Laser-plasma Interaction Physics Research Articles

Calibration of the sub-aperture backscatter system on OMEGA EP

The sub-aperture backscatter (SABS) diagnostic on the OMEGA EP Laser System [Waxer etal., Opt. Photonics News 16, 30 (2005)] is a diagnostic that is used to measure the backscattered and sidescattered light during laser-plasma interaction experiments [W. L. Kruer, The Physics of Laser Plasma Interactions, Frontiers in Physics Vol. 73, edited by D. Pines (Addison-Wesley, Redwood City, CA, 1988) and Myatt etal., Phys. Plasmas 21, 055501 (2014)] that are relevant to high-energy-density physics and inertial confinement fusion. The diagnostic collects stimulated Brillouin scattering (SBS) UV light at around 351nm and stimulated Raman scattering (SRS) in the visible-light regime in the 420-720-nm-wavelength range and provides spectrally and temporally resolved information. Five 1-in. light collectors, composed of a lens, ground glass diffuser, and coupling into a 300-μm fiber, are positioned behind the last steering mirror on one of the four beamlines to catch a portion of the beam cross section (∼1.5%) of the emission that is scattered into the beamline. The SRS light is collected in two light collectors, combined, and transported via graded index fibers to a streaked spectrometer. The SABS-SRS streak spectrometer has a temporal and spectral resolution of 100ps and 1nm, respectively. Three other light collectors collect, combine, and transport the SBS signal to a Hamamatsu high-voltage photodiode, where an oscilloscope digitizes the data, providing a time resolution of better than 1ns. To obtain an absolute energy calibration of SRS measurements, light signals of known energy and wavelength were injected into the light collectors one at a time. The resulting counts on the streak camera charge-coupled device for SRS are then correlated with the incident fluence of scattered light at the light collector in order to allow a quantitative assessment of streak camera sensitivity to determine the energy of the scattered light during experiments. The measurements were performed in situ from the light collectors to the detectors. Additional offline measurements provided the transmission of the optics between the target chamber center and the light collectors.

Second harmonic generation of high power Cosh-Gaussian beam in cold collisionless plasma

The purpose of this study is to explore the second harmonic generation (SHG) of a high power Cosh-Gaussian beam in cold collisionless plasma. The ponderomotive force causes carrier redistribution from high field to low field region in presence of a Cosh-Gaussian beam thereby producing density gradients in the transverse direction. The density gradients so produced the results in electron plasma wave (EPW) generation at the frequency of the input beam. The EPW interacts with the input beam resulting in the production of 2nd harmonics. WKB and paraxial approximations are employed for obtaining the 2nd order differential equation describing the behavior of the beam’s spot size against normalized distance. The impact of well-established laser-plasma parameters on the behavior of the beam’s spot size and SHG yield are also analyzed. The focusing behavior of the beam and SHG yield is enhanced with an increase in the density of plasma, the radius of the beam and the decentred parameter, and with a decrease in the intensity of the beam. The results of the current problem are really helpful for complete information of laser-plasma interaction physics.

Ionization states for the multipetawatt laser-QED regime

A paradigm shift in the physics of laser-plasma interactions is approaching with the commissioning of multipetawatt laser facilities worldwide. Radiation reaction processes will result in the onset of electron-positron pair cascades and, with that, the absorption and partitioning of the incident laser energy, as well as the energy transport throughout the irradiated targets. To accurately quantify these effects, one must know the focused intensity on target in situ. In this work, a way of measuring the focused intensity on target is proposed based upon the ionization of xenon gas at low ambient pressure. The field ionization rates from two works [Phys. Rev. A 59, 569 (1999)1050-294710.1103/PhysRevA.59.569 and Phys. Rev. A 98, 043407 (2018)2469-992610.1103/PhysRevA.98.043407], where the latter rate has been derived using quantum mechanics, have been implemented in the particle-in-cell code SMILEI [Comput. Phys. Commun. 222, 351 (2018)0010-465510.1016/j.cpc.2017.09.024]. A series of one- and two-dimensional simulations are compared and shown to reproduce the charge states without presenting visible differences when increasing the simulation dimensionality. They provide a way to accurately verify the intensity on target using in situ measurements.

Analytic solutions for delocalized heat transport

Comprehensive understanding of nonlocal transport is mandatory for many applications of laser–plasma interaction physics, such as inertial confinement fusion and modeling of astrophysical phenomena in the laboratory. Theoretical description is important for guiding numerical simulations and experiments. In this article, an analytic approach is developed: by using a simple integral differential model, we calculate the preheating of plasma upstream of the heat front associated with nonlocal transport. A detailed comparison of various transport configurations as a function of the boundary conditions and the nonlinearity parameter is presented in the context of laser–plasma interaction. Three important results are demonstrated in this paper. First, analytical expressions for all possible self-similar solutions of local nonlinear transport are obtained. Second, a systematic comparison of various nonlocal kernels and forms of the delocalization length is performed. Third, an analytic expression for the temperature profile upstream of the heat front is obtained.

Studies of laser-plasma interaction physics with low-density targets for direct-drive inertial confinement fusion on the Shenguang III prototype

The physics of laser-plasma interaction is studied on the Shenguang III prototype laser facility under conditions relevant to inertial confinement fusion designs. A sub-millimeter-size underdense hot plasma is created by ionization of a low-density plastic foam by four high-energy (3.2 kJ) laser beams. An interaction beam is fired with a delay permitting evaluation of the excitation of parametric instabilities at different stages of plasma evolution. Multiple diagnostics are used for plasma characterization, scattered radiation, and accelerated electrons. The experimental results are analyzed with radiation hydrodynamic simulations that take account of foam ionization and homogenization. The measured level of stimulated Raman scattering is almost one order of magnitude larger than that measured in experiments with gasbags and hohlraums on the same installation, possibly because of a greater plasma density. Notable amplification is achieved in high-intensity speckles, indicating the importance of implementing laser temporal smoothing techniques with a large bandwidth for controlling laser propagation and absorption.

High-power, low-coherence laser driver facility.

We report the first (to the best of our knowledge) high-power, low-coherence Nd:glass laser delivering kilojoule pulses with a coherent time of 249 fs and a bandwidth of 13 nm, achieving the 63%-efficiency second-harmonic conversion of the large-aperture low-coherence pulse and good beam smoothing effect. It provides a new type of laser driver for laser plasma interaction and high energy density physics research.

Direct-drive laser fusion: status, plans and future.

Laser-direct drive (LDD), along with laser indirect (X-ray) drive (LID) and magnetic drive with pulsed power, is one of the three viable inertial confinement fusion approaches to achieving fusion ignition and gain in the laboratory. The LDD programme is primarily being executed at both the Omega Laser Facility at the Laboratory for Laser Energetics and at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. LDD research at Omega includes cryogenic implosions, fundamental physics including material properties, hydrodynamics and laser-plasma interaction physics. LDD research on the NIF is focused on energy coupling and laser-plasma interactions physics at ignition-scale plasmas. Limited implosions on the NIF in the 'polar-drive' configuration, where the irradiation geometry is configured for LID, are also a feature of LDD research. The ability to conduct research over a large range of energy, power and scale size using both Omega and the NIF is a major positive aspect of LDD research that reduces the risk in scaling from OMEGA to megajoule-class lasers. The paper will summarize the present status of LDD research and plans for the future with the goal of ultimately achieving a burning plasma in the laboratory. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Progress and opportunities for inertial fusion energy in Europe.

In this paper, I consider the motivations, recent results and perspectives for the inertial confinement fusion (ICF) studies in Europe. The European approach is based on the direct drive scheme with a preference for the central ignition boosted by a strong shock. Compared to other schemes, shock ignition offers a higher gain needed for the design of a future commercial reactor and relatively simple and technological targets, but implies a more complicated physics of laser–target interaction, energy transport and ignition. European scientists are studying physics issues of shock ignition schemes related to the target design, laser plasma interaction and implosion by the code developments and conducting experiments in collaboration with US and Japanese physicists, providing access to their installations Omega and Gekko XII. The ICF research in Europe can be further developed only if European scientists acquire their own academic laser research facility specifically dedicated to controlled fusion energy and going beyond ignition to the physical, technical, technological and operational problems related to the future fusion power plant. Recent results show significant progress in our understanding and simulation capabilities of the laser plasma interaction and implosion physics and in our understanding of material behaviour under strong mechanical, thermal and radiation loads. In addition, growing awareness of environmental issues has attracted more public attention to this problem and commissioning at ELI Beamlines the first high-energy laser facility with a high repetition rate opens the opportunity for qualitatively innovative experiments. These achievements are building elements for a new international project for inertial fusion energy in Europe.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

Azimuthal decomposition study of a realistic laser profile for efficient modeling of Laser WakeField Acceleration

The advent of ultra short high intensity lasers has paved the way to new and promising, yet challenging, areas of research in the laser-plasma interaction physics. The success of constructing petawatt femtosecond lasers, for instance the Apollon laser in France, will help understanding and designing future particle accelerators and next generation of light sources. Achieving this goal intrinsically relies on the combination between experiments and massively parallel simulations. So far, Particle-In-Cell (PIC) codes have been the ultimate tool to accurately describe the laser-plasma interaction especially in the field of Laser WakeField Acceleration (LWFA). Nevertheless, the numerical modelling of laser plasma accelerators in 3D can be a very challenging task. This is due to the large dispersity between the scales involved in this process. In order to make such simulations feasible with a significant speed up, we need to use reduced numerical models which simplify the problem while retaining a high fidelity. Among these models, Fourier field decomposition in azimuthal modes for the cylindrical geometry [1] is a promising reduced model especially for physical problems that have close to cylindrical symmetry which is the case in LWFA. This geometry has been implemented in the open-source code SMILEI [2] in Finite Difference Time Domain (FDTD) discretization scheme for the Maxwell solver. In this paper we will study the case of a realistic laser measurement from Apollon facility, the ability of this method to describe it correctly and the determination of the necessary number of modes for this purpose. We will also show the importance of higher modes inclusion in the case of realistic laser profiles to insure fidelity in simulation.

Innovative education and training in high power laser plasmas (PowerLaPs) for plasma physics, high power laser matter interactions and high energy density physics: experimental diagnostics and simulations

The second and final year of the Erasmus Plus programme ‘Innovative Education and Training in high power laser plasmas’, otherwise known as PowerLaPs, is described. The PowerLaPs programme employs an innovative paradigm in that it is a multi-centre programme, where teaching takes place in five separate institutes with a range of different aims and styles of delivery. The ‘in-class’ time is limited to 4 weeks a year, and the programme spans 2 years. PowerLaPs aims to train students from across Europe in theoretical, applied and laboratory skills relevant to the pursuit of research in laser plasma interaction physics and inertial confinement fusion. Lectures are intermingled with laboratory sessions and continuous assessment activities. The programme, which is led by workers from the Hellenic Mediterranean University and supported by co-workers from the Queen’s University Belfast, the University of Bordeaux, the Czech Technical University in Prague, Ecole Polytechnique, the University of Ioannina, the University of Salamanca and the University of York, has just finished its second and final year. Six Learning Teaching Training activities have been held at the Queen’s University Belfast, the University of Bordeaux, the Czech Technical University, the University of Salamanca and the Institute of Plasma Physics and Lasers of the Hellenic Mediterranean University. The last of these institutes hosted two 2-week-long Intensive Programmes, while the activities at the other four universities were each 5 days in length. In addition, a ‘Multiplier Event’ was held at the University of Ioannina, which will be briefly described. In this second year, the work has concentrated on training in both experimental diagnostics and simulation techniques appropriate to the study of plasma physics, high power laser matter interactions and high energy density physics. The nature of the programme will be described in detail, and some metrics relating to the activities carried out will be presented. In particular, this paper will focus on the overall assessment of the programme.

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.

Innovative Education and Training in high power laser plasmas (PowerLaPs) for plasma physics, high power laser–matter interactions and high energy density physics – theory and experiments

The Erasmus Plus programme ‘Innovative Education and Training in high power laser plasmas’, otherwise known as PowerLaPs, is described. The PowerLaPs programme employs an innovative paradigm in that it is a multi-centre programme where teaching takes place in five separate institutes with a range of different aims and styles of delivery. The ‘in class’ time is limited to four weeks a year, and the programme spans two years. PowerLaPs aims to train students from across Europe in theoretical, applied and laboratory skills relevant to the pursuit of research in laser–plasma interaction physics and inertial confinement fusion (ICF). Lectures are intermingled with laboratory sessions and continuous assessment activities. The programme, which is led by workers from the Technological Educational Institute (TEI) of Crete, and supported by co-workers from the Queen’s University Belfast, the University of Bordeaux, the Czech Technical University in Prague, Ecole Polytechnique, the University of Ioannina, the University of Salamanca and the University of York, has just completed its first year. Thus far three Learning Teaching Training (LTT) activities have been held, at the Queen’s University Belfast, the University of Bordeaux and the Centre for Plasma Physics and Lasers (CPPL) of TEI Crete. The last of these was a two-week long Intensive Programme (IP), while the activities at the other two universities were each five days in length. Thus far work has concentrated upon training in both theoretical and experimental work in plasma physics, high power laser–matter interactions and high energy density physics. The nature of the programme will be described in detail and some metrics relating to the activities carried out to date will be presented.

Physics of laser plasma interaction and particle transport in the context of inertial confinement fusion

Lasers are unique tools for transporting extremely high powers over large distances, but transfer of such a power from photons to matter in small volumes is a very complicated problem. First of all, the interaction proceeds very far from equilibrium, as with photons having energy of a few electron-volts one would like to heat plasma to temperatures thousand times higher. Second, these processes are strongly nonlinear, as they correspond to transfer energies of a large number of photons to a much smaller number of charged particles in extremely small volumes and in very short time scales. Research in inertial confinement fusion (ICF) gave a strong push for studying all these processes in detail, and now, although many issues remain to be resolved, we have quite a good understanding of how they operate in ICF conditions and what limitations and advantages they offer.In this short review, I share my personal recollections of almost 50 years history of the physics of laser plasma interaction. Understanding of highly nonlinear microscopic processes allowed us to improve the hydrodynamic performance of ICF targets and to foresee future developments. The key point is that multiscale modeling allowed for the retainment of major elements of microscopic physics in macroscopic hydrodynamic codes and make them more accurate and predictive.

Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments.

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length (∼500 to 700 μm), electron temperature (∼3 to 5keV), and laser intensity (6 to 16×10^{14} W/cm^{2}) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is ∼0.7% to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from ∼4×10^{14} to ∼6×10^{14} W/cm^{2}. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

A flexible, on-line magnetic spectrometer for ultra-intense laser produced fast electron measurement

We have developed an on-line magnetic spectrometer to measure energy distributions of fast electrons generated from ultra-intense laser-solid interactions. The spectrometer consists of a sheet of plastic scintillator, a bundle of non-scintillating plastic fibers, and an sCMOS camera recording system. The design advantages include on-line capturing ability, versatility of detection arrangement, and resistance to harsh in-chamber environment. The validity of the instrument was tested experimentally. This spectrometer can be applied to the characterization of fast electron source for understanding fundamental laser-plasma interaction physics and to the optimization of high-repetition-rate laser-driven applications.

Physics of laser-plasma interaction for shock ignition of fusion reactions

The shock ignition scheme is an alternative approach, which aims to achieve ignition of fusion reactions in two subsequent steps: first, the target is compressed at a low implosion velocity and second, a strong converging shock is launched during the stagnation phase and ignites the hot spot. In this paper we describe the major elements of this scheme and recent achievements concerning the laser-plasma interaction, the crucial role of hot electrons in the shock generation, the shock amplification in the imploding shell and the ignition conditions.

Studies on laser–plasma interaction physics for shock ignition

We realized a series of experiments to study the physics of laser–plasma interaction in an intensity regime of interest for the novel “Shock Ignition” approach to Inertial Fusion. Experiments were performed at the Prague Asterix Laser System laser in Prague using two laser beams: an “auxiliary” beam, for pre-plasma creation, with intensity around 7 × 1013 W/cm2 (250 ps, 1ω, λ = 1315 nm) and the “main” beam, up to 1016 W/cm (250 ps, 3ω, λ = 438 nm), to launch a shock. The main goal of these experiments is to study the process of the formation of a very strong shock and the influence of hot electrons in the generation of very high pressures. The shock produced by the ablation of the plastic layer is studied by shock breakout chronometry. The generation of hot electrons is analyzed by imaging Kα emission.

Physics issues for shock ignition

The paper presents theoretical analysis and experimental results concerning the major physical issues in the shock-ignition approach. These are the following: generation of a high amplitude shock in the imploding target, laser–plasma interaction physics under the conditions of high laser intensities needed for high amplitude shock excitation, symmetry and stability of the shock propagation, role of fast electrons in the symmetrization of the shock pressure and the fuel preheat. The theoretical models and numerical simulations are compared with the results of specially designed experiments on laser plasma interaction and shock excitation in plane and spherical geometries.

Experimental Approach to Interaction Physics Challenges of the Shock Ignition Scheme Using Short Pulse Lasers

An experimental program was designed to study the most important issues of laser-plasma interaction physics in the context of the shock ignition scheme. In the new experiments presented in this Letter, a combination of kilojoule and short laser pulses was used to study the laser-plasma coupling at high laser intensities for a large range of electron densities and plasma profiles. We find that the backscatter is dominated by stimulated Brillouin scattering with stimulated Raman scattering staying at a limited level. This is in agreement with past experiments using long pulses but laser intensities limited to 2×10(15) W/cm2, or short pulses with intensities up to 5×10(16) W/cm2 as well as with 2D particle-in-cell simulations.

Laser plasma interaction physics on the LIL facility

We present an overview of the interpretation of laser plasma interaction (LPI) experiments carried out on the LIL facility. These multikilojoule experiments have been done using underdense foam targets at 0.351 μm laser light leading to high temperature and large plasmas. We discuss the interpretation using our different numerical tools: hydrodynamics simulations carried out with the code FCI2 to characterize the plasma, linear gain estimates with the postprocessor Piranah and paraxial simulations with the code HERA.