Liang Zhang

In this paper, we present an improved method for synthesizing smooth-profiled horns. A direct connection between the mode compositions and far-field parameters was established by utilizing a parameterized aperture field distribution and the field decomposition method. It speeds up the optimization process by a factor of 5. The requirements for the aperture field to maintain a constant directivity were studied based on the method. A feedhorn was optimized for a W-band gyro-TWA with a 0.7 dB variation of the directivity within a bandwidth of 10%.

A novel double nonparallel staggered grating (DNPSG) slow wave structure (SWS) is proposed to enhance the coupling impedance in a ${W}$ -band backward wave oscillator (BWO) driven by a pseudospark-sourced sheet electron beam. The DNPSG SWS has been shown a broadband of 72–125 GHz and a higher coupling impedance compared with the traditional double staggered grating (DSG) SWS in simulation. The DNPSG BWO structure consisting of ten SWS units and a broadband output structure is designed and fabricated. In the cold test of the DNPSG BWO, the measured ${S}_{11}$ (double of the losses in the DNPSG BWO) is above −10 dB in most of the band, which is satisfactory in the pseudospark-driven high-power device. The hot-test performance of the DNPSG BWO is analyzed by particle-in-cell (PIC) simulation in a beam voltage range of 14–90 kV and a current density range of 1.5– $5\times10$ 7 A/m2, obtaining a high output power (max. 190 kW) over an ultrawide tuning band of 38 GHz (75–113 GHz) due to enhanced coupling impedance.

This article presents the dispersion curve study of the helically corrugated waveguide (HCW) using the 2-D finite element method (FEM) method based on the helicoidal coordinate transform. Calculation of the dispersion curve is 60 times faster compared with 3-D FEM simulation while maintaining similar accuracy. The 2-D method was used to investigate the relative error between it and a 1-D analytical method based on perturbation theory. Over a narrow parametric range, the 1-D method has a similar dispersion (within a relative error of 3%) in comparison to the 2-D/3-D methods but the error greatly increased when the parameter space was expanded. Correction factors were determined to greatly enhance the 1-D method accuracy over the wider parametric range. Assisted by the improved 1-D method, it allows identifying the desired operating mode automatically within the multiple eigenvalues from the 2-D simulation results. Therefore, it can be integrated into an optimization routine to explore the applications of the HCWs at wider parameter ranges.

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 TE _1,2 for fundamental harmonic, TE _2,3 and TE _2,4 for second-harmonic, TE _3,5 and TE _3,6 for third-harmonic, and TE _4,8 for fourth-harmonic interactions. As the main operating mode, the TE _4,8 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.

Microwave undulators (MUs) have great potential to be an alternative solution to permanent magnet undulators in a free electron laser (FEL) when shorter undulator periods are required. In this paper, the factors that affect the choice of the high-power drive sources were studied via a Ka-band cavity-type MU with a corrugated waveguide proposed for the CompactLight X-ray FEL. They include the technology of the high-power vacuum electronic devices, the quality factor of the MU cavity that was demonstrated by prototyping a short section of the MU structure, and the beam dynamic study of the electrons’ trajectories inside the MU. It showed that at high beam energy, a high-power oscillator is feasible to be used as the drive source. At low beam energy, the maximum transverse drift distance becomes larger therefore an amplifier has to be used to minimize the drift distance of the electrons by controlling the injection phase.

A free-electron-based vortex Smith-Purcell radiation with the higher-order topological charge is proposed in this paper based on uniform helical grating. Due to the spiral constraint of azimuth, the evanescent field of the electron beam will be diffracted and possess the orbital angular momentum. When the wave vectors of the diffraction wave match those of the free space, the vortex beam will be generated with different topological charges. Moreover, the higher-order topological charge can be affirmatively generated by utilizing the higher spatial harmonics according to Bloch’s theorem of the helical periodic system. Moreover, the vortex beam with mixed orbital angular momentum can also be achieved. Furthermore, and the mode purities of these vortex beams are analyzed. The proposed scheme may facilitate the journey of the vortex beam from optical into terahertz band.

Free electron laser (FEL) facilities provide broadly tunable and highly coherent photon beams. These machines still have an unexplored potential and develo

The design, simulation, manufacture, and measurement of a W-band fivefold (5F) helically corrugated waveguide (HCW) is reported. The 5F HCW is based on the coupling of the traveling TE₃₁ and near cutoff TE₂₁ modes to create an operating eigenwave. The fabricated test structure has circular waveguide ports and features elliptical polarizer sections and broadband TE₁₁ to TE₃₁ mode converters on either side of the 5F HCW. The optimized mode converter design, based on a fourfold (4F) HCW, has a predicted power conversion efficiency greater than 90% from 89 to 102.5 GHz, and 96% peak efficiency at 94 GHz. The optimization of the 5F HCW geometry produced an eigenwave suitable for gyro-devices, but the optimization could equally well have been directed to applications such as pulse compression and microwave undulators. Analysis of simulated electric field profiles showed that the propagating power in the 5F HCW was increased by a factor of 6 over that in the 3F HCW at equivalent peak electric field strength. This is due to the larger diameter of the waveguide. Test structures were manufactured through a combination of precision machining of a sacrificial mandrel, copper growth by electroforming followed by removal of the aluminum mandrel by chemically etching. Measurements of the 5F HCW structure's dispersion showed excellent agreement with the prediction over the design range of 90-98 GHz.

Using pseudospark discharge sourced electron beams for the generation of high-peak-power millimeter and terahertz radiation has attracted increasing research interest in recent years. However, one of the crucially important and hitherto unanswered questions is “what is the upper-frequency limit at which millimeter-wave devices can be driven by pseudospark discharge sourced electron beams?”. In this paper, we studied this question from the perspective of beam transportation in a plasma background, more specifically an ion channel using particle-in-cell simulations to find the limitations. The parameter ranges of the beam transportation with small oscillations in the beam diameter were investigated and summarized, through simulations of beam propagation in a large diameter drift tube with different ion densities, plasma electron densities, beam density distributions, and beam energies. The beam transportation in a small diameter beam tunnel was also simulated. It showed the maximum beam current with a small velocity spread that can be transported in the beam tunnel was determined by the diameter of the beam tunnel and the ion density. High injected current will cause significant beam loss and reduce the overall efficiency. The simulation results indicate a minimum diameter of the beam tunnel in a millimeter-wave circuit that can be effectively driven by a pseudospark-sourced electron beam. The equivalent upper limit in the operating frequency is about 400 GHz.

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.