Energy Distribution Of Fast Electrons Research Articles

A flexible, on-line magnetic spectrometer for ultra-intense laser produced fast electron measurement

We have developed an on-line magnetic spectrometer to measure energy distributions of fast electrons generated from ultra-intense laser-solid interactions. The spectrometer consists of a sheet of plastic scintillator, a bundle of non-scintillating plastic fibers, and an sCMOS camera recording system. The design advantages include on-line capturing ability, versatility of detection arrangement, and resistance to harsh in-chamber environment. The validity of the instrument was tested experimentally. This spectrometer can be applied to the characterization of fast electron source for understanding fundamental laser-plasma interaction physics and to the optimization of high-repetition-rate laser-driven applications.

Energy distribution of fast electrons accelerated by high intensity laser pulse depending on laser pulse duration

The dependence of high-energy electron generation on the pulse duration of a high intensity LFEX laser was experimentally investigated. The LFEX laser (λ = 1.054 and intensity = 2.5 – 3 x 1018 W/cm2) pulses were focused on a 1 mm3 gold cubic block after reducing the intensities of the foot pulse and pedestal by using a plasma mirror. The full width at half maximum (FWHM) duration of the intense laser pulse could be set to either 1.2 ps or 4 ps by temporally stacking four beams of the LFEX laser, for which the slope temperature of the high-energy electron distribution was 0.7 MeV and 1.4 MeV, respectively. The slope temperature increment cannot be explained without considering pulse duration effects on fast electron generation.

Accuracy evaluation of a Compton X-ray spectrometer with bremsstrahlung X-rays generated by a 6 MeV electron bunch.

A Compton-scattering-based X-ray spectrometer is developed to obtain the energy distribution of fast electrons produced by intense laser and matter interactions. Bremsstrahlung X-rays generated by fast electrons in a material are used to measure fast electrons' energy distribution in matter. In the Compton X-ray spectrometer, X-rays are converted into recoil electrons by Compton scattering in a converter made from fused silica glass, and a magnet-based electron energy analyzer is used to measure the energy distribution of the electrons that recoil in the direction of the incident X-rays. The spectrum of the incident X-rays is reconstructed from the energy distribution of the recoil electrons. The accuracy of this spectrometer is evaluated using a quasi-monoenergetic 6 MeV electron bunch that emanates from a linear accelerator. An electron bunch is injected into a 1.5 mm thick tungsten plate to produce bremsstrahlung X-rays. The spectrum of these bremsstrahlung X-rays is obtained in the range from 1 to 9 MeV. The energy of the electrons in the bunch is estimated using a Monte Carlo simulation of particle-matter interactions. The result shows that the spectrometer's energy accuracy is ±0.5 MeV for 6.0 MeV electrons.

Electron energy distributions through superdense matter by Monte-Carlo simulations

We have studied energy distribution of fast electrons passing through a highly compressed core plasma for fast ignition research in inertial confinement fusion. Recent PIC calculations indicate that the collective effect of electric and magnetic fields on the transport may be less significant than the binary collisions in the case of a high density fusion pellet. In order to understand the net effect of binary collisions in dense plasma, we calculate electron energy distributions at several viewing angles using an electromagnetic cascade Monte-Carlo simulation, EGS5, for estimation of the contribution of multi collisional process. Here, the construction of physical parameters in the code were taken from the calculation results given by 2 dimensional particle-in-cell simulations. In the result, the number of electrons detected on the laser axis within the range to 15 MeV significantly decreases for the superdense region (max: 1.6 · 10 25 (/cm 3 )) compared with the low density plasma. The reduction on the electron number decreases with increase of observation angles gradually and finally the number almost coincides more than 40 degrees.

Investigation of lower hybrid physics through power modulation experiments on Alcator C-Mod

Lower hybrid current drive (LHCD) is an attractive tool for off-axis current profile control in magnetically confined tokamak plasmas and burning plasmas (ITER), because of its high current drive efficiency. The LHCD system on Alcator C-Mod operates at 4.6 GHz, with ~ 1 MW of coupled power, and can produce a wide range of launched parallel refractive index (n||) spectra. A 32 chord, perpendicularly viewing hard x-ray camera has been used to measure the spatial and energy distribution of fast electrons generated by lower hybrid (LH) waves. Square-wave modulation of LH power on a time scale much faster than the current relaxation time does not significantly alter the poloidal magnetic field inside the plasma and thus allows for realistic modeling and consistent plasma conditions for different n|| spectra. Inverted hard x-ray profiles show clear changes in LH-driven fast electron location with differing n||. Boxcar binning of hard x-rays during LH power modulation allows for ~ 1 ms time resolution which is sufficient to resolve the build-up, steady-state, and slowing-down phases of fast electrons. Ray-tracing/Fokker-Planck modeling in combination with a synthetic hard x-ray diagnostic shows quantitative agreement with the x-ray data for high n|| cases. The time histories of hollow x-ray profiles have been used to measure off-axis fast electron transport in the outer half of the plasma, which is found to be small on a slowing down time scale.

Angular and energy distribution of fast electrons emitted from a solid surface irradiated by femtosecond laser pulses in various conditions

Angular and energy distributions of fast electrons generated from the interaction of 60 fs, 795 nm laser pulses with aluminum targets have been experimentally investigated in various conditions. Increasing laser intensities from the nonrelativistic to the relativistic, a transition of the angular distribution of outgoing fast electrons from the specular reflection direction to the target normal has been observed for p-polarized laser irradiation. The fast electrons’ energy spectrum at high laser intensity, e.g., ∼2.6×1018 W/cm2, consists of two peaks, which are found to originate from the target normal with low energy and specular reflection direction with high energy, respectively. By adding a prepulse to generate preplasma, the electron yields at the direction of the reflected laser can be greatly enhanced, and a double-peak angular distribution is observed. Besides, a more collimated electron emission peak in the specular reflection direction can be obtained by employing a larger f-number focusing system.

Calculation of electron-molecule reactions of NO decomposition in pulsed corona discharges

In recent years there has been increasing research interest in the removal of nitrogen-oxides from exhaust gases using a pulsed corona discharge reactor. The pulsed streamer corona produces energetic electrons that excite, ionize and dissociate gas molecules, and by forming radicals that enhance the gas-phase chemical reactions which reduce the pollutant's concentration. The decomposition ratio of these substances is dependant on the gas composition, concentration, energy distribution of fast electrons, and other parameters. For a detailed analysis of the phenomena, knowledge of chemical reaction mechanisms is essential. The reaction rates of possible molecule-molecule reactions can in most cases be found in the literature, but the rates of the electron-molecule collision reactions depend on the energy of free electrons. With the knowledge of the field distribution, the energy of the free electrons and the reaction rates can be found. In this paper the authors present a method, in which the reaction rates of the electron- molecule collision are determined. The model is based on a calculation of: the energy of free electrons in the time and space varying field, considering the mean free path and the energy-dependent reaction cross sections of molecules. Knowing the rates, it is possible to solve the reaction kinetic equations, and to get the time-evolution of byproducts, and the decomposition ratio of the pollutant gases.

Multi-peak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of < 60°. This multi-peak characteristic is found to be independent of the polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.

Multipeak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of &amp;lt;60°. This multipeak characterization is found to be independent of laser polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.

Phenomenological model of pulsed corona reactor

As it is well known, the non-thermal plasma discharge is an effective way to clean the flue and exhaust gases of hazardous pollutants, like sulphur-oxides, nitrogen-oxides. The decomposition ratio of these substances depends on the gas composition, concentration, energy distribution of fast electrons, and other parameters. For a detailed analysis of the phenomena, the first step is the mathematical description of corona pulses which determines the electric field. The second step is the determination of the electric field distribution inside the reaction chamber.In this paper the authors give an easy model for the pulsed corona current and a simplified model for the electric field by using the corona current and the displacement current pulse shapes. The result of the model is the space and time dependent field distribution, which is suitable for calculating the rate coefficients of chemical reactions.

Neutron production in picosecond laser-generated plasma on a be target

Experimental data on neutron production in a plasma generated on a Be target by a picosecond laser of intensity 2 × 1018 W/cm2 are presented. In contrast to previous measurements, a Ta converter is not used in this study to generate γ rays. The neutron yield is equal to 2 × 103 over a solid angle of 4π steradians per laser pulse. A simultaneous measurement of the maximum energy of hard x rays gave Eγmax ∼ 6 MeV, the number of these photons being 5 × 108 over an angle of 4π steradians per laser pulse. The energy distributions of fast electrons and photons are estimated theoretically.

Effects of the energy distribution of fast electrons on H2 vibrational excitation in a tandem negative ion source

The population distribution of vibrationally excited hydrogen molecules H2(v″), and the H− production are investigated theoretically by solving numerically a set of particle balance equations in a steady-state pure hydrogen plasma. In particular, the enhancement of the H2(v″) distribution is discussed for different energy distributions of fast electrons ef. Whether the energy distribution for ef is a delta function type or a plateau type, collisional excitation of H2(v″) caused by ef is very effective to enhance the H− production and a resultant vibrational distribution becomes the plateau distribution. Besides these, if ef with energies higher than 30–40 eV are present, both H2(v″) excitation and then H− yield hardly depend on the form of the energy distribution of ef.

Energy degradation of fast electrons in hydrogen gas

An equation is derived for calculating the energy distribution of fast electrons in a partially ionized gas and a method is provided to solve for the electron degradation spectrum and the energy deposition in different forms (ionization, excitation, or heating). As an example, the energy degradation of fast electrons in a gas of pure hydrogen is calculated, considering excitations to the lowest 10 atomic levels. The Bethe approximation and the continuous slowing-down approximation are discussed and it is concluded that these approximations are accurate to the order of 20 percent for electrons with initial energy of greater than about keV. The method and results can be used to determine heating, excitations, and ionizations by high-energy photoelectrons or cosmic-ray particles in various astrophysical circumstances, such as the interstellar medium, supernova envelopes, and QSO emission-line clouds.

On the energy distribution of fast electrons and the intensities of lines in the hollow cathode discharge

On the energy distribution of fast electrons and the intensities of lines in the hollow cathode discharge

Analysis of the change in the energy distribution of fast electrons behind barriers by means of a magnetic beta-spectrometer

A method of analyzing the energy spectra of fast electrons behind barriers using a betatron and a magnetic beta-spectrometer is described. The fundamental parameters of the apparatus are given. Measurements of the energy distributions of electrons traveling in a forward direction behind barriers of thickness up to 500 mg/cm2 are reported. The range of thicknesses for electron kinetic energies of 1–6 MeV in which the experimental and theoretical results agree is established. For a comparative description of the results for different barriers, the effective atomic number of the barrier material for ionization processes is used. The connection between the barrier thickness, its effective atomic number, the initial electron energy, and the parameters of the secondary distribution is established. The extent to which the crystal structure affects the deformation of the energy spectrum of the electrons behind barriers of single crystals of alkalihalide salts is estimated.

Theory of deka-keV solar X-ray bursts

In this paper an attempt has been made to investigate theoretically the time-profile of an X-ray burst observed at photon energies well below 0.5 MeV. Following DeJager (1967) this type of X-bursts is called deka-keV X-ray bursts. The energy distribution of fast electrons which emit the hard X-ray burst has been computed as a function of time. On the basis of these expressions the time-profile of a deka-keV burst has been calculated. In this paper two plausible initial electron distributions were chosen, a mono-energetic distribution and a maxwellian distribution of electron energies. It has been proved that the process of energy loss of an electron is completely governed by losses due to magnetic bremsstrahlung emission. This implies that the decay shape of a deka-keV X-ray burst is determined by the value of the magnetic-field strength existing in the plasma. A typical decay time of an X-ray burst, which is about 3 min, can be expected theoretically from a thermal plasma of temperature 109 °K confined by a magnetic field of about 750 gauss. The theory developed in this paper indicates that the soft X-ray burst accompanying the deka-keV burst lasts much longer than the deka-keV burst itself.