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Interaction of circularly polarized laser pulses with spherical nano-droplets generates nanometer-size magnets with lifetime on the order of hundreds of femtoseconds. Such magnetic dipoles are close enough in a cluster target and magnetic interaction takes place. We investigate such system of several magnetic dipoles and describe their rotation in the framework of Lagrangian formalism. The semi-analytical results are compared to particle-in-cell simulations, which confirm the theoretically obtained terrahertz frequency of the dipole oscillation.

An analytical model has been developed for the generation of a superstrong quasi-stationary magnetic field (up to several GG) in the focal waist of an ultra-intense short circularly polarised laser pulse interacting with a gas-cluster target. The rotation of relativistic electrons around the ionised core of the cluster has been shown to produce a magnetic moment and a quasi-stationary magnetic field. With a large number of nanoclusters, the magnetic field occupies the entire focal volume with a characteristic spatial scale of tens of micrometers and exists during the cluster expansion time (i.e., several picoseconds).

It has been shown that the irradiation of a gas target with nanoclusters by a short (tens of femtoseconds), relativistically intense, circularly polarized laser pulse results in the appearance of magnetic dipoles, which are oriented in the direction of the propagation of the laser pulse and produce a quasi-static regular magnetic field with an amplitude to tenths of the field of the initial laser pulse. At a sufficient concentration of clusters, after the end of the laser pulse, the magnetic moments of the dipoles undergo vibrations and the magnetic field oscillates, which is accompanied by the appearance of a short (few-cycle) secondary terahertz radiation. The condition for the appearance of such a short terahertz pulse is that the cyclotron frequency of an electron on average over the volume of the magnetic field is higher than the inverse lifetime of the cluster.

An optical diagnostic based on resonant absorption of laser light in a plasma is introduced and is used for the determination of density scale lengths in the range of 10 nm to >1 μm at the critical surface of an overdense plasma. This diagnostic is also used to extract the plasma collisional frequency, allowing inference of the temporally evolving plasma composition on the tens of femtosecond timescale. This is found to be characterized by two eras: the early time and short scale length expansion (L 0.1λ); this is consistent with a hydrogen plasma decoupling from the bulk target material. Density gradients and plasma parameters on this scale are of importance to plasma mirror optical performance and comment is made on this theme.

The photon yield obtained during the betatron motion of electrons in the ion cavity is proportional to the number of such oscillations (or wiggling). In the case of high repetition rate laser systems the pulse energy is very limited, thus the laser pulse has to be tightly focused in order to achieve high peak intensity in the focus. In this way narrow gas jets are used, which results in short acceleration length of electrons, thus in low number of betatron oscillations. In order to increase the number of x-ray photons a mixture of clusters and gas target can be used, where the space-charge of nano-meter size droplets forces the electrons to oscillate at much higher frequency leading to the emission of more photons. In this work we present the interaction of such cluster targets with linearly polarized laser pulses and describe some possible positive effects when circular polarization is used.

A hybrid mechanism of ion acceleration is investigated which demonstrates the higher spectral density of protons at high energies. The interaction of few-cycle terrawatt laser pulses with near-critical density gas target is studied with the help of two-dimensional particle-in-cell simulation. The generation of few MeV protons with high spectral concentration near cutoff is attributed to the propagation of solitary waves in the decaying density profile of the gas jet. Plasma dynamics at longer time scale is explained by semianalytical modeling and conditions for solitary wave breaking are presented.

It is known that in the small quantum parameter regime the Breit-Wheeler electron-positron pair creation cross section is extremely sensitive on the photon energy and on the background field. We present the dependence of positron yield on laser intensity in shaped laser under/over dense (gas/foil) plasma interactions with the help of analytical modeling and PIC simulations. This allows for establishing in situ laser intensity diagnostic by measuring the positron yield in experiments where multi PW lasers are used.

The characteristic lines K-α, Ly-α and He-α X-ray source for diagnostics of the compressed state of laser plasma was proposed, and its optimal parameters were calculated under the conditions required for nuclear fusion [1]. Such a source operating in a pulsed regime can be used for determining the spatial distribution of laser- plasma density with high temporal resolution by means of multi-frame backlight imaging of the main target by X-ray pulses obtained by picosecond laser pulses irradiation of a secondary target. In the present report, we analyze the scheme with Ly-α/He-α line emission, found quantitative parameters of a multi-frame X-ray source for radiography of compressed state nuclear-fusion target, and describe images of the target in the case of spherically (elliptically) symmetric compression.

We analyse the possibility of directed scattering and angular filtering of intense short wavelength radiation (laser harmonics) in the XUV range by Mie scattering in the suitable cluster jet

The process of high energy electron acceleration along the surface of grating targets (GTs) that were irradiated by a relativistic, high-contrast laser pulse at an intensity $I=2.5\times 10^{20}~\text{W}/\text{cm}^{2}$ was studied. Our experimental results demonstrate that for a GT with a periodicity twice the laser wavelength, the surface electron flux is more intense for a laser incidence angle that is larger compared to the resonance angle predicted by the linear model. An electron beam with a peak charge of ${\sim}2.7~\text{nC}/\text{sr}$ , for electrons with energies ${>}1.5~\text{MeV}$ , was measured. Numerical simulations carried out with parameters similar to the experimental conditions also show an enhanced electron flux at higher incidence angles depending on the preplasma scale length. A theoretical model that includes ponderomotive effects with more realistic initial preplasma conditions suggests that the laser-driven intensity and preformed plasma scale length are important for the acceleration process. The predictions closely match the experimental and computational results.

Profound effects of spaceflight on the physiology of humans and non-human animals are well-documented but incompletely explored. Current goals to undertake interplanetary missions increase the urgency to learn more about adaptation to prolonged spaceflight and readaptation to Earth-normal conditions, especially with the inclusion of radiation exposures greater than those confronted in traditional, orbital flights. The 30-day-long Bion M-1 biosatellite flight was conducted at a relatively high orbit, exposing the mice to greater doses of radiation in addition to microgravity, a combination of factors relevant to Mars missions. Results of the present studies with mice provide insights into the consequences on brain function of long-duration spaceflight. After landing, mice showed profound deficits in vestibular responses during aerial drop tests. Spaceflown mice displayed reduced grip strength, rotarod performance, and voluntary wheel running, each, which improved gradually but incompletely over the 7-days of post-flight testing. Continuous monitoring in the animals’ home cage activity, in combination with open-field and other tests of motor performance, revealed indices of altered affect, expressed as hyperactivity, potentiated thigmotaxis, and avoidance of open areas which, together, presented a syndrome of persistent anxiety-like behavior. A learned, operant response acquired before spaceflight was retained, whereas the acquisition of a new task was impaired after the flight. We integrate these observations with other results from Bion-M1’s program, identifying deficits in musculoskeletal and cardiovascular systems, as well as in the brain and spinal cord, including altered gene expression patterns and the accompanying neurochemical changes that could underlie our behavioral findings.

An analytical model is presented, which calculates recombination radiation in He and Ly characteristic plasma lines formed as a result of irradiating a solid target with an intense picosecond laser pulse. For calibration, this model is compared with the computer FLYCHK code. According to the results of the comparison, the region of the model applicability lies in the temperature range of the main plasma volume of 200–2000 eV and the electron density of 1023–1024 cm–3. The advantage of this analytical model is its simplicity, which makes it possible to be used for quick estimates of the luminosity of the characteristic plasma lines at a given temperature and density, or, vice versa, to determine the density and temperature of a plasma source from the relative height and structure of the emitted lines.

The potential of laser-driven ion beam applications is limited by high quality requirements. The excellent "point-source" characteristics of the laser accelerated proton beam in a broad energy range were found by using proton radiographs of a mesh. The "virtual source" of protons, the point where the proton trajectories are converging and form a waist, gradually decreases and moves asymptotically to the target with increasing particles' energy. Computer simulations confirmed that the beam profile at the center is fully conserved, the virtual source of higher energy protons gradually moves closer to the target, and if the particle energy is further increased, the virtual source will be located on the target front surface (for portions above 13 MeV, in this case) with a size comparable to the laser spot size. The laser ponderomotive force pushes the electrons deep into the target creating a bipolar charge structure, i.e., an electron cavity and spike which produces strong accelerating field, realizing a point-size source of accelerated protons. This behavior has not previously been predicted. These results contribute to the development of next generation laser-accelerators suitable for many applications.

Interaction of an ultrashort and ultraintense laser pulse with overdense/underdense plasmas is considered. Efficient conversion of fundamental laser radiation into sub-femtosecond XUV/X-ray radiation and its significant amplification in laser plasmas are obtained. The results of the simulations were compared with the experimental data and have shown a good coexistence.

Femtosecond lasers are a proven source of hard X-rays for ultrafast probing applications; however, such X-ray sources do not offer significant advantages over accelerator-based photon sources. In this paper, the use of the proposed 100-kHz two-cycle ALPS-HR laser at the ELI-ALPS facility to drive a femtosecond hard X-ray source is investigated and target-related issues for high repetition rates are also discussed. A particle-in-cell simulation is used to determine the optimal plasma scale length for the investigated laser parameters to generate the necessary hot electron spectrum. Electron transport and photon creation are simulated using a Monte Carlo method in liquid droplet targets. The generated photon characteristics are postprocessed to build the synthetic X-ray spectrum and temporal pulse shape. The X-ray source characteristics are compared with other similar laser plasma X-ray sources. The parameters for the preliminary experiments are also discussed.

Numerical simulations were made of the interaction of a relativistically intense laser pulse with a target consisting of nanometre fibres. Fast electrons were shown to execute forced betatron oscillations in the electrostatic fibre field and the laser field. The fibre diameter was determined whereby the amplitude of betatron electron oscillations is resonantly increased. The power of coherent X-ray betatron radiation of the electron bunch was calculated outside of the resonance domain and in the resonance case. We showed that the laser-to-X-ray betatron radiation conversion coefficient in the resonance case amounts to a few percent and the target made up of nanometre fibres may be regarded as an efficient laser-driven source of coherent X- and gamma-ray radiation.

High frequency modulations appearing in the kinetic energy distribution of laser accelerated ions are proposed for retrieving the acceleration field dynamics at femtosecond time scale. Such an approach becomes possible if the laser-cycling field modulates the particle density in the ion spectra and produces quasi time stamps for analysis. We investigate target and laser parameters determining this effect and discuss the dependencies of the observed modulation. Our findings refine a basic mechanism, the Target Normal Sheath Acceleration, where an intense and ultrafast laser pulse produces a very strong electrical field at a plasma-vacuum interface. The field decays rapidly due to energy dissipation and forms a characteristic spectrum of fast ions streaming away from the interface. We show that the derived decay function of the field is in accordance with model predictions of the accelerating field structure. Our findings are supported by 2-dimensional particle-in-cell simulations. The knowledge of the femtosecond field dynamics helps to rerate optimization strategies for laser ion acceleration., 7 figures

Increasing the laser energy absorption into energetic particle beams represents a longstanding quest in intense laser-plasma physics. During the interaction with matter, part of the laser energy is converted into relativistic electron beams, which are the origin of secondary sources of energetic ions, γ-rays and neutrons. Here we experimentally demonstrate that using multiple coherent laser beamlets spatially and temporally overlapped, thus producing an interference pattern in the laser focus, significantly improves the laser energy conversion efficiency into hot electrons, compared to one beam with the same energy and nominal intensity as the four beamlets combined. Two-dimensional particle-in-cell simulations support the experimental results, suggesting that beamlet interference pattern induces a periodical shaping of the critical density, ultimately playing a key-role in enhancing the laser-to-electron energy conversion efficiency. This method is rather insensitive to laser pulse contrast and duration, making this approach robust and suitable to many existing facilities., Enhanced coupling of laser energy to the target particles is a fundamental issue in laser-plasma interactions. Here the authors demonstrate increased photon absorption leading into higher laser to electron and proton energy transfer through the interference of multiple coherent beamlets.

In this work, we present the analytical model, which can calculate laser plasma emission in Ly and He lines. Comparison of our model with FLYCHK simulation code shows their good agreement. This model is easy to learn and it could be used for fast estimation to determine the plasma parameters from its emission spectrum.

An analytical formula describing the temporal profile of the ultrashort chirped pulse after reflection from a preformed overdense plasma has been presented. The frequency chirp is noticed to act as damping factor which ultimately reduces the intensity (power) of the pulse after reflection.

The electron-positron pair creation occurring near the laser-plasma interface is studied with the help of Particle-in-Cell 1D and 2D simulations. We present the angular distribution of positrons which can help in their detection in real experiments. This technique can be used to measure the intensity and duration of the laser pulses with higher precision.

In the present report, we discussed our recent results concerning the investigations of high energy density matter. By using of high-resolution X-ray spectroscopic measurements and kinetic simulations, we demonstrate that the energy of femtosecond laser pulses is efficiently converted to X-ray radiation and produces exotic states in solid density plasma regions. It is shown that exotic hollow multicharged ions states can be obtained and studied not only by ultra-intense X-ray radiation of XFEL lasers, but also upon using optical laser technology. We have concluded that observable features of the hollow-ion spectra are sensitive to such plasma parameters as density, temperature, hot-electron fraction, and intensity of the X-ray pumping radiation and could be used for effective diagnostics of warm dense matter parameters.

The dependence of energy of an ion accelerated from a thin laser target on its charge is investigated numerically and analytically. It is demonstrated that this dependence is determined by the values of ionization potentials of the target atoms and has a sharper character than those obtained in previous models of ion acceleration. The physical origin of sharper dependence is the relation of ion charge to its initial position in the ionized target. As a result, more highly charged ions turn out to be in the region of stronger accelerating field and thus attain higher energy. For a laser target made of a gold foil, the results obtained with an analytical model are compared to those obtained by means of a two-dimensional simulation, which demonstrated agreement between theoretical and calculated dependences of ion energy on its charge.

The efficient transfer of angular orbital momentum from circularly polarized laser pulses into ions of solid density targets is investigated with different geometries using particle-in-cell simulations. The detailed electron and ion dynamics presented focus upon the energy and momentum conversion efficiency. It is found that the momentum transfer is more efficient for spiral targets and the maximum value is obtained when the spiral step is equal to twice the laser wavelength. This study reveals that the angular momentum distribution of ions strongly depends up on the initial target shape and density.

A mathematical model of proton image formation is developed, and minimum size of objects visible under proton irradiation (resolution of proton radiography) is estimated. Due to ion flux being laminar, micron-sized objects smaller than the ion emission spot size are detected. The results of simulation are in good agreement with the experiment.

A new method to determine the peak intensity of focused relativistic laser pulses is experimentally justified. It is based on the measurement of spectra of electrons, accelerated in the beam waist. The detected electrons were emitted from the plasma, generated by nonlinear ionization of low-density gases (helium, argon, and krypton) in the focal area of a laser beam with the peak intensity >1020 W/cm2. The measurements revealed generation of particles with the maximum energy of a few MeV, observed at a small angle relative to the beam axis. The results are supported by numerical particle-in-cell simulations of a laser–low-density plasma interaction. The peak intensity in the focal region derived from experimental data reaches the value of 2.5 × 1020 W/cm2.

Experimental studies on the formation of highly charged ions of medium-Z elements using femtosecond laser pulses with different contrast levels were carried out. Multiply charged Fe ions were generated by laser pulses with 35 fs duration and an intensity exceeding 1021 W/cm2. Using high-resolution X-ray spectroscopic methods, bulk electron temperature of the generated plasma has been identified. It is shown that the presence of a laser pre-pulse at a contrast level of 105–106 with respect to the main pulse drastically decreases the degree of Fe ionization. We conclude that an effective source of energetic, multiply charged moderate and high-Z ions based on femtosecond laser–plasma interactions can be created only using laser pulses of ultra-high contrast.

A method, how electrons can be directly accelerated in intense laser fields, is investigated experimentally and discussed with numerical and analytical simulation. When ultrathin foil targets are exposed with peak laser intensities of ~ 1x10 20 W/cm 2 , slow electrons ( ~ keV kinetic energy), that are emitted from the ultrathin foil target along laser propagation direction, are post-accelerated in the transmitted laser field. They received significant higher kinetic energies (MeV), when this interaction was limited in duration and an enhanced number of fast electrons were detected. The decoupling of the light field from the electron interaction we realized with a second separator foil, blocking the transmitted laser light at a particular distance and allowing the fast electrons to pass. Variation of the propagation distance in the laser field results in different energy gains for the electrons. This finding is explained with electron acceleration in the electromagnetic field of a light pulse and confirms a concept being discussed for some time. In the experiments the effect manifests in an electron number amplification of about 3 times around a peak at 1 MeV electron energy. Measurements confirmed that the overall number in the whole bunch is enhanced to about 10 9 electrons covering kinetic energies between 0.5 to 5 MeV. The method holds promise for ultrashort electron bunch generation at MeV energies for direct application, e.g. ultra-fast electron diffraction, or for injection into post accelerator stages for different purposes.

Mechanical loading is necessary for the development and maintenance of the musculoskeletal system. Removal of loading via microgravity, paralysis, or bed rest leads to rapid loss of muscle mass and function; however, the molecular mechanisms that lead to these changes are largely unknown, particularly for the spaceflight (SF) microgravity environment. Furthermore, few studies have explored these effects on the shoulder, a dynamically stabilized joint with a large range of motion; therefore, we examined the effects of microgravity on mouse shoulder muscles for the 15-d Space Transportation System (STS)-131, 13-d STS-135, and 30-d Bion-M1 missions. Mice from STS missions were euthanized within 4 h after landing, whereas mice from the Bion-M1 mission were euthanized within 14 h after landing. The motion-generating deltoid muscle was more sensitive to microgravity than the joint-stabilizing rotator cuff muscles. Mice from the STS-131 mission exhibited reduced myogenic (Myf5 and -6) and adipogenic (Pparg, Cebpa, and Lep) gene expression, whereas either no change or an increased expression of these genes was observed in mice from the Bion-M1 mission. In summary, muscle responses to microgravity were muscle-type specific, short-duration SF caused dramatic molecular changes to shoulder muscles and responses to reloading upon landing were rapid.-Shen, H., Lim, C., Schwartz, A. G., Andreev-Andrievskiy, A., Deymier, A. C., Thomopoulos, S. Effects of spaceflight on the muscles of the murine shoulder.

In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. We used laser-induced periodic surface structures generated in-situ as a very fast and economic way to produce nanostructured targets capable of high-repetition rate applications. Both in experiment and theory, we investigate the impact of nanostructuring on the proton spectrum for different laser–plasma conditions. Our experimental data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, i.e. too steep plasma gradients, nanostructuring is found to significantly enhance the proton cutoff energy and conversion efficiency. In contrast, if the plasma gradient is optimized for laser absorption of the plane target, the nanostructure-induced absorption increase is not reflected in higher cutoff energies. Both, simulation and experiment point towards the energy transfer from the laser to the hot electrons as bottleneck.

Direct acceleration of electrons in a coherent, intense light field is revealed by a remarkable increase of the electron number in the MeV energy range. Laser irradiation of thin polymer foils with a peak intensity of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ releases electron bunches along the laser propagation direction that are postaccelerated in the partly transmitted laser field. They are decoupled from the laser field at high kinetic energies, when a second foil target at an appropriate distance prevents their subsequent deceleration in the declining laser field. The scheme is established with laser pulses of high temporal contrast (${10}^{10}$ peak to background ratio) and two ultrathin polymer foils at a distance of $500\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. 2D particle in cell simulations and an analytical model confirm a significant change of the electron spectral distribution due to the double foil setup, which leads to an amplification of about 3 times of the electron number around a peak at 1 MeV electron energy. The result verifies a theoretical concept of direct electron bunch acceleration in a laser field that is scalable to extreme acceleration potential gradients. This method can be used to enhance the density and energy spread of electron bunches injected into postaccelerator stages of laser driven radiation sources.

The use of a plasma mirror to improve the temporal contrast of few cycle laser pulses has been considered. Pre-plasma features, prior to the main pulse, have been evaluated using an analytical model that has been verified using hydrodynamic code. The temporal/ spectral profile, reflectivity, and broadening of the reflected pulse have been parametrically analysed using an analytical formulation that describes the reflection of broadband ultra-short pulses from the plasma gradient. The analytical estimate for the pulse reflectivity is in good agreement with experimental measurements. The consistency of the analytical expressions for the collisionless case has been validated via comparison with a 1D particle in cell simulations.

It was shown (Faenov et al., 2015b) that the energy of femtosecond laser pulses with relativistic intensity approaching to ~1021 W/cm2 is efficiently converted to X-ray radiation and produces exotic states in solid density plasma periphery. We propose and show by one-dimensional two-temperature hydrodynamic modeling, that applying two such unique ultra-bright X-ray sources with intensities above 1017 W/cm2 – allow to generate shock waves with strength of up to some hundreds Mbar, which could give new opportunities for studies of matter in extreme conditions.

Nano-plasma photonics is a field that deals with optical processes in a plasma at the nanoscale, much shorter than the wavelength of laser radiation. The laser-matter interactions, which include nano-scale confinement of radiation or matter and its transformation, present numerous opportunities for both fundamental research and technological applications.

SPIE Optics + Optoelectronics, Prague, Czech Republic; Proceedings of SPIE 10237, 102370S (2017). doi:10.1117/12.2270552, Simulations of experiments at modern light sources, such as optical laser laboratories, synchrotrons, and free electron lasers, become increasingly important for the successful preparation, execution, and analysis of these experiments investigating ever more complex physical systems, e.g. biomolecules, complex materials, and ultra–short lived states of matter at extreme conditions. We have implemented a platform for complete start–to–end simulations of various types of photon science experiments, tracking the radiation from the source through the beam transport optics to the sample or target under investigation, its interaction with and scattering from the sample, and registration in a photon detector. This tool allows researchers and facility operators to simulate their experiments and instruments under real life conditions, identify promising and unattainable regions of the parameter space and ultimately make better use of valuable beamtime. In this paper, we present an overview about status and future development of the simulation platform and discuss three applications: 1.) Single–particle imaging of biomolecules using x–ray free electron lasers and optimization of x–ray pulse properties, 2.) x–ray scattering diagnostics of hot dense plasmas in high power laser–matter interaction and identification of plasma instabilities, and 3.) x–ray absorption spectroscopy in warm dense matter created by high energy laser–matter interaction and pulse shape optimization for low–isentrope dynamic compression., Published by SPIE, Bellingham, Wash.

We have investigated the effect of the laser spot shape on the spatial distribution of accelerated ions on the front and back sides of a thin target irradiated by a picosecond laser pulse having the intensity of (3 – 4) × 1018 W cm-2. Experimental data are compared with numerical calculations. It is shown that the spatial structure of the ion bunch on the front side of the target resembles the laser spot structure rotated by 90°.

The possibility of improving the efficiency of X-ray generation under the interaction of femtosecond laser pulses (exhibiting high repetition rate and subrelativistic intensity) with a thin copper foil by changing the angle of incidence of the laser radiation is studied both experimentally and theoretically. The brightness of such an X-ray source in the conducted experiments considerably exceeded that of a conventional X-ray tube. The structure of the target surface before and after interaction is described in detail, and specific features of the influence of this structure on absorption of laser pulses are analyzed.

The set of equations describing the motion of a thin (compared to the wavelength) target in the field of a laser pulse that takes into consideration separate motion of the electron and ion layers is derived. In the case of strong Coulomb coupling between the layers, the set of equation of motions of the layers is reduced to the well-known light-sail equation containing a self-consistent coefficient of nonlinear reflection of laser radiation by a moving target. The optimal thickness of the laser target at which the target acquires maximum energy for given laser-pulse parameters is determined. It is shown that this thickness depends not only on laser intensity, but also on laser-pulse duration and the ratio of electron and ion masses. The growth rates of transverse instability of optimal targets under their intense acceleration are analyzed. It is demonstrated that instability does not develop in the currently experimentally accessible range of laser intensities and pulse durations between 100 and 200 fs.

Generation and propagation of fast electrons in laser targets consisting of thin nanofilaments are studied numerically and analytically. Such targets completely absorb laser radiation and exhibit a large coefficient of laser-energy conversion to kinetic energy of a flow of fast electrons. Analytical estimates show that the optimal thickness of the filament is on the order of the skin depth of the laser plasma, while an optimal distance between filaments is on the order of the Debye radius of hot electrons. A bunch of relativistic electrons can propagate as far as several hundred micrometers in such targets, while the fastest electrons can propagate several millimeters. Upon bending of filaments, the flow of electrons propagates along the filaments and can be focused by bringing the filaments together. Laser targets of the discussed composition are used as sources of dense bunches of relativistic electrons and subsequent generation of high-intensity X-ray radiation with their help.

Absorption of an intense short laser pulse by targets consisting of a bungle of carbon nanofilaments either parallel or converging to the vertex of a cone has been numerically simulated. Such targets efficiently generate a flux of relativistic electrons propagating along filaments and following their bends. The focusing of fluxes from several filaments to one flux makes it possible to reduce the transverse size of an electron bunch as compared to the diameter of the laser beam. The energy flux density of electrons in such a bunch propagating along one nanofilament is several times higher than the energy flux density of the laser pulse.

A new two-step approach for frequency conversion of laser radiation towards hard X-rays is developed and examined experimentally. Fast electrons are produced in a form of thin jets at the first stage, as an intense femtosecond laser pulses impinges on a micrometer water target. In the second stage the accelerated electrons hit a secondary metal target and generate characteristic K-shell radiation with a duration down to sub-femtosecond. It is shown that counter propagating laser radiation experiences very strong up-shift with up to 6 × 103 times of fundamental frequency by reflection from the electron jets.

We demonstrated experimentally the formation of monoenergetic beams of accelerated electrons by focusing femtosecond laser radiation with an intensity of $2\times 1{0}^{17} ~\mathrm{W} / {\mathrm{cm} }^{2} $ onto the edge of an aluminum foil. The electrons had energy distributions peaking in the 0.2–0.8 MeV range with energy spread less than 20%. The acceleration mechanism related to the generation of a plasma wave as a result of self-modulation instability of a laser pulse in a dense plasma formed by a prepulse (arriving 12 ns before the main pulse) is considered. One-dimensional and two-dimensional Particle in Cell (PIC) simulations of the laser–plasma interaction showed that effective excitation of a plasma wave as well as trapping and acceleration of an electron beam with an energy on the order of 1 MeV may occur in the presence of sharp gradients in plasma density and in the temporal shape of the pulse.

It is shown that the relief structure with optimum parameters can significantly increase the short-pulse laser absorption, which is connected with the enhancement of moving electrons between relief ledges. Analytical modeling and numerical simulations confirm this argumentation. In the considered cases, degradation of a structure by a laser pre-pulse is the most important factor and for this scheme to work, one needs a very high-contrast laser-pulse and a nanosecond laser pre-pulse duration. The limitation on laser pulse duration is not so strong because after destruction of a first relief a secondary dynamic structure of ion density appears. Thus, high absorption connected with a relief existence continues during a long time that gives a possibility for structure targets to be more efficient compared to a plane one. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Animals are an essential component of space exploration and have been used to demonstrate that weightlessness does not disrupt essential physiological functions. They can also contribute to space research as models of weightlessness-induced changes in humans. Animal research was an integral component of the 30-day automated Russian biosatellite Bion-M 1 space mission. The aim of the hemodynamic experiment was to estimate cardiovascular function in mice, a species roughly 3000 times smaller than humans, during prolonged spaceflight and post-flight recovery, particularly, to investigate if mice display signs of cardiovascular deconditioning. For the first time, heart rate (HR) and blood pressure (BP) were continuously monitored using implantable telemetry during spaceflight and recovery. Decreased HR and unchanged BP were observed during launch, whereas both HR and BP dropped dramatically during descent. During spaceflight, BP did not change from pre-flight values. However, HR increased, particularly during periods of activity. HR remained elevated after spaceflight and was accompanied by increased levels of exercise-induced tachycardia. Loss of three of the five mice during the flight as a result of the hardware malfunction (unrelated to the telemetry system) and thus the limited sample number constitute the major limitation of the study. For the first time BP and HR were continuously monitored in mice during the 30-day spaceflight and 7-days of post-flight recovery. Cardiovascular deconditioning in these tiny quadruped mammals was reminiscent of that in humans. Therefore, the loss of hydrostatic pressure in space, which is thought to be the initiating event for human cardiovascular adaptation in microgravity, might be of less importance than other physiological mechanisms. Further experiments with larger number of mice are needed to confirm these findings.

Recent advances in the production of high repetition, high power, and short laser pulse have enabled the generation of high-energy proton beam, required for technology and other medical applications. Here we demonstrate the effective laser driven proton acceleration from near-critical density hydrogen plasma by employing the short and intense laser pulse through three-dimensional (3D) particle-in-cell (PIC) simulation. The generation of strong magnetic field is demonstrated by numerical results and scaled with the plasma density and the electric field of laser. 3D PIC simulation results show the ring shaped proton density distribution where the protons are accelerated along the laser axis with fairly low divergence accompanied by off-axis beam of ring-like shape.

We have studied particle acceleration in different nanostructured targets irradiated by high intensity laser pulses of high contrast. We find that the maximum energy of emitted particles and their directionality is significantly enhanced in the case of nanostructured targets with respect to plane targets. We have studied theoretically in detail the generation and propagation of fast electrons in nanowire targets. Such targets exhibit an extraordinary high conversion efficiency of laser energy into electron kinetic energy. We observe guiding of electron bunches along the wires. Results from theory and simulation compare reasonably well with the experimental data.

A significant level of back reflected laser energy was measured during the interaction of ultra-short, high contrast PW laser pulses with solid targets at 30° incidence. 2D PIC simulations carried out for the experimental conditions show that at the laser-target interface a dynamic regular structure is generated during the interaction, which acts as a grating (quasi-grating) and reflects back a significant amount of incident laser energy. With increasing laser intensity above 1018 W/cm2 the back reflected fraction increases due to the growth of the surface modulation to larger amplitudes. Above 1020 W/cm2 this increase results in the partial destruction of the quasi-grating structure and, hence, in the saturation of the back reflection efficiency. The PIC simulation results are in good agreement with the experimental findings, and, additionally, demonstrate that in presence of a small amount of pre-plasma this regular structure will be smeared out and the back reflection reduced.

We report on the first observation of high-n hollow ions (ions having no electrons in the K or L shells) produced in Si targets via pumping by ultra-intense x-ray radiation produced in intense laser-plasma interactions reaching the radiation dominant kinetics regime (RDKR). The existence of these new types of hollow ions in high-energy density plasma has been found via observation of highly resolved x-ray emission spectra of silicon plasma. This has been confirmed by plasma kinetics calculations, underscoring the ability of powerful radiation sources to fully strip electrons from the innermost shells of light atoms. Hollow-ions spectral diagnostics provide a unique opportunity to characterize powerful x-ray radiation of laboratory and astrophysical plasmas. With the use of this technique we provide evidence for the existence of the RDKR via observation of asymmetry in the observed radiation of hollow ions from the front and rear sides of the target.

We present a detailed study of the electric and magnetic fields, which are created on plasma vacuum interfaces as a result of highly intense laser-matter-interactions. For the field generation ultra-thin polymer foils were irradiated with high intensity femtosecond and picosecond laser pulses with ultra-high contrast. To determine the temporal evolution and the spatial distribution of these fields the proton streak deflectometry method has been developed further and applied in two different imaging configurations. It enabled us to gather complementary information about the investigated field structure, in particular about the influence of different field components (parallel and normal to the target surface) and the impact of a moving ion front. The applied ultra-high laser contrast significantly increased the reproducibility of the experiment and improved the accuracy of the imaging method. In order to explain the experimental observations, which were obtained by applying ultra-short laser pulses, two different analytical models have been studied in detail. Their ability to reproduce the streak deflectometry measurements was tested on the basis of three-dimensional particle simulations. A modification and combination of the two models allowed for an extensive and accurate reproduction of the experimental results in both imaging configurations. The controlled change of the laser pulse duration from 50 femtoseconds to 2.7 picoseconds led to a transition of the dominating force acting on the probing proton beam at the rear side of the polymer foil. The applied proton deflectometry method allowed for an unambiguous determination of the magnetic field polarity at the rear side of the ultra-thin foil., Comment: 20 pages, 6 figures