Collimated Electron Beam Research Articles

Stable Field Emissions from Zirconium Carbide Nanoneedle Electron Source.

In this study, a single zirconium carbide (ZrC) nanoneedle structure oriented in the <100> direction was fabricated by a dual-beam focused ion beam (FIB-SEM) system, and its field emission characteristics and emission current stability were evaluated. Benefiting from controlled fabrication with real-time observation, the ZrC nanoneedle has a smooth surface and a tip with a radius of curvature smaller than 20 nm and a length greater than 2 μm. Due to its low work function and well-controlled morphology, the ZrC nanoneedle emitter, positioned in a high-vacuum chamber, was able to generate a single and collimated electron beam with a current of 1.2 nA at a turn-on voltage of 210 V, and the current increased to 100 nA when the applied voltage reached 325 V. After the treatment of the nanoneedle tip, the field emission exhibited a stable emission for 150 min with a fluctuation of 1.4% and an emission current density as high as 1.4 × 1010 A m-2. This work presents an efficient and controllable method for fabricating nanostructures, and this method is applicable to the transition metal compound ZrC as a field emission emitter, demonstrating its potential as an electron source for electron-beam devices.

Use of 3D Printing Technology to Improve Lead Shield Fabrication for Electron Therapy of the Face.

Superficial lesions of the face are often treated with an electron beam and surface collimation utilizing a conformal lead shield with an opening around the region of treatment (ROT). To fabricate the lead shield, an imprint of the patient face is needed. Historically, this was achieved using a laborious and time-consuming process that involved a gypsum imprinted model (GIM) of the patient topography. We propose utilization of 3-dimentional (3D) printing technology to create a 3-dimensional printed custom model (3D-PCM) of the patient facial topography as a more accurate and more efficient alternative to GIM. GIM and 3D-PCM were generated for three patients requiring en face electron therapy of the nose. The models for both methods were then CT-scanned and fused rigidly to the CT of the patient. The accuracy of the models was compared with the CT image of the patient via visual inspection and the Sørensen-Dice similarity coefficient (DSC). The efficiency of the two methods was evaluated by the average time needed to complete each process based on user-reported experience. The average DSC between the patient and GIM is 0.95336 (standard deviation (SD) = 0.0099479), while the average DSC of the patient and 3D-PCM is 0.97886 (SD = 0.0037441). With respect to efficiency, the average time to fabricate and dry GIM is 54.5 hours with hands-on time of 2.5 hours, while generation of 3D-PCM takes about 6.5 hours, with hands on time of approximately 2.5 hours. 3D-PCMs based on CT scan images are found to be an excellent substitute for GIMs by exhibiting a higher degree of fidelity with patient's anatomy, requiring significantly less time to complete, being less labor intensive, and allowing for greater patient comfort. The disadvantage of exposing the patient to radiation associated with the CT scan image acquisition for designing a 3D-PCM could be eliminated by employing 3D-camera scanning technology.

Monte Carlo source verification of Elekta Synergy for pMLC collimated electron beams

IntroductionIn previous works, an electron beam source was developed that models its energy spectrum with a Lévy distribution. This source is used in BEAMnrc Monte Carlo simulation. It was benchmarked against water tank measurements for electrons delivered through an applicator collimation system, and it was found to agree with 2%/2 mm. This study uses this electron source further by investigating its ability to simulate electrons collimated by the photon multi-leaf collimators for MERT applications. Simulated and Gafchromic EBT3 depth dose and profile data were compared. MethodMeasured data were obtained from film exposures in a mini phantom with water-equivalent slabs at SSDs of 60 and 70 cm, respectively. Output factors and R90 values were calculated from film strip data taken in the depth direction. Corresponding dose data were simulated in BEAMnrc to obtain phase-space files for each beam and used in DOSXYZnrc to obtain dose data in a 3D water tank with 2 mm side length subvoxels. ResultsSimulation data corresponded with film data within 3 %/2 mm in most cases. Uncertainties in film measurements prevent a more accurate agreement. The output factors showed good agreement and could be fitted satisfactorily with an analytic function. A clinical example of a scalp treatment showed good agreement between simulated and film data. ConclusionIt is concluded that the electron source with the Lévy-based energy distribution can model electron dose for photon multi-leaf collimated electron beams within 3 %/2 mm.

Thrust Generation Through a Collimated Electron Beam in a Magnetic Mirror and its Possible Applications in Plasma Thrusters

In this article, we apply some basic concepts of electric thrusters, using Monte Carlo simulations. One such concept is the thrust, which we calculate for the simulated case of a simple thruster, consisting of two stages. The goal is to study the thrust in a magnetic bottle as a function of the collimation of an assembly of particles. In the first stage, a voltage applied between a pair of electrodes accelerates the particles. We perform simulations of the behavior of the assembly of charged particles. A normal distribution is used for the initial velocities of the assembly, with a given standard deviation. The resulting velocity distribution and its dependence on the applied voltage are analyzed. Further, the set of particles, whose pitch angles follow a normal distribution with a given standard deviation, enters a magnetic bottle where it is collimated to reduce its dispersion upon the exit of the thruster. In this second stage, we study the conditions for which the collimation produced, by the magnetic bottle, leads to the least thrust reduction. For the simulations, we assume electrons; However, the concepts apply to any charged particle species. The simulations show that the standard deviation of the pitch angle distribution has an important influence on the confinement of charged particles and on the thrust that can be generated when collimating the particle beam in a magnetic bottle.

THz transition radiation of electron bunches laser-accelerated in long-scale near-critical-density plasmas

Direct laser electron acceleration in near-critical density plasma produces collimated electron beams with high charge Q (up to µC). This regime could be of interest for high-energy THz radiation generation, as many of the mechanisms have a scaling ∝Q2 . In this work, we focus specifically on the challenges that arise during numerical investigations of transition radiation in such interactions. Detailed analytical calculations that include both the diffraction and decoherence effects of the characteristics of transition radiation in the THz range were conducted with the input parameters obtained from 3D particle-in-cell and hydrodynamic simulations. The calculated characteristics of THz radiation are in good agreement with the experimentally measured ones. Therefore, this approach can be used both to optimize the properties of THz radiation and to distinguish the transition radiation contribution if several mechanisms of THz radiation generation are considered.

Laser Plasma‐Accelerated Ultra‐Intense Electron Beam for Efficiently Exciting Nuclear Isomers

AbstractUtilizing a laser plasma wakefield to accelerate ultrahigh charge and current electron beams is critical for many pioneering applications, for example, to efficiently produce nuclear isomers with short lifetimes that may be widely used. However, because of the beam loading effect, the electron charge in a single plasma bubble is limited to hundreds of picocoulombs. Herein, via a tightly focused intense laser pulse self‐guiding in dense gas, a twenty‐nanocoulomb, tens‐of‐MeV, hundred‐kiloampere collimated electron beam is produced from a chain of plasma bubbles in the saturated injection regime. This intense electron beam is utilized to excite indium nuclear isomers with an ultrahigh peak rate of particles·s‐1 via photonuclear reaction. This efficient isomer production method can be widely used for pumping isotopes with excited state lifetimes on the picosecond scale, which is beneficial for deeply understanding nuclear transition mechanisms or stimulating γ‐ray lasers.

Study of a low-energy collimated beam electron source and its application in a stable ionisation gauge

Stable ionisation vacuum gauges obtain stable sensitivity by controlling both the well-defined electron trajectory and electron energy in the ionisation volume. To achieve this goal, a collimated electron beam and large ionisation cross-section are required as the electrons enter the ionisation volume. In this paper, a low-energy electron source based on carbon nanotubes is designed to realise a collimated electron beam by the addition of specialised deceleration and focus electrodes. The experimental transmission is approximately 25%, which is in good agreement with the simulation results. Furthermore, a simulation study is carried out combining the electron source with the components of a novel stable ionisation gauge. The numerical simulation results show a sensitivity of 0.250 Pa-1. It provides an important reference for the development of stable ionisation gauge based on the field emission cathode.

Valley-polarized and supercollimated electronic transport in an 8-Pmmn borophene superlattice

Analogous to real spins, valleys as carriers of information can play significant roles in physical properties of two-dimensional Dirac materials. On the other hand, utilizing external periodic potential is an efficient method to manipulate their band structures and transport properties. In this work, we investigate the valley dependent optics-like behaviors based on an 8-Pmmn borophene superlattice with the transfer matrix method and effective band approach. Firstly, it is found that the band structure is renormalized, more tilted Dirac cones are generated, and the group velocities are modified by the periodic potentials. Secondly, due to the exotic tilted Dirac cones in 8-Pmmn borophene, a perfect valley selected angle filter can be realized. The electrons with a specific incident angle can transmit completely in an energy window, which is flexibly tunable by changing the periodic potential. Thirdly, by using the Green’s function to simulate the time evolution of wave packets, electrons can be shown to propagate without any diffraction, valley electron beam supercollimation happens by modulating the potential parameters. Different from the graphene superlattice, the electron supercollimation here is valley dependent and can be used as a valley electron beam collimator. Fourthly, we can tune the polarization and supercollimation angles by changing the superlattice direction. These intriguing results in an 8-Pmmn borophene-based superlattice offer more opportunities in diverse electronic transport phenomena and may facilitate the devices applications in valleytronics and electron-optics.

Ultra-relativistic electron beams deflection by quasi-mosaic crystals

This paper provides an explanation of the key effects behind the deflection of ultra-relativistic electron beams by means of oriented ‘quasi-mosaic’ Bent Crystals (qmBC). It is demonstrated that accounting for specific geometry of the qmBC and its orientation with respect to a collimated electron beam, its size and emittance is essential for an accurate quantitative description of experimental results on the beam deflection by such crystals. In an exemplary case study, a detailed analysis of the recent experiment at the SLAC facility is presented. The methodology developed has enabled to understand the peculiarities in the measured distributions of the deflected electrons. Also, this achievement constitutes an important progress in the efforts toward the practical realization of novel gamma-ray crystal-based light sources and puts new challenges for the theory and experiment in this research area.

Collimation of fast electron beams by strong external magnetic field in laser–solid interaction using Monte Carlo simulation

In this paper, the effect of the external magnetic field on the fast electron beam angular spread and spatial divergence in laser–solid interaction were investigated and energy deposition of these electrons into the high-density fuel was evaluated using Monte Carlo simulation by Geant4 simulation toolkit. In our simulation, two types of the energy distribution function, exponential energy distribution, and quasi-two temperature energy distribution function, for electrons were considered. Our simulations in the presence of an external magnetic field show that the dynamics and spatial divergence of the high energy electrons are strongly affected by the external magnetic field. It was shown that in the case with the two-temperature energy distribution, the number of electrons (electron number density) with a large angular spread in the momentum space which were trapped by the magnetic fields lines and collimated toward the fuel were more populated than that of the exponential function. Comparison of the results indicate that the acceptable condition is obtained for the density in the presence of a magnetic field of about with a laser wavelength and laser intensity . It was presented that in the case with the sufficiently strong external fields, , it seems that the optimal results for suppression of electron spatial divergence, electron beam collimation, and energy deposition rate are obtained.

Super-Klein tunneling and electron-beam collimation in the honeycomb superlattice

The honeycomb superlattice model from the nearly commensurate charge-density-wave phase of $1\mathrm{T}\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ has been shown to host multiple robust flat bands that depends on the number $(m)$ of extra atoms residing on each edge of the hexagon [Lee et al., Phys. Rev. Lett. 124, 137002 (2020)]. In this work we numerically study the transport phenomenon of the single-valley Dirac electrons around the band center $E=0$ where a flat band crosses. A super-Klein tunneling (SKT) phenomenon is found for $m=1$ that the Dirac electrons can tunnel through a rectangle barrier with a unity transmission irrespective of the incident angle of electrons. For the case of $m=3$, a super electron-beam collimation instead of the SKT phenomenon is identified that the Dirac electrons are allowed to tunnel through the barrier only within nearly zero incident angle, and such perpendicular transmission probability is optimal in the low-energy regime but suppressed in the high-energy regime.

High-endurance micro-engineered LaB6 nanowire electron source for high-resolution electron microscopy

The size tunability and chemical versatility of nanostructures enable electron sources of high brightness and temporal coherence, both of which are important characteristics for high-resolution electron microscopy1-3. Despite intensive research efforts in the field, so far, only conventional field emitters based on a bulk tungsten (W) needle have been able to yield atomic-resolution images. The absence of viable alternatives is in part caused by insufficient fabrication precision for nanostructured sources, which require an alignment precision of subdegree angular deviation of a nanometre-sized emission area with the macroscopic emitter axis4. To overcome this challenge, in this work we micro-engineered a LaB6 nanowire-based electron source that emitted a highly collimated electron beam with good lateral and angular alignment. We integrated a passive collimator structure into the support needle tip for the LaB6 nanowire emitter. The collimator formed an axially symmetric electric field around the emission tip of the nanowire. Furthermore, by means of micromanipulation, the support needle tip was bent to align the emitted electron beam with the emitter axis. After installation in an aberration-corrected transmission electron microscope, we characterized the performance of the electron source in a vacuum of 10-8 Pa and achieved atomic resolution in both broad-beam and probe-forming modes at 60 kV beam energy. The natural, unmonochromated 0.20 eV electron energy loss spectroscopy resolution, 20% probe-forming efficiency and 0.4% probe current peak-to-peak noise ratio paired with modest vacuum requirements make the LaB6 nanowire-based electron source an attractive alternative to the standard W-based sources for low-cost electron beam instruments.

Electron acceleration in intense laser – solid interactions at parallel incidence

Using multidimensional particle-in-cell simulations, we show that an electron beam with a huge space charge can be accelerated to high energies by irradiating the edge of a solid density target with an intense femtosecond laser pulse at parallel incidence. The process of energy gain of each electron is divided into two parts: the transverse laser field and the longitudinal field of the excited surface plasma wave (SPW). It is shown that the longitudinal field of the SPW dominates the acceleration of the major part of electrons. This process leads to generation of a highly collimated electron beam with a huge space charge.

Role of contrast of a relativistic femtosecond laser pulse interacting with solid and structured targets

We consider the effect of a pre-plasma layer inevitably present in experiments on the acceleration of electrons and ions during interaction of a relativistic femtosecond laser pulse with a dense plasma. The interaction regimes are identified in which the presence of such a layer can significantly increase the average and maximum energies of electrons. The regimes are discussed in which an artificial nanosecond prepulse makes it possible to produce a collimated electron beam with a high charge and an average energy of up to 10 ponderomotive energies in the direction of the reflected or incident laser beam. It is shown that the acceleration of ions, as a rule, requires an ultrahigh contrast of the laser pulse, since the parameters of the accelerated ion beams deteriorate significantly in the presence of preplasma or due to the evaporation of a thin-film target. The regimes of interaction of laser pulses with thick targets, in which heavy multiply charged ions can be accelerated by cleaning the surface with a prepulse, are also discussed. An essential part of the review is devoted to the interaction of radiation with micro- and nanostructured targets. Both the methods of their fabrication and the issues related to the interaction of a femtosecond laser pulse and its contrast with such structures are considered.

MULTI-PURPOSE MODES OF FORMATION OF ELECTRON BEAMS IN THE CYLINDRICAL MAGNETIC FIELD OF THE MAGNETRON GUN

The results of the study on the formation of electron beams by the magnetron gun at various configurations of the magnetic field in the beam transport channel are presented. A technique for modeling the processes of formation of electron flows and control of the distribution of beams by collimation is presented. Numerical simulation of the dynamics of electron beams in the magnetic field of the gun for its various configurations has been carried out. Experimental data on the transportation and collimation of electron beams are presented. The possibility of stable formation of an electron beam in the axial direction during its transportation is shown. Imprints of the collimated electron beam were obtained on metal targets. The possibility of controlling the beam diameter by varying the magnetic field is shown. Comparison of the results of numerical modeling and experimental data on the motion and collimation of the tubular electron flow is carried out.

Extremely Collimated Electron Beams in the High Latitude Magnetosphere Observed by Arase

AbstractThis report describes electron beams observed at high latitudes in the magnetosphere by the Arase satellite. According to fine angular channel measurements, beam electrons are mainly flowing away from the Earth, and are well collimated to within a few degrees. A statistical study indicates that the beams are seen when Arase is located at Lm &gt; ∼5, and more frequently as the magnetosphere becomes active. The field lines of the beam events are traced back to the typical auroral ovals. The statistical properties of the beams are consistent with the interpretation that ionospheric electrons are accelerated by a parallel electric field of an auroral potential structure and are streaming upward to the high latitude magnetosphere.

Transparent mirror effect in twist-angle-disordered bilayer graphene

When light is incident on a medium with spatially disordered index of refraction, interference effects lead to near-perfect reflection when the number of dielectric interfaces is large, so that the medium becomes a "transparent mirror." We investigate the analog of this effect for electrons in twisted bilayer graphene (TBG), for which local fluctuations of the twist angle give rise to a spatially random Fermi velocity. In a description that includes only spatial variation of Fermi velocity, we derive the incident-angle-dependent localization length for the case of quasi-one-dimensional disorder by mapping this problem onto one dimensional Anderson localization. The localization length diverges at normal incidence as a consequence of Klein tunneling, leading to a power-law decay of the transmission when averaged over incidence angle. In a minimal model of TBG, the modulation of twist angle also shifts the location of the Dirac cones in momentum space in a way that can be described by a random gauge field, and thus Klein tunneling is inexact. However, when the Dirac electron's incident momentum is large compared to these shifts, the primary effect of twist disorder is only to shift the incident angle associated with perfect transmission away from zero. These results suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.

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.

Towards MR-guided electron therapy: Measurement and simulation of clinical electron beams in magnetic fields

In the current era of MRI-linac radiotherapy, dose optimization with arbitrary dose distributions is a reality. For the first time, we present new and targeted experiments and modeling to aid in evaluating the potential dose improvements offered with an electron beam mode during MRI-linac radiotherapy. Small collimated (1 cm diameter and 1.5 × 1.5 cm2 square) electron beams (6, 12 and 20MeV) from a clinical linear accelerator (Varian Clinac 2100C) are incident perpendicular and parallel to the strong and localized magnetic fields (0-0.7T) generated by a permanent magnet device. Gafchromic EBT3 film is placed inside a slab phantom to measure two-dimensional dose distributions. A benchmarked and comprehensive Monte Carlo model (Geant4) is established to directly compare with experiments. With perpendicular fields a 5% narrowing of the beam FWHM and a 10mm reduction in the 15% isodose penetration is seen for the 20MeV beam. In the inline setup the penumbral width is reduced by up to 20%, and a local central dose enhancement of 100% is observed. Monte Carlo simulations are in agreement with the measured dose distributions (2% or 2mm). A new range of experiments have been performed to offer insight into how an electron beam mode could offer additional choices in MRI-linac radiotherapy. The work extends on historic studies to bring a successful unified experimental and Monte Carlo modeling approach for studying small field electron beam dosimetry inside magnetic fields. The results suggest further work, particularly on the inline magnetic field scenario.

Effect of laser polarization and target location on acceleration of electrons generated during ionization of gases by a laser pulse

We investigate the effect of the target position, laser polarization, and focusing on the energy spread and the angle of emittance for the acceleration of electrons generated during the ionization of rarefied gases, neon ions Ne8+, krypton ions Kr32+, and argon ions Ar16+ by a laser pulse. The electrons generated from the ions at the position after laser focus interact with the laser pulse for a longer duration and gain more energy than those electrons generated from the ions at the position before laser focus. There are two peaks in the energy spectrum for linear polarization and only one peak for circular polarization. The energy spectrum peak is sharper for circular polarization than that for linear polarization. The energy gained by the electrons increases with the laser spot size due to the increase in the laser energy. The spectrum of the angle of emittance for electrons shows the sharpest peak at the lowest angle for linear polarization for ions at the position after laser focus. The circular polarization is good to obtain quasi-monoenergetic electron beams and linearly polarized laser pulse is good to generate collimated electron beams. The required laser intensity to ionize electrons from the ions Ne8+, Kr32+, and Ar16+ increases and the electron energy peaks are at higher energies and scattering angles are at lower values for these gases, in their respective orders.