Laser Plasma Interaction Studies Research Articles

Nanoplasma resonance condition in the middle-infrared spectral range.

The resonance-absorption condition in the laser-nanoplasma interactions has been considered to follow the wavelength dependence of the critical plasma density. We experimentally demonstrate that this assumption fails in the middle-infrared spectral range, while it is valid for visible and near-infrared wavelengths. A thorough analysis supported by molecular dynamic (MD) simulations indicates that the observed transition in the resonance condition is caused by the reduction of the electron scattering rate and the associated increase of the cluster outer-ionization contribution. An expression for the nanoplasma resonance density is derived based on experimental results and MD simulations. The findings are important for a broad range of plasma experiments and applications, since the extension of the laser-plasma interaction studies to longer wavelengths has become increasingly topical.

Algorithm for computing the electron-positron yield from the linear Breit-Wheeler process in high-intensity laser-plasma interactions

High-intensity laser-plasma interactions have been shown to generate dense populations of gamma-rays, so these interactions are expected to generate electron-positron pairs via binary photon collisions (linear Breit-Wheeler process). However, particle-in-cell (PIC) codes that are used for studies of laser plasma interactions are not yet equipped to compute the yield from the linear Breit-Wheeler process. We present a post-processing algorithm that allows one to quickly calculate the yield of the linear Breit-Wheeler process inside a photon-emitting plasma using PIC simulation data. The algorithm splits the PIC computational domain into smaller sub-domains whose shape and size are determined based on a specific problem. The photons emitted within each sub-domain are grouped into collimated mono-energetic beams called beamlets. The algorithm computes the yield by evaluating beamlet-beamlet collisions without the spatial integration over the interaction region. Presented benchmarking shows that the computational time is reduced by two orders of magnitude compared to the direct approach that involves the spatial integration, while the resulting error in the total yield remains around 10%. We also show how the algorithm can be leveraged to compute the density of the pair-producing events and the positron momentum distribution at the time of their creation. The ability of our algorithm to quickly compute the pair yield makes it a useful tool for studies of high-intensity laser-plasma interactions. It can also be useful for testing future implementations of the linear Breit-Wheeler process into plasma simulation codes.

Separation of ion component from solid hydrocarbon materials by laser ablation

Plasma usually consists of multiple ion component. Ion-component separation occurs in various conditions, and profoundly affects the plasma dynamic evolution. In this work, ion-component separation in two-ion-component plasma is investigated in the hydrodynamic condition. Starting from the Landau-Fokker-Planck equations of two-ion-component plasma, the ion transport equations are reduced through the Chapman-Enskog approach. The transport equations are then transformed into a set of linear algebraic equations and solved by expanding the perturbed ion distribution functions into the series of Sonine polynomials. The diffusive ion mass flows with inclusion of baro-diffusion, thermo-diffusion and electro-diffusion are thus obtained. With these efforts, the complete ion fluid equations are presented, which can be used to describe the processes of ion-component separation. We evaluate ion-component separation in the case of a solid CH plate target ablated with a laser pulse, by solving the ion diffusion equation with the hydro states output from the one-dimensional radiative hydro code Multi-1D. The simulation results show that ion-component separation mainly occurs around ablation front and under-dense region, and that the effect of ion-species separation on plasma hydrodynamic evolution is minor and can be neglected. For those physical processes sensitive to ion concentration such as Thomson scattering, however, the effect of ion-component separation is significant, which means that ion-component separation should be included in the study of laser plasma interaction.

Helium as a Surrogate for Deuterium in LPI Studies

Helium or neopentane can be used as surrogate gas fill for deuterium (D2) or deuterium-tritium (DT) in laser-plasma interaction studies. Surrogates are convenient to avoid flammability hazards or the integration of cryogenics in an experiment. To test the degree of equivalency between deuterium and helium, experiments were conducted in the Pecos target chamber at Sandia National Laboratories. Observables such as laser propagation and signatures of laser-plasma instabilities (LPI) were recorded for multiple laser and target configurations. It was found that some observables can differ significantly despite the apparent similarity of the gases with respect to molecular charge and weight. While a qualitative behaviour of the interaction may very well be studied by finding a suitable compromise of laser absorption, electron density, and LPI cross sections, a quantitative investigation of expected values for deuterium fills at high laser intensities is not likely to succeed with surrogate gases.

Particle integrator for particle-in-cell simulations of ultra-high intensity laser-plasma interactions

Particle-in-cell codes are the most widely used simulation tools for kinetic studies of ultra-intense laser-plasma interactions. Using the motion of a single electron in a plane electromagnetic wave as a benchmark problem, we show surprising deterioration of the numerical accuracy of the PIC algorithm with increasing normalized wave amplitude for typical time-step and grid sizes. Two significant sources of errors are identified: strong acceleration near stopping points and the temporal field interpolation. We propose adaptive electron sub-cycling coupled with a third order temporal interpolation of the magnetic field and electric field as an efficient remedy that dramatically improves the accuracy of the particle integrator.

Method for Measuring the Laser Field and the Opacity of Spectral Lines in Plasmas

In experimental studies of laser-plasma interactions, the laser radiation can exist inside plasma regions where the electron density is below the critical density (“underdense” plasma), as well as at the surface of the critical density. The surface of the critical density could exhibit a rich physics. Namely, the incident laser radiation can get converted in transverse electromagnetic waves of significantly higher amplitudes than the incident radiation, due to various nonlinear processes. We proposed a diagnostic method based on the laser-produced satellites of hydrogenic spectral lines in plasmas. The method allows measuring both the laser field (or more generally, the field of the resulting transverse electromagnetic wave) and the opacity from experimental spectrum of a hydrogenic line exhibiting satellites. This spectroscopic diagnostic should be useful for a better understanding of laser-plasma interactions, including relativistic laser-plasma interactions.

Characterization of residual inhomogeneities in a plasma created by laser ionization of a low-density foam

Low density foams are widely used for laser plasma interaction studies. Here, we present the results of first direct measurements of a residual level of inhomogeneity of a foam plasma by using a pump-probe technique. It is demonstrated that large scale density modulations in such a plasma can survive a time larger than 1 ns, much longer than the plasma formation time.

The nonlinear wave in semiconductor quantum plasma for laser beam in a self-consistent plasma channel

In this paper investigating the current task, our emphasis is on distributed a serious laser beam in a self-made plasma channel. As the intensity of the laser beam increases, the medium displays the neuromuscular electrical conductivity functioning as a result of the mass effectiveness of the effectiveness of the electrons as well as the intensity of the electrons. Based on Wentzel–Kramers–Brillouin and paraxial ray theory, the steady-state solution of an intense, Gaussian electromagnetic beam is studied. The differential equation of the distance with the beam width parameter is derived, including the relativistic effects of self-focusing (SF) and self-channeling ponder motive. Plasma propagation is a radial dynamical force, depending on the width of the beams and σp greater, plasma ratio frequency screws. Once the distribution regimes, the beam power beamwidth is obtained in plane. The σp is a particular value characterized by standard deviation, oscillation and diffusion regimes such as self focusing. The corresponding center parameters are intended for introduction of the plasma density curve and the laser beam is spatially analyzed to the spatial plasma. Performing this margin can lead to a long distance laser beam guide. The disadvantage of numerical predictions is done for laser plasma interaction studies for common factors.

Characterization of a 100 micrometer-scale cryogenically cooled gas jet for near-critical density laser-plasma experiments

We present the design and characterization of a thin, high density pulsed gas jet for use in the study of near critical laser plasma interactions with ultrashort Ti:sapphire laser pulses. The gas jet uses a range of capillary nozzles with inner diameters between 50 and 150 μm and is operated in the sonic regime. Cryogenic cooling of the gas valve body to −160 °C provides the necessary density enhancement for reaching overcritical plasma densities at λ = 800 nm (Ncr ≈ 1.7 × 1021 cm−3) using hydrogen gas at jet backing pressures below 1000 psi. Under certain conditions, fast expansion of the gas from a nozzle can lead to formation of clusters; here, we use our previously demonstrated all-optical method to estimate the cluster mean size and density. For the jets studied here, we find that cluster formation only begins at distances from the nozzle exit greater than a few times the nozzle orifice diameter.

Studies of laser-plasma interaction physics with low-density targets for direct-drive inertial confinement schemes

Comprehensive understanding and possible control of parametric instabilities in the context of inertial confinement fusion (ICF) remains a challenging task. The details of the absorption processes and the detrimental effects of hot electrons on the implosion process require as much effort on the experimental side as on the theoretical and simulation side. This paper describes a proposal for experimental studies on nonlinear interaction of intense laser pulses with a high-temperature plasma under conditions corresponding to direct-drive ICF schemes. We propose to develop a platform for laser-plasma interaction studies based on foam targets. Parametric instabilities are sensitive to the bulk plasma temperature and the density scale length. Foam targets are sufficiently flexible to allow control of these parameters. However, investigations conducted on small laser facilities cannot be extrapolated in a reliable way to real fusion conditions. It is therefore necessary to perform experiments at a multi-kilojoule energy level on medium-scale facilities such as OMEGA or SG-III. An example of two-plasmon decay instability excited in the interaction of two laser beams is considered.

Effects of ultraintense magnetic fields due to relativistic laser-plasma interactions on Langmuir-wave-caused dips in x-ray spectral line profiles

GigaGauss (GG) or even multi-GG magnetic fields are expected to develop during relativistic laser-plasma interactions. In our previous paper we proposed a method for measuring GG magnetic fields based on the phenomenon of Langmuir-wave-cased dips (L-dips) in x-ray line profiles. The L-dips were observed in several experimental spectroscopic studies of relativistic laser-plasma interactions. Ultra-strong magnetic fields affect the separation of the L-dips from one another, so that this separation can be used to measure such fields. In the present paper we provide some new results concerning the effect of the ultra-strong magnetic fields on the separation between the L-dips. But the primary focus of the present paper is at another effect of the GG magnetic fields on the L-dips: we study how these ultra-intense magnetic fields affect the halfwidth of the L-dips. One of the results turns out to be counterintuitive: for the case of the two-quantum resonance, the halfwidth of the L-dips is a non-monotonic function of the magnetic field. By advancing the results of the previous work, the present paper expands the possibilities for measuring super-strong magnetic fields up to ~ 10 GG expected to arise during relativistic laser-plasma interactions.

Nm-sized cryogenic hydrogen clusters for a laser-driven proton source.

A cryogenic hydrogen cluster-jet target is described which has been used for laser-plasma interaction studies. Major advantages of the cluster-jet are, on the one hand, the compatibility to pulsed high repetition lasers as the target is operated continuously and, on the other hand, the absence of target debris. The cluster-jet target was characterized using the Mie-scattering technique allowing to determine the cluster size and to compare the measurements with an empirical formula. In addition, an estimation of the cluster beam density was performed. The system was implemented at the high power laser system ARCTURUS, and the measurements show the acceleration of protons after irradiation of the cluster target by high intensity laser pulses with a repetition rate of 5 Hz.

X-ray Spectroscopy Based Diagnostic of GigaGauss Magnetic Fields during Relativistic Laser-Plasma Interactions

GigaGauss (GG), and even multi-GG magnetic fields are expected to be developed during relativistic laser-plasma interactions. Sub-GG magnetic fields were previously measured by a method using the self-generated harmonics of the laser frequency, and the fact that the magnetized plasma is birefringent and/or optically active depending on the propagation direction of the electromagnetic wave. In the present short communication, we outline an idea for a method of measuring GG magnetic fields based on the phenomenon of Langmuir-wave-caused dips (L-dips) in X-ray line profiles. The L-dips were observed in several experimental spectroscopic studies of relativistic laser-plasma interactions. Ultrastrong magnetic fields affect the separation of the L-dips from one another, so that this relative shift can be used to measure such fields.

Modeling the ponderomotive interaction of high-power laser beams with collisional plasma: the FDTD-based approach.

The ponderomotive interaction of high-power laser beams with collisional plasma is modeled in the nonrelativistic regime and is simulated using the powerful finite-difference time-domain (FDTD) method for the first time in literature. The nonlinear and dissipative dielectric constant function of the collisional plasma is deduced that takes the ponderomotive effect into account and is implemented in the discrete framework of FDTD algorithms. Maclaurin series expansion approach is applied for implementing the obtained physical model and the time average of the square of light field is extracted by numerically evaluating an integral identity based on the composite trapezoidal rule for numerical integration. Two numerical examples corresponding to two different types of laser beams, Gaussian beam and vortex Laguerre-Gaussian beam, propagating in collisional plasma, are presented for specified laser and plasma parameters to verify the validity of the proposed FDTD-based approach. Simulation results show the anticipated self-focusing and attenuation phenomena of laser beams and the deformation of the spatial density distributions of electron plasma along the beam propagation path. Due to the flexibility of FDTD method in light beam excitation and accurate complex material modeling, the proposed approach has a wide application prospect in the study of the complex laser-plasma interactions in a small scale.

Detailed calibration of the PI-LCX:1300 high performance single photon counting hard x-ray CCD camera**Project supported by the Science Foundation of China Academy of Engineering Physics (Grant Nos. 2013A0103003 and 2012B0102008) and the National High-Tech Inertial Confinement Fusion Committee of China.

X-ray charge-coupled-device (CCD) camera working in single photon counting mode is a type of x-ray spectrometer with high-sensitivity and superior signal-to-noise performance. In this study, two single photon counting CCD cameras with the same mode (model: PI-LCX: 1300) are calibrated with quasi-monochromatic x-rays from radioactive sources and a conventional x-ray tube. The details of the CCD response to x-rays are analyzed by using a computer program of multi-pixel analyzing and event-distinguishing capability. The detection efficiency, energy resolution, fraction of multi-pixel events each as a function of x-ray energy, and consistence of two CCD cameras are obtained. The calibrated detection efficiency is consistent with the detection efficiency from Monte Carlo calculations with XOP program. When the multi-pixel event analysis is applied, the CCDs may be used to measure x-rays up to 60 keV with good energy resolution (E/ΔE ≈ 100 at 60 keV). The difference in detection efficiency between two CCD cameras is small (5.6% at 5.89 keV), but the difference in fraction of the single pixel event between them is much larger (25% at 8.04 keV). The obtained small relative error of detection efficiency (2.4% at 5.89 keV) makes the high accurate measurement of x-ray yield possible in the laser plasma interaction studies. Based on the discrete calibration results, the calculated detection efficiency with XOP can be used for the whole range of 5 keV–30 keV.

Role of Density Profiles for the Nonlinear Propagation of Intense Laser Beam through Plasma Channel

In this work role of density profiles for the nonlinear propagation of intense laser beam through plasma channel is analyzed. By employing the expression for the dielectric function of different density profile plasma, a differential equation for beamwidth parameter is derived under WKB and paraxial approximation. The laser induces modifications of the dielectric function through nonlinearities. It is found that density profiles play vital role in laser-plasma interaction studies. To have numerical appreciation of the results the propagation equation for plasma is solved using the fourth order Runge-Kutta method for the initial plane wave front of the beam, using boundary conditions. The spot size of the laser beam decreases as the beam penetrates into the plasma and significantly adds self-focusing in plasma. This causes the laser beam to become more focused by reduction of diffraction effect, which is an important phenomenon in inertial confinement fusion and also for the understanding of self-focusing of laser pulses. Numerical computations are presented and discussed in the form of graphs for typical parameters of laser-plasma interaction.

Distribution regimes of intense laser beam in a self-consistent plasma channel

In the present work, we investigate the distributed regimes of an intense laser beam in a self-consistent plasma channel. As the intensity of the laser beam increases, the relativistic mass effect as well as the ponderomotive expulsion of electrons modifies the dielectric function of the medium due to which the medium exhibits nonlinearity. Based on Wentzel–Kramers–Brillouin and paraxial ray theory, the steady-state solution of an intense, Gaussian electromagnetic beam is studied. A differential equation of the beamwidth parameter with the distance of propagation is derived, including the effects of relativistic self-focusing (SF) and ponderomotive self-channeling. The nature of propagation and radial dynamics of the beam in plasma depend on the power, width of the beam, and Ωp, the ratio of plasma to wave frequency. For a given value of Ωp (<1), the distribution regimes have been obtained in beampower–beamwidth plane, characterizing the regimes of propagation as steady divergence, oscillatory divergence, and SF. The related focusing parameters are optimized introducing plasma density ramp function, and spot size of the laser beam is analyzed for inhomogeneous plasma. This results in overcoming the diffraction and guiding the laser beam over long distance. Numerical computations are performed for typical parameters of relativistic laser–plasma interaction studies.

Study of laser plasma interactions using one-dimensional particle-in-cell code in kinetic regime

Stimulated Raman scatting (SRS), which is one of the parametric processes of laser-plasma interactions, is examined by an explicit, electromagnetic, relativistic kinetic particle-in-cell code in one dimension. The code algorithm and implementation details are discussed. It is found that kinetic effects are important to SRS instability. Time-averaged reflectivity onsets at threshold intensity, and saturates at higher intensity. Backward SRS bursts in sub-picosecond, periodically. Kinetic ions initially delay the growth of SRS. Electron trapping results in the SRS bursts. The saturation of SRS results from the nonlinear frequency shift of Langmuir wave. Work is underway to add binary Coulomb collision to parallelize it, and to extend the code to 2D3V.

Laser–plasma interaction studies in the context of shock ignition: the regime dominated by parametric instabilities

The shock ignition concept for inertial confinement fusion includes launching a strong shock with a high-intensity laser spike into an imploding shell. The laser intensity in the plasma corona is above the threshold for parametric instabilities, thus providing conditions for strong non-linear effects. Here we present a series of one-dimensional kinetic simulations of laser–plasma interactions in such a regime. After a transient period of strong non-stationary scattering, the laser–plasma interaction enters an asymptotic regime where a significant part of the incident laser flux is absorbed in the plasma and is transformed into hot electrons. The repartition of the absorbed energy and spectral characteristics of the scattered radiation are presented for laser intensities in the range 2.4–24 PW cm−2. For a laser intensity of 8 PW cm−2, the total absorption is 69% ; about 50% of absorption takes place at quarter critical density and the remaining 19% at 1/16th of the critical density. 52% of the total laser pulse energy are absorbed due to stimulated Raman scattering, which produces electrons with a temperature of about 30 keV, and 17% is absorbed due to cavitation, which produces a more isotropic distribution of hot electrons with a temperature of about 10 keV.

Development and characterization of very dense submillimetric gas jets for laser-plasma interaction

We report on the characterization of recently developed submillimetric He gas jets with peak density higher than 10(21) atoms/cm(3) from cylindrical and slightly conical nozzles of throat diameter of less than 400 μm. Helium gas at pressure 300-400 bar has been developed for this purpose to compensate the nozzle throat diameter reduction that affects the output mass flow rate. The fast-switching electro-valve enables to operate the jet safely for multi-stage vacuum pump assembly. Such gaseous thin targets are particularly suitable for laser-plasma interaction studies in the unexplored near-critical regime.