Distribution Of High-energy Electrons Research Articles

Observation of ultrahigh-energy electrons by resonance absorption of high-power microwaves in a pulsed plasma.

The interaction of high power microwave with collisionless unmagnetized plasma is studied. Investigation on the generation of superthermal electrons near the critical layer, by the resonance absorption phenomenon, is extended to very high microwave power levels (eta=E(2)(0)/4 pi n(e)kT(e) approximately 0.3). Here E0, n(e), and T(e) are the vacuum electric field, electron density, and electron temperature, respectively. Successive generation of electron bunches having maximum energy of about 2 keV, due to nonlinear wave breaking, is observed. The electron energy epsilon scales as a function of the incident microwave power P, according to epsilon proportional to P0.5 up to 250 kW. The two-dimensional spatial distribution of high energy electrons reveals that they are generated near the critical layer. However, the lower energy component is again produced in the subcritical density region indicating the possibility of other electron heating mechanisms.

The location of very small particles in silane RF discharge

The size and location of silicon particles that grow in a pure silane, capacitively coupled RF discharge, are measured by laser light scattering. The discharge conditions were similar to those typically used to produce amorphous silicon devices, except the temperatures is 300 K. At early discharge time, when the particles are small (D/spl sim/15 nm), they are located at the middle of the discharge. The larger ones that occur at later discharge times form a double layer nearer the electrodes. Surprisingly, the particles are not concentrated at the region of brightest discharge-light, which represents the distribution of high-energy electrons. Yet as expected, the distribution of film deposition on the electrodes fits radical diffusion with a source proportional to light intensity. It is also shown, by tilting the substrate, that a small gradient in plasma potential can have a major effect on particle positions.

Dosimetric measurements of electron and photon yields from solid targets irradiated with 30 fs pulses from a 14 TW laser

A 14 TW Ti:Sa laser furnishing pulses with a duration of 30 fs (full width at half maximum) at a repetition rate of 10 Hz was used to expose solid targets to intensities of up to 3x10(18) W/cm(2). Dosimetric techniques were employed to study the total x-ray yield, the spectral and angular distribution of the x-ray photons and the energy distribution of high-energy electrons injected into the solid target and emitted into the vacuum. Scans of laser pulse energy and duration were carried out to study the dependence of the x-ray generation efficiency on these parameters. The radiation transport processes in the target were modeled using the Monte Carlo method. The results of these calculations were used to interpret the measurement results and to critically discuss the applicability of dosimetric methods to the investigation of photon and electron emission from laser-produced plasmas.

Influence of the wave characteristics on the electron radiation belt distribution

The three dimensional Salammbô diffusion code is used to study the influence of whistler mode wave scattering inside the plasmasphere on the high energy electron distribution. A combination of radial diffusion and pitch angle scattering leads to the formation of a pronounced high energy tail population in the outer radiation zone. The results are interpreted in terms of a recirculation process which produces a peak in the electron phase space density just inside the plasmapause.

Semi-empirical studies on electronic structures of a boron-doped graphene layer — implications on the oxidation mechanism

The electronic structures of pure and boron-doped graphene layers have been investigated using the semi-empirical Molecular Orbital Package (MOPAC) and large clusters of carbon atoms. It is shown that boron-doping on the edge and internal lattice sites of the graphene layer produces very different effects on the electronic structure around the edges. It is found that the substitutional boron atoms on the edges dramatically alter the density distribution of high energy electrons along the edges and the substitutional boron atoms in the deep internal lattice sites do not produce any significant effect on the density distribution along the edges. Based on the results obtained, a model is proposed for describing the oxidation process in boron-doped graphite. The mechanism of oxidation inhibition due to boron-doping of a graphene layer is chemical inhibition via the reduction of electron density with high energy at surface sites, and consequently, a reduction in the total number of active sites for gasification of the carbon.

Contribution to studies on hollow cathode discharges

The electron temperature measured in a hollow cathode by the optogalvanic method, discloses the fact that at lower gas pressure (order 100 Pa) its value is of the order of 0.1 eV, not sufficient to sustain the discharge. The role of high energy electrons, that are responsible for maintaining of the discharge in such cases, was investigated. A simple Monte Carlo simulation, based on the concept of beam electrons, containing only the most important processes has been applied in order to obtain the population of the two lowest excited states of neon. At the same time, the energy distribution of high energy electrons was obtained. The results of calculated densities of 1s5 and 1s4 neon atom states were found to be in a favourable agreement with values measured by the laser induced fluorescence method.

The Monte Carlo Calculations for the Distribution Function of Energetic Electrons in a Plane-Parallel Hollow Cathode

The energy distribution of high-energy electrons was calculated from the plane-parallel hollow-cathode glow discharge of helium gas. Calculations showed an energy distribution function similar to that of a cylindrical cathode whose function had already been obtained by the authors. However, few electrons whose energy equalled the cathode fall potential existed in the central region, contrary to the many electrons in that region in the cylindrical cathode.

High-energy electron distribution in electron beam excited Ar/Kr and Ne/Xe mixtures

The electron energy distribution in electron beam (e beam) excited Ar/Kr and Ne/Xe gas mixtures is examined in detail. The binary rare-gas mixtures are similar to those used in excimer lasers. Cooling processes for the secondary electrons generated in the gas mixture plasma by the e beam are calculated using a reduced Boltzmann equation in which elastic and electron-electron collisions for electron energy distributions above the first excitation threshold of the rare gas are ignored. During the calculations for the Ar/Kr and Ne/Xe mixtures, all electron-related reactions and the interaction between the two different rare gases in the mixture are simultaneously considered. The high-energy secondary electrons produce a steady-state distribution within a very short time; however, it is found that the distribution is not Maxwellian. W values [eV/electron-ion pair] and yields of rare-gas excited states calculated from the steady-state high-energy electron distribution show a dependence on the mixture composition, especially for mixtures with low concentrations of the minor rare gas. This implies that the practice in excimer kinetics models of using the W values determined from pure rare gases is not entirely accurate.

An analytic representation of the high energy electron distribution in atomic or molecular plasmas produced by particle beams

Analytic formulae are given for the distribution functions pertaining to medium and high energy electrons in time independent atomic or molecular plasmas that are generated by particle beams. The formulae are derived by making use of those features of inelastic collision cross sections which are responsible for significant departures from maxwellian in the “tail” of the electron distribution. Use of the present formulation, together with measurements of the high energy part of the distribution, provides a novel means for obtaining information about the energy dependence of cross sections for vibrational transitions in molecules.

The variably spaced superlattice energy filter

A new superlattice device concept which provides for high energy injection of electrons into a semiconductor layer is presented. The device is based on resonant tunneling of electrons between adjacent aligned quantum well levels in a variably spaced superlattice structure. By a judicial choice of well and barrier widths the energy levels under reverse bias become aligned such that resonant tunneling of electrons through the structure can occur. Thus, electrons are injected into a semiconductor layer at an energy corresponding to the energy of the first subband in the last quantum well. This structure has significant advantages over the conventional method of producing hot electrons in that a nearly monoenergetic high-energy electron distribution is created at low reverse bias and with high efficiency, since energy loss to phonons is inhibited as a consequence of the channeling of electrons through a narrow band of quantum states. Applications of the VSSEF structure to avalanche photodiodes, IMPATT diodes and electroluminescent devices are discussed.

Microwave imaging of a solar limb flare - Comparison of spectra and spatial geometry with hard X-rays

A solar limb flare was mapped using the Very Large Array (VLA) together with hard X-ray (HXR) spectral and spatial observations of the Solar Maximum Mission satellite. Microwave flux records from 2.8 to 19.6 GHz were instrumental in determining the burst spectrum, which has a maximum at 10 GHz. The flux spectrum and area of the burst sources were used to determine the number of electrons producing gyrosynchrotron emission, magnetic field strength, and the energy distribution of gyrosynchrotron-emitting electrons. Applying the thick target model to the HXR spectrum, the number of high energy electrons responsible for the X-ray bursts was found to be 10 to the 36th, and the electron energy distribution was approximately E exp -5, significantly different from the parameters derived from the microwave observations. The HXR imaging observations exhibit some similiarities in size and structure o the first two burst sources mapped with the VLA. However, during the initial burst, the HXR source was single and lower in the corona than the double 6 cm source. The observations are explained in terms of a single loop with an isotropic high-energy electron distribution which produced the microwaves, and a larger beamed component which produced the HXR at the feet of the loop.

High-energy kinetic theory of a particle beam generated plasma

The equilibrium high-energy electron distribution in a particle beam generated plasma is calculated. The tail of the distribution, above the first excited state, is derived from a Boltzmann equation which contains inelastic collisional processes and a continuous source term. This equation is analytically solved by a Laplace transformation method and, in a numerical application for the case of an argon plasma created by a high-energy electron beam between 103 and 106eV, the branching ratios for energy deposition in ionization and excitation states are calculated. We compare the results with those of the laser group of Orsay.

A modified radiotherapy technique in the treatment of medulloblastoma.

Craniospinal irradiation is a standard treatment technique in patients who receive surgery for medulloblastoma. In most centers megavoltage photon irradiation is used, resulting in significant irradiation exposure to critical organs. In order to overcome this difficulty, we recently modified the technique applied in our center, by using high energy electrons (20 MeV) for irradiation of the spinal cord. The reliability of this technique was checked by performing dosimetry in a specially constructed wax phantom. Attention was focused upon dose variations at the junction of fields. Furthermore, the influence of vertebrae on the absorbed dose distribution of high energy electrons is presented. This technique seems to be safe and reliable in selected patients (children and teenagers).

4948. High-energy electron distribution in an electron-beam-generated argon plasma: J Bretagne et al, J Phys D: Appl Phys, 14 (7), 1981, 1225–1239

4948. High-energy electron distribution in an electron-beam-generated argon plasma: J Bretagne et al, J Phys D: Appl Phys, 14 (7), 1981, 1225–1239

Impulsive phase of solar flares. I - Characteristics of high energy electrons

The variation along a magnetic field line of the energy and pitch angle distribution of high energy electrons injected into a cold hydrogen plasma containing either an open or closed magnetic field structure was investigated. The problem is formulated as a time independent Fokker-Planck Equation for the electron number distribution as a function of the electron energy, electron pitch angle, and the structure of the global magnetic field. Simple analytic solution valid in the small pitch angle regime and for slowly varying magnetic field is presented. For the more general situation a numerical code for solving the Fokker-Planck Equation was used and it was found that the analytic expression agrees well with the numerical results to values of the pitch angle much larger than expected. For most practical applications, one many confidently use the analytic expression instead of having to resort to lengthy numerical computations. These results are useful in the study of the nonthermal models of the impulsive phase of solar flares.

High-energy electron distribution in an electron-beam-generated argon plasma

The time evolution of high-energy electron distribution in an electron-beam-generated argon plasma is calculated. The distribution is derived for energy values above the threshold value of the first excited state (11.56 eV) from a reduced Boltzmann equation with no electron-neutral and electron-electron collisions. This equation can be numerically solved with a continuous source term taking account of all the new plasma electrons produced over the total energy range by primary electrons. As a result, the distribution reaches a steady state within a very short time tau s and its shape is pressure independent for a given current density, and a given energy of the beam, the evolution time tau s being inversely proportional to the pressure. Moreover, the energy distributions for given beam energy and pressure are in the same ratio as the primary-electron currents. An analytical approximation for the distribution tail is given as a function of the beam parameters (energy and current) and may be used for an electron-beam-generated Ar plasma as soon as these parameters are known. The branching ratios for energy deposition with electron-beam energy ranging between 103 and 106 eV is calculated. The relative influence of primary and secondary electrons is also discussed.

A comparison of the radio data and model calculations of Jupiter's synchrotron radiation, 1. The high energy electron distribution in Jupiter's inner magnetosphere

An electron distribution in Jupiter's magnetosphere as a function of energy, pitch angle and spatial coordinates is derived from a comparison of model calculations of the planet's synchrotron radiation with the radio data; the distribution is consistent with the information available from the Pioneer data. The model calculations are based upon the magnetic field configurations as derived by Acuna and Ness [1976a, b] (the O4 model) and Davis, Jones and Smith [quoted in Smith and Gulkis, 1979] (the P11 (3,2) A model) from the Pioneer data. The electrons are assumed to diffuse radially inwards with a rate D=3 × 10−9 L³ s−1 (where L is McIlwain's value) and have a lifetime against local losses τ=4 × 106 L−0.6 s, in agreement with the values derived by Goertz et al. [1979] from the Pioneer data. The number density of electrons with E ≥20 MeV at L=3 is in agreement with the values measured by the spacecraft, but the energy distribution appears to be much flatter. Electrons are absorbed with survival probabilities of ∼0.5 diffusing inwardly past Amalthea's sweeping region (for 20‐MeV electrons) and ∼0.35 past the ring, respectively. An energy dependent pitch angle scattering is found to occur at Amalthea's orbit, and there is no pitch angle scattering present near the ring.

Depth absorbed dose distributions for electrons

The influence of energy spread on the depth dose distribution of high energy electrons, which is complicated by the different methods used in the specification of electron beam energy, is discussed.

Energy Distribution at Large Angles of High-Energy Electrons in Bremsstrahlung

The energy distribution of high-energy electrons, that have been scattered by the nuclear potential and have lost energy by bremsstrahlung, has been investigated. It has been found that there is a peak in the energy distribution which occurs at energies of the order of $m{c}^{2}$. The nature of and reasons for the peak are discussed. Formulas for the energy distribution near the peak and for the area under the peak are given. Similar results apply also to pair production.

The Angular Distribution of High Energy Electrons in Air Showers. I. Landau Approximation

The angular distribution of high energy (> 5 X 108 e V) electrons in air showers is calculated on a track length basis, using approximation A of Rossi and Greisen (1941) (no ionization loss) and the Landau (1940) multiple scattering approximation. We start with a discussion of the approximations used and an estimate of their validity. The basic equations are written down; qualitative results and very rough solutions are developed and applied to the Furry model of a cascade. For the Furry cascade the qualitative arguments lead directly to an Ansatz which yields an exact solution. In the actual cascade the corresponding Ansatz does not yield an exact solution. We then perform an iteration, employing a general method of Friedman. The final (iterated) solution is compared with the exact (in the Landau approximation) solution by means of their moments, and appears to be within 10 per cent. of the correct solution for E6fEs <l. Our solution compares well with earlier work on this problem. Appendices contain a short derivation of the angular moments, a general inversion formula for going from the distribution-in-projected-angle to the distribution-in-angle-with-the-showeraxis, and a derivation of the Friedman variation principle in vector space terminology.