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We present optical Thomson scattering measurements of electron density and temperature in a large-scale (~2 cm) exploding laser plasma produced by irradiating a solid target with a high energy (5-10 J) laser pulse at high repetition rate (1 Hz). The Thomson scattering diagnostic matches this high repetition rate. Unlike previous work performed in single shots at much higher energies, the instrument allows point measurements anywhere inside the plasma by automatically translating the scattering volume using motorized stages as the experiment is repeated at 1 Hz. Measured densities around 4$\times 10^{16}$ cm$^{-3}$ and temperatures around 7 eV result in a scattering parameter near unity, depending on the distance from the target. The measured spectra show the transition from collective scattering close to the target to non-collective scattering at larger distances. Densities obtained by fitting the weakly collective spectra agree to within 10% with an irradiance calibration performed via Raman scattering in nitrogen.

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.

With the wavelength of a short-pulse laser driver shifted to the midinfrared range, the low-frequency output of laser-induced plasmas can be drastically enhanced, as our experiments show, providing a source of ultrabroadband radiation with a spectrum spanning across the entire terahertz, millimeter-wave, and microwave bands. At low gas pressures, such ultrabroadband field waveforms are shown to rapidly build up their coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. As counterintuitive as it may seem, this behavior of the intensity, coherence, and polarization of the low-frequency plasma output is shown to be consistent with a physical scenario of Cherenkov-type radiation emission by ponderomotively driven plasma currents.

In this work, simulations of multipetawatt lasers at irradiances ${\sim }10^{23} \ \mathrm {W}\ \mathrm {cm}^{-2}$ , striking solid targets and implementing two-dimensional particle-in-cell code was used to study particle acceleration. Preformed plasma at the front surface of a solid target increases both the efficiency of particle acceleration and the reached maximum energy by the accelerated charged particles via nonlinear plasma processes. Here, we have investigated the preformed plasma scale length effects on particle acceleration in the presence and absence of nonlinear quantum electrodynamic (QED) effects, including quantum radiation reaction and multiphoton Breit–Wheeler pair production, which become important at irradiances ${\sim } 10^{23}\ \mathrm {W}\ \mathrm {cm}^{-2}$ . Our results show that QED effects help particles gain higher energies with the presence of preformed plasma. In the results for all cases, preplasma leads to more efficient laser absorption and produces more energetic charged particles, as expected. In the case where QED is included, however, physical mechanisms changed and generated secondary particles ( $\gamma$ -rays and positrons) reversing this trend. That is, the hot electrons cool down due to QED effects, while ions gain more energy due to different acceleration methods. It is found that more energetic $\gamma$ -rays and positrons are created with increasing scale length due to high laser conversion efficiency ( ${\sim }$ 24 % for $\gamma$ -rays and $\sim$ 4 % for positrons at $L = 7\ \mathrm {\mu }\textrm {m}$ scale length), which affects the ion and electron acceleration mechanisms. It is also observed that the QED effect reduces the collimation of angular distribution of accelerated ions because the dominant ion acceleration mechanism is changing when QED is involved in the process.

Magnetic reconnection driven by laser plasma interactions attracts great interests in the recent decades. Motivated by the rapid development of the laser technology, the ultra strong magnetic field generated by the laser-plasma accelerated electrons provides unique environment to investigate the relativistic magnetic field annihilation and reconnection. It opens a new way for understanding relativistic regimes of fast magnetic field dissipation particularly in space plasmas, where the large scale magnetic field energy is converted to the energy of the nonthermal charged particles. Here we review the recent results in relativistic magnetic reconnection based on the laser and collisionless plasma interactions. The basic mechanism and the theoretical model are discussed. Several proposed experimental setups for relativistic reconnection research are presented.

The dynamical behaviors of electromagnetic (EM) solitons formed due to nonlinear interaction of linearly polarized intense laser light and relativistic degenerate plasmas are studied. In the slow motion approximation of relativistic dynamics, the evolution of weakly nonlinear EM envelope is described by the generalized nonlinear Schr{ö}dinger (GNLS) equation with local and nonlocal nonlinearities. Using the Vakhitov-Kolokolov criteria, the stability of an EM soliton solution of the GNLS equation is studied. Different stable and unstable regions are demonstrated with the effects of soliton velocity, soliton eigenfrequency, as well as the degeneracy parameter $R=p_{Fe}/m_ec$, where $p_{Fe}$ is the Fermi momentum and $m_e$ the electron mass, and $c$ is the speed of light in vacuum. It is found that the stability region shifts to an unstable one and is significantly reduced as one enters from the regimes of weakly relativistic $(R\ll1)$ to ultrarelativistic $(R\gg1)$ degeneracy of electrons. The analytically predicted results are in good agreement with the simulation results of the GNLS equation. It is shown that the standing EM soliton solutions are stable. However, the moving solitons can be stable or unstable depending on the values of soliton velocity, the eigenfrequency or the degeneracy parameter. The latter with strong degeneracy $(R>1)$ can eventually lead to soliton collapse., 10 pages, 5 figures; Revised version to appear in J. Plasma Physics (2020)

We compare experimental and theoretical studies of the quasi-phase matching (QPM) of high-order harmonic generation (HHG) during propagation of two-color pulses through the indium multi-jet plasmas. Role of the sizes of plasma jets and distance between them on the HHG efficiency at QPM conditions is analyzed. The results of the QPM conditions’ studies provide the reliable interpretation of this phenomenon.

Errors in the Epperlein & Haines [PoF (1986)] transport coefficients were recently found at low electron magnetizations, with new magnetic transport coefficients proposed simultaneously by two teams [Sadler, Walsh & Li, PRL (2021) and Davies, Wen, Ji & Held, PoP (2021)]; these two separate sets of updated coefficients are shown in this paper to be in agreement. The importance of these new coefficients in laser-plasmas with either self-generated or applied magnetic fields is demonstrated. When an external magnetic field is applied, the cross-gradient-Nernst term twists the field structure; this twisting is reduced by the new coefficients in the low magnetization regime. For plasmas where only self-generated magnetic fields are present, the new coefficients are found to result in the magnetic field moving with the Righi-Leduc heat-flow, enhancing the impact of MHD. Simulations of Biermann Battery magnetic fields around ICF hot-spot perturbations are presented, with cross-gradient-Nernst transport increasing spike penetration.

Relativistic mirrors can be realized with strongly nonlinear Langmuir waves excited by intense laser pulses in underdense plasma. On reflection from the relativistic mirror the incident light affects the mirror motion. The corresponding recoil effects are investigated analytically and with particle-in-cell simulations. It is found that if the fluence of the incident electromagnetic wave exceeds a certain threshold, the relativistic mirror undergoes a significant back reaction and splits into multiple electron layers. The reflection coefficient of the relativistic mirror as well as the factors of electric field amplification and frequency upshift of the electromagnetic wave are obtained., 8 pages, 5 figures, submitted to Physics of Plasmas

Ultrafast ionization of a gas medium driven by ultrashort midinfrared laser pulses provides a source of bright ultrabroadband radiation whose spectrum spans across the entire microwave band, reaching for the sub-gigahertz range. We combine multiple, mutually complementary detection techniques to provide an accurate polarization-resolved characterization of this broadband output as a function of the gas pressure. At low gas pressures, the lowest-frequency part of this output is found to exhibit a drastic enhancement as this field builds up its coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. This behavior of the intensity, coherence, and polarization of the microwave output is shown to be consistent with Cherenkov-type radiation by ponderomotively driven plasma currents.

Various laser-produced metal plasmas were used for high-order sum and difference harmonic generation using the mixing of tunable mid-infrared pulses from optical parametric amplifier and 810 nm ultrashort pulses. We show that, regardless of non-optimal spatio-temporal overlap of two sources of radiation in the plasmas, the application of proposed technique allows a significant growth of harmonic yield and broadening of the number of different combinations of interacting waves in the extreme ultraviolet region, which could be useful for observation of the resonance induced enhancement of some frequency components.

We investigate relativistic high-order harmonic generation from intense few-cycle laser and plasma interactions via particle-in-cell simulations. Differences of spectral and temporal structures in XUV beam resulted from few-cycle and non-few-cycle driver pulses are compared.

This work is dedicated to the multiparametric numerical simulations of the dynamics of electron vortices - one of the coherent structures that can form due to the interaction of high-intensity laser pulses with plasmas. Using a two-dimensional Particle-in-Cell simulations it is demonstrated that the postsoliton stage of the evolution of the electron vortex is described well by the ”snow plow” model. The dependence between the parameters of the vortex and the characteristic time of the vortex boundary disintegration is absorbed.

Electron vortices appear in the wake of a finite length laser pulse propogating in the underdense plasma. Usually they form two chains of vortices with opposite signs of the magnetic fields locked inside an electron cavity. Using 2D PIC simulations, we discuss the effects of evolution of single and binary electron vortices. Single electron vortices, though being in a quasistationary state on electron timescales, evolve on ion timescales, leading to anisotropic multishell ion motion. Binary electron vortices may be subject to complex motions, which can be described by the point-vortex solutions of Hasegawa-Mima equation. When the finite radius effects come into play, we observe effects as magnetic field annihilation with the subsequent fast electron bunch generation and secondary vortex formation.

The future availability of high-intensity laser facilities capable of delivering tens of petawatts of power (e.g. ELI-NP) into small volumes of matter at high repetition rates will give the unique opportunity to investigate nuclear reactions and fundamental interactions process under extreme plasma conditions [1]. In this context, use of targets with nanostructured surfaces is giving promising indications to reproduce plasma conditions suitable for measurements of thermonuclear reactions rates, in the domain of nanosecond laser pulses.

Laser-plasma interactions in the novel regime of relativistically induced transparency (RIT) have been harnessed to generate intense ion beams efficiently with average energies exceeding 10 MeV/nucleon (>100 MeV for protons) at “table-top” scales in experiments at the LANL Trident Laser. By further optimization of the laser and target, the RIT regime has been extended into a self-organized plasma mode. This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency. This mode involves self-generation of persistent high magnetic fields (∼104 T, according to particle-in-cell simulations of the experiments) at the rear-side of the plasma. These magnetic fields trap the laser-heated multi-MeV electrons, which generate a high localized electrostatic field (∼0.1 T V/m). After the laser exits the plasma, this electric field acts on a highly structured ion-beam distribution in phase space to reduce the energy spread, thus separating acceleration and energy-spread reduction. Thus, ion beams with narrow energy peaks at up to 18 MeV/nucleon are generated reproducibly with high efficiency (≈5%). The experimental demonstration has been done with 0.12 PW, high-contrast, 0.6 ps Gaussian 1.053 μm laser pulses irradiating planar foils up to 250 nm thick at 2–8 × 1020 W/cm2. These ion beams with co-propagating electrons have been used on Trident for uniform volumetric isochoric heating to generate and study warm-dense matter at high densities. These beam plasmas have been directed also at a thick Ta disk to generate a directed, intense point-like Bremsstrahlung source of photons peaked at ∼2 MeV and used it for point projection radiography of thick high density objects. In addition, prior work on the intense neutron beam driven by an intense deuterium beam generated in the RIT regime has been extended. Neutron spectral control by means of a flexible converter-disk design has been demonstrated, and the neutron beam has been used for point-projection imaging of thick objects. The plans and prospects for further improvements and applications are also discussed.

We compare the results of experimental and theoretical studies of the quasi-phase matching (QPM) of high-order harmonic generation (HHG) during propagation of two-color femtosecond pulses through the indium multi-jet plasmas. The role of number of coherent zones and size of plasma jets on the HHG efficiency at QPM conditions is analysed. We demonstrate the enhancement of the HHG efficiency due to QPM during laser-plasma interaction.

Fundamental investigations into the atomic structure of actinide atoms and ions provide a wealth of information on the physical and chemical behaviour of these elements. In this work, thorium, the second actinide element is investigated. Inner shell photoabsorption features in the 6p and 5d spectra of the free thorium atom and thorium ions produced in a laser plasma plume have been recorded using the space and time resolved Dual Laser Plasma (DLP) spectroscopic technique. Studies focus, in particular, on the investigation of the 6p —» 6d and the 5d —> 5, si' giant dipole resonance features residing in the VUV (15 to 40eV) and XUV (80 to 150eV) inner shell photoabsorption spectra of atomic and ionic thorium. The evolution of both giant dipole resonance profiles with increasing ionisation along the thorium isonuclear sequence Th to Th6+ has been studied. Results indicate that the 6d wavefunction is collapsed in neutral thorium, whereas the 5f wavefunction is to a large extent un-collapsed in neutral thorium but collapses with increasing ionisation along the thorium isonuclear sequence. A picosecond XUV continuum laser plasma light source has been developed in this work and is characterised both spectrally and temporally using time integrated flat field spectroscopy and time resolved streak camera detection. XUV continuum emission pulsewidths of less than 200ps in the 100 to 200eV energy region were recorded from the high-Z metal plasmas of tungsten, gold, lead and samarium. When incorporated into the DLP technique, the picosecond XUV continuum source permits probing of the absorbing plasma plume with superior temporal resolution than previously achieved using XUV laser plasma sources produced using nanosecond laser pulses. The newly developed source was used to the study the collapse of the broad asymmetric 5d -» 5, rf giant dipole resonance feature to discrete Rydberg-like structure along the thorium isonuclear sequence.

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.

In this contribution we will present the future activities that our collaboration will carry out at ELI-NP (Extreme Light Infrastructure Nuclear Physics), the new multi peta-watt Laser facility, currently under construction at Bucharest (Romania). The activities concerns the study of nuclear reactions in laser plasmas. In this framework we proposed the construction of a new, general-purpose experimental set-up able to detect and identify neutrons and charged particles.

Investigation of nuclear reactions in a solid-state plasma generated by modern high-intensity sources of ultrashort-wavelength laser radiation is undoubtedly of interest for establishing nuclear-reaction yields versus the intensity of laser radiation and for efficiently exciting new nuclear and photonuclear reactions—in particular, in the resonance energy region. The present article is devoted to describing, for laser plasmas, various mechanisms of electron acceleration, which triggers the acceleration of atomic ions and the excitation of subsequent nuclear reactions.

A survey on the mechanisms of powerful terahertz (THz) radiation from laser plasmas is presented. Firstly, an analytical model is described, showing that a transverse net current formed in a plasma can be converted into THz radiations at the plasma oscillation frequency. This theory is applied to explain THz generation in a gas driven by two-color laser pulses. It is also applied to THz generation in a tenuous plasma driven by a chirped laser pulse, a few-cycle laser pulse, a DC/AC bias electric field. These are well verified by particle-in-cell simulations, demonstrating that THz radiations produced in these approaches are nearly single-cycles and linear polarized. In the chirped laser scheme and the few-cycle laser scheme, THz radiations with the peak field strength of tens of MV/cm and the peak power of gigawatt can be achieved with the incident laser intensity less than 10^{17} W/cm2.

A novel technique for laser produced plasmas (LPPs) investigation is proposed. It combines high time resolved measurements using a compact Langmuir probe (LP) with simulated data obtained with a numerical model designated as hybrid laser ablation simulations (HYBLAS). The code simulates charged particle collection with what we will refer to as a virtual LP. With the use of an appropriate experimental setup and with a MATLAB software, which analyzes the experimental I-V curves, LPPs can be investigated properly even if the probe is placed very close to the target surface. The method permits to study the plume expansion with a high temporal resolution and to correctly estimate the self-generated Coulomb electric field inside the plume. It permits to detect the inner structure of the first upcoming expanding plasma. HYBLAS is able to describe the plume expansion at relatively low power densities if the initial conditions are set properly. A direct comparison of the theoretical data with the experimental ones realized on different metal targets shows that this method is able to predict properly the overall plasma expansion in the nanosecond laser pulse duration regime. The virtual probe method was moreover tested by comparing the numerical results with the numerical code called MULTI.

A review is given of processes fundamental for current relativistic regimes of laser matter interaction such as the generation and evolution of relativistic coherent nonlinear electromagnetic structures; generation of extremely high intensity electromagnetic waves and extremely high energy charged particle beams; novel regimes of electromagnetic wave interaction with matter and of the prospects for using laser accelerators for the high energy physics and for the laboratory relativistic astrophysics with relativistic laser plasmas.

We have investigated the polarization of attosecond light pulses generated from relativistic few-cycle laser pulse interaction with the surface of overdense plasmas using particle-in-cell simulation. Under suitable conditions, a desired polarization state of the generated attosecond pulse can be achieved by controlling the polarization of the incident laser. In particular, an elliptically polarized laser pulse of suitable ellipticity can generate an almost circularly polarized attosecond pulse without compromising the harmonic generation efficiency. The process is thus applicable as a new tabletop circularly-polarized XUV radiation source for probing attosecond phenomena with high temporal resolution.

WOS:000373585800006 PubMed:27176413 An analysis of an electron spectrometer used to characterize fast electrons generated by ultraintense (10(20) W cm(-2)) laser interaction with a preformed plasma of scale length measured by shadowgraphy is presented. The effects of fringing magnetic fields on the electron spectral measurements and the accuracy of density scale-length measurements are evaluated. 2D EPOCH PIC code simulations are found to be in agreement with measurements of the electron energy spectra showing that laser filamentation in plasma preformed by a prepulse is important with longer plasma scale lengths (> 8 mu m). RFBR Research Project [15-32-21121]; RAS Presidium Program for Basic Research [13]; UK EPSRCEngineering & Physical Sciences Research Council (EPSRC) [EP/G054950/1, EP/G056803/1, EP/G055165/1, EP/M022463/1]; Engineering and Physical Sciences Research CouncilEngineering & Physical Sciences Research Council (EPSRC) [EP/M022463/1, EP/G056803/1] The authors gratefully acknowledge the assistance of laser operations, target preparation, and engineering staff at the Central Laser Facility of RAL. Part of the research was supported by RFBR Research Project No. 15-32-21121 and RAS Presidium Program for Basic Research No. 13. This work was in part funded by the UK EPSRC Grants No. EP/G054950/1, No. EP/G056803/1, No. EP/G055165/1, and No. EP/M022463/1.

The interaction of high intensity laser radiation with an underdense plasma may lead to the formation of electron vortices. Though being quasistationary on the electron timescales, these structures tend to expand on a proton timescale due to Coulomb repulsion of ions. Using a simple analytical model of a stationary vortex as an initial condition, 2D PIC simulations are performed. A number of effects are observed such as vortex boundary field intensification, multistream instabilities at the vortex boundary, and bending of the vortex boundary with the subsequent transformation into smaller electron vortices.

Relativistic flying mirrors in plasmas are realized as thin dense electron (or electron-ion) layers accelerated by high-intensity electromagnetic waves to velocities close to the speed of light in vacuum. The reflection of an electromagnetic wave from the relativistic mirror results in its energy and frequency changing. In a counter-propagation configuration, the frequency of the reflected wave is multiplied by the factor proportional to the Lorentz factor squared. This scientific area promises the development of sources of ultrashort X-ray pulses in the attosecond range. The expected intensity will reach the level at which the effects predicted by nonlinear quantum electrodynamics start to play a key role., Comment: 24 pages, 8 figures, This review article is based on the materials of the plenary lecture presented at the ICPIG-2015 conference (Iasi, Romania, 2015) by S.V.B