Free Electron Beam Research Articles

Construction of Dirac spinors for electron vortex beams in background electromagnetic fields

Exact solutions of the Dirac equation, a system of four partial differential equations, are rare. The vast majority of them are for highly symmetric stationary systems. Moreover, only a handful of solutions for time dependent dynamics exists. Given the growing number of applications of high energy electron beams interacting with a variety of quantum systems in laser fields, novel methods for finding exact solutions to the Dirac equation are called for. We present a method for building up solutions to the Dirac equation employing a recently introduced approach for the description of spinorial fields and their driving electromagnetic fields in terms of geometric algebras. We illustrate the method by developing several stationary as well as non-stationary solutions of the Dirac equation with well defined orbital angular momentum along the electron's propagation direction. The first set of solutions describe free electron beams in terms of Bessel functions as well as stationary solutions for both a homogeneous and an inhomogeneous magnetic field. The second set of solutions are new and involve a plane electromagnetic wave combined with a generally inhomogeneous longitudinal magnetic field. Moreover, the developed technique allows us to derive general physical properties of the dynamics in such field configurations, as well as provides physical predictions on the self-consistent electromagnetic fields induced by the dynamics.

Polarization out of the vortex

The virtual photons that are exchanged when a free-electron vortex beam interacts with a nanoscopic target unlock an explicit connection between polarized optical spectroscopy and the inelastic scattering of scalar electron waves.

Information’s Relativistic Convey with Matter Wave’s Non-Dispersive Propagation

The Wave-Particle Duality is a basic property of microscopic particles. As a basic concept of quantum mechanics, the wave-particle duality theory from elementary particles to big molecules had been verified by lots of experiments. Different from electromagnetic wave, the matter wave’s propagation is not only fast but also adjustable. According to the special relativity theory, the group velocity with which the overall envelope shape of the wave, namely the related particle’s propagation and information convey speed is changeable with its energy and related wavelength, among which only the energy exceeds over the minimum value, the propagation can be starting and the velocity is not allowed to surpass the maximum value i.e. the light speed in vacuum. Take electron as an example, if the free electron beam gains energy higher than around 8.187×10ˉᴵ⁴J and the related wavelength is shorter than around 5.316×10ˉ³nm, the matter wave with information can start to propagate.

Coherent terahertz emission from free-electron-driven Fabry–Pérot resonators with coupled grooves

Fabry–Pérot (FP) resonators with metallic subwavelength grooves or holes, which have been extensively applied in optical region, have recently attracted increasing interest for terahertz wave generation and manipulation. However, the practices of such resonators in the terahertz region are greatly restricted by the low energy-capacity and by the difficulty of manufacturing high-aspect-ratio grooves/holes. Here we illustrate a kind of terahertz FP resonator formed by a cluster of coupled grooves with low-aspect-ratio, which has much higher energy-capacity, higher quality-factor and is much easier to fabricate than one formed by a single uncoupled-groove. Using free-electron beams to excite an array of such FP resonators, coherent terahertz emission can be achieved, the intensity of which is more than an order of magnitude higher than that from an array of uncoupled-grooves, affording a promising way for high-power and coherent terahertz wave generation.

SARS-CoV-2 Rapid Detection System using Electric Current-Spectrum for Human Exhaled Air Samples

The SARS-COV-2 Rapid Detection System is a SARS-COV-2 Electro-Spectroscopy detection system. The preliminary design of this system was studied theoretically in this paper. This system can detect the existence of sub-micro impurity particles in the human exhaled air, based on the unique shape, dimensions and density of these sub-micro impurities, such as viral particles, including CORONA viruses. This information is carried out by electron current buildup forming Electric Current-Spectrum (ECS) distinguishing the contents of the exhaled air. The design is based on Flashing Ratchet Potential (FRP) and beam of free electrons passing through the electrodes of the FRP. The ECS is characterized by curve deviations caused by interaction of low energy electrons’ (-β radiation) with matter, which depends on the matter’s shape, dimension and density. This interaction causes a scattering (delaying) or absorption of the electrons by matter, by elastic or inelastic collisions, respectively. The ability of FRP to drift back the delayed electrons to their initial points can be used to characterize the produced ECS. The effect of delayed electrons can be amplified to form a visualized deviation in the ECS by electron multiplication system and can be interpreted into distinguishable SARS-COV-2 Barcodes or Fingerprints.

Erratum: Radiation emitted by transverse-gradient undulators [Phys. Rev. Accel. Beams 19 , 090704 (2016)

Received 28 August 2020DOI:https://doi.org/10.1103/PhysRevAccelBeams.23.099901Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSingle-particle dynamicsPhysical SystemsPlasma acceleration & new acceleration techniquesSynchrotron radiation & free-electron lasersAccelerators & Beams

Electrostatic and electromagnetic waves driven by current free electron beam instability: Effects of positron, background composition, and ion-to-electron mass ratio

Counterstreaming electron beams drifting along the magnetic field in electron–proton plasmas may lead to the generation of electrostatic (ES) waves and structures widely observed in planetary magnetospheres. In electron–positron plasmas, streaming electron/positron beams along the background magnetic field may first result in ES waves followed by the electromagnetic (EM) waves with substantial magnetic field perturbations, as shown by Jao and Hau [Phys. Rev. E 98, 013203 (2018)]. In this study, we examine the formation of ES and EM waves driven by current free counterstreaming electron beams in electron–ion background plasmas and current free electron–positron beams in electron–proton–positron background plasmas based on one-dimensional EM particle-in-cell simulations with the drift velocity being along the background magnetic field. For the former cases, the effects of ion-to-electron mass ratios are examined, which have implications for two- and three-dimensional particle in cell simulations with reduced mass ratios. For the latter cases, the effects of positrons are examined, which have applications to astrophysical plasma environments and laboratory experiments. It is shown that the presence of positrons or light ions may shorten the occurrence times between the ES and EM waves driven by current free electron beam instability. The EM waves have relatively longer wavelengths as compared to the ES waves and the magnetic field evolution resembles parallel proton firehose instability. The effects of positrons, background compositions, and ion-to-electron mass ratios on the occurrence of ES and EM waves are consistent with the linear fluid theory.

Controlling free electrons with optical whispering-gallery modes.

Free-electron beams are versatile probes of microscopic structure and composition1,2, and have revolutionized atomic-scale imaging in several fields, from solid-state physics to structural biology3. Over the past decade, the manipulation and interaction of electrons with optical fields have enabled considerable progress in imaging methods4, near-field electron acceleration5,6, and four-dimensional microscopy techniques with high temporal and spatial resolution7. However, electron beams typically couple only weakly to optical excitations, and emerging applications in electron control and sensing8-11 require large enhancements using tailored fields and interactions. Here we couple a free-electron beam to a travelling-wave resonant cavity mode. The enhanced interaction with the optical whispering-gallery modes of dielectric microresonators induces a strong phase modulation on co-propagating electrons, which leads to a spectral broadening of 700 electronvolts, corresponding to the absorption and emission of hundreds of photons. By mapping the near-field interaction with ultrashort electron pulses in space and time, we trace the lifetime of the the microresonator following a femtosecond excitation and observe the spectral response of the cavity. The natural matching of free electrons to these quintessential optical modes could enable the application of integrated photonics technology in electron microscopy, with broad implications for attosecond structuring, probing quantum emitters and possible electron-light entanglement.

Single-particle dynamics of microbunching

The mechanism of steady-state microbunching (SSMB) has been proposed [D. F. Ratner and A. W. Chao, Phys. Rev. Lett. 105, 154801 (2010)] to generate high-power coherent radiation at a high repetition rate or in continuous-wave mode using electron storage rings. In this paper, the related single-particle dynamics are theoretically and numerically studied, and important results are presented. The investigated effects are longitudinal quantum radiation excitation, nonlinear momentum compaction, and linear and nonlinear coupling of the transverse and longitudinal motion. Although this analysis is oriented toward SSMB, some of the analyzed effects are also crucial in cases such as coherent harmonic generation, bunch slicing, bunch compression, free-electron laser beam transport lines, and quasi-isochronous rings, which involve precise longitudinal phase-space manipulations.

Transforming terahertz plasmonics within subwavelength hole arrays into enhanced terahertz mission via Smith-Purcell effect.

We illustrate the transformation of terahertz plasmonics within an array of rectangular sub-wavelength holes (RSHs) into coherent and enhanced terahertz emission via Smith-Purcell effect. The radiative plasmonic modes within each RSH of the array are successively excited by an free-electron beam, which then generate coherent radiation by constructive interference. Compared with the case without taking plasmonics into consideration, the radiation field intensity is enhanced by more than an order of magnitude, affording a promising way of developing high-power terahertz radiation. We perform detailed analysis of the plasmonic modes within the RSH by using the dielectric waveguide theory, and the results are verified by numerical simulations. The influences of the RSH parameters on the radiation properties are revealed and discussed.

Density matrix reconstructions in ultrafast transmission electron microscopy: uniqueness, stability, and convergence rates

In the recent paper Priebe et al (2017 Nat. Photon. 11 793–7) the first experimental determination of the density matrix of a free electron beam has been reported. The employed method leads to a linear inverse problem with a positive semidefinite operator as unknown. The purpose of this paper is to complement the experimental and algorithmic results in the work mentioned above by a mathematical analysis of the inverse problem concerning uniqueness, stability, and rates of convergence under different types of a priori information.

Optical confinement in TiO2 waveguides fabricated by resist free electron beam lithography

The performance of TiO2 waveguides fabricated by a novel resist free electron beam lithography technique is reported. The precursor films were first spin coated on substrates followed by patterning and heating under the electron beam. Subsequent dissolution of the pattern in a suitable solvent and annealing leads to the formation of rectangular TiO2 waveguides. Dimensional control of the waveguides is achieved during the lithography step. Phase and morphology studies confirm the high quality of the waveguides. The TiO2 films crystallize in the anatase structure with transmission >80% and refractive index of 1.95–2.00 in the visible-NIR region. Optical field confinement in the TiO2 waveguides was investigated by orthogonal illumination followed by scanning the waveguide structure with a self-customized optical microscopy system. The optical losses are relatively low as evidenced by the intensity of the light beams propagating through the waveguides. It is further reduced by coupling the light from one waveguide into another waveguide located at close proximity. It is demonstrated that this technique reduces the number of steps while yielding waveguides with performance comparable to any other fabrication technique.

Transverse diagnostics based on dipole mode signal fitting method in TESLA-type accelerating cavities at the free-electron laser FLASH

TESLA cavities are used at the free-electron laser in Hamburg (FLASH) to accelerate electron bunches for generating intense free-electron laser beams. Two specially designed couplers mounted at both ends of each cavity damp the higher order modes (HOMs) excited by the beam in these cavities. By using a specific dipole mode signal extracted from these couplers, one can determine the transverse beam position as well as the dipole mode polarizations and center in each cavity. We introduce a new method based on fitting the dipole mode signal to correlate the dipole mode amplitude and phase to the measured beam position. With this method, the TESLA cavities can be used as HOM-based beam position monitors (HOMBPMs), delivering consistent results over several months, with a resolution better than $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}\text{ }$ RMS. We implemented this method for a whole accelerating module for beam orbit monitoring. The method also delivers the polarization axes and the center of the dipole mode for all eight cavities in this module. Our measurements show that the cavities did not move inside the module within almost one year.

Spectral analysis of the EBT3 radiochromic films for clinical photon and electron beams.

The purpose of this study was to investigate the spectral response of the EBT3 radiochromic films to different beam qualities for radiation therapy dosimetry. Dose, dose rate, and interbatch dependencies on the spectral response of the films were investigated as well. Pieces of EBT3 films placed between layers of solid water phantoms were irradiated with 6 and 15MV photon beams, 6 and 10MV-flattening filter free (FFF) photon beams, and 6 and 20MeV electron beams at dose levels between 0.4 and 50Gy. Net absorbance was measured as a function of wavelength from the spectra acquired in the wavelength range of 400-800nm using a fiber-coupled spectrometer and broadband light source. No significant change was observed in the absorption spectra of the EBT3 film from the same batch irradiated with the same amount of dose using different beam qualities. Also, no spectral change with dose rate was observed. The measured net absorbance per Gy was independent of beam quality in the 1-50Gy dose range. Slight differences in the spectral shape and absorption band positions were observed in film samples from different batches. The net absorbance spectra showed two absorption bands centered around 634-636nm (primary) and 583-585nm (secondary). However, depending on the film batch, for doses above a certain level the primary absorption band appears to "split" into two bands centered around ~624-628 and ~641-645nm. The spectral shape of the EBT3 radiochromic films irradiated with photons (including FFF) and electron beams is beam quality and dose rate independent; however, it varies with dose level, batch, and spectroscopy system used.

Generation and attosecond shaping of high coherence free-electron beams for ultrafast TEM

We demonstrate the generation and optical control of ultrashort high-coherence electron pulses. The free-electron quantum state is phase-modulated in the longitudinal and transverse dimensions, and the formation of attosecond electron pulse trains is quantitatively probed.

Threshold-less and focused Cherenkov radiations using sheet electron-beams to drive sub-wavelength hole arrays.

Cherenkov radiation (CR) was one of the most famous discoveries in the last century and still has broad applications in modern physics. Recently, threshold-less and reversed CRs have attracted even more attention thanks to their unique characteristics and application prospects. Here we illustrated a threshold-less CR in vacuo by using a sheet free-electron beam (FEB) to excite an oblique-lined sub-wavelength hole array. It is achieved by setting the effective velocity of emitters-resonant modes successively excited by the sheet FEB-to be greater than the speed of light in vacuo. By letting the sub-wavelength holes line up along a designed curve, we further demonstrated a focused CR with radiation being convergent to specific focusing spots, which can be located at any designed positions in space, achieving backward (reversed) as well as forward (normal) CRs in effect. This focused CR does not have the conventional Cherenkov cone, and its intensity at the focusing spot is greatly enhanced. These newly revealed threshold-less and focused CRs may lead to broad interest and attractive applications, especially for developing integrated and focused light sources in the terahertz region.

Multicolor Terahertz Frequency Mixer Using Multibunching of Free-Electron Beams

Terahertz electromagnetic waves have applications in fields as diverse as biological imaging, materials science, and astrophysics. However, the development of compact, high-power, broadly tunable terahertz sources is challenging. The authors propose a frequency mixer that uses a free-electron beam to drive two cascaded gratings, producing multicolor coherent terahertz radiation via the superradiant Smith-Purcell effect. This device can simultaneously generate several terahertz emissions of high average power, and its frequency can be tuned from 0.8 to 1.8 THz, offering a compact, efficient terahertz source.

Sensing and cooling of a nanomechanical resonator with an electron beam stimulated internal feedback and a capacitive force

A model for the cooling properties of a nanocantilever by a free electron beam is presented for a capacitive interaction. The optimal parameters for position sensing and cooling applications are estimated from previous experimental conditions. In particular, we demonstrate that a purely capacitive force and an electron beam stimulated internal feedback can lower the temperature of a nanocantilever by several orders of magnitude, in striking contrast with the conventional electrostatic damping regime. We propose a step by step protocol to extract the interdependent parameters of the experiments. This work will aid future developments of ultra-sensitive force sensors in electron microscopes.

Single-pass high-gain tapered free-electron laser with transverse diffraction in the postsaturation regime

It has been well known that the resonant interaction of an ultrarelativistic electron beam and the radiation field in the single-pass high-gain free electron laser (FEL) amplifier leads to the optical gain guiding. The transverse Laplacian term of the slowly varying wave equation in the linear regime can be approximated as a constant detuning parameter, i.e., |∇⊥2|∼kR/zR where kR is the resonant wave number and zR is the Rayleigh range of the laser. In the post-saturation regime, the radiation power begins to oscillate about an equilibrium for the untapered case while continues to grow by undulator tapering. Moreover, in this regime the gain guiding decreases and the simple constant detune is no longer valid. In this paper we study the single-pass high-gain FEL performance in the post-saturation regime with inclusion of diffraction effect and undulator tapering. Our analysis relies upon two constants of motion, one from the energy conservation and the other from the adiabatic invariant of the action variable. By constructing a two-dimensional axisymmetric wave equation and the coupled one-dimensional electron dynamical equations, the performance of a tapered FEL in the postsaturation regime can be analyzed, including the fundamental mode profile, the power efficiency and the scaled energy spread. We begin the analytical investigation with two different axisymmetric electron beam profiles, the uniform and bounded parabolic ones. It is found that the tapered FEL power efficiency can be smaller but close to the taper ratio provided the resonant phase remains constant and the beam-wave is properly matched. Such a tapered efficiency is nearly independent of transverse electron beam size before significant electron detrapping occurs. This is essentially different from the untapered case, where the power extraction efficiency is around the essential FEL gain bandwidth (or ρ, the Pierce or FEL parameter) and depends on the beam size. It is also found that the power enhancement due to undulator tapering is attributed more by the field increase outside the transverse electron beam than that inside the transverse electron beam. Several scaling properties on the taper ratio and the transverse electron beam size are also discussed in this paper.3 MoreReceived 29 January 2018DOI:https://doi.org/10.1103/PhysRevAccelBeams.21.060702Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsSynchrotron radiation & free-electron lasersAccelerators & Beams

Tunable Free-Electron-Driven Terahertz Diffraction Radiation Source

In this paper, we propose a tunable terahertz (THz) radiation source that utilizes a free electron beam to generate THz diffraction radiation coupled with the guided modes in a cylindrical corrugated waveguide. Detailed theoretical analyses and computer simulations are conducted, and the results demonstrate the characteristics of the tunable radiation frequency. In particular, the coherent diffraction radiation excited by a prebunched electron beam is discussed both theoretically and numerically with regard to the enhancement of the radiation power and efficiency. This paper may offer attractive prospects for the development of compact and tunable radiation sources in the THz frequency regime.