Electron density on the surface of a dielectric interacting with a plasma

The density functional method is used to calculate the electron density on the surface of a dielectric interacting with a gas-discharge plasma.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 48, No. 2, pp. 284–289, February, 1985.     

Electron and photon fields for dosimetric metrology generated by electron beams from a microtron

Final results of investigation of collimating systems for the generation of electron and photon fields for dosimetric metrology are presented together with basic information concerning the circular microtron used as a source of relativistic electrons for metrological purposes. Isodose plots of the photon and electron fields and absorbed depth dose curves for photon energies of 10 and 19 MeV and electron energies of 9.8, 14.6 and 18.7 MeV demonstrate the possibility of using a circular microtron as a source of homogeneous and symmetrical fields for calibration of dosimetric devices.     

Enhanced linear polarization for tagged photon beams

The Illinois photon tagging facility has been modified to provide beams with a high degree of linear polarization. The method allows the polarization of an off-axis tagged photon beam to be greatly enhanced by means of a kinematic selection of the post-bremsstrahlung residual electrons that are used for tagging. As an initial test of the system, photon scattering was observed from the strong 1⁺ levels in magnesium and silicon.     

Electron transitions and radiation of atoms interacting with ultrashort electromagnetic pulses

Excitation and ionization of an atom interacting with a short electromagnetic pulse is studied within the framework of the sudden perturbation approximation. The excitation and ionization probabilities and the spectra and cross sections of reradiation of the pulse by the atom are calculated. It is suggested that the process of reradiation of ultrashort electromagnetic pulses by multielectron atoms possesses a coherent character.     

Bending moment and axial force interacting on solid timber beams

This paper describes the results of bending moment (M) and axial force (N) interaction tests on 220 solid spruce specimens with cross-section 80×160 mm. Depending on the timber strength, existing design models for M/N-interaction are fitted to the test results and different analytical approaches for the magnification or reduction of the initial moment due to beam deflections are evaluated. Interaction graphs of timber columns subjected to simultaneous action of bending moment and axial force exhibit convex curvilinearity viewed from the basis of the diagram. The degree of convexity depends on the slenderness of the column and on the strength of the timber. In the case of stocky columns subjected to high bending moments and small axial forces, the highest deviation from linearity can be observed. Design models published in standards and in literature describe the non-linearity of the interaction curve on the basis of an elastic stress-strain relationship in the tension portion of the cross-section and by a non-linear, “plastic” behaviour of the compression zone. Majority of former tests were performed on timber beams with small cross-section. For the case of interaction of bending and tension stress, only a limited number of results are available, because of the difficulty to apply high tension forces.     

Electron acceleration by single and double laser beams

The longitudinal electric field of single and double Gaussian laser beams are used to accelerate electrons. The longitudinal field of the single beam is concentrated on the axis and is favourable for acceleration. A set of two beams is considered. Beams run parallel, collinearly, overlap partially and have a phase difference iπ in between. As a result, the transverse components of fields cancel each other while the longitudinal components are double-fold. In both schemes, the electrons are accelerated in lengths of the Rayleigh range, which is common to the plasma-based accelerators.     

Proposal for a magnetic-film memory accessed by combined photon and electron beams

A magnetic-film memory accessed by combined photon and electron beams is proposed. The electron beam is used to heat a selected bit, which results in lowering the switching threshold so that information can be written selectively into that bit by means of an external magnetic field. Reading is accomplished by simultaneously illuminating a bit with an electron and a photon beam. Then a thermally modulated magnetic-optical signal is generated by intensity modulation of the electron beam. This arrangement is advantageous since a high-resolution photon beam and photon deflector are not required. The frequency response for thermally modulating a 1-μm bit is calculated to be ∼500 MHz; the necessary temperature dependence of the magneto-optical coefficient and the coercive force can be obtained by using composite films made from layers having different Curie points. Various magneto-optical configurations are readily devised which yield the value of a bit (one or zero) in terms of the phase (0 or π) between the magneto-optical signal and the electron-beam intensity modulation. The shot noise limited signal-to-noise ratio (SNR) is determined by heating of a bit from the photon beam. It is calculated that for a low-loss magnetooptical material such as EuO a 1-μm bit can be read in 1 μs when illuminated with a 1000-μm photon beam. The base-line temperature rise due to heating from the photon beam can be kept small by using narrow pulses of light (widthsim10^{-11}second), as, for example, from a mode-locked laser.     

Electron entrainment current in gases bombarded with beams of fast highly-charged ions

It is shown that asymmetry in the ejection of electrons in an elementary collision event between an atom and a fast highly-charged ion may lead to a macroscopic effect, the electron entrainment current, when a gas target is bombarded by a beam of fast highly-charged ions. The entrainment current is calculated for the bombardment of a helium target by a 25 MeV/u Mo⁴⁰⁺ beam. Pis’ma Zh. Tekh. Fiz. 24, 62–65 (September 26, 1998)     

Response of silicon diode dosemeters to scattered radiation from megavoltage photon beams

Dose response effects of diodes due to the high atomic number of silicon relative to water are investigated. While quality chances in the primary component of a megavoltage beam with depth are minimal. Compton scattered photons are shown to have a substantial effect on the quality leading to their enhanced absorption in silicon via the photoelectric effect. Monte Carlo methods were used to study and model this phenomenon. Measurements of dose rate, depth and field size dependence are examined for commercially available diode detectors and ionisation chambers.     

Using photon funnels based on metamaterial cloaks to compress electromagnetic wave beams

Based on the metamaterial cloaking technique, we propose the use of a new photon funnel to compress a plane electromagnetic (EM) wave. The theoretical analysis and numerical simulations indicate that the compression ratio can be designed optionally and the compressed wave beam remains the original wave shape without any distortions. Here we apply the method to EM waves but it can be applied to acoustic waves and other fields as well.     

Electron beams formed in a diode filled with air or nitrogen at atmospheric pressure

We have studied the electron beam formation in a diode filled with a molecular gas at atmospheric pressure. A beam current amplitude of up to ∼20 A at an electron energy of ∼70 keV was obtained in an air-filled diode. It is suggested that the main fraction of runaway electrons at low initial values of the parameter E/p (∼0.1 kV/(cm Torr)) is formed in the space between cathode plasma and anode. As the plasma spreads from cathode to anode, the electric field strength between the plasma front and anode increases and the E/p value reaches a critical level.     

Low energy proton beams from laser-generated plasma

Low energy proton beams are produced by ns pulsed Nd:Yag laser operating in repetition rate at intensities of the order of 10¹⁰ W/cm². Laser pulses interacting with thick and thin solid hydrogenated targets, placed in high vacuum, produce non-equilibrium plasmas with ion emission in backward and forward directions along the normal to the target surface.The ion emission is analyzed with time-of-flight techniques by using ion collectors and ion deflecting spectrometers. The spectra analysis permits the evaluation of the plasma temperature, density, proton energy and current, ion energy and charge state distributions.Special targets, based on thin polymers coupled to metals or to conducting nanostructures, induce high electric field in the plasma and confer high kinetic energy to the protons, up to about 200 eV.By using a post-acceleration system, placed along the target normal direction, it is possible to accelerate further the protons up to the energy depending on the applied acceleration voltage. The maximum proton energy of 30 keV, the current density of about10 nA/cm² and the beam quality can be improved, as discussed.     

Evaluation of the EDR-2 film for relative dosimetry of high-energy photon and electron beams

A sensitometric study of Kodak XV and EDR-2 radiographic films (Eastman Kodak Company, Rochester, NY) was performed using photons ranging from 75 kV to 18 MV and electrons ranging from 6 to 20 MeV. To investigate the applicability of the EDR-2 film for clinical radiation dosimetry, percentage depth-doses, profiles and distributions in open and dynamically wedged fields were measured using film and compared with data from a linear diode. Moreover, conventional quality assurance dose parameters were measured, including open-field dose profiles to determine flatness and symmetry of photon and electron beams. Finally, film was employed to validate dose distributions produced by complex computerised treatment planning techniques. Our conclusion is that the EDR-2 film is an effective tool for relative dosimetry of photon and electron beams.     

Teravolt-per-meter beam and plasma fields from low-charge femtosecond electron beams

Recent initiatives in ultra-short, GeV electron beam generation have been aimed at achieving sub-femtosecond (fs) pulses capable of driving X-ray free-electron lasers (FELs) in single-spike mode. This scheme foresees the use of very low charge beams, which may allow existing FEL injectors to produce few-100 as pulses, with very high brightness. Towards this end, recent experiments at SLAC have produced ∼2 fs rms, low transverse emittance, 20 pC electron pulses. Here we examine the use of such pulses to excite plasma wakefields exceeding 1 TV/m, permitting a table-top TeV accelerator. We present a scheme for focusing the beam to very small dimensions, where the surface Coulomb fields are also at the TV/m level. These conditions access a new regime for high field for atomic physics, allowing frontier atomic physics experiments such as barrier suppression regime ionization. They also, critically, permit well-sub-fs plasma formation for subsequent wake excitation. We examine the use of such ultra-short beams for creating coherent sub-cycle IR radiation at unprecedented high power levels.     

An investigation of energy spectrum and lineal energy variations in mega-voltage photon beams used for radiotherapy

Megavoltage photon beams are routinely used for external-beam radiotherapy. Recently, new treatment modalities based on dynamic and intensity modulated (IM), beam delivery systems are increasingly used in clinical practice. The purpose of this work is to investigate the energy spectrum and microdosimetric features of these photon beams. A Monte Carlo technique was first used to simulate beam lines of medical accelerators and to compute photon fluence and spectrum per unit dose-to-water inside the irradiated medium. Subsequently, a track structure code was used to compute the lineal energy and its distribution in a 1 micron sphere based on the individual photon spectrum. Results showed that the low energy photon component varied significantly with field size and location within the field due to the presence of the scattered photons. The calculated dose-mean lineal energy ranged from 2.3 keV.micron-1 at a depth of 1.5 cm along the central axis of a 4 cm x 4 cm field, to 3.7 keV.micron-1 at a depth of 20 cm of the field edge in a 10 cm x 10 cm field. In the tested IM fields at a depth of 10 cm, ranged from 2.6 to 3.5 keV.micron-1, which was inversely related to the dose intensity in the field. The values for the clinical photon beams were also significantly greater than that of a reference 60Co beam (1.8 keV.micron-1 from the calculation). The beam quality factor was estimated to vary within 20% due to the change of the energy spectrum for the radiotherapy photon fields.     

A study of molecular correlations observed in the small-angle photon scattering distributions of 60 keV photons interacting with low-atomic-number media

A variant of the multisection filter and annular target geometry, with a designed angular acceptance of ±0.5°, has been utilised in measuring accurate. O(5%), absolute total differential scattering cross sections of 60 keV photons for H₂O, methyl methacrylate (C₅H₈O₂)_n and nylon-6 (C₁₂H₂₂O₃N₂)_n in the angular scattering range of 2°–10°. The effects of molecular correlations manifest, to varying degree, in strong forward peaking of the scattered photon distribution. Comparison is made with available experiment and theory.