Number Of Fast Electrons Research Articles

New synergy effects of the lower hybrid wave and the high harmonic fast wave current drive

The combined drive current of the lower hybrid wave (LHW) and the high harmonic fast wave (HHFW) was studied theoretically for the first time, based on the use of low and higher β e operational parameters in EAST. Broad and significant synergistic effects are found in the simulation, the current drive (CD) efficiency is greatly improved, and the current profile is modified as a result of the synergy between the two types of waves. The LHW interacted with the resonant electrons in low parallel velocity region and pushes them into the adjacent resonance region of the high phase velocity wave (HHFW), thus the number of fast electrons resonant with the HHFW is increased dramatically, and the driven current is enhanced. Therefore the synergy effect strongly depends on the positional relation between the velocity resonance regions of the two waves. Moreover, the effects of the parallel refractive index, the wave frequency and the wave power on the synergy effect are examined. Some problems well known in the single LHW CD or the HHFW CD may be overcome by the combined CD.

Flare energy release and avalanche ionization of plasma by runaway electrons in lower solar atmosphere

Abstract The analysis of the electron acceleration by the quasi-stationary sub-Dreiser electric fields in the lower solar atmosphere has been done. It has been shown that the Dreiser electric field turned out to be several orders of magnitude larger than coronal values due to the inelastic collisions between electrons and hydrogen atoms. The ionization of hydrogen atoms gives rise to the resulting secondary electrons, which become runaway under the action of sub-Dreiser electric fields. This causes an further avalanche-like ionization of the plasma and leads to the acceleration of the large number of fast electrons up to relativistic energies at small (≲ 100 km) distances.

Effect of ‘wandering’ and other features of energy transfer by fast electrons in a direct-drive inertial confinement fusion target

The heating of an inertial confinement fusion (ICF) target by fast electrons, which are generated as a result of laser interaction with the expanding plasma (corona) of a target, is investigated theoretically. It is shown that due to remoteness of the peripheral region, where electrons are accelerated, a significant portion of these particles, moving in corona and repeatedly crossing it due to reflection in a self-consistent electric field, will not hit into the compressed part of target. Using the modern models of fast electron generation, it is shown that in a typical target designed for spark ignition, the fraction of fast electrons that can pass their energy to the compressed part of the target is small. Only 12% of the total number of fast electrons can do it. Such an effect of ‘wandering’ of fast electrons in corona leads to a significant decrease in the negative effect of fast electrons on target compression. Taking into account the wandering effect, the distribution of energy transmitted by fast electrons to different parts of target and the resulting reduction of deuterium–tritium (DT) fuel compression are established.

The Effect of the Mode of Gas Preionization on the Parameters of Runaway Electrons in High-Pressure Discharges

The results of theoretical modeling of the process of formation of a nanosecond discharge in a coaxial discharge gap filled with a high-pressure gas are presented. Two cardinally different evolution scenarios of the nanosecond discharge are addressed: A) in a uniformly volume pre-ionized gas medium and B) in a strongly spatially-nonuniform initially-ionized region near the cathode with a small curvature radius. Relying on the minimal mathematical model of a high-voltage discharge and the description of the physical kinetics of runaway electrons, it is shown using the Boltzmann kinetic equation that the amplitude and duration of a current pulse of runaway electrons and their energy spectrum strongly depend on the mode of gas preionization in the gap. In particular, the other conditions being equal, near-cathode initiation gives rise to the generation of a large group of low-energy runaway electrons within the late current-switching stage. The volume-homogeneous gas preionization can reduce the number of fast electrons by nearly two orders of magnitude compared to the regime without preionization.

Effect of inner-surface roughness of conical target on laser absorption and fast electron generation

The effect of inner-surface roughness of conical targets on the generation of fast electrons in the laser—cone interaction is investigated using particle-in-cell simulation. It is found that the surface roughness can reduce the fast-electron number (in the energy range E > 1 MeV) and energy, as compared to that from a cone with smooth inner wall. A scaling law for the laser reflectivity based on the vacuum-heating model is derived. Both theory and simulation indicate that laser reflection increases with the height-to-width ratio of the periodic inner surface structure and approaches that of a smooth cone as this ratio becomes zero.

SIGNIFICANT CONTRIBUTION OF THE CERENKOV LINE-LIKE RADIATION TO THE BROAD EMISSION LINES OF QUASARS

The Cerenkov line-like radiation in a dense gas (N H > 1013 cm–3) is potentially important in the exploration of the optical broad emission lines of quasars and Seyfert 1 galaxies. With this quasi-line emission mechanism, some long standing puzzles in the study of quasars could be resolved. In this paper, we calculate the power of the Cerenkov line-like radiation in dense gas and compare with the powers of other radiation mechanisms by a fast electron to confirm its importance. From the observed gamma-ray luminosity of 3C 279, we show that the total number of fast electrons is sufficiently high to allow effective operation of the quasi-line emission. We present a model calculation for the luminosity of the Cerenkov Lyα line of 3C 279, which is high enough to compare with observations. We therefore conclude that the broad line of quasars may be a blend of the Cerenkov emission line with the real line produced by the bound-bound transition. A new approach to the absorption of the Cerenkov line is presented with the method of escape probability, which markedly simplifies the computation in the optically thick case. The revised set of formulae for the Cerenkov line-like radiation is more convenient in applications.

Trapping of Free Electrons in Multipole System

In this paper, the effective Parameters in the confinement and trapping of fast electrons in plasma source Such as; plasma pressure, wall material of plasma chamber and magnetic mirror rate have been investigated with using Comsol & Geant4 code. The calculations are shown that the Multicusp magnetic field was effective the pressure less than 5 mTor, and the confinement effect becomes stronger with decreasing pressure. It is equivalent to a higher yield of output ions of plasma source. The number of fast electrons trapped in the magnetic field increases with increasing magnetic field intensity and using aluminum for wall material. Optimum conditions of confinement plasma, leading to increased the hot electron density, and ionization efficiency is increased. The results of investigations have demonstrated good correspondence with theoretical calculations, therefore there is the adequacy of the developed approach and the possibility to build more effective source ion on this basis.

Relativistic kinetic model for energy deposition of intense laser-driven electrons in fast ignition scenario

One of the most crucial steps for a fast ignition scenario is the energy deposition into the highly compressed deuterium-tritium core plasmas via intense laser-produced relativistic electrons. Based on fundamental principles, a kinetic model is developed by considering both binary collisions and the contribution due to collective process. The collision operator is exactly simplified by taking into account relativistic effects within the context of fast ignition. It is expressed in a differential form with the help of two analogous Rosenbluth potentials. The explicit formulation of a relativistic kinetic equation in three-dimensional momentum space is obtained by expanding the potential functions in terms of spherical harmonics, in which only simple differentiations and integrations are involved. Fast electron number is well conserved in this model. The range and penetration depth are also discussed.

A model experiment of a double-cone target using a gap target

We have conducted preliminary experiments to prove the vacuum-shielding effect using an Al-Cu double-foil (both Al and Cu foils are 1 mm × 1 mm × 10 μmt) target with a vacuum-gap. Particle-in-cell simulation results have shown that the double-cone confines the electrons for hundreds of femtoseconds by the sheath electric field inside the vacuum-gap [T. Nakamura, et al., Phys. Plasmas 14, 103105 (2007)]. Cu-Kα intensity decreased with the vacuum-gap, i.e., the number of fast electrons that reached the Cu foil decreased with the gap. In the stack measurement, the number of electrons detected in the target surface direction increased with the gap. These results indicate that when the double-cone target is used, fast electrons created by an ignition laser can be reflected from the vacuum-gap, move along the cone surface, and be transferred towards the cone tip.

Effect of density profile on beam control of intense laser-generated fast electrons

Divergent relativistic electrons are produced during the intense laser interacting with overdense or solid targets. When these energetic electrons propagate through a structured two-layer target, a strong interface magnetic field will be generated due to the plasma density difference near the interface. This field will affect the motions of the electrons and can be utilized to control the beam divergence. In this paper, the effect of the target density profile on beam control is explored via a physical model as well as two-dimensional particle-in-cell simulations. It is shown that the fast electron number and forward current can be greatly concentrated and enhanced if the plasma density of the inner layer is suitably lower than that of the outer layer.

Enhancement of the Number of Fast Electrons Generated in a Laser Inverse Cone Interaction

Enhancement of the energy-conversion efficiency from laser to target electrons is demonstrated by two-dimensional particle-in-cell simulations in a laser-inverse cone interaction. When an intense short-pulse laser illuminates the inverse cone target, the electrons at the cone end are accelerated by the ponderomotive force. Then these electrons are guided and confined to transport along the inverse cone walls by the induced electromagnetic fields. A device consisting of inverse hollow-cone and multihole array plasma is proposed in order to increase the energy-conversion efficiency from laser to electrons. Particle-in-cell simulations present that the multiholes transpiercing the cone end help to enhance the number of fast electrons and the maximum electron energy significantly.

Local re-acceleration and a modified thick target model of solar flare electrons

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (${\cal E}$) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local ${\cal E}$ fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfv\'{e}n speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.

Fast electron generation under proton interaction with a silicon surface

The computational procedure for a number of fast electrons generated by ion impact with a surface is proposed. The ionization of 2s and 2p atomic shells is shown to be the main source of fast electrons under proton interaction with a silicon surface. The number of fast electrons does not depend on the angle of ion incidence on a surface with a small number of reflected ions.

Numerical Simulation of Ne–Tl and Ne–Ga Ion Hollow-Cathode Lasers

Results of theoretical and experimental studies of the behavior of the level populations and gain factors for the np1,3P – ns1,3S and np1,3P –> (n-1)d1,3D transitions in Tl II (n = 7) and Ga II (n = 5) are presented. These levels are filled due to the charge exchange in thermal collisions of neon ions with thallium and gallium atoms in thallium--neon and gallium--neon mixtures, respectively. The total pumping rate of all levels of a metal due to the charge exchange in the hollow-cathode discharge plasma at the operating concentrations of metal vapors was shown to be equal to the gas ionization rate. The latter is defined by the number of fast electrons and is independent of the charge-exchange cross section. The exciting and deexciting collisions with slow electrons and gas atoms were taken into account for all Tl II and Ga II levels related to the laser ones, and the radiation trapping by the resonance transitions of metal ions was considered. The partial pumping rates by the charge exchange were found for levels with 0 < ΔE(∞) < 1.5 eV. Theoretical results are compared with experimental data. The behavior of the lasing characteristics is explained for the known laser transitions. The parameters of the as yet unknown transitions in the IR range are predicted.

Influence of angles of incidence of laser radiation on the generation of fast ions

It was established experimentally that the number and energy of fast ions in laser plasma increased with increasing angle of focusing laser radiation onto a flat target. Numerical calculations showed that the increase in angle of focusing brought the mean angle of incidence of laser radiation closer to the optimal angle corresponding to the maximal efficiency of the resonance absorption mechanism and, as a result, increased the fraction of absorbed laser energy in the energy of fast electrons and increased the number of fast electrons. In turn, the increase in the energy and number of fast electrons resulted in an increase in the number of fast electrons involved in the formation of a self-consistent electric field at the target edge and led to the growth of the field strength, which, eventually, was the reason for the increase in the number and energy of fast ions.

Theory and simulations of electron beam ion source in reflex mode

The electron beam ion sources (EBIS) proposed in JINR are applied for generation of extremely highly charged ions. The intensity of extracted ion beams and ion charge states are determined by the current and current density of used electron beams. In the normal mode of EBIS operation each electron passes through the region of an ion trap only one time and then it picks up by the collector. The number of extracted ions during one pulse is proportional to the number of fast electrons in the trap region. The idea of EBIS operating in reflex mode, when electron passes many times through the trap region is directed to the increase of the number of the electrons in the trap region and consequently to the increase of an ion output. The experimental results of an EBIS operation in the reflex mode are obtained during several of the last years. The theoretical simulations of EBIS in this mode are discussed in this article.

First observations of runaway breakdown phenomenon in laboratory experiments

Microwave electron cyclotron resonance discharge in air is studied in a time-varying magnetic mirror trap. The obtained experimental data demonstrate a rapid increase in the number of fast electrons with wide energy spectrum (1–300 keV). It is followed by strong flashes of γ photons and associated secondary optic emission. The experimental results are interpreted as the first observation in a laboratory of the runaway breakdown phenomenon, which plays a significant role in a thunderstorm atmosphere.

Hybrid model of a rectangular hollow cathode discharge

A rectangular hollow cathode discharge was investigated by means of a two-dimensional self-consistent hybrid model combining Monte Carlo simulation of the motion of fast electrons and a fluid description of slow electrons and positive ions. Our calculations were carried out for a cold-cathode dc abnormal glow discharge in helium, operating at moderate (${\mathrm{\ensuremath{\sim}}1\phantom{\rule{0ex}{0ex}}\mathrm{m}\mathrm{A}/\mathrm{c}\mathrm{m}}^{2})$ current densities and at low (\ensuremath{\sim}1 mbar) pressures. The results demonstrate the existence of the hollow cathode effect in the discharge. On the other hand the energy distribution function of electrons indicates that a considerable number of fast electrons is absorbed by the anode, representing a major loss for the maintenance of the discharge. A significant fraction of primary electrons $(\ensuremath{\approx}5\ensuremath{-}20%)$ was absorbed by the anode before they were able to produce any ions. Due to the loss of high-energy electrons at the absorbing anodes, ionization is less efficient than that in conventional (e.g., cylindrical) hollow cathodes, thereby explaining the increasing voltage---linear current density characteristics of the discharge. Backscattering of high-energy electrons from the anodes significantly affects the discharge characteristics: a backscattering coefficient of 0.2 resulted in $\ensuremath{\approx}90%$ increase of the ${\mathrm{He}}^{+}$ density and $\ensuremath{\approx}60%$ increase of the linear current density at 2 mbar pressure and 300 V voltage.

Short-pulse high-intensity laser-generated fast electron transport into thick solid targets

The transport of fast electrons generated by 1 ps, 1 $\ensuremath{\mu}\mathrm{m}$ wavelength laser pulses focused to spot diameters of 20 $\ensuremath{\mu}\mathrm{m}$ and peak intensities of up to $2\ifmmode\times\else\texttimes\fi{}{10}^{18}$ W ${\mathrm{cm}}^{\ensuremath{-}2}$ on to solid aluminum targets is considered using a relativistic Fokker-Planck equation, which is solved by reducing it to an equivalent system of stochastic differential equations. The background is represented by $\mathbf{E}=\ensuremath{\eta}{\mathbf{j}}_{b},$ where $\ensuremath{\eta}$ is the resistivity and ${\mathbf{j}}_{b}$ is the background current density. Collisions, electric and magnetic fields, and changes in resistivity due to heating of the background are included. Rotational symmetry is assumed. The treatment is valid for fast electron number densities much less than that of the background, fast electron energies much greater than the background temperature, and time scales short enough that magnetic diffusion and thermal conduction are negligible. The neglect of ionization also limits the validity of the model. The intensities at which electric and magnetic fields become important are evaluated. The electric field lowers the energy of fast electrons penetrating the target. The magnetic field reduces the radial spread, increases the penetration of intermediate energy fast electrons, and reflects lower energy fast electrons. Changes in resistivity significantly affect the field generation. The implications for $K\ensuremath{\alpha}$ emission diagnostics are discussed.

Analysis of electron cyclotron emission by lower-hybrid current drive plasmas

Electron cyclotron emission may be used for diagnosing non-thermal electron distribution functions generated by radio-frequency current drive. Our attention is concentrated in theX-mode-downshifted second-harmonic range of frequencies, by inspecting at the radiation collected in the horizontal low-magnetic-field side of a tokamak. The cut-off presence eliminates the thermal first-harmonic emission and the non-thermal one may be observed without other inference. The electron distribution function is modelled by a suitable sum of drifting Maxwellians for reproducing the flat tails usually obtained in the presence of lower-hybrid power injection. This model is time saving as far as the computer simulations are concerned and permits a lot of runs in order to accomplish a wide parameters exploitation. A sensitivity analysis of ECE intensity is performed with respect to the main moments of the suprathermal electrons distribution function. The analysis shown that the radiation spectra are very sensitive to the perpendicular energy and to the number of fast electrons, while only a weak dependence on the parallel energy is found.