Effect of Small Focus on Electron Heating and Proton Acceleration in Ultrarelativistic Laser-Solid Interactions.

Acceleration of particles from the interaction of ultraintense laser pulses up to 5×10^{21} W cm^{-2} with thin foils is investigated experimentally. The electron beam parameters varied with decreasing spot size, not just laser intensity, resulting in reduced temperatures and divergence. In particular, the temperature saturated due to insufficient acceleration length in the tightly focused spot. These dependencies affected the sheath-accelerated protons, which showed poorer spot-size scaling than widely used scaling laws. It is therefore shown that maximizing laser intensity by using very small foci has reducing returns for some applications.

Using X-ray spectroscopy of relativistic laser plasma interaction to reveal parametric decay instabilities: a modeling tool for astrophysics.

By analyzing profiles of experimental x-ray spectral lines of Si XIV and Al XIII, we found that both Langmuir and ion acoustic waves developed in plasmas produced via irradiation of thin Si foils by relativistic laser pulses (intensities ~1021 W/cm2). We prove that these waves are due to the parametric decay instability (PDI). This is the first time that the PDI-induced ion acoustic turbulence was discovered by the x-ray spectroscopy in laser-produced plasmas. These conclusions are also supported by PIC simulations. Our results can be used for laboratory modeling of physical processes in astrophysical objects and a better understanding of intense laser-plasma interactions.

Mie scattering from submicron-sized CO2 clusters formed in a supersonic expansion of a gas mixture.

A detailed mathematical model is presented for a submicron-sized cluster formation in a binary gas mixture flowing through a three-staged conical nozzle. By measuring the angular distribution of light scattered from the clusters, the size of CO(2) clusters, produced in a supersonic expansion of the mixture gas of CO(2)(30%)/H(2)(70%) or CO(2)(10%)/He(90%), has been evaluated using the Mie scattering method. The mean sizes of CO(2) clusters are estimated to be 0.28 ± 0.03 µm for CO(2)/H(2) and 0.26 ± 0.04 µm for CO(2)/He, respectively. In addition, total gas density profiles in radial direction of the gas jet, measuring the phase shift of the light passing through the target by utilizing an interferometer, are found to be agreed with the numerical modeling within a factor of two. The dryness (= monomer/(monomer + cluster) ratio) in the targets is found to support the numerical modeling. The apparatus developed to evaluate the cluster-gas targets proved that our mathematical model of cluster formation is reliable enough for the binary gas mixture.

Soft-x-ray harmonic comb from relativistic electron spikes.

We demonstrate a new high-order harmonic generation mechanism reaching the "water window" spectral region in experiments with multiterawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving µJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

Effect of plastic coating on the density of plasma formed in Si foil targets irradiated by ultra-high-contrast relativistic laser pulses.

The formation of high energy density matter occurs in inertial confinement fusion, astrophysical, and geophysical systems. In this context, it is important to couple as much energy as possible into a target while maintaining high density. A recent experimental campaign, using buried layer (or "sandwich" type) targets and the ultrahigh laser contrast Vulcan petawatt laser facility, resulted in 500 Mbar pressures in solid density plasmas (which corresponds to about 4.6×10^{7}J/cm^{3} energy density). The densities and temperatures of the generated plasma were measured based on the analysis of x-ray spectral line profiles and relative intensities.

Wide-field-of-view phase-contrast imaging of nanostructures with a comparatively large polychromatic soft x-ray plasma source.

Polychromatic soft x-ray plasma sources were not previously considered to be among the sources suitable for the propagation based phase contrast imaging because of their comparatively large emission-zone size. In the current work a scheme based on the combination of soft x-ray emission of multicharged ions, generated by the interaction of femtosecond laser pulses with an ultrasonic jet of gas clusters, and an LiF crystal detector was used to obtain phase-enhanced high-resolution images of micro- and nanoscale objects in a wide field of view.

Energy increase in multi-MeV ion acceleration in the interaction of a short pulse laser with a cluster-gas target.

An approach for accelerating ions, with the use of a cluster-gas target and an ultrashort pulse laser of 150-mJ energy and 40-fs duration, is presented. Ions with energy 10-20 MeV per nucleon having a small divergence (full angle) of 3.4 degrees are generated in the forward direction, corresponding to approximately tenfold increase in the ion energies compared to previous experiments using solid targets. It is inferred from a particle-in-cell simulation that the high energy ions are generated at the rear side of the target due to the formation of a strong dipole vortex structure in subcritical density plasmas.

SNOM images of X-ray radiographs at nano-scale stored in a thin layer of lithium fluoride.

In this work, we report a method to observe soft X-ray radiographs at nanoscale of various kind of samples, biological and metallic, stored in a thin layer of lithium fluoride, employing scanning near-field optical microscopy with an optical resolution that reaches 50 nm. Lithium fluoride material works as a novel image detector for X-ray nano-radiographs, due to the fact that extreme ultraviolet radiation and soft X-rays efficiently produce stable point defects emitting optically stimulated visible luminescence in a thin surface layer. The bi-dimensional distribution of the so-created defects depends on the local nanostructure of the investigated sample.

Advanced high resolution x-ray diagnostic for HEDP experiments.

High resolution X-ray imaging is crucial for many high energy density physics (HEDP) experiments. Recently developed techniques to improve resolution have, however, come at the cost of a decreased field of view. In this paper, an innovative experimental detector for X-ray imaging in the context of HEDP experiments with high spatial resolution, as well as a large field of view, is presented. The platform is based on coupling an X-ray backligther source with a Lithium Fluoride detector, characterized by its large dynamic range. A spatial resolution of 2 µm over a field of view greater than 2 mm2 is reported. The platform was benchmarked with both an X-ray free electron laser (XFEL) and an X-ray source produced by a short pulse laser. First, using a non-coherent short pulse laser-produced backlighter, reduced penumbra blurring, as a result of the large size of the X-ray source, is shown. Secondly, we demonstrate phase contrast imaging with a fully coherent monochromatic XFEL beam. Modeling of the absorption and phase contrast transmission of X-ray radiation passing through various targets is presented.

Scintillator-based transverse proton beam profiler for laser-plasma ion sources.

A high repetition rate scintillator-based transverse beam profile diagnostic for laser-plasma accelerated proton beams has been designed and commissioned. The proton beam profiler uses differential filtering to provide coarse energy resolution and a flexible design to allow optimisation for expected beam energy range and trade-off between spatial and energy resolution depending on the application. A plastic scintillator detector, imaged with a standard 12-bit scientific camera, allows data to be taken at a high repetition rate. An algorithm encompassing the scintillator non-linearity is described to estimate the proton spectrum at different spatial locations.

Experimental evidence for short-pulse laser heating of solid-density target to high bulk temperatures.

Heating efficiently solid-density, or even compressed, matter has been a long-sought goal in order to allow investigation of the properties of such state of matter of interest for various domains, e.g. astrophysics. High-power lasers, pinches, and more recently Free-Electron-Lasers (FELs) have been used in this respect. Here we show that by using the high-power, high-contrast "PEARL" laser (Institute of Applied Physics-Russian Academy of Science, Nizhny Novgorod, Russia) delivering 7.5 J in a 60 fs laser pulse, such coupling can be efficiently obtained, resulting in heating of a slab of solid-density Al of 0.8 µm thickness at a temperature of 300 eV, and with minimal density gradients. The characterization of the target heating is achieved combining X-ray spectrometry and measurement of the protons accelerated from the Al slab. The measured heating conditions are consistent with a three-temperatures model that simulates resistive and collisional heating of the bulk induced by the hot electrons. Such effective laser energy deposition is achieved owing to the intrinsic high contrast of the laser which results from the Optical Parametric Chirped Pulse Amplification technology it is based on, allowing to attain high target temperatures in a very compact manner, e.g. in comparison with large-scale FEL facilities.

Burst intensification by singularity emitting radiation in multi-stream flows.

Burst Intensification by Singularity Emitting Radiation (BISER) is proposed. Singularities in multi-stream flows of emitting media cause constructive interference of emitted travelling waves, forming extremely localized sources of bright coherent emission. Here we for the first time demonstrate this extreme localization of BISER by direct observation of nano-scale coherent x-ray sources in a laser plasma. The energy emitted into the spectral range from 60 to 100 eV is up to ~100 nJ, corresponding to ~1010 photons. Simulations reveal that these sources emit trains of attosecond x-ray pulses. Our findings establish a new class of bright laboratory sources of electromagnetic radiation. Furthermore, being applicable to travelling waves of any nature (e.g. electromagnetic, gravitational or acoustic), BISER provides a novel framework for creating new emitters and for interpreting observations in many fields of science.

X-ray spectroscopy of 1 cm plasma channels produced by self-guided pulse propagation in elongated cluster jets.

We diagnose the self-channeled propagation of intense femtosecond pulses over an extended distance in a N2O cluster gas target using high resolution kilovolt x-ray pinhole images of the channel and spatially resolved x-ray spectra. The x-ray images are consistent with femtosecond optical scattering, shadowgraphy, and interferometry images. We observe extended plasma channels (approximately 9 mm) limited either by the cluster jet length or by absorption, for injected laser intensities in the range of 10(16)-10(17) W/cm2. Spectral line shapes for the OVII 1s2-1s3p and OVIII 1s-2p transitions (at 1.8627 and 1.8969 nm, respectively) show significant broadening to the blue side and with truncated emission on the red side. We attribute this effect to Doppler blueshifted emission from fast ions from exploding clusters moving toward the spectrometer; redshifted emission from the opposite side of the cluster is absorbed.

Spectroscopic characterization of an ultrashort-pulse-laser-driven Ar cluster target incorporating both Boltzmann and particle-in-cell models.

A model that solves simultaneously both the electron and atomic kinetics was used to generate a synthetic He alpha and satellite x-ray spectra to characterize a high intensity ultrashort laser driven Ar cluster target experiment. In particular, level populations were obtained from a detailed collisional-radiative model where collisional rates were computed from a time varying electron distribution function obtained from the solution of the zero-dimensional Boltzmann equation. In addition, a particle-in-cell simulation was used to model the laser interaction with the cluster target and provided the initial electron energy distribution function (EEDF) for the Boltzmann solver. This study suggests that a high density average, high, of 3.2 x 10(20) cm(-3) was held by the system for a time, delta tau, of 5.7 ps, and during this time the plasma was in a highly nonequilibrium state in both the EEDF and the ion level populations.

Measurement of 2l-nl' x-ray transitions from approximately 1 microm Kr clusters irradiated by high-intensity femtosecond laser pulses.

X-ray line emission from 2l-nl' transitions in Ne-like Kr and nearby ions has been observed from approximately 1 microm Kr clusters irradiated by fs-scale laser pulses at the JAERI facility in Kyoto, Japan. The incident laser intensity reached 10(19) W/cm2, with pulse energies from 50 to 300 mJ and pulse durations from 30 to 500 fs. The dependence of the x-ray spectral features and intensity on the incident laser intensity is rather weak, indicating that the 1-2 ps cluster lifetimes limit the number of ions beyond Ne-like Kr that can be produced by collisional ionization. Lines from F- to Al-like Kr emitted from the cluster plasmas have been identified using data from the relativistic multiconfiguration flexible atomic code. A collisional-radiative model based on these data has been constructed and used to determine that the cluster plasma has electron densities near 10(22) cm(-3), temperatures of a few hundred eV, and hot electron fractions of a few percent.

Temperature determination using Kalpha spectra from M -shell Ti ions.

The compact multipulse terawatt (COMET) laser facility at LLNL was used to irradiate Al-coated 2-50 microm Ti foils with approximately 10(19) W cm(-2) , 500 fs, 3-6 J laser pulses. Laser-plasma interactions on the front side of the target generate hot electrons with sufficient energy to excite inner-shell electrons in Ti, creating Kalpha emission which has been measured using a focusing spectrometer with spatial resolution aimed at the back surface of the targets. The spatial extent of the emission varies with target thickness. The high spectral resolution (lambda/Deltalambda approximately equal to 3800) is sufficient to measure broadening of the Kalpha emission feature due to the emergence of blueshifted satellites from ionized Ti in a heated region of the target. A self-consistent-field model is used to spectroscopically diagnose thermal electron temperatures up to 40 eV in the strongly coupled Ti plasmas.

Nonlinear increase of X-ray intensities from thin foils irradiated with a 200 TW femtosecond laser.

We report, for the first time, that the energy of femtosecond optical laser pulses, E, with relativistic intensities I > 10(21) W/cm(2) is efficiently converted to X-ray radiation, which is emitted by "hot" electron component in collision-less processes and heats the solid density plasma periphery. As shown by direct high-resolution spectroscopic measurements X-ray radiation from plasma periphery exhibits unusual non-linear growth ~E(4-5) of its power. The non-linear power growth occurs far earlier than the known regime when the radiation reaction dominates particle motion (RDR). Nevertheless, the radiation is shown to dominate the kinetics of the plasma periphery, changing in this regime (now labeled RDKR) the physical picture of the laser plasma interaction. Although in the experiments reported here we demonstrated by observation of KK hollow ions that X-ray intensities in the keV range exceeds ~10(17) W/cm(2), there is no theoretical limit of the radiation power. Therefore, such powerful X-ray sources can produce and probe exotic material states with high densities and multiple inner-shell electron excitations even for higher Z elements. Femtosecond laser-produced plasmas may thus provide unique ultra-bright X-ray sources, for future studies of matter in extreme conditions, material science studies, and radiography of biological systems.

Influence of optical thickness and hot electrons on Rydberg spectra of Ne-like and F-like copper ions.

Spectra in the 7.10 to 8.60 A range from highly charged copper ions are observed from three different laser-produced plasmas (LPPs). The LPPs are formed by a 15-ns Nd:glass laser pulse (type I: E(pulse)=1-8 J, lambda=1.064 microm), a 1-ps Nd:glass laser pulse (type II: E(pulse)=1 J, lambda=1.055 microm), and a 60-fs Ti:sapphire laser pulse (type III: E(pulse)=800 mJ, lambda=790 nm). The spectra of high-n (n

Analysis of high- n dielectronic Rydberg satellites in the spectra of Na -like Zn XX and Mg -like Zn XIX.

We have observed spectra from highly charged zinc ions in a variety of laser-produced plasmas. Spectral features that are Na - and Mg -like satellites to high- n Rydberg transitions in the Ne -like Zn XXI spectrum are analyzed and modeled. Identifications and analysis are made by comparison with highly accurate atomic structure calculations and steady state collisional-radiative models. Each observed Zn XX and Zn XIX feature comprises up to approximately 2 dozen individual transitions, these transitions are excited principally by dielectronic recombination through autoionizing levels in Na - and Mg -like Zn19+ and Zn18+. We find these satellites to be ubiquitous in laser-produced plasmas formed by lasers with pulse lengths that span four orders of magnitude, from 1 ps to approximately 10 ns. The diagnostic potential of these Rydberg satellite lines is demonstrated.

Charge-exchange-induced two-electron satellite transitions from autoionizing levels in dense plasmas.

Order-of-magnitude anomalously high intensities for two-electron (dielectronic) satellite transitions, originating from the He-like 2s(2) 1S0 and Li-like 1s2s(2) (2)S(1/2) autoionizing states of silicon, have been observed in dense laser-produced plasmas at different laboratories. Spatially resolved, high-resolution spectra and plasma images show that these effects are correlated with an intense emission of the He-like 1s3p 1P-1s(2) 1S lines, as well as the K(alpha) lines. A time-dependent, collisional-radiative model, allowing for non-Maxwellian electron-energy distributions, has been developed for the determination of the relevant nonequilibrium level populations of the silicon ions, and a detailed analysis of the experimental data has been carried out. Taking into account electron density and temperature variations, plasma optical-depth effects, and hot-electron distributions, the spectral simulations are found to be not in agreement with the observations. We propose that highly stripped target ions (e.g., bare nuclei or H-like 1s ground-state ions) are transported into the dense, cold plasma (predominantly consisting of L- and M-shell ions) near the target surface and undergo single- and double-electron charge-transfer processes. The spectral simulations indicate that, in dense and optically thick plasmas, these charge-transfer processes may lead to an enhancement of the intensities of the two-electron transitions by up to a factor of 10 relative to those of the other emission lines, in agreement with the spectral observations.