Narrow Electron Beam Research Articles

Injection and acceleration of quasimonoenergetic relativistic electron beams using density gradients at the edges of a plasma channel

The injection of quasimonoenergetic electron beams into a laser wakefield accelerator is demonstrated experimentally using density gradients at the edges of a plasma channel. In the experiment, two laser pulses are used; the main laser pulse drives a wakefield, while a second countercrossing laser beam produces a plasma whose expansion creates a channel with significant density gradients. Local injection of electrons in the wakefield is triggered by wave breaking in the density ramp. The injection is localized spatially and leads to the generation of collimated and narrow energy spread relativistic electron beams at the 100 MeV level, with charges in the range of 20–100 pC. The stability of this injection process is compared to the stability of the colliding pulse injection process and is found to be inferior for our experimental conditions. On the other hand, it is found that as the electron beam divergence is smaller in the case of gradient injection, the transverse emittance might be better.

Small-signal theory of a grating-based free-electron laser in three dimensions

We present an analytic theory for the small-signal operation of a grating-based free-electron laser that includes the effects of transverse diffraction on the evanescent wave. In this device, the electron beam interacts with an evanescent wave of the grating that bunches the beam and creates superradiant Smith-Purcell radiation. We find that the evanescent wave is guided by the electron beam, giving an optical-mode width that depends on the gain. We consider the cases of very wide and very narrow electron beams. For a wide electron beam, the cubic dispersion relation previously found for slow-wave structures is recovered. When the electron beam is narrow, so that gain guiding is important, a fifth-order dispersion relation is found instead. Diffraction in a system where the group velocity is very different (sometimes negative) from the phase velocity leads to unexpected results. The Brillouin zone subdivides into four regions; only two physically allowed (gain-guided) roots are obtained in the regions near the center of the Brillouin zone, but three are found in the regions away from the center. In the left half of the Brillouin zone, corresponding to high electron energy, the device operates on a convective instability, as an amplifier. In the right half of the Brillouin zone, where the group velocity is negative, the device operates on an absolute instability, as an oscillator. In the region where only two guided modes exist, oscillator operation will be more difficult.

The effect of electron collimator leaf shape on the build-up dose in narrow electron MLC fields

Previously, we have found that the build-up dose from abutting narrow electron beams formed with unfocussed electron multi-leaf collimator (eMLC) steal leaves was higher than with the respective open field. To investigate more closely the effect of leaf material and shape on dose in the build-up region, straight, round (radius 1.5 cm) and leaf ends with a different front face angle of α (leaf front face pointing towards the beam axis at an angle of 90 − α) made of steel, brass and tungsten were modelled using the BEAMnrc code. Based on a treatment head simulation of a Varian 2100 C/D linac, depth–dose curves and profiles in water were calculated for narrow 6, 12 and 20 MeV eMLC beams (width 1.0 cm, length 10 cm) at source-to-surface distances (SSD) of 102 and 105 cm. The effects of leaf material and front face angle were evaluated based on electron fluence, angle and energy spectra. With a leaf front face angle of 15°, the dose in the build-up region of the 6 MeV field varied between 91 and 100%, while for straight and round leaf shapes the dose varied between 89 and 100%. The variation was between 94 and 100% for 12 and 20 MeV. For abutting narrow 6 MeV fields with total field size 5 × 10 cm2, the build-up doses at 5 mm depth for the face angle 15° and straight and round leaf shapes were 96% and 86% (SSD 102 cm) and 89% and 85% (SSD 105 cm). With higher energies, the effect of eMLC leaf shape on dose at 5 mm was slight (3–4% units with 12 MeV) and marginal with 20 MeV. The fluence, energy and angle spectra for total and leaf scattered electrons were practically the same for different leaf materials with 6 MeV. With high energies, the spectra for tungsten were more peaked due to lower leaf transmission. Compared with straight leaf ends, the face angle of 15° and round leaf ends led to a 1 mm (for 6 MeV) and between 1 and 5 mm (12 and 20 MeV at a SSD of 105 cm) decrease of therapeutic range and increase of the field size, respectively. However, profile flatness was better for abutting 6 MeV beams with round (2.5%) and face angle 15° leaves (3.0%) compared to straight leaf shape (5.2%). The eMLC leaves with a face angle of 15° resulted in a marked increase in the build-up dose for the single narrow eMLC beam and thus in the dose in the build-up region from matched abutting fields.

A method of determining narrow energy spread electron beams from a laser plasma wakefield accelerator using undulator radiation

In this paper a new method of determining the energy spread of a relativistic electron beam from a laser-driven plasma wakefield accelerator by measuring radiation from an undulator is presented. This could be used to determine the beam characteristics of multi-GeV accelerators where conventional spectrometers are very large and cumbersome. Simultaneous measurement of the energy spectra of electrons from the wakefield accelerator in the 55–70 MeV range and the radiation spectra in the wavelength range of 700–900 nm of synchrotron radiation emitted from a 50 period undulator confirm a narrow energy spread for electrons accelerated over the dephasing distance where beam loading leads to energy compression. Measured energy spreads of less than 1% indicates the potential of using a wakefield accelerator as a driver of future compact and brilliant ultrashort pulse synchrotron sources and free-electron lasers that require high peak brightness beams.

Interaction of surface electromagnetic waves with an electron beam moving along the interface between metamaterials

Excitation of surface polaritons by an infinitely narrow electron beam propagating in the vacuum gap between a metal-like medium and an artificial dielectric with a negative permeability is studied theoretically. A dispersion relation is derived for the waves excited by the beam for an arbitrary thickness of the gap. The possibility of the emergence of absolute instability is demonstrated for an infinitely narrow vacuum gap, and the corresponding increments are calculated with allowance for small dissipative losses.

Numerical solution of the FEL correlation function equation

The equation for two-particle correlation function in FEL was derived recently to provide a new way of noise calculations in SASE FELs [O.A. Shevchenko, N.A. Vinokurov, Nucl. Instr. and Meth. A 507 (2003) 84]. In this paper, this equation is solved numerically for the simplest case of narrow electron beam. Time-independent solution with saturation is obtained. It is compared with the results of quasilinear theory and results of previous SASE linewidth estimates.

Dosimetry of Narrow Electron Beams used for the Treatment of Keloids by Postoperative Radiotherapy

Dosimetry of Narrow Electron Beams used for the Treatment of Keloids by Postoperative Radiotherapy

A theory of narrow vortex relativistic electron beams with an elliptic section

Narrow vortex relativistic beams with an elliptic section are considered. The beams propagate along an arbitrary smooth spatial curve in the presence of an external nonuniform magnetic field. A model is proposed for description of such beams in a coordinate frame associated with current tubes. It is shown that, contrary to known results, the axial distributions of the potential and magnetic field and the shape of a beam can be specified independently. Solution of the problem is reduced to integration of a single second-order ordinary differential equation.

Formation of narrow low-energy high-intensity electron beams

The process of formation of high-density low-energy (5–10 keV) pulsed electron beams of small diameter (on the order of a few millimeters) in a gun of the “channel-spark” type is studied. It is shown that beams with a rate of rise of the current exceeding 1011 A/s and an amplitude exceeding the Alfven current by a factor of 1.5–2.0 can be obtained in experiments with intense preionisation of the transport channel combined with a pulsed supply of the accelerating voltage to the cathode. In the optimal pressure mode, the current density at a distance of 2–3 cm from the gun outlet is 40–25 kA/cm2, which will ensure ablation of most solid targets.

Characterisation of mega-voltage electron pencil beam dose distributions: viability of a measurement-based approach

The concept of electron pencil-beam dose distributions is central to pencil-beam algorithms used in electron beam radiotherapy treatment planning. The Hogstrom algorithm, which is a common algorithm for electron treatment planning, models large electron field dose distributions by the superposition of a series of pencil beam dose distributions. This means that the accurate characterisation of an electron pencil beam is essential for the accuracy of the dose algorithm. The aim of this study was to evaluate a measurement based approach for obtaining electron pencil-beam dose distributions. The primary incentive for the study was the accurate calculation of dose distributions for narrow fields as traditional electron algorithms are generally inaccurate for such geometries. Kodak X-Omat radiographic film was used in a solid water phantom to measure the dose distribution of circular 12 MeV beams from a Varian 21EX linear accelerator. Measurements were made for beams of diameter, 1.5, 2, 4, 8, 16 and 32 mm. A blocked-field technique was used to subtract photon contamination in the beam. The "error function" derived from Fermi-Eyges Multiple Coulomb Scattering (MCS) theory for corresponding square fields was used to fit resulting dose distributions so that extrapolation down to a pencil beam distribution could be made. The Monte Carlo codes, BEAM and EGSnrc were used to simulate the experimental arrangement. The 8 mm beam dose distribution was also measured with TLD-100 microcubes. Agreement between film, TLD and Monte Carlo simulation results were found to be consistent with the spatial resolution used. The study has shown that it is possible to extrapolate narrow electron beam dose distributions down to a pencil beam dose distribution using the error function. However, due to experimental uncertainties and measurement difficulties, Monte Carlo is recommended as the method of choice for characterising electron pencil-beam dose distributions.

A theory of narrow relativistic electron beams

Various models describing narrow electron beams are analyzed. The models are based on the theory proposed by Danilov, the theory developed from Ovcharov’s ideas, and the recent results obtained with the use of the geometrized theory of dense beams. The possibilities provided by different approaches are assessed and compared.

Three-dimensional theory of the Cerenkov free-electron laser

We describe an analytical theory for the gain of a Cerenkov free-electron laser including diffraction of the optical beam in the direction transverse to the electron beam, parallel to the surface of the dielectric. Since the width of the optical beam depends on the gain, the usual cubic dispersion relation of two-dimensional slow-wave structures is replaced by a 5∕2-power dispersion relation, but three of the five roots are extraneous. The results show that for a narrow electron beam, the optical beam is much wider than the electron beam. This reduces the gain by an order of magnitude. Moreover, in the three-dimensional theory the allowed roots of the dispersion relation have positive real parts, so they correspond to slow waves; when transverse diffraction is included, fast waves are forbidden.

Hydrogen-induced Nitrogen Passivation in Dilute Nitrides: A Novel Approach to Defect Engineering

AbstractThe capability of hydrogen to passivate nitrogen in dilute nitrides is exploited to in-plane engineer the electronic properties of Ga(AsN)/GaAs heterostructures. Two methods are presented: i) by deposition of hydrogen-opaque metallic masks on Ga(AsN) and subsequent hydrogen irradiation, we artificially create zones of the crystal having the band gap of untreated Ga(AsN) surrounded by GaAs-like barriers; ii) by employing an intense (∼100 nA) and narrow (∼100 nm) beam of electrons, we dissociate the complexes formed by N and H in a spatially delimited part of a hydrogenated Ga(AsN) sample. As a consequence, in the spatial regions irradiated by the electron beam, hydrogenated Ga(AsN) recovers the smaller energy gap it had before hydrogen implantation.

Model of secondary emission and its application on the charging of gold dust grains

Dust grains of various shapes and materials can be found anywhere in the space where the temperature decreases under a melting point. Many plasma processes are driven by the grain charge but, on the other hand, dust grain charging can be influenced by several different processes, too. Among others, secondary emission can play a dominant role wherever the plasma is hot. In the present paper, a further development of our previous simple numerical secondary emission model, now using techniques for a beam interaction modelling applied in electron microscopies, is sketched and its application onto charging of spherical dust grains by a narrow electron beam is discussed. The model fits experimental data rather well and, thus, our conclusion suggests that the model is a very useful tool for considerations on the dust charging under different conditions in the space.

Short Rayleigh length free electron lasers

Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. We model this interaction using a coordinate system that expands with the rapidly diffracting optical mode from the ends of the undulator to the mirrors. Simulations show that the interaction of the strongly focused optical mode with a narrow electron beam inside the undulator distorts the optical wave front so it is no longer in the fundamental Gaussian mode. The simulations are used to study how mode distortion affects the single-pass gain in weak fields, and the steady-state extraction in strong fields.

Radiotherapy with laser‐plasma accelerators: Monte Carlo simulation of dose deposited by an experimental quasimonoenergetic electron beam

The most recent experimental results obtained with laser-plasma accelerators are applied to radio-therapy simulations. The narrow electron beam, produced during the interaction of the laser with the gas jet, has a high charge (0.5 nC) and is quasimonoenergetic (170 +/- 20 MeV). The dose deposition is calculated in a water phantom placed at different distances from the diverging electron source. We show that, using magnetic fields to refocus the electron beam inside the water phantom, the transverse penumbra is improved. This electron beam is well suited for delivering a high dose peaked on the propagation axis, a sharp and narrow tranverse penumbra combined with a deep penetration.

Interaction of electromagnetic waves with a radially bounded electron beam

The resonant interaction of a narrow electron beam spiraling in a cold homogeneous magnetized plasma with electromagnetic cylindrical waves is considered. The conclusions derived in recent papers about the formation, in the presence of effective electromagnetic energy dissipation out of the radially bounded beam, of dynamically stable and longtime living bunches of resonant electrons in the developed stage of their nonlinear interaction with the waves are discussed in this paper for more general physical cases and for wider ranges of characteristic parameters. General analytical formulas for the excited electromagnetic fields, the radiated electromagnetic power flux and the linear growth rate as well as relevant numerical simulations results are presented.

Generations and properties on narrow electron flows in mesoscopic solid-state structures

A method for the evaluation of the angular width of an electron beam generated by a nanoconstriction is proposed and demonstrated. The approach is based on analysis of a narrow-width electron flow that quantizes into modes inside a confining constriction, which is described in the adiabatic approximation, evolving into a freely propagating electronic state after exiting the constriction. The method that we developed allows us to find the parameters and the shape of the constriction that are optimal for generation of extremely narrow electron beams. In the case of a constriction characterized by a linear widening shape, an asymptotically exact solution for the injection problem is found. That solution verifies semiquantitative results related to the angular characteristics of the beam, and it opens the way for determination of the distribution function of the electrons in the beam. We have found the relationship between the angular distribution of the electron density in the beam and the quantum states of the electrons inside the constriction. Such narrow electron beams may be employed in investigations of electronic systems and in data manipulations in electronic and spintronic devices.

MCM41 and silica supported MoVTe mixed oxide catalysts for direct oxidation of propane to acrolein

Bulk Mo 1.3V 0.3Te 0.3 mixed oxide and MCM41 and amorphous silica supported Mo 1.3V 0.3Te 0.3 mixed oxide (3 and 8 mol%, i.e. 6 and 15% monolayer coverage on MCM41 and 19 and 51% monolayer coverage on silica, respectively) catalysts have been synthesized and studied for direct oxidation of propane (DOP) to acrolein (ACR). After calcination at 853 K for 2 h under flowing nitrogen, characterization results (XRD, Raman and TEM-EDX) show that Mo species, MoV, MoTe and VTe mixed oxides with various compositions are present in the samples before and after testing, but after testing the EDX analyses indicate more domains rich in Mo when using a narrow electron beam (Ø∼5–10 nm in size), particularly for bulk and 8 mol% supported samples. At variance, broad electron beam (Ø∼50–100 nm in size) EDX analyses give values close to chemical compositions. This shows that under catalytic conditions, solid-state reaction has occurred leading to restructuring of several phases present in the samples, with extraction of some volatile MoO 3. Catalytic study at reaction temperature 773 K shows that amorphous silica and MCM41 supports exhibit catalytic activity in DOP to ACR, respectively. This means that homogeneous reaction occurs in the gas phase under the studied conditions after initiation on the solid surface. Mo 1.3V 0.3Te 0.3 mixed oxide is shown to be highly dispersed on MCM41 and silica supports, particularly for low mixed oxide content (3 mol%) leading to higher selectivity and yield in ACR obtained than that for bulk samples, in particular for MCM41 support. This is indicated to arise from a better dispersion of the active phases, which favors redox properties of the catalytic materials and prevents deep oxidation of propane.

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

Large‐amplitude (up to ∼50 mV/m) solitary waves, identified as electron holes, have been observed during waveform captures on two of the four Cluster satellites during several plasma sheet encounters that have been identified as the passage of a magnetotail reconnection x line. The electron holes were seen near the outer edge of the plasma sheet, within and on the edge of a density cavity, at distances on the order of a few ion inertial lengths from the center of the current sheet. The electron holes occur during intervals when there were narrow electron beams but not when the distributions were more isotropic or contained beams that were broad in pitch angle. The region containing the narrow beams (and therefore the electron holes) can extend over thousands of kilometers in the x and y directions, but is very narrow in the z direction. The association with electron beams and the density cavity and the location along the separatrices are consistent with simulations shown herein. The velocities and scale sizes of the electron holes are consistent with the predictions of Drake et al. [2003]. Particle simulations of magnetic reconnection reproduce the observed Cluster data only with the addition of a small (0.2 of the reversed field) ambient guide field. The results suggest that electron holes may sometimes be an intrinsic feature of magnetotail reconnection and that in such cases the traditional neglect of the guide field may not be justified. Very large amplitude lower hybrid waves (hundreds of millivolts per meter), as well as waves at frequencies up to the electron plasma frequency, were also observed during this interval.