Simulations Of Laser-plasma Interaction Research Articles

SPRAY: A smoothed particle radiation hydrodynamics code for modeling high intensity laser-plasma interactions

Here we report the development of SPRAY, a massively parallel GPU accelerated, smoothed particle hydrodynamics (SPH)-based, radiation hydrodynamics (RHD) code designed specifically for simulating high intensity laser-plasma interactions. When a target is irradiated by an intense laser, highly complex fluid deformation occurs due to instabilities, which is challenging to study numerically. SPRAY is particle-based, mesh-free, and Lagrangian, which addresses numerical issues that posed difficulties to existing methods. Its SPH formulations for RHD governing equations are tailored toward accurate and reliable simulations of laser-target irradiation phenomena, and are solved via a time-dependent, flux-limited diffusion method. A new laser energy coupling module, which is based on the Wentzel-Kramers-Brillouin (WKB) approximation, is implemented with a totally mesh-free ray-tracing scheme that is applicable for arbitrary geometry and dimensions. The accuracy and reliability of the code are demonstrated with a series of benchmark problems. To the authors' knowledge, this is the first attempt to employ SPH method for simulations of laser-plasma interactions in high energy density physics research. Possible expansions to the code, such as laser beam-beam interaction modeling and more sophisticated multi-group radiation transport are left for future development.

SRS-SBS competition and nonlinear laser energy absorption in a high temperature plasma

Stimulated Raman and Brillouin scattering of laser radiation in a plasma corona are outstanding issues for the inertial confinement fusion. Stimulated Raman scattering may produce absorption of a significant fraction of laser energy near the plasma quarter critical density associated with plasma cavitation and generation of hot electrons. By contrast, stimulated Brillouin scattering operates in a lower density plasma and prevents the laser light access to the absorption region. In the present paper, we report the results of analysis of competition of these two parametric instabilities with a series of one-dimensional kinetic simulations of laser-plasma interactions. We revised several flexible methods for controlling the SRS-SBS competition in inertial fusion plasmas. Suppression of SBS favors SRS excitation, electron acceleration and cavitation. The latter process makes an important contribution to the laser absorption by trapping the backscattered light. Consequently, by controlling the Brillouin backscattering through variation such plasma parameters as ion acoustic wave damping, divergence of the plasma expansion velocity or the laser bandwidth, we demonstrate the possibility of controlling the level of nonlinear laser absorption and scattering in a hot, weakly collisional plasma.

Suppressing cross-beam energy transfer with broadband lasers

The scattering of laser light due to cross-beam energy transfer (CBET) is an undesirable process in direct-drive inertial confinement fusion (ICF) that degrades both the compression and symmetry of the imploding target. Here, we present results from laser-plasma interaction simulations performed with the wave-based code LPSE that explore two techniques for suppressing CBET in frequency-tripled, Nd:glass laser beams crossing in a transonic plasma: a.) frequency detuning using two or three discrete “colors” of narrowband laser light; and b.) broad laser bandwidth. We find that for beams modeled with random speckle patterns, distributed phase plates and polarization smoothing, and for plasma conditions similar to those on the National Ignition Facility, the former method reduces CBET to an extent, but the degree of mitigation plateaus once the frequency separation greatly exceeds the resonance width of the CBET instability. Broadband lasers, on the other hand, are predicted to suppress CBET completely at a bandwidth of about 8 THz (Δ ω/ω0 ≃ 1%, where Δ ω/2π and ω0 are the laser bandwidth and angular frequency, respectively) for the same conditions. Although the Nd:glass lasers used for ICF research today have bandwidths far below this value, the spectra from such lasers could likely be broadened to multi-terahertz levels by utilizing stimulated rotational Raman scattering in a gaseous diatomic medium. Alternatively, the required bandwidth could be obtained with an excimer laser driver such as argon-fluoride, which has a native bandwidth in excess of 7 THz. Either of these two options would enable higher and more symmetric ablation pressures in future, direct-drive, ICF target designs.

Generation of a Bessel beam in FDTD using a cylindrical antenna.

Bessel beams are becoming a very useful tool in many areas of optics and photonics, because of the invariance of their intensity profile over an extended propagation range. Finite-Difference-Time-Domain (FDTD) approach is widely used for the modeling of the beam interaction with nanostructures. However, the generation of the Bessel beam in this approach is a computationally challenging problem. In this work, we report an approach for the generation of the infinite Bessel beams in three-dimensional FDTD. It is based on the injection of the Bessel solutions of Maxwell's equations from a cylindrical hollow annulus. This configuration is compatible with Particle In Cell simulations of laser plasma interactions. This configuration allows using a smaller computation box and is therefore computationally more efficient than the creation of a Bessel-Gauss beam from a wall and models more precisely the analytical infinite Bessel beam. Zeroth and higher-order Bessel beams with different cone angles are successfully produced. We investigate the effects of the injector parameters on the error with respect to the analytical solution. In all cases, the relative deviation is in the range of 0.01-7.0 percent.

Simulation study of positron production by picosecond laser-driven electrons

Positron production by picosecond laser-driven electrons has been studied via simulations. The laser parameters were chosen according to a typical 100 J/1 ps laser system, such as the XingGuang III laser. A near-critical density plasma was used to accelerate electrons. Then the positrons were generated by these electrons interacting with a high-Z converter. Particle-in-cell simulations of laser-plasma interaction indicate that the picosecond laser-accelerated electrons have a temperature up to 12 MeV with a beam charge of 800 nC (>5 MeV). The positron generation was simulated by a Monte Carlo toolkit. The simulation results show that with an optimal converter thickness the generated positron beam has a yield of 5 × 1010 that is over one order of magnitude higher than that from direct laser-solid interaction.

High-order Particle-In-Cell simulations of laser-plasma interaction

The simulation of laser-plasma interaction via three- dimensional Particle-In-Cell methods based on higher-order schemes is considered. High-order methods allow for drastically reducing the required number of degrees of freedom while still capturing the complex physical nature of non-linear processes. The suitability of high-order methods for the acceleration of protons from thin films by intense short-pulse lasers in terms of accuracy and efficiency is shown.

Investigation and modeling of optics damage in high-power laser systems caused by light backscattered in plasma at the target

High-power laser facilities may be restricted in operations by the risk of damage to optics due to the backscattering of laser light from the intended target. A portion of this backscattered light can re-enter the beam line and at sufficient fluence may damage (burn) optical apparatus. Here, the observed cumulative burn pattern on mirrors at the National Ignition Facility (NIF) due to stimulated Brillouin scattering (SBS) at the target is explained using detailed simulations. Our methodology involves using a measurement of a phase plate profile installed at the NIF to provide the laser input to a three-dimensional simulation of laser-plasma interaction in a target typical of that fielded at the NIF. Light scattered by SBS at the target is then followed back to the final aperture of the beam line, back through the phase plate, and to a mirror where damage typically occurs. We find that mirror damage patterns are largely dictated by the phase imprint of the phase plate on the returning SBS light. Our simulations show that the instantaneous SBS signal at the mirror varies on a picosecond time scale and is also highly modulated in space. Temporal averaging of the simulated SBS light pattern on the mirror reveals a pattern in close agreement with the experimentally observed damage. Understanding these burn patterns (distribution, modulation depth) may lead to future phase plate designs that limit damage throughout the optics assembly of high-powered laser facilities.

Suppressing Two-Plasmon Decay with Laser Frequency Detuning.

Three-dimensional laser-plasma interaction simulations show that laser frequency detuning by an amount achievable with current laser technology can be used to suppress the two-plasmon decay (TPD) instability and the corresponding hot-electron generation. For the plasma conditions and laser configuration in a direct-drive inertial confinement fusion implosion on the OMEGA laser, the simulations show that ∼0.7% laser frequency detuning is sufficient to eliminate TPD-driven hot-electron generation in current experiments. This allows for higher ablation pressures in future implosion designs by using higher laser intensities.

Convergence analysis of an explicit splitting method for laser plasma interaction simulations

The convergence of a triple splitting method originally proposed by Tückmantel, Pukhov, Liljo, and Hochbruck for the solution of a simple model that describes laser plasma interactions with overdense plasmas is analyzed. For classical explicit integrators it is the large density parameter that imposes a restriction on the time step size to make the integration stable. The triple splitting method contains an exponential integrator in its central component and was specifically designed for systems that describe laser plasma interactions and overcomes this restriction. We rigorously analyze a slightly generalized version of the original method. This analysis enables us to identify modifications of the original scheme such that a second-order convergent scheme is obtained.

Relativistic quasi-solitons and embedded solitons with circular polarization in cold plasmas

The existence of localized electromagnetic structures is discussed in the framework of the 1-dimensional relativistic Maxwell-fluid model for a cold plasma with immobile ions. New partially localized solutions are found with a finite-difference algorithm designed to locate numerically exact solutions of the Maxwell-fluid system. These solutions are called quasi-solitons and consist of a localized electromagnetic wave trapped in a self-generated plasma density cavity with oscillations at its tails. They are organized in families characterized by the number of nodes p of the vector potential and exist in a continuous range of parameters in the plane, where V is the velocity of propagation and ω is the vector potential angular frequency. A parametric study shows that the familiar fully localized relativistic solitons are special members of the families of partially localized quasi-solitons. Soliton solution branches with p > 0 are therefore parametrically embedded in the continuum of quasi-solitons. On the other hand, geometric arguments and numerical simulations indicate that p = 0 solitons exist only in the limit of either small amplitude or vanishing velocity. Direct numerical simulations of the Maxwell-fluid model indicate that the p > 0 quasi-solitons (and embedded solitons) are unstable and lead to wake excitation, while p = 0 quasi-solitons appear stable. This helps explain the ubiquitous observation of structures that resemble p = 0 solitons in numerical simulations of laser-plasma interaction.

Laser-plasma interactions with a Fourier-Bessel particle-in-cell method

A new spectral particle-in-cell (PIC) method for plasma modeling is presented and discussed. In the proposed scheme, the Fourier-Bessel transform is used to translate the Maxwell equations to the quasi-cylindrical spectral domain. In this domain, the equations are solved analytically in time, and the spatial derivatives are approximated with high accuracy. In contrast to the finite-difference time domain (FDTD) methods, that are used commonly in PIC, the developed method does not produce numerical dispersion and does not involve grid staggering for the electric and magnetic fields. These features are especially valuable in modeling the wakefield acceleration of particles in plasmas. The proposed algorithm is implemented in the code PLARES-PIC, and the test simulations of laser plasma interactions are compared to the ones done with the quasi-cylindrical FDTD PIC code CALDER-CIRC.

Laser–plasma interactions for fast ignition

In the electron-driven fast-ignition (FI) approach to inertial confinement fusion, petawatt laser pulses are required to generate MeV electrons that deposit several tens of kilojoules in the compressed core of an imploded DT shell. We review recent progress in the understanding of intense laser–plasma interactions (LPI) relevant to FI. Increases in computational and modelling capabilities, as well as algorithmic developments have led to enhancement in our ability to perform multi-dimensional particle-in-cell simulations of LPI at relevant scales. We discuss the physics of the interaction in terms of laser absorption fraction, the laser-generated electron spectra, divergence, and their temporal evolution. Scaling with irradiation conditions such as laser intensity are considered, as well as the dependence on plasma parameters. Different numerical modelling approaches and configurations are addressed, providing an overview of the modelling capabilities and limitations. In addition, we discuss the comparison of simulation results with experimental observables. In particular, we address the question of surrogacy of today's experiments for the full-scale FI problem.

Three-wavelength scheme to optimize hohlraum coupling on the National Ignition Facility

By using three tunable wavelengths on different cones of laser beams on the National Ignition Facility, numerical simulations show that the energy transfer between beams can be tuned to redistribute the energy within the cones of beams most prone to backscatter instabilities. These radiative hydrodynamics and laser-plasma interaction simulations have been tested against large-scale hohlraum experiments with two tunable wavelengths and reproduce the hohlraum energetics and symmetry. Using a third wavelength provides a greater level of control of the laser energy distribution and coupling in the hohlraum, and could significantly reduce stimulated Raman scattering losses and increase the hohlraum radiation drive while maintaining a good implosion symmetry.

Simple collision operators for direct Vlasov simulations of laser plasma interaction and transport

Non-local electron transport effects have a direct influence on the compression of cryogenic targets in laser driven ICF and target heating in high energy density experiments. There is a growing need for self-consistent models of laser plasma interactions coupled to nonlocal transport. We present a direct Vlasov solver that includes multiple species and a simple collision operator. This BGK model operator – which conserves particle density, energy and momentum – is fully implicit. For collisionless plasmas it has been shown that a double layer may be formed in which an accelerated, kinetic ion population satisfies the zero current condition. Here we extend this result to collisionalities of interest to laser driven ignition to assess the validity of nonlocal electron transport models based on fluid ions.

A code to solve the Vlasov–Fokker–Planck equation applied to particle transport in magnetic turbulence

We present a novel code which solves the Vlasov–Fokker–Planck (VFP) equation in three-dimensional magnetic turbulence using finite difference methods. The approach is distinct from particle tracking codes. The angular component of the velocity space distribution function is represented by a spherical harmonic expansion drawing on an approach pioneered by Bell et al (2006 Plasma Phys. Control. Fusion 48 R37) for laser–plasma interaction simulations. This method enables the accurate representation of magnetic fields and of the effect of angular scattering effects on a particle distribution function. The code has been verified against both collisional and quasi-linear turbulent transport theories. It is shown to address successfully the physics of cross-field transport in regimes of magnetic field field perturbation amplitude and collisionality that are difficult to study using other approaches.

The transition from plasma gratings to cavitons in laser-plasma interactions

One-dimensional Vlasov–Maxwell simulations of laser-plasma interactions are presented. It is shown that plasma gratings and density cavitons are formed sequentially. There are strong electromagnetic fields in the cavitons and the electromagnetic structures are nearly standing and long-lived. The formation of gratings and cavitons can be explained by a nonlinear second-order differential equation. The electromagnetic fields trapped in cavitons have both subcycle and cycle structures. Plasma whose density is higher than the critical density can be formed around the cavitons. Gratings and high density plasmas can reflect light in a very high level. This may be detrimental to the inertial confinement fusion.

Observation of the Density Threshold Behavior for the Onset of Stimulated Raman Scattering in High-Temperature Hohlraum Plasmas

We show that the measured stimulated Raman scattering (SRS) in a large-scale high-temperature plasma scales strongly with the plasma density, increasing by an order of magnitude when the electron density is increased by 20%. This is consistent with linear theory, including pump depletion, in a uniform plasma and, as the density is typically constrained by other processes, this effect will set a limit on drive laser beam intensity for forthcoming ignition experiments at the National Ignition Facility. Control of SRS at laser intensities consistent with 285 eV ignition hohlraums is achieved by using polarization smoothing which increases the intensity threshold for the onset of SRS by 1.6 +/- 0.2. These results were quantitatively predicted by full beam three-dimensional numerical laser-plasma interaction simulations.

Energetics of multiple-ion species hohlraum plasmas

A study of the laser-plasma interaction processes has been performed in multiple-ion species hohlraum plasmas at conditions similar to those expected in indirect drive inertial confinement fusion targets. Gas-filled hohlraums with electron densities of 5.5×1020 and 9×1020cm−3 are heated by 14.3kJ of laser energy (wavelength 351nm) to electron temperatures of 3keV and backscattered laser light is measured. Landau damping of the ion acoustic waves is increased by adding hydrogen to a CO2 or CF4 gas. Stimulated Brillouin backscattering of a 351nm probe beam is found to decrease monotonically with increasing Landau damping, accompanied by a comparable increase in the transmission. More efficient energy coupling into the hohlraum by suppression of backscatter from the heater beams results in an increased hohlraum radiation temperature, showing that multiple-ion species plasmas improve the overall hohlraum energetics. The reduction in backscatter is reproduced by linear gain calculations as well as detailed full-scale three-dimensional laser-plasma interaction simulations, demonstrating that Landau damping is the controlling damping mechanism in inertial confinement fusion relevant high-electron temperature plasmas. These findings have led to the inclusion of multiple-ion species plasmas in the hohlraum point design for upcoming ignition campaigns at the National Ignition Facility.

Two-dimensional particle-in-cell simulations of plasma cavitation and bursty Brillouin backscattering for nonrelativistic laser intensities

Two-dimensional particle-in-cell simulations of laser-plasma interaction using a plane-wave geometry show strong bursty stimulated Brillouin backscattering, rapid filamentation, and subsequent plasma cavitation. It is shown that the cavitation is not induced by self-focusing. The electromagnetic fields below the plasma frequency that are excited are related to transient soliton-like structures. At the origin of these solitons is a three-wave decay process exciting new modes in the plasma. The cavitation is responsible for a strong local reduction of the reflectivity and goes along with an efficient but transient heating of the electrons. Once heating ceases, transmission starts to increase. Local as well as global average reflectivities attain a very low value due to strong plasma density variations brought about by the cavitation process. On the one hand, the simulations confirm the existence of a new mechanism of cavity and soliton formation in nonrelativistic laser-plasma interaction in two dimensions, which was shown to exist in one-dimensional simulations [S. Weber, C. Riconda, and V. T. Tikhonchuk, Phys. Rev. Lett. 94, 055005 (2005)]. On the other hand, new aspects are introduced inherently related to the additional degree of freedom.

Study of laser plasma interactions using an Eulerian Vlasov code

We present a one-dimensional Eulerian Vlasov code for performing kinetic simulations of laser-plasma interactions. We use the code to study parametric instabilities, in particular stimulated Raman scattering. For conditions similar to those of single-hot-spot experiments, we find that kinetic effects are important in the saturation of this instability. Work is underway to extend the code to 1 1 2 and 2D (resolving y and p y ) and to parallelize it.