Laser-generated Electrons Research Articles

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures.

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Investigation of hard x-ray emissions from terawatt laser-irradiated foils at the Matter in Extreme Conditions instrument of the Linac Coherent Light Source

In this technical report, we investigate the hard x-ray background produced at the Matter in Extreme Conditions (MEC) instrument of the Linac Coherent Light Source (LCLS) from the interaction of a high-intensity (∼1019 W/cm2) femtosecond laser with solid μm-thick aluminum and polypropylene targets. This background is dominated by bremsstrahlung from laser-generated relativistic electrons, and a measurement of the broadband x-ray spectrum via differential x-ray energy filtering was used to infer the existence of two electron distributions with electron temperatures of Thot = 500 ± 300 keV and Tcold = 5.0 ± 0.5 keV. Simultaneous single-shot measurements of the proton energies accelerated from laser-irradiated solid targets could be correlated with these measurements to further constrain the on-target laser parameters. Measurements of the hard x-ray photon background generated from laser-irradiated foils can be used to directly monitor and test the signal-to-background limits of silicon-based hybrid pixel array x-ray detectors at laser intensities approaching 1019 W/cm2.

Photo-transmutation of long-lived radionuclide 135Cs by laser–plasma driven electron source

AbstractLaser-driven relativistic electrons can be focused onto a high-Z convertor for generating high-brightness γ-rays, which in turn can be used to induce photonuclear reactions. In this work, photo-transmutation of long-lived radionuclide 135Cs induced by laser–plasma–interaction-driven electron source is demonstrated using Geant4 simulation (Agostinelli et al., 2003 Nucl. Instrum. Meth. A506, 250). High-energy electron generation, bremsstrahlung, as well as photonuclear reaction are observed at four different laser intensities: 1020, 5 × 1020, 1021, and 5 × 1021 W/cm2. The transmutation efficiency depends on the laser intensity and target size. An optimum laser intensity, namely 1021 W/cm2, was found, with the corresponding photonuclear reaction yield reaching 108 J−1 of the laser energy. Laser-generated electrons can therefore be a promising tool for transmutation reactions. Potential application in nuclear waste management is suggested.

Characterizing the energy distribution of laser-generated relativistic electrons in cone-wire targets

Transport of relativistic electrons in a solid Cu wire target has been modeled with the implicit hybrid particle-in-cell code LSP to investigate the electron energy distribution and energy coupling from the high-intensity, short-pulse laser to electrons entering to the wire. Experiments were performed on the TITAN laser using a 1.5 mm long Cu wire attached to a Au cone tip at the laser intensity of 1 × 1020 W/cm2 which was irradiated into the cone. The simulated Cu Kα wire profile and yields matched the measurements using a two-temperature energy distribution. These modeling results show that the cold component of the energy spectrum can be determined with ±100 keV accuracy from the fit to the initial experimental fall-off of the Kα emission while the simulated profiles were relatively insensitive to the hotter component of the electron distribution (>4 MeV). The slope of measured escaped electrons was used to determine the hotter temperature. Using exponential energy distributions, the laser-to-electron-in-wire coupling efficiencies inferred from the fits decreased from 3.4% to 1.5% as the prepulse energy increases up to 1 J. The comparison of the energy couplings using the exponential and Relativistic Maxwellian distribution functions showed that the energy inferred in the cold component is independent of the type of the distribution function.

Laser-driven electron beamlines generated by coupling laser-plasma sources with conventional transport systems

Laser-driven electron beamlines are receiving increasing interest from the particle accelerator community. In particular, the high initial energy, low emittance, and high beam current of the plasma based electron source potentially allow generating much more compact and bright particle accelerators than what conventional accelerator technology can achieve. Using laser-generated particles as injectors for generating beamlines could significantly reduce the size and cost of accelerator facilities. Unfortunately, several features of laser-based particle beams need still to be improved before considering them for particle beamlines and thus enable the use of plasma-driven accelerators for the multiple applications of traditional accelerators. Besides working on the plasma source itself, a promising approach to shape the laser-generated beams is coupling them with conventional accelerator elements in order to benefit from both a versatile electron source and a controllable beam. In this paper, we perform start-to-end simulations to generate laser-driven beamlines using conventional accelerator codes and methodologies. Starting with laser-generated electrons that can be obtained with established multi-hundred TW laser systems, we compare different options to capture and transport the beams. This is performed with the aim of providing beamlines suitable for potential applications, such as free electron lasers. In our approach, we have analyzed which parameters are critical at the source and from there evaluated different ways to overcome these issues using conventional accelerator elements and methods. We show that electron driven beamlines are potentially feasible, but exploiting their full potential requires extensive improvement of the source parameters or innovative technological devices for their transport and capture.

Focusing of Relativistic Electrons in Dense Plasma Using a Resistivity-Gradient-Generated Magnetic Switchyard

A method for producing a self-generated magnetic focussing structure for a beam of laser-generated relativistic electrons using a complex array of resistivity gradients is proposed and demonstrated using numerical simulations. The array of resistivity gradients is created by using a target consisting of alternating layers of different Z material. This new scheme is capable of effectively focussing the fast electrons even when the source is highly divergent. The application of this technique to cone-guided fast ignition inertial confinement fusion is considered, and it is shown that it may be possible to deposit over 25% of the fast electron energy into a hot spot even when the fast electron divergence angle is very large (e.g., 70° half-angle).

An empirical model of collective electrostatic effects for laser-beam channeling in long-scale-length relativistic plasmas

This work investigates the capability of ultraintense lasers with irradiance from 1018 to 1021 W cm−2 to produce highly energetic electron beams from a Gaussian focus in a low-density plasma. A simple particle simulation code including a physical model of collective electrostatic effects in relativistic plasmas has been developed. Without electrostatic fields, free electrons escape from the Gaussian focal region of a 10-ps petawatt laser pulse very quickly, well before the laser field reaches its maximum amplitude. However, it has been demonstrated that the electrostatic field generated by the electron flow is able to strongly modify the range and direction of the laser-generated MeV electrons by allowing trapped electrons to experience much higher laser-intensity peaks along their trajectories. This modeling predicts some collimation but not enough to meet the requirements of fast ignition.

Transport of energy by ultraintense laser-generated electrons in nail-wire targets

Nail-wire targets (20 μm diameter copper wires with 80 μm hemispherical head) were used to investigate energy transport by relativistic fast electrons generated in intense laser-plasma interactions. The targets were irradiated using the 300 J, 1 ps, and 2×1020 W⋅cm−2 Vulcan laser at the Rutherford Appleton Laboratory. A spherically bent crystal imager, a highly ordered pyrolytic graphite spectrometer, and single photon counting charge-coupled device gave absolute Cu Kα measurements. Results show a concentration of energy deposition in the head and an approximately exponential fall-off along the wire with about 60 μm 1/e decay length due to resistive inhibition. The coupling efficiency to the wire was 3.3±1.7% with an average hot electron temperature of 620±125 keV. Extreme ultraviolet images (68 and 256 eV) indicate additional heating of a thin surface layer of the wire. Modeling using the hybrid E-PLAS code has been compared with the experimental data, showing evidence of resistive heating, magnetic trapping, and surface transport.

Fast ignition relevant study of the flux of high intensity laser-generated electrons via a hollow cone into a laser-imploded plasma

An integrated experiment relevant to fast ignition . A Cu-doped deuterated polymer spherical shell target with an inserted hollow Au cone is imploded by a six-beam 900-J, 1-ns laser. A 10-ps, 70-J laser pulse is focused into the cone at the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of CuKα fluorescence by comparison with a Monte Carlo model. The electrons are estimated to carry about 15% of the laser energy. Collisional and Ohmic heating are modeled, and Ohmic effects are shown to be relatively unimportant. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is calculated in the model. Enhanced scattering by instability-induced magnetic fields is suggested. An extension of this fluor-based technique to measurement of coupling efficiency to the ignition hot spot in future larger-scale fast ignition experiments is outlined.

Simulation of heating-compressed fast-ignition cores by petawatt laser-generated electrons

In this work, unique particle-in-cell simulations to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas are reported. The simulations yield heated core parameters in good agreement with the GEKKO-PW experimental measurements, given reasonable assumptions of laser-to-electron coupling efficiency and the distribution function of laser-produced electrons. The classical range of the hot electrons exceeds the mass density-core diameter product pL by a factor of several. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-EMF impeding hot electrons on the injection side of the density maxima.This methodology is then applied to the design of experiments for the ZR machine coupled to the Z-Beamlet/PW laser. Sandia National Laboratories is also developing a combination of experimental and theoretical capabilities useful for the study of pulsed-power-driven fast ignition physics. In preparation for these fast ignition experiments, the theory group at Sandia is modeling various aspects of fast ignition physics. Numerical simulations of laser/plasma interaction, electron transport, and ion generation are being performed using the LSP code. LASNEX simulations of the compression of deuterium/tritium fuel in various reentrant cone geometries are being performed. Analytic and numerical modeling has been performed to determine the conditions required for fast ignition breakeven scaling. These results indicate that to achieve fusion energy output equal to the deposited energy in the core will require about 5% of the laser energy needed for ignition and might be an achievable goal with an upgraded Z-beamlet laser in short pulse mode.

Simulation of Heating-Compressed Fast-Ignition Cores by Petawatt Laser-Generated Electrons

We report on unique particle-in-cell simulations to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas. The simulations yield heated core parameters in good agreement with the GEKKO-PW experimental measurements, given reasonable assumptions of laser-to-electron coupling efficiency and the distribution function of laser-produced electrons. The classical range of the hot electrons exceeds the mass density-core diameter product rhoL by a factor of several. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-EMF impeding hot electrons on the injection side of the density maxima.

Fast electron transport and heating in solid-density matter

Two experiments have been performed to investigate heating by high-intensity laser-generated electrons, in the context of studies of the fast ignitor approach to inertial confinement fusion (ICF). A new spectrometer and layered targets have been used to detect Kα emission from aluminum heated by a fast electron beam. Results show that a temperature of about 40 eV is reached in solid density aluminum up to a depth of about 100 μm.

Filamented transport of laser-generated relativistic electrons penetrating a solid target

The paraxial propagation of a relativistic electron beam in a solid target is examined, within a three-dimensional model of particles interacting with the target electron return current via a diffusive electromagnetic field. Simulations of a modulated beam show amplification of the modulation seed, with growth rates comparing reasonably well with the linear analysis of the model. Scenarios of beam fragmentation are observed and discussed in more realistic conditions, when beam collisions on both target ions and electrons and the resulting solid heating and ionization are taken into account.

Experimental measurements of deep directional columnar heating by laser-generated relativistic electrons at near-solid density.

In our experiments, we irradiated solid CH targets with a 400 J, 5 ps, 3 x 10(19) W/cm(2) laser, and we used x-ray imaging and spectroscopic diagnostics to monitor the keV x-ray emission from thin Al or Au tracer layers buried within the targets. The experiments were designed to quantify the spatial distribution of the thermal electron temperature and density as a function of buried layer depth; these data provide insights into the behavior of relativistic electron currents which flow within the solid target and are directly and indirectly responsible for the heating. We measured approximately 200-350 eV temperatures and near-solid densities at depths ranging from 5 to 100 microm beneath the target surface. Time-resolved x-ray spectra from Al tracers indicate that the tracers emit thermal x rays and cool slowly compared to the time scale of the laser pulse. Most intriguingly, we consistently observe annular x-ray images in all buried tracer-layer experiments, and these data show that the temperature distribution is columnar, with enhanced heating along the edges of the column. The ring diameters are much greater than the laser focal spot diameter and do not vary significantly with the depth of the tracer layer for depths greater than 30 microm. The local temperatures are 200-350 eV for all tracer depths. We discuss recent simulations of the evolution of electron currents deep within solid targets irradiated by ultra-high-intensity lasers, and we discuss how modeling and analytical results suggest that the annular patterns we observe may be related to locally strong growth of the Weibel instability. We also suggest avenues for future research in order to further illuminate the complex physics of relativistic electron transport and energy deposition inside ultra-high-intensity laser-irradiated solid targets.

Tunneling of laser-generated free electrons in semiconductor double wells.

A theoretical study of the dynamics of free photogenerated carriers in resonance-biased asymmetric GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double wells is presented. We consider a situation where photogeneration takes place on a subpicosecond time scale and, initially, produces a coherent ensemble of free electron-hole pairs in the wide well. The simultaneous thermalization and tunneling of electrons between the two wells is analyzed within the density matrix approach. The interplay between tunneling and Coulomb scattering is explored at several levels of approximation regarding free-carrier screening. We find that the Coulomb interaction represents an effective agent to destroy phase coherence and to damp out coherent charge oscillations in the system. Our calculations predict that, if the free-carrier Coulomb interaction provides the dominant dephasing mechanism in the system, coherent charge density oscillations associated with free carriers should persist up to electron sheet densities of about 3\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. It is shown that, under present conditions, dynamic screening within the plasmon-pole approximation and static screening give virtually identical results.

LASER-INDUCED PIEZOEXCITATION OF LOVE WAVES: THEORY AND POSSIBLE REALIZATION

Abstract A full theoretical analysis of the optical excitation of Love waves is presented. The explicit formulae relating the frequency spectrum of the Love modes to the space-time modulation of laser radiation are derived. The photoacoustic transformation is assumed to be caused by the inverse piezoeffect in the electric field induced by the spatial separation of laser-generated electrons and holes. It is demonstrated that information on the parameters of the film can be extracted from the measurements of the relative amplitudes of the different order Love modes. As the structure of the Love modes is simpler than that of pseudo-Rayleigh waves, we consider optical monitoring of the Love modes to be a promising method for remote diagnostics of thin coatings when the film or the substrate, or both, are piezoelectrics.

Laser-induced piezoexcitation of acoustic shear horizontal waveguide modes

A full theoretical analysis of the optical generation of acoustic shear horizontal (SH) waveguide modes in ultrathin piezoelectric films is presented. The explicit formulae relating the frequency spectrum of the SH wave to the space-time modulation of laser radiation intensity are derived. The optoacoustic transformation is supposed to be caused by the inverse piezoeffect in the electric field induced by the spatial separation of laser-generated electrons and holes. As the structure of the SH waves is much simpler in comparison with that of Lamb waves we consider optical monitoring of the SH waves to be a promising method for piezoelectric thin film remote diagnostics.

Long pulse x-ray emission from laser-irradiated exploding cylinders

A CO2 laser was used to irradiate hollow polystyrene cylinders along their axis. The laser-generated high-energy electrons appear to uniformly heat the cylinder throughout its volume and the cylinder explodes. The imploding and stagnation character of the exploding cylinder gives rise to a long x-ray pulse, much longer than the laser pulse, and to a peak emission that occurs after the laser pulse. The fast ions that are generated appear not to interact with the thermal plasma. Two-dimensional calculations that reproduce much of the experimental x-ray signature and observed hydrodynamic flow are presented.