilizing PYTHON script, aiding faster data processing and mathematical computations. For this purpose, a 3-D cost function has been formulated to optimize the position of the anode with respect to the filament which aims at maximizing the beam power density, maximizing beam parallelism or laminar flow and optimizing beam shape. CST Studio-Particle Tracking Module is used for gun design, magnetic field design, and for the study and analysis of generated electron beam trajectories by performing both electrostatic and electromagnetic 3-D gun simulations. Unlike the conventional 270° bent transverse electron guns using Helmholtz coils for generating uniform magnetic fields, here permanent magnets have been used for generating nonuniform magnetic fields making the overall system compact along with electromagnets and magnetic shunts for precise beam maneuvering. Magnetic fields are experimentally measured by using Gauss meter which match well with that of the simulated values. The electron gun was fabricated and trial experiments were conducted under high vacuum environment. Experimental validation was done by puncturing a stainless steel plate kept at 90° and 270° positions above the crucible and the results match well with the simulated results. [ABSTRACT FROM AUTHOR]

A multitube configuration for a gyrotron traveling-wave amplifier (gyro-TWA) that is based on the use of one multibeam electron gun and several parallel helically corrugated interaction waveguides operating with axis-encircling-like electron beamlets at the second cyclotron harmonic is proposed. This new gyro-TWA concept, similar to that for multibeam or clustered linear-beam klystrons, can provide a multifold decrease in the thermal loading and increase in the RF breakdown strength for each tube at the same average or pulse output powers or a significant increase in the output power at a moderate level of the mentioned factors. It is shown that the multibeam electron flow appropriate for such a multitube gyro-TWA can be formed by a single magnetron-injection gun (MIG) using selectively emitting regions on the cathode. In some cases, such a gun can be less sensitive to the influence of space-charge forces and other disturbing factors and generate a beam of better quality than a single-beam cusp gun. In particular, a gun for the ten-tube W-band gyro-TWA, which ensures generation of a beam with a total power of about 1.3 MW, a pitch factor from 1.2 to 1.5, and a root-mean-square (rms) transverse velocity spread of about 8%, was designed and numerically modeled. [ABSTRACT FROM AUTHOR]

This article presents a design of the triode coaxial magnetron injection gun (CMIG) for a 170-GHz MW-class gyrotron. The trade-off equations have been used to initialize the design of gun geometry and beam parameters. The operating mode of gyrotron is TE34,11 at fundamental harmonic. The 3-D electron tracing code CST Particle Studio has been employed to simulate and optimize the CMIG design. An optimum electron beam with transverse to longitudinal velocity ratio $\alpha = 1.3 $ and transverse velocity spread $\delta \beta _{\bot }= 3.29\%$ has been obtained. The sensitivity analysis of electron gun parameters are also investigated including geometric deviations and misalignments. [ABSTRACT FROM AUTHOR]

In this article, the design and simulation of an electron injector based on a thermionic RF-modulated electron gun for particle accelerator applications is presented. The electron gun is based on a gridded thermionic cathode with the geometry based on a Pierce-type configuration. Both theory and numerical simulation were used to explore the relationship between the bunch length and the charge. The reasons for the pulse widening were also analyzed. The beam dynamics simulations showed that a minimum pulselength of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. The average transverse emittance was about 17 mm·mrad from the particle-in-cell simulations. Operating at a higher RF frequency did not significantly reduce the micro pulselength. [ABSTRACT FROM AUTHOR]

We propose an unconventional structure of an electron gun to produce an annular beam for the frequency-tunable gyrotrons. The proposed gun takes advantage of the nonadiabatic electron motion during the acceleration of electrons by placing an emitter on the concave region of the cathode with a relatively weak electric field. The commonly employed adiabatic theory fails to predict the simulated transverse velocity spread, which suggests that the nonadiabatic effect plays an important role during the acceleration of electrons. Simulations are carried out by the EGUN code and verified with the commercial Computer Simulation Technology (CST) Particle Studio. From the change in the kinetic energy and the magnetic moment, we can define a section of the nonadiabatic electron motion. The rapid change in the beam-quality parameters within the nonadiabatic section is associated with the significant change in the electric field near the cathode. By considering the nonadiabatic electron motion, simulations predict appropriate parameters, namely, a pitch factor of 1.5 and a transverse velocity spread of 2.8%, over a wide range of the magnetic field (7.4–8.0 T) and the beam voltage (12–22 kV) with a high structural tolerance on the cathode geometry. The promising results enable the development of frequency-tunable gyrotrons. [ABSTRACT FROM AUTHOR]

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800-MHz dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic (ω = 2ωc) of the electron cyclotron frequency. It produces up to 9.3-W continuous microwave (CW) power at 527.2-GHz frequency using a diode type electron gun operating at V = 16.65 kV, Ib = 110 mA in a TE11,2,1 mode, corresponding to an efficiency of ~0.5%. The gyrotron is tunable within ~0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE11 mode of an internal 12-mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12- to 16-mm i.d. waveguide followed by 3 m of the 16-mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ~84% HE11 mode content. [ABSTRACT FROM AUTHOR]

We developed a new prototype micro-multifocus X-ray source based on electron beam scanning (MMFEBS) for multi-view stationary Micro Computed Tomography (multi-view stationary Micro-CT). This micro-multifocus X-ray source includes an electron gun, electron beam deflection coils, a dynamic condensor, a flat panel transmission target and dynamic vacuum system. The electron beam is controlled by the deflection coils and sweeps back and forth on the flat panel transmission target. It hits a series of accurately-defined positions on the target, resulting in two-dimensional X-ray multi-focus array. A dynamic condensor is utilized to overcome the electron beam defocusing caused by the deflection. It guarantees the same high spatial resolution (better than $4~\mu \text{m}$) at any deflection angle. The MMFEBS, together with a rotational stage and a flat panel detector, produces a multi-view stationary Micro-CT which can obtain multi-view X-ray projection data for CT images in static condition. Micron resolution CT images obtained in our experiments showed that the MMFEBS has high resolution and good repeatability. [ABSTRACT FROM AUTHOR]

In this article, a nonlinear self-consistent, multimode analysis, and particle-in-cell (PIC) simulation of a uniformly dielectric-loaded (UDL) millimeter-wave gyrotron traveling-wave tube (gyro-TWT) amplifier has been discussed. A single-anode magnetron injection electron gun is designed to produce a relativistic electron beam of 100 kV and 5 A with a unity velocity ratio and 5% of axial velocity spread. The desired operating mode TE01 is fed by converting the TE10 rectangular waveguide mode through the Y-shaped power transducer. Both the multimode theory and PIC simulation results are agreed and predicted the saturated RF power of ~140 and ~145 kW at 92 GHz, respectively. A saturated gain of ~51 dB, a 3-dB bandwidth of ~5%, and a power conversion efficiency of ~28% are obtained. To collect the spent electrons, a collector is designed by using electron optics, E-GUN. The maximum heat wall loading at the collector surface due to a spent beam is calculated as 0.255 kW/cm2. [ABSTRACT FROM AUTHOR]

We report the enhancement of carbon nanotube (CNT) field emitters by growing them directly on stripe-patterned alloy substrates. For a gate voltage of 1300 V, CNT field emitters grown on full and stripe-patterned alloy substrates achieved respective emission currents of 0.22 and 2.32 mA, corresponding to tenfold performance enhancement. Performance enhancement is attributed to the reduced screening effect (as a result of the spacing between strips) during field emission and a damage-free patterning process. To verify the ability of the stripe-patterned CNT field emitters to generate X-rays, we designed and assembled an electron gun through brazing. With the open type X-ray system, an X-ray image of human teeth was successfully obtained, verifying the potential of the stipe-patterned CNT emitters in X-ray applications. [ABSTRACT FROM AUTHOR]

The cathode power supply is one of the most important ancillary devices of gyrotrons. Some interesting transient phenomena about the cathode voltage and the cathode current were found in the gyrotron operation in the electron cyclotron (EC) resonance heating system of the Experimental Advanced Superconducting Tokamak. The cathode voltage drops to about 10% of the original value in about $90~\mu \text{s}$ in the overcurrent case, which is much longer than the $25~\mu \text{s}$ in the normal case. In order to explain these phenomena, an equivalent circuit model of the magnetron injection gun was proposed. The equivalent circuit is composed of parallel resistors and capacitors, and it can explain the test results very well. Using the equivalent circuit model to analyze gyrotrons can be an effective means of developing and operating an EC heating system. [ABSTRACT FROM AUTHOR]

This paper deals with optimum design and development of a thermionic electron gun to meet specified beam requirements within defined electric and geometric constraints for a Ka-band traveling wave tube (TWT) for space applications. The electron gun design is based on the Pierce method and carried out according to the iterative process indicated by Vaughan. The design of a periodic permanent magnet (PPM) beam focusing system for the stability of the beam is also required. A sensitivity analysis, by varying electric parameters and geometric parameters, is presented and taken into account as a fundamental role to the aim of optimizing the design of the Pierce gun. A cathode current value of 550 mA at a cathode voltage value of 20 KV with a beam diameter value of less than 0.55 mm, as a result of the analysis, is imposed as a goal design parameter, to obtain a TWT output power value more than 500 W. The simulated results show very good agreement with the beam tester experimental results. The body current measured value, in excellent agreement with the simulated one, shows the effectiveness of the design procedure. Data from the experimental beam tester demonstrate 98% beam transmission to the collector. [ABSTRACT FROM AUTHOR]

Measuring the charge on a nearby space object during close proximity, servicing, and rendezvous and docking operations without requiring physical touch remains challenging despite decades worth of data on spacecraft charging and its risks. This paper proposes the means to identify the charge on a closely neighboring space object and its elemental composition by examining the X-ray spectrum generated by energetic electrons impacting the target. In particular, deconvolution of the bremsstrahlung X-ray continuum provides an estimate of the landing energy of the electrons. Knowing the initial electron energy, the potential difference between the source and the target is determined. Additionally, characteristic X-rays emitted during the process of energetic electron–matter impact allows the relative abundance of elements in the target to be determined. Spatial separations in the order of tens of meters are required between an electron gun and the corresponding detector to maximize the collection of the bremsstrahlung spectrum. This could be achieved with either the single-craft and deployable booms or through the use of two spacecraft. Electron beam energies of 40 kV are found to generate sufficient levels of X-rays for potential determination at over 10 m from the target and to determine the landing energy of the beam to within 0.14% using commercial X-ray detectors. [ABSTRACT FROM AUTHOR]

High-voltage electron guns can commonly be damaged by surface flashover, which can occur across the insulator surface of the gun as a result of electron emission around the triple junction structure. Using secondary electron emission avalanche (SEEA) theory as a basis, the triple junction in a 90 kV electron gun is optimized in this work to avoid damage from the effects of surface discharges. A metal shielding structure is used to reduce the surface flashover caused by electron emission and ensure that the electron gun provides reliable high voltage performance. The aim of the proposed design method is to reduce the field strength that occurs within the electron gun structure. Simultaneously, it is shown that SEEA theory can provide a reasonable explanation for and a solution to the flashover phenomenon that occurs on the insulator surface. In addition, the metal shielding structure used to address the high voltage vacuum discharge phenomenon proves to be an effective design method. Experiments performed on the electron gun for a 90 kV X-ray tube demonstrate that the proposed design method has a favorable effect on control of the flashover phenomenon. [ABSTRACT FROM AUTHOR]

A magnetic-focused X-band traveling wave tube (TWT) based on carbon nanotube (CNT) cathode is presented. The CNT TWT has a simple structure of the electron gun, consisting of a CNT emitter electrode and a ring-shaped anode, with no metallic mesh extracting electrode, nor electrostatic focusing electrode. The field emission electrons are extracted directly by the ring-shaped anode, while the electron beam focusing is realized by an extra magnetic field. Apart from the CNT electron gun, the TWT mainly has a high-frequency interaction circuit and a collector. All these components are totally immersed in a uniform axial magnetic field, and thus, electrons are confined by the Lorentz force during their traveling process from the cathode surface toward the collector. The fabricated CNT TWT was tested in a superconducting system providing a uniform axial magnetic field. The dc mode testing showed about 86% of the emission electrons can reach collector, which is better than a common gate-controlled CNT electron gun. [ABSTRACT FROM AUTHOR]

The nondominated sorting genetic algorithm II and the electron optics simulator were combined to form a multiobjective optimization design tool for electron optics systems of traveling-wave tubes. After determining the objective functions that vary with the macroscopic electrical parameters that need to optimize the electron optics system, the optimization tool is capable of obtaining the optimal solution set automatically. Then, the microscopic electron trajectories of the individuals in the optimal solution set are analyzed to get the final scheme. In the collector optimization, the maximum collector efficiency and the minimum back-streaming current are used as optimization goals. While for the demonstrated electron gun, the maximum waist position is set to be the optimization goal, together with constraints of emitting current and beam waist radius. After analyzing the energy density distribution on the collector inner surface or the laminar flow of the electron gun, the final scheme is determined from the optimal solution set. [ABSTRACT FROM AUTHOR]

The design of a hollow electron beam (HEB) electron optics system for a 10-kW Ka-band extended interaction klystron is presented. To lower the maximum cathode current density and realize the fast switch of the current emission, an HEB with two focus electrodes (FEs) is chosen, which attains to a high area compression ratio of over 100. A 25-kV, 2.5-A beam is produced by a curved spherical segment cathode with the area of 0.321 cm2 and an average beam loading of 8 A/cm2, namely, the high area compression ratio is necessary to reduce the cathode emission current density. The HEB with an area compression ratio of 105 can propagate 40 mm long with good laminarity confined by a 0.7-T uniform magnetic field. The beam current can be quickly shut off when the voltages of the FEs are biased to −4100 V. The results were simulated with two simulation codes: EGUN and OPERA 3-D. A sensitivity analysis with respect to critical parameters such as the displacement of FE1 and FE2 in the radial direction is provided. The simulation results of OPERA 3-D/SCALA and EGUN show an excellent agreement. [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 Hybrid Illinois Device for Research and Applications (HIDRA) is a toroidal fusion device at the University of Illinois at Urbana–Champaign, Urbana, IL, USA. HIDRA is the former WEGA stellarator that was operated at the Max Planck Institut für Plasmaphysik, Greifswald, Germany. The machine is a five-period, $l=2, m=5$ stellarator, with major radius $R_{0}=0.72$ m, and minor radius $a=0.19$ m. Initial heating is achieved with 2.45-GHz electron cyclotron resonance heating and an on-axis magnetic field of $B_{0}=0.087$ T that can go as high as $B_{0}=0.5$ T. HIDRA has the ability to operate as both a stellarator and a tokamak, initially operating in the stellarator mode. The focus of research on HIDRA will be doing dedicated studies on plasma-material interactions (PMIs) using the wealth of knowledge and experience at the Center for Plasma Material Interactions, Urbana, IL, USA. In early 2016, the first experiments were performed on HIDRA. This paper presents some of the first results obtained from the machine such as initial magnetic fields’ measurements and plasma discharges. It also shows the development of the control system being currently implemented and introduces HIDRA-materials analysis tool, the in situ PMI facility that will be mounted on HIDRA in the near future to further enhance the diagnostics and material testing experiments meant to be conducted on the machine. [ABSTRACT FROM AUTHOR]

This paper presents the results of a newly developed 2-D electron beam analyzing code (EBAC) and its capabilities. This finite element cylindrical axis-symmetric code tracks particles by the fourth-order Runge–Kutta method in the local coordinate of elements and calculates steady-state results for both particle trajectories and the emitted current under space-charge limited condition. Two analytical test cases, along with an experimental case are presented to validate the EBAC results. Some techniques have also been developed to augment the basic algorithm of gun codes, dealing with computation of both the emitted current and particle trajectories. These proposed techniques in EBAC have proven to be efficient for reducing convergence time of the basic steady-state algorithm in Pierce-type structures for producing rectilinear electron beam. [ABSTRACT FROM AUTHOR]

Traveling-wave tubes (TWTs) in broadband power-combining applications should have closely matched gain and phase to achieve high system-level efficiency. A gain- and phase-matched TWT operating over a frequency band of 7.5–18 GHz with a minimum output power of 60 W is currently under production at Teledyne e2v. The tube design is based on an existing mini-TWT that incorporates focus electrode switching gun, phase velocity tapered helix slow wave structure, single-stage depressed collector, subminiature A connector at the input, and threaded Neill–Concelman connector at the output. During development and production stages, a number of devices were manufactured and their performances were matched. In this paper, we have described gain- and phase-matching techniques of such devices. Experimental results of 132 matched devices have been collated that show a gain match within ±3 dB and phase match within ±15°. [ABSTRACT FROM AUTHOR]