Annular Beam Research Articles

A high efficiency Ku-band radial line relativistic klystron amplifier

To achieve the gigawatt-level microwave amplification output at Ku-band, a radial-line relativistic klystron amplifier is proposed and investigated in this paper. Different from the annular electron beam in conventional axial relativistic klystron amplifiers, a radial-radiated electron beam is employed in this proposed klystron. Owing to its radially spreading speciality, the electron density and space charge effect are markedly weakened during the propagation in the radial line drift tube. Additionally, the power capacity, especially in the output cavity, is enhanced significantly because of its large volume, which is profitable for the long pulse operation. Particle-in-cell simulation results demonstrate that a high power microwave with the power of 3 GW and the frequency of 14.25 GHz is generated with a 500 kV, 12 kA electron beam excitation and the 30 kW radio-frequency signal injection. The power conversion efficiency is 50%, and the gain is about 50 dB. Meanwhile, there is insignificant electron beam self-excitation in the proposed structure by the adoption of two transverse electromagnetic reflectors. The relative phase difference between the injected signals and output microwaves keeps stable after the amplifier saturates.

Vortex operation in Er:LuYAG crystal laser at ∼1.6 μm

An Er3+-doped Lu1.5Y1.5Al5O12 (Er:LuYAG) solid-state laser with direct generation of optical vortex is reported. The vortex laser operation was realized through being pumped by an annular beam at 1532 nm, which was reformatted by a specially fabricated optical mirror. With two different laser output couplers of 10% and 20% transmissions, pure LG01 mode lasers with right-handedness at 1647.7 nm and 1619.5 nm were yielded from a simple two-mirror cavity, respectively, without any helicity control optical element. Furthermore, stable pulse trains at 1647.7 nm have been achieved via employing an acousto-optic Q-switch, and ∼0.66 mJ pulsed energy and ∼65 ns pulse duration were finally obtained at 1 kHz repetition rate, corresponding to a peak power of ∼10.2 kW. The generated pulse vortex maintained LG01 mode with well-determined right-handedness, as in the case of cw laser operation.

~1 mJ pulsed vortex laser at 1645 nm with well-defined helicity.

We report on a millijoule level Er:YAG ceramic pulsed vortex laser resonantly pumped by an annular beam at 1532 nm. By means of an uncoated YAG crystal plate inserting into the cavity and a proper thermal gradient on the ceramic, 1.03 mJ LG0,-1 mode and 0.97 mJ LG0, + 1 mode were respectively produced, and no vortex mode instability problem was observed. We believe the directly generated millijoule level optical vortex with well-defined helicity will have potential applications in material modification and processing, high-field laser physics, and nonlinear optics, etc.

Excitation of THz hybrid modes in an elliptical dielectric rod waveguide with a cold collisionless unmagnetized plasma column by an annular electron beam

The dispersion relation of electromagnetic waves propagating in an elliptical plasma waveguide with a cold collisionless unmagnetized plasma column and a dielectric rod is studied analytically. The frequency spectrum of the hybrid waves and the growth rate for excitation of the waves by a thin annular relativistic elliptical electron beam (TAREEB) is obtained. The effects of relative permittivity constant of dielectric rod, geometrical dimensions, plasma frequency, accelerating voltage, and current density of TAREEB on the growth rate and frequency spectra of the waveguide will be investigated.

Design and characterization of the annular cathode high current pulsed electron beam source for circular components

In order to irradiate circular components with high current pulsed electron beam (HCPEB), an annular cathode based on carbon fiber bunches was designed and fabricated. Using an acceleration voltage of 25kV, the maximum pulsed irradiation current and energy of this annular cathode can reach 7.9kA and 300J, respectively. The irradiation current density distribution of the annular cathode HCPEB source measured along the circumferential direction shows that the annular cathode has good emission uniformity. In addition, four 9310 steel substrates fixed uniformly along the circumferential direction of a metal ring substrate were irradiated by this annular cathode HCPEB source. The surface and cross-section morphologies of the irradiated samples were characterized by scanning electron microscopy (SEM). SEM images of the surface reveal that crater and surface undulation have been formed, which hints that the irradiation energy of the HCPEB process is large enough for surface modification of 9310 steel. Meanwhile, SEM cross-section images exhibit that remelted layers with a thickness of about 5.4μm have been obtained in all samples, which proves that a good practical irradiation uniformity can be achieved by this annular cathode HCPEB source.

Tight focusing properties of cylindrically polarized annular multi-Gaussian beam

Focusing properties of cylindrically polarized annular multi gaussian beam with high numerical aperture lens system is investigated theoretically by vector diffraction theory. Results show that intensity distribution in focal region can be tailored considerably by appropriately adjusting the polarization angle. Peak-centered, donut and flattop focal shapes with extended focal depth, potentially useful in optical tweezers, material processing and laser printing can be obtained using this technique.

Experimental Study of 50-kV/3.5-A Hollow Electron Beam Produced by Ferroelectric Cathode

An annular electron beam emitted from a flat ferroelectric cathode is demonstrated in this paper. The cathode with a pierced ring-shaped front electrode was operated in two different electron gun setups. In the first electron gun, the cathode was operated in low voltages and under different magnetic fields. The beam shape was photographed, and the beam radii were controlled by the magnetic field. In the second electron gun, with a high voltage of 50 kV, an annular beam was also obtained, with a current of $\sim 3.5$ A. The configuration presented here may be the basis for an annular electron beam source for a gyrotron.

Linear analysis of a backward wave oscillator with triangular corrugated slow wave structure

In this work, a backward wave oscillator (BWO) with triangularly corrugated periodic metallic slow wave structure (TrCSWS) driven by an infinitely thin annular electron beam is studied using linear theory. The electron beam is assumed to be guided by a strong magnetic field. The triangular axial profile of the SWS is approximated by a Fourier series in order to apply the linear Rayleigh-Fourier (R-F) theory that has long been used in the theoretical analysis of BWOs with sinusoidally corrugated SWS (SCSWS). The dispersion equation for various beam parameters has been solved and the temporal growth rate (TGR) of the electromagnetic wave for the fundamental $ TM_{01}$ mode is calculated numerically. The TGR values for different beam parameters have been compared with those of the BWO with SCSWS, semi-circularly corrugated SWS (SCCSWS) and trapezoidally corrugated SWS (TCSWS). In order to compare the TGR values, the amplitude of corrugation of the TrCSWS is varied so that its dispersion curve of $ TM_{01}$ mode almost coincides with that of the SCSWS and TCSWS. The study reveals that the performance (in terms of TGR) of the proposed BWO with TrCSWS is comparable to that of other BWOs with SCSWS and TCSWS for the same set of beam parameters and it provides significantly better performance than SCCSWS. So, the proposed TrCSWS that can easily be constructed may replace SCSWS, SCCSWS or TCSWS as their viable alternative.

Gyro-TWT Using a Metal PBG Waveguide as Its RF Circuit—Part II: PIC Simulation and Parametric Analysis

In this paper, 3-D electromagnetic particle-in-cell (PIC) simulation of gyro-TWT amplifier using a metal photonic bandgap (PBG) waveguide as its RF interaction circuit is presented. A commercial finite integration-based PIC simulation code CST Particle Studio has been used to study the beam-wave interaction mechanism. The mode selectivity of a PBG waveguide has been investigated in the absence of an electron beam, so as to obtain the single-mode operation in a gyro-TWT. The simulated PBG gyro-TWT produced a stable RF output power of ~90 kW at 35 GHz for a 72-keV, 10-A annular electron beam with a velocity ratio of 1.05. The instantaneous bandwidth has been obtained as ~14% with respect to the operating frequency. Furthermore, the performance of the simulated PBG gyro-TWT has also been investigated through the parametric variation of the velocity spread, beam voltage, beam current, axial magnetic field, beam pitch factor, and the RF input power to validate the conceptual design of the device.

A Compact THz Source: 100/200 GHz Operation of a Cylindrical Smith–Purcell Free-Electron Laser

We report first operation in the terahertz regime of a cylindrical grating Smith–Purcell free-electron laser. Propagation of an annular electron beam in proximity to a cylindrical grating causes strong electron bunching due to a beam-surface wave interaction. Electromagnetic radiation results from the bunching (fundamental) and, at bunch harmonics, the Smith–Purcell effect. In the experiment, over 2.5 kW was generated at 100 GHz (fundamental) and over 100 W at 200 GHz (Smith–Purcell). The results illustrate the potential of this configuration for generation of high-power terahertz radiation.

Collinear Acousto-Optical Transformation of Bessel Light Beams in Biaxial Gyrotropic Crystals

The collinear acousto-optical transformation of Bessel light beams in biaxial gyrotropic crystals into two annular, internal conical refraction beams with orthogonal elliptical polarization is studied. It is found that the diffraction efficiency is maximal (~50–60%) for low ultrasound intensities and varies slightly with further increases in acoustic power. At high ultrasound intensities, the intensities of the transmitted and diffracted annular beams differ insignificantly. The possible use of this acousto-optical interaction for creating collinear tuneable narrow-band acousto-optical filters at low ultrasonic frequencies is demonstrated.

Numerical Modeling of Annular High-Current Relativistic Beam Forming in a Toroidal Chamber with a Magnet

A scheme of forming an annular high-current relativistic beam (HCRB) from a directly propagating HCRB in a diode with magnetic insulation and toroidal chamber with a constant magnet is described. The code KARAT is used to analyze numerically the HCRB dynamics. It is demonstrated that for a proper relationship of the system parameters the directly propagating HCRB is rolled up into a torus.

Single-exposure multiphoton fabrication of polygonized structures by an SLM-modulated Fresnel zone lens

Recently, annular beams have been developed to rapidly fabricate microscope tubular structures via two-photon polymerization, but the distribution of the light field is limited to a ring pattern. Here a Fresnel lens is designed and applied to modulate the light field into a uniform quadrangle or hexagon shape with controllable diameters. By applying a spatial light modulator to load the phase information of the Fresnel lens, quadrangle and hexagon structures are achieved through single exposure of a femtosecond laser. A 3×6 array of structures is made within 9 s. Comparing with the conventional holographic processing, this method shows higher uniformity, high efficiency, better flexibility, and easy operation. The approach exhibited a promising prospect in rapidly fabricating structures such as tissue engineering scaffolds and variously shaped tubular arrays.

Focal construct geometry for high intensity energy dispersive x-ray diffraction based on x-ray capillary optics.

We presented a focal construct geometry (FCG) method for high intensity energy dispersive X-ray diffraction by utilizing a home-made ellipsoidal single-bounce capillary (ESBC) and a polycapillary parallel X-ray lens (PPXRL). The ESBC was employed to focus the X-rays from a conventional laboratory source into a small focal spot and to produce an annular X-ray beam in the far-field. Additionally, diffracted polychromatic X-rays were confocally collected by the PPXRL attached to a stationary energy-resolved detector. Our FCG method based on ESBC and PPXRL had achieved relatively high intensity diffraction peaks and effectively narrowed the diffraction peak width which was helpful in improving the potential d-spacing resolution for material phase analysis.

Electron-Beam-Driven Enhanced Terahertz Coherent Smith-Purcell Radiation Within a Cylindrical Quasi-Optical Cavity

In this paper, an electron-beam-driven enhanced terahertz (THz) coherent Smith-Purcell radiation (SPR) from the interaction between cylindrical surface wave and annular electron beam within a cylindrical quasi-optical cavity is presented. The results show that the quasi-optical cavity plays a very important role in the coherent SPR resonance and energy feedback which leads to an enhanced coherent SPR with relatively high efficiency and low current density. Moreover, it is found that the radiated field intensity and field energy of the coherent SPR with the quasi-optical cavity has been improved five times and almost one order of magnitude, respectively, compared with those without the cavity. This system provides an alternative way for high-performance THz sources.

Annular beam high-intensity X-ray diffraction based on an ellipsoidal single-bounce monocapillary

This short communication presents a study of the use of an annular X-ray beam produced by an ellipsoidal single-bounce monocapillary (ESBC) to perform focal construct geometry (FCG) high-intensity angular-dispersive X-ray diffraction (ADXRD) in transmission mode. The ESBC optic effectively focused a large focal spot X-ray source into a smaller focal spot and produced a narrowed X-ray ring in the far-field pattern when combined with a beam stop. A CCD imaging detector was linearly translated along the principal axis of the ESBC-FCG and obtained the corresponding sequential images of diffraction concentric circular caustics and convergence points, which were formed by the constructive interference of a continuous set of Debye cones arising from the annular interrogation volume. Pixels from the central region of an approximately 0.6 mm2 area were interrogated on each sequential image; as a result, a one-dimensional diffractogram of an aluminium oxide sample was revealed. The presented ESBC-FCG ADXRD technique shows potential for increasing the diffracted intensity and streamlining the operation of crystallographic analysis.

Adaptive Flattop Beam Shaping With a Spatial Light Modulator Controlled by the Holographic Tandem Method

This paper reports the realization of a closed-loop feedback system, which consists of a spatial light modulator controlled by the holographic tandem method, for adaptive far-field flattop beam shaping. The holographic tandem method iteratively adjusts the phase distributions to be written into the two different regions of a spatial light modulator so that the Gaussian beam can be converted into an annular flattop beam for better uniformity. The technique described in this paper can provide a promising convenient way to create the desired flattop intensity profile of a beam on a focal plane, which could be applied in applications involving laser fusion and laser precision machining.

Beam Interactions With Surface Waves and Higher Order Modes in Oversized G-band Slow-Wave Structure

Beam interactions with surface waves and higher order modes in oversized $G$ -band slow-wave structures (SWSs) are studied in a coaxial SWS. The coaxial SWS reduces the number of higher order mode and eases the mode competition problem. The SWS is excited by an annular electron beam. The slow cyclotron (SC) interaction as well as the Cherenkov interaction occurs due to 3-D beam perturbations of the electron beam. Growth rates for the surface waves are larger than those for the higher order modes and increase with increasing beam current. The SC interaction merges with the Cherenkov interaction for a sufficiently high beam current and a relatively low magnetic field. And the merged growth rate also increases with the beam current.

Overmoded subterahertz surface wave oscillator with pure TM01 mode output

Overmoded O-type Cerenkov generators using annular electron beams are facing the problem of multi-modes output due to the inevitable structural discontinuities. A simple but effective method to achieve the pure TM01 mode output is applied on the 0.14 THz overmoded surface wave oscillator (SWO) in this paper. In spite of still using an overmoded slow wave structure to ensure the easy fabrication, the followed smooth circular waveguide is shrinkingly tapered to the output waveguide with appropriate radius that it cuts off other higher modes except TM01 mode. Moreover, the modified device here has the same power capacity as the previous one according to the numerical analysis. By optimized lengths of the transition waveguide and tapered waveguide, particle-in-cell simulation results indicate that the subterahertz wave with output power increased 14.2% at the same frequency is obtained from the proposed SWO under the previous input conditions, and importantly, the output power is all carried by TM01 mode as expected. Further simulation results in the pulse regime confirm the feasibility of the optimized structure in the actual experiments. This simple and viable design is also applicable to overmoded devices in the lower frequency band of subterahertz wave.

Nonlinear space-time focusing and filamentation of annular femtosecond pulses in dielectrics

Filamentation during simultaneous space-time focusing in bulk fused silica is investigated numerically. We model the use of a pair of concentric gratings to transform a common femtosecond laser pulse with a Gaussian spatial profile into a radially chirped, annular beam shape that is focused into the bulk of silica by a lens. By varying the energy and/or time-chirp of the incident pulse, we capture the pulse dynamics and material response that yields material modification and damage in the nonlinear focus. The results show rich pulse dynamics, enhanced damage/modification site localization, and suggest novel approaches to laser machining of solids.