Fast Electron Generation Research Articles

Effects of preformed plasma of CH foam on fast electron generation

In fast ignition, the intensity of an unavoidable pre-pulse of a heating laser is still high to generate plasmas even if the contrast ratio is high. So, a preformed plasma (pre-plasma) is created before a main pulse of the heating laser irradiates a target. The recent research suggests that coating an inner surface of the cone of the cone-guided target with the low-density foam enhances fast electron generation. But the pre-plasma and the foam structure, which consists of dense CH plasmas and voids, are ignored in that research. In this paper, effects of the pre-plasma and the foam structure on the fast electron generation are investigated with the use of an one-dimensional Particle-In-Cell code. It is found that effects of the pre-plasma on the fast electron generation are small and the foam structure weakens the generation of fast electrons with the moderate energy.

Fast electron propagation in high-density plasmas created by 1D shock wave compression: Experiments and simulations

We present results from an experimental characterization of fast electron transport in high density plasmas created by 1D shock wave compression. The Kα fluorescence from a Cu layer embedded in Al or CH foil targets is measured. We use long laser pulses (LP) with 180 J, 1.5 ns, 0.53μm to compress the foils by shock wave propagation to 2-3 times their solid density and heat them to ∼ 4eV (close to the Fermi temperature). A counter-propagating high-intensity short laser pulse (SP), with 40 J, 1 ps, 57×1019 Wcm−2, generates intense currents of fast electrons which propagate through the deep regions of the target just before shock breakthrough. The results are compared to the uncompressed, solid density case (without the LP beam). The complete set of measurements is compared to numerical results, including a 2D hydrodynamic description of the compression and pre-pulse effects, 2D PIC simulations of the SP beam interaction and both hybrid and PIC simulations of the electron transport in the target depth and sheaths. In the case of the non-compressed targets we need to take fast electron refluxing into account to reproduce the experimental results. By exploring the domain of warm temperatures, we identify a regime for the incident fast electron current density, 1010 < jh < 1012 Acm−2, for which the collective mechanisms of electron transport differs appreciably between solid density and compressed matter.

Lateral propagation of fast electrons at the laser-irradiated target surfaces

Lateral propagation of fast electrons at the target surfaces irradiated by femtosecond intense laser pulses is measured by kα x-ray imaging technique when a preplasma is presented. An annular halo surrounding a bright spot is observed in the x-ray images when the scale length of the electron density is large. For an incidence angle of 70° the x-ray images show a non-symmetrical distribution peaked to the laser propagation direction. The x-ray photons in the halo are mainly excited by the fast electrons that flow in the preplasma when their paths intersect the high density regions near the target surface.

Micro-radiography with laser plasma X-ray source operating in air atmosphere

AbstractK-shell emission from copper target was observed by focusing femtosecond laser pulses very close to the target surface-air interface. It was shown that mechanism of X-ray emission is connected with generation of fast electrons in the air plasma area. Experiments demonstrated that moderate intensity of laser radiation (IL &lt; 1015 W/cm2) was enough to produce considerable flux of X-ray photons of at least 10 keV energy. The parameters of generated X-ray emission were studied. It was found that after propagation through 40 cm thick air layer X-ray spectra consisted of pronounced Kα and Kβ characteristic lines and relatively small Bremsstrahlung continuum. Since transversal source size has an order of a few tens of micrometers, such a source can be used for absorption imaging of micro-objects in standard laboratory conditions. That can be particularly important for diagnostic of medical and biological samples in vivo.

Transport study of intense-laser-produced fast electrons in solid targets with a preplasma created by a long pulse laser

The effect of preplasma on fast electron generation and transport has been studied using an intense-laser pulse (I=2×1018 W/cm2) at the Osaka University. An external long pulse laser beam (E&amp;lt;1.5 J) was used to create various levels of preplasmas in front of a planar target for a systematic study. Kα x-ray emission from a fluorescence layer (copper) was absolutely counted and its spatial distribution was monitored. Experimental data show Kα x-ray signal reduction (up to 60%) with an increase in the preplasma level. In addition, a ring structure of Kα x rays was observed with a large preplasma. The underlying physics of the ring structure production was studied by integrating the modeling using a radiation hydrodynamics code and a hybrid particle-in-cell code. Modeling shows that the ring structure is due to the thermoelectric magnetic field excited by the long pulse laser irradiation and an electrostatic field due to the fast electrons in the preplasma.

Formation of Optical Bullets in Laser-Driven Plasma Bubble Accelerators

Electron density bubbles--wake structures generated in plasma of density n(e) approximately 10(19) cm(-3) by the light pressure of intense ultrashort laser pulses--are shown to reshape weak copropagating probe pulses into optical "bullets." The bullets are reconstructed using frequency-domain interferometric techniques in order to visualize bubble formation. Bullets are confined in three dimensions to plasma-wavelength size, and exhibit higher intensity, broader spectrum and flatter temporal phase than surrounding probe light, evidence of their compression by the bubble. Bullets observed at 0.8 approximately < n(e) approximately < 1.2x10(19) cm(-3) provide the first observation of bubble formation below the electron capture threshold. At higher n(e), bullets appear with high shot-to-shot stability together with relativistic electrons that vary widely in spectrum, and help relate bubble formation to fast electron generation.

Integrated kinetic simulation of laser-plasma interactions, fast-electron generation, and transport in fast ignition

We present new results on the physics of short-pulse laser-matter interaction of kilojoule-picosecond pulses at full spatial and temporal scale using a new approach that combines a three-dimensional collisional electromagnetic particle-in-cell code with a magnetohydrodynamic-hybrid model of high-density plasma. In the latter, collisions damp out plasma waves, and an Ohm’s law with electron inertia effects neglected determines the electric field. In addition to yielding orders of magnitude in speed-up while avoiding numerical instabilities, this allows us to model the whole problem in a single unified framework: the laser-plasma interaction at subcritical densities, energy deposition at relativistic critical densities, and fast- electron transport in solid densities. Key questions such as the multipicosecond temporal evolution of the laser energy conversion into hot electrons, the impact of return currents on the laser-plasma interaction, and the effect of self-generated electric and magnetic fields on electron transport will be addressed. We will report applications to current experiments.

Spherically bent crystal for X-ray imaging of laser produced plasmas

In the fast ignition approach to inertial confinement fusion (ICF), the ignition is expected to be triggered by an ultra-high intensity laser-produced fast electron beam, propagating and releasing its energy at the core of the shock-compressed deuterium–tritium (DT) capsule. Therefore the study of fast electron generation and transport is crucial for fast ignition science. As a consequence of fast electron transport, K-shell ionization of the medium atoms is produced with the subsequent emission of a K-alpha photon. The imaging of K-α sources is therefore very useful to obtain both quantitative and qualitative information on fast electron transport. Spherically bent crystals can provide 2-D spatially resolved and mono-energetic images of the K-α source and are widely used in several fast ignition oriented experiments.

Filling the vacuum chamber of a technological system with homogeneous plasma using a stationary glow discharge

Experimental study of a glow discharge with electrostatic confinement of electrons is carried out in the vacuum chamber volume V ≈ 0.12 m3 of a technological system “Bulat-6” in argon pressure range 0.005–5 Pa. The chamber is used as a hollow cathode of the discharge with the inner surface area S ≈ 1.5 m2. It is equipped with two feedthroughs, which make it possible to immerse in the discharge plasma interchangeable anodes with surface area S a ranging from ∼0.001 to ∼0.1 m2, as well as floating electrodes isolated from both the chamber and the anode. Dependences of the cathode fall U c = 0.4−3 kV on the pressure p at a constant discharge current in the range I = 0.2−2 A proved that aperture of the electron escape out of the electrostatic trap is equal to the sum S o = S a + S f of the anode surface S a and the floating electrode surface S f . The sum S o defines the lower limit p o of the pressure range, in which U c is independent of p. At p < p o the cathode fall U c grows up dramatically, when the pressure decreases, and the pressure p tends to the limit p ex, which is in fact the discharge extinction pressure. At p ≈ p ex electrons emitted by the cathode and the first generation of fast electrons produced in the cathode sheath spend almost all their energy up to 3 keV on heating the anode and the floating electrode up to 600–800°C and higher. In this case the gas in the chamber is being ionized by the next generations of electrons produced in the cathode sheath, their energy being one order of magnitude lower. When S a < (2m/M)1/2 S, where m is the electron mass and M is the ion mass, the anode may be additionally heated by plasma electrons accelerated by the anode fall of potential U a up to 0.5 kV.

X-ray Emission of Multiwire $Z$-Pinches in Energy Range Over 20 keV

The results concerning the emission of hard X-rays (HXRs) with an energy over 20 keV from the plasma of dense radiating <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinches are considered. The work aims at identifying the nature of the fast-electron (electron-beam) generation at the implosion of cylindrical and conical multiwire arrays (liners) at a current up to 3 MA on the Angara-5-1 installation. An effect of zippering on a time profile of HXRs at the implosion of conical arrays that is caused by an inclination of wires was revealed. It was shown that the generation of HXRs always correlates with the disruption of a plasma column near the cathode in the final phase of pinch compression. The pinch images in HXRs are attributed to the emission of bremsstrahlung of fast electrons generated at the plasma column disruption on the plasma ions and in the anode target. It was also found that using conical arrays makes it possible to control the plasma zippering direction, to regulate the space-temporal and power characteristics of X-ray emission, and, in particular, to change the HXR yield. For 12 mm-diameter multiwire arrays with a linear mass 200-400 mug/ cm the current and energy of the fast electrons is estimated to be ~ 20 kA and ~ 60 J, respectively, which make ~ 0.002 of the energy of the soft-X-ray pulse.

Fast Electron Generation by Coulomb Scattering on Spatially Correlated Ions in a Strong Laser Field

AbstractElectrons colliding with spatially fixed ions in strong laser fields are investigated by solving the time‐dependent Schrödinger equation. Considering first simple one‐dimensional model systems, the mechanisms and energy spectra of fast electrons are analyzed, starting from collisions on a single ion. By using these electrons as projectiles for a second and third collision, the maximum possible energy obtained can be significantly increased. We then generalize the analysis to 2D systems where additional angular degrees of freedom lead to a drastic loss of efficiency. This problem can be overcome by introducing external confinements, which allow to focus the electrons and increase the intensity of high‐energy electrons (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Recent results at LULI on fast electron transport with and without guiding cone in the context of fast ignitor

We present experimental and numerical results obtained at LULI (Laboratoire pour l’Utilisation des Lasers intenses) on propagation and energy deposition of laser-generated fast electrons into conical targets. The experimental measurements were performed by means of several diagnostics in order to assess the predicted benefit of conical targets over standard planar ones. Various configurations have been tried, regarding the laser parameters with the aim of optimizing the laser-to-target coupling. Our best results have been obtained when the laser was frequency-doubled at 0.53 μm, corresponding to interaction conditions without laser pedestal due to the ASE (Amplified Spontaneous Emission). Our data pinpoint the detrimental influence of the pre-plasma generated by the laser pedestal at 1.057 μm, whose confinement is enhanced in conical geometry as evidenced by shadowgraphic measurements which is also confirmed by 2D Cu-Ka transverse images obtained from Cu cones. The consequence is the filling of the cone, preventing the laser beam from efficiently reaching the cone tip. These experimental results are compared to 2D PIC simulations modeling of the laser-cone interaction.

Plasmon induced electron transfer at gold–TiO 2 interface under femtosecond near-IR two-photon excitation

Charge-transfer process from photoexcited gold nanoparticles to TiO 2 is significant due to the very strong surface plasmon absorption band of gold nanoparticles and the very high photocatalytic activity of TiO 2. Bioapplication of gold–TiO 2 systems utilizing near-infrared (NIR) light based on the two photon excitation of the surface plasmon band will be interesting. In this study, TiO 2 particles with a 30 nm average diameter loaded with 10 nm gold spherical nanoparticles were used to explore the electron injection kinetics by observing electrons in TiO 2 using femtosecond IR-probe transient absorption (TA) spectroscopy. Optical excitation of the gold–TiO 2 system using ~ 150 fs light at 1200 nm led to very fast (< 240 fs) electron generation in the conduction band of TiO 2. Excitation intensity dependence of the initial TA signals gave a second order nonlinear relation, confirming the occurrence of two photon induced electron injection. Two photon absorption cross section of gold nanoparticle at 1200 nm was estimated to be very high, around 10 8 GM (1 GM = 10 − 50 cm 4 s photon − 1 molecule − 1 ).

Excitation of promising nuclear fusion reactions in picosecond laser plasmas

The results of experiments devoted to studying the excitation of the promising nuclear fusion reactions 6Li(d, α)4He, 3He(d, p)4He, 11B(p, 3α), and 7Li(p, α)4He, along with the standard reaction D(d, n)3He, in picosecond laser plasmas are presented. For the first time, it was shown that these reactions may proceed at a moderate laser-radiation intensity of 2 × 1018 W/cm2, the respective yield being 2 × 103 to 105 per laser pulse. A brief survey of the main processes responsible for the generation of fast electrons and fast ions (protons) at the front surface of the target and for the excitation of nuclear fusion reactions is given. The calculated and experimental results on the yield from nuclear fusion reactions in picosecond laser plasmas are compared. The possibilities for optimizing the yield from the promising fusion reactions excited in femto- and picosecond laser plasmas are discussed.

LASER-driven fast electron dynamics in gaseous media under the influence of large electric fields

We present the results of experiments performed at the LULI laboratory, using the 100 TW laser facility, on the study of the propagation of fast electrons in gas targets. The implemented diagnostics included chirped shadowgraphy and proton imaging. Proton images showed the presence of very large fields in the gas (produced by charge separation). In turn, these imply a strong inhibition of propagation, and a slowing down of the fast electron cloud as it penetrates in the gas. Indeed chirped shadowgraphy images show a reduction in time of the velocity of the electron cloud from the initial value, of the order of a fraction of c, over a time scale of a few picoseconds.

Guiding and confining fast electrons by transient electric and magnetic fields with a plasma inverse cone

The fast electron propagation in an inverse cone target is investigated computationally and experimentally. Two-dimensional particle-in-cell simulation shows that fast electrons with substantial numbers are generated at the outer tip of an inverse cone target irradiated by a short intense laser pulse. These electrons are guided and confined to propagate along the inverse cone wall, forming a large surface current. The propagation induces strong transient electric and magnetic fields which guide and confine the surface electron current. The experiment qualitatively verifies the guiding and confinement of the strong electron current in the wall surface. The large surface current and induced strong fields are of importance for fast ignition related researches.

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.

Spatiotemporal behavior of X-ray emission above 20 keV from a Z-pinch produced by wire-array implosion

Results are presented from experimental studies of hard X-ray (HXR) emission in the photon energy range above 20 keV from dense radiating Z-pinch plasmas. The work is aimed at revealing the nature of fast-electron (electron beam) generation during the implosion of cylindrical and conical wire arrays in the Angara-5-1 facility at currents of up to 3 MA. It is found that the plasma implosion zippering caused by the inclination of wires affects the parameters of the HXR pulse emitted during the implosion of a conical array. It is shown that HXR emission correlates well with the decay of the plasma column near the cathode in the stagnation phase. HXR images of the pinch are produced by the bremsstrahlung of fast electrons generated during plasma column decay and interacting with plasma ions and the anode target. It is found that the use of conical arrays makes it possible to control the direction of plasma implosion zippering and the spatiotemporal and energy parameters of the pinch X-ray emission, in particular the X-ray yield. For wire array with diameters of 12 mm and linear masses of 200–400 μg/cm, the current of the fast electron beam is 20 kA and its energy is 60 J, which is about 1/500 of the energy of the main soft X-ray pulse.

Review of Energetic Particle Generation and Electromagnetic Radiation from Intense Laser-Plasma Interactions at the Institute of Physics, Chinese Academy of Sciences

This review covers recent developments in multi-hundred-TW femtosecond laser systems and progress in intense laser-plasma interactions at the Institute of Physics, Chinese Academy of Sciences. The peak power of the institute’s Xtreme Light III (XL-III) laser system has been upgraded from 350 to 725TW, and the system used to observe the lateral transport of fast electrons due to magnetic and electrostaticfields at target surfaces. The surface fields cause fast-electron beams to be emitted along the front and rear target surfaces. Confinement and guiding of fast-electron propagation is demonstrated with wire- or wedge-shaped targets. Longitudinal transport is detected by optical and ion emission. Numerical simulations show that quasi-monoenergetic ion beams can be generated by collisionless electrostatic shock acceleration and phase-stable acceleration with a circularly polarized laser field. A new mechanism for high-power THz emission with plasmas as media is proposed; it can be used to obtain a single-cycle THz emission. The conversion efficiency from laser energy to Kα X-ray emission in a laser-copper target interaction is increased to 10 −4 using high-contrast laser pulses. c

Fast electron propagation in high density plasmas created by shock wave compression

We present one of the first results of relativistic laser intensities of the transport of fast electrons in high density and warm plasmas. The fast electrons are produced by the interaction of 40 J, 1 ps, 5 × 1019 W cm−2 laser pulses with solid foil targets. A 200 J, 1.5 ns laser focalized over a 500 µm diameter zone on the opposite side of the foil is used to create a shock propagating through and compressing the target to 2–3 times its solid density before the relativistic interaction. For both the solid and the compressed cases, the fast electron transport divergence and range are investigated, via the Kα emission from an embedded copper layer, for a conducting (aluminium) and an insulating (plastic) target material.