Shaped Electron Beam Research Articles

“Beam à la carte”: Laser heater shaping for attosecond pulses in a multiplexed x-ray free-electron laser

Electron beam shaping allows the control of the temporal properties of x-ray free-electron laser pulses from femtosecond to attosecond timescales. Here, we demonstrate the use of a laser heater to shape electron bunches and enable the generation of attosecond x-ray pulses. We demonstrate that this method can be applied in a selective way, shaping a targeted subset of bunches while leaving the remaining bunches unchanged. This experiment enables the delivery of shaped x-ray pulses to multiple undulator beamlines, with pulse properties tailored to specialized scientific applications.

Nanometer-scale electron beam shaping with thickness controlled and stacked nanostructured graphite

The generation of small electron probes is the basis for various techniques in which such a probe is scanned across a sample, and special probe shapes like vortices can be desirable, e.g., to gain insight into magnetic properties. Micron-scale phase plates or holographic masks, in combination with demagnifying optics, are usually used for creating such special probe wave functions. Here, we present the fabrication of nanometer-sized phase plates based on thickness-selected and stacked graphite layers as well as an analysis of their performance. First, a spiral phase plate is demonstrated that creates a vortex beam with an orbital angular momentum of 1 and an outer radius of 2.5 nm. Second, a three-level Fresnel lens built from two nanopatterned graphite membranes is presented, which achieves a focal spot with a full width at half maximum of 5.5 nm. Third, an array of electron sieves is demonstrated, each of which creates a focal spot with a radius of 2 nm, and the array is applied as a Shack–Hartmann wavefront detector. These elements allow the generation of few-nanometer sized focused probes or vortices without the need for additional optical elements.

Numerical simulation of electron beam depth dose distribution in water phantom passed through modified plastic materials

The aim of the study is to evaluate the feasibility of using modified plastics for 3D printing of boluses for electron beam therapy. Filaments for 3D printing made from plastic materials with an admixture of copper were studied. Test samples were made from these filaments and investigated. Mathematical models of such products were created based on the obtained data. The thicknesses of the boluses made from each modified material were calculated to achieve the same electron beam depth dose distribution in the water phantom as natural plastic. Using a the developed model of a therapeutic electron beam and newly created models of test object materials, the electron beam depth dose distributions in the water phantom that passed through the materials under study were obtained in bolus geometry. The results of the work demonstrate the potential of using modified plastics for creating forming elements in electron beam therapy. The application of this plastic samples for electron beam shaping with individual configuration allows to modify the irradiation field based on the clinical tasks quickly and efficiently and to increase the effectiveness of electron beam radiotherapy procedures.

The Atomic Drill Bit: Precision Controlled Atomic Fabrication of 2D Materials.

The ability to deterministically fabricate nanoscale architectures with atomic precision is the central goal of nanotechnology, whereby highly localized changes in the atomic structure can be exploited to control device properties at their fundamental physical limit. Here, an automated, feedback-controlled atomic fabrication method is reported and the formation of 1D-2D heterostructures in MoS2 is demonstrated through selective transformations along specific crystallographic orientations. The atomic-scale probe of an aberration-corrected scanning transmission electron microscope (STEM) is used, and the shape and symmetry of the scan pathway relative to the sample orientation are controlled. The focused and shaped electron beam is used to reliably create Mo6 S6 nanowire (MoS-NW) terminated metallic-semiconductor 1D-2D edge structures within a pristine MoS2 monolayer with atomic precision. From these results, it is found that a triangular beam path aligned along the zig-zag sulfur terminated (ZZS)direction forms stable MoS-NW edge structures with the highest degree of fidelity without resulting in disordering of the surrounding MoS2 monolayer. Density functional theory (DFT) calculations and ab initio molecular dynamic simulations (AIMD) are used to calculate the energetic barriers for the most stable atomic edge structures and atomic transformation pathways. These discoveries provide an automated method to improve understanding of atomic-scale transformations while opening a pathway toward more precise atomic-scale engineering of materials.

3-D analysis of electron beam shape at target plane through parametric variation of an indirectly heated cathode-based electron gun

In this paper, a detailed 3-D analysis of the electron beam shape at the target plane of high power (60 kW, 60 kV, 1 A) 2700-bent indirectly heated cathode-based electron gun has been successfully carried out. This work will help analyse individual electron beam trajectories in the actual 3-D plane to ensure optimum electron beam bending and focusing at the target plane. Moreover, due to the presence of an additional electrode in an indirectly heated cathode-based electron gun i.e., a filament which is resistively heated and an indirectly heated solid cathode (heated through electron bombardment), this analysis becomes even more imperative. The work carried out in this paper will ensure better performance, characterization, and optimization of the 2700-bent indirectly heated cathode-based electron gun. Rigorous simulation work has been carried out by performing 3-D electrostatic, electromagnetic, and particle tracking simulations using CST Studio Particle Tracking Module. Adaptive mesh refinement has been utilized to ensure high accuracy of the results obtained. An optimum filament-solid cathode interspacing was found, using strip-shaped filament by providing a higher negative potential to the grid cup. It led to a well-focused 2700-bent electron beam, exhibiting minimum electron beam emittance at the target plane.

Optical-cavity mode squeezing by free electrons.

The generation of nonclassical light states bears a paramount importance in quantum optics and is largely relying on the interaction between intense laser pulses and nonlinear media. Recently, electron beams, such as those used in ultrafast electron microscopy to retrieve information from a specimen, have been proposed as a tool to manipulate both bright and dark confined optical excitations, inducing semiclassical states of light that range from coherent to thermal mixtures. Here, we show that the ponderomotive contribution to the electron-cavity interaction, which we argue to be significant for low-energy electrons subject to strongly confined near-fields, can actually create a more general set of optical states, including coherent and squeezed states. The postinteraction electron spectrum further reveals signatures of the nontrivial role played by A 2 terms in the light-matter coupling Hamiltonian, particularly when the cavity is previously excited by either chaotic or coherent illumination. Our work introduces a disruptive approach to the creation of nontrivial quantum cavity states for quantum information and optics applications, while it suggests unexplored possibilities for electron beam shaping.

Globular surface layer in electron beam powder-bed fusion Ti-6Al-4V after standard powder removal blasting and hot isostatic pressing

A globular α recrystallization has been discovered at the surface of net shape electron beam powder-bed fusion (PBF-EB) Ti-6Al-4V alloy parts. This recrystallization is the result of powder recovery blasting followed by hot isostatic pressing treatment (HIP), which mimics the thermomechanical treatment necessary to achieve globularization in wrought Ti-6Al-4V. The thickness of the globular surface α is variable depending on the blasting intensity as well as the presence of surface protrusions that block the line-of-sight of the blasting media. The globular layer contains a 0001 orientation relationship parallel to the blasting direction, plus a low intragranular orientation deviation, when compared to the bulk. The results indicate that this globular α at the surface may be present for all net-shape Ti-6Al-4V components that have been manufactured via PBF-EB and subjected to a HIP treatment. Tunable fatigue and wear performance may be possible through controlled blasting parameters and globular surface layer thickness.

Machine learning-based direct solver for one-to-many problems on temporal shaping of relativistic electron beams

To control the temporal profile of a relativistic electron beam to meet requirements of various advanced scientific applications like free-electron-laser and plasma wakefield acceleration, a widely-used technique is to manipulate the dispersion terms which turns out to be one-to-many problems. Due to their intrinsic one-to-many property, current popular stochastic optimization approaches on temporal shaping may face the problems of long computing time or sometimes suggesting only one solution. Here we propose a real-time solver for one-to-many problems of temporal shaping, with the aid of a semi-supervised machine learning method, the conditional generative adversarial network (CGAN). We demonstrate that the CGAN solver can learn the one-to-many dynamics and is able to accurately and quickly predict the required dispersion terms for different custom temporal profiles. This machine learning-based solver is expected to have the potential for wide applications to one-to-many problems in other scientific fields.

Ultrafast Transverse Modulation of Free Electrons by Interaction with Shaped Optical Fields.

Spatiotemporal electron-beam shaping is a bold frontier of electron microscopy. Over the past decade, shaping methods evolved from static phase plates to low-speed electrostatic and magnetostatic displays. Recently, a swift change of paradigm utilizing light to control free electrons has emerged. Here, we experimentally demonstrate arbitrary transverse modulation of electron beams without complicated electron-optics elements or material nanostructures, but rather using shaped light beams. On-demand spatial modulation of electron wavepackets is obtained via inelastic interaction with transversely shaped ultrafast light fields controlled by an external spatial light modulator. We illustrate this method for the cases of Hermite-Gaussian and Laguerre-Gaussian modulation and discuss their use in enhancing microscope sensitivity. Our approach dramatically widens the range of patterns that can be imprinted on the electron profile and greatly facilitates tailored electron-beam shaping.

Transverse Electron-Beam Shaping with Light

Transverse Electron-Beam Shaping with Light

Electron beam shaping for actively outcoupling radiation from an x-ray regenerative amplifier free-electron laser

We present a method to actively outcouple radiation from an x-ray regenerative amplifier. The method uses an undulator within a cavity formed by the diamond mirror and is driven by a high repetition rate linac to build up a high power, narrow bandwidth seed within the cavity. Shaping the electron beam via laser heater modulation on a subsequent pass increases the bandwidth of the x-ray free-electron laser pulse beyond the bandwidth of the diamond mirrors, transmitting the radiation from the cavity. A fraction of the power from the shaped pulse is returned to the cavity, reducing the subsequent number of passes needed to rebuild the seed and increasing the duty factor. The cavity seeding may be used to produce short pulses that are significantly more stable than those produced via self-amplified spontaneous emission (SASE) or enhanced SASE.

Theoretical and practical aspects of the design and production of synthetic holograms for transmission electron microscopy

Beam shaping—the ability to engineer the phase and the amplitude of massive and massless particles—has long interested scientists working on communication, imaging, and the foundations of quantum mechanics. In light optics, the shaping of electromagnetic waves (photons) can be achieved using techniques that include, but are not limited to, direct manipulation of the beam source (as in x-ray free electron lasers and synchrotrons), deformable mirrors, spatial light modulators, mode converters, and holograms. The recent introduction of holographic masks for electrons provides new possibilities for electron beam shaping. Their fabrication has been made possible by advances in micrometric and nanometric device production using lithography and focused on ion beam patterning. This article provides a tutorial on the generation, production, and analysis of synthetic holograms for transmission electron microscopy. It begins with an introduction to synthetic holograms, outlining why they are useful for beam shaping to study material properties. It then focuses on the fabrication of the required devices from theoretical and experimental perspectives, with examples taken from both simulations and experimental results. Applications of synthetic electron holograms as aberration correctors, electron vortex generators, and spatial mode sorters are then presented.

Inhibition of current-spike formation based on longitudinal phase space manipulation for high-repetition-rate X-ray FEL

The formation of a double-horn current profile is a challenging issue in the achievement of electron bunch with high peak current, especially for high-repetition-rate X-ray free-electron lasers (XFELs) where more nonlinear and intricate beam dynamics propagation is involved. Here, we propose to correct the nonlinear beam longitudinal phase space distortion in the photoinjector section with a dual-mode buncher. In combination with the evolutionary many-objective beam dynamics optimization, this method is shown to be effective in manipulating the longitudinal phase space at the injector exit. Furthermore, the formation of the current spike is avoided after the multi-stage charge density modulation and electron bunch with a peak current of 1.6 kA is achieved for 100-pC bunch charge. Start-to-end simulations based on the Shanghai high-repetition-rate XFEL and extreme light facility demonstrate that the proposed scheme can increase the FEL pulse energy by more than 3 times in a cascading operation of echo-enabled harmonic generation and high-gain harmonic generation. Moreover, this method can also be used for longitudinal phase space shaping of electron beams operating at a high repetition rate to meet the specific demands of different researches.

Field emission and electron beam transmission characteristics of microtips array on the (100) plane of single-crystal Gadolinium hexaboride ceramic

A field-emission microtips array with a radius of curvature, height, and separation distance of 0.5, 5, and 4 μm, respectively, was processed on the single-crystal Gadolinium hexaboride ceramic (GdB6) (100) plane using a femtosecond laser. The height and morphology of the microtips array were uniform, and the crystal structure and phase composition of GdB6 did not change after the processing. According to the field-emission performance test of a GdB6 microtips array, the turn-on electric field intensity was 1.9 V/μm, and the field-emission current density reached 0.49 A/cm2 when the applied electric field strength was 6.7 V/μm; meanwhile, it had outstanding field-emission stability. Subsequently, the transmission process of the field-emission current of the GdB6 between the cathode and the anode was simulated using COMSOL Multiphysics 5.6 software. The transmission process of the electron beam from emitters to the anode surface was divided into three stages due to the interaction between emission electrons and a magnetic field generated by its motion, including divergence, pinch, and drift. The increase of electric field strength reduced the radial displacement of the electron beam while moving the beam waist further away from the anode surface. The simulation results provide theoretical guidance for regulating the shape of the field-emission electron beam, size, and energy.

Shaping of Electron Beams Using Sculpted Thin Films.

Electron beam shaping by sculpted thin films relies on electron–matter interactions and the wave nature of electrons. It can be used to study physical phenomena of special electron beams and to develop technological applications in electron microscopy that offer new and improved measurement techniques and increased resolution in different imaging modes. In this Perspective, we review recent applications of sculpted thin films for electron orbital angular momentum sorting, improvements in phase contrast transmission electron microscopy, and aberration correction. For the latter, we also present new results of our work toward correction of the spherical aberration of Lorentz scanning transmission electron microscopes and suggest a method to correct chromatic aberration using thin films. This review provides practical insight for researchers in the field and motivates future progress in electron microscopy.

Focused ion beam fabrication of Janus bimetallic cylinders acting as drift tube Zernike phase plates for electron microscopy

Modern nanotechnology techniques offer new opportunities for fabricating structures and devices at the micrometer and sub-micrometer level. Here, we use focused ion beam techniques to realize micrometer-sized Janus bimetallic cylinders acting as drift tube devices, which are able to impart a controlled phase shift to an electron wave. The phase shift results from the presence of contact potentials in the cylinders, in a similar manner to the electrostatic Aharonov–Bohm effect in bimetallic wires. We use electron Fraunhofer interference to demonstrate that such bimetallic structures introduce phase shifts that can be tuned to desired values by varying the dimensions of the pillars, in particular their heights. Such devices are promising for electron beam shaping and for the realization of electrostatic Zernike phase plates (i.e., devices that are able to impart a constant phase shift between an unscattered and a scattered electron wave) in electron microscopy, in particular, cryo-electron microscopy.

Alignment of electron optical beam shaping elements using a convolutional neural network

A convolutional neural network is used to align an orbital angular momentum sorter in a transmission electron microscope. The method is demonstrated using simulations and experiments. As a result of its accuracy and speed, it offers the possibility of real-time tuning of other electron optical devices and electron beam shaping configurations.

Characterization and dosimetry of photon multileaf collimated electron beams using Gafchromic film measurements

Previous studies indicated that clinical electron beam shaping involving a photon multileaf collimator (MLC) can be achieved. It is usually applied in Modulated Electron Radiation Therapy or MERT. This technique requires accurate clinical electron beam characterization before implementation in advanced treatment planning techniques. This study presents the dosimetric characterization of clinical electron beams shaped by an Elekta Synergy linear accelerator equipped with a 160 leaf Agility MLC for electron beams energies of 4, 6, 8, 10, 12, and 15 MeV. Dosimetric parameters of regular fields ranging from 1 × 1 to 20 × 20 cm2 were investigated to reflect those fields used in electron beam therapy at two source-to-surface distances (SSD) of 60 and 70 cm. Percent depth doses (PDDs), in-line, and cross-line beam profiles and output factors (OFs) were measured. Beam profiles were found to be wider compared to electron beams collimated by standard applicators. Penumbrae formed by the pMLC in cross-line direction was wider compared to penumbrae formed by the back-up jaws in in-line direction. The jaws are located below the MLC and are closer to the plane of measurement. The beams produced at 60 cm SSD also produced smaller penumbra. Beam spacing was also investigated and a clinical case was investigated for total scalp treatment in a head phantom and measured with Gafchromic film. The clinical investigation result showed good dose distribution over the therapeutic region and the treatment time is 3 times faster compared to the use of standard applicators for field greater than 5 × 5 cm2. All the measurements for beam dosimetric characterization were performed using Gafchromic EBT3 film and solid phantoms.

Tungsten carbide and LMPA electron cutouts: comparison and validation using Monte Carlo modelling and measurement of dose

Electron applicator cutouts for radiation therapy electron beam shaping are typically cast from Low Melting Point Alloys (LMPA), such as cerrobend. In this work, we describe the Monte Carlo modelling of novel 3D printed cutouts based on tungsten carbide powder and the resulting dose profiles subject to Elekta Agility electron beams. Cerrobend cutouts were also modelled using the Monte Carlo code EGSnrc. Cerrobend and tungsten carbide cutouts were found to have the same dose profiles within model variance. Computed profiles and percentage depth dose (PDDs) curves of the Elekta Agility accelerator model using standard cutouts in water were found to agree with water tank measurements using gamma criteria of 2%/2mm. The Monte Carlo computed dose profiles of the tungsten carbide cutouts in a polystyrene phantom were also found to agree with liquid-filled ionization chamber array measurements using gamma criteria of 2%/2mm. We conclude that the tungsten carbide cutouts are clinically equivalent to LMPA cutouts.

PO-1485: Comparison of electron beam shaping efficiency with metal and 3D-printed plastic collimators

PO-1485: Comparison of electron beam shaping efficiency with metal and 3D-printed plastic collimators