Session JP17: Poster Session: HED: Warm Dense Matter, Nonlinear Optics, and Short-Pulse Laser-Plasma Interactions (2:00pm - 5:00pm)

Development of Ion Wave Plasma Optics for NIF and Other Lasers.

The success of combining beam energies and increasing fluences on target with an ion wave plasma optic produced by Cross Beam Energy Transfer (CBET) [1,2,3] motivates its development for new applications. It has been shown that the linear plasma response observed in present plasma optics [4] would allow a 1 ns, high energy plasma-combined beam to act as a pump for a second stage of amplification and compression of a \textless \textasciitilde 0.1 ns seed beam [5] if non-linear plasma wave effects remain benign. However the suppression of secondary instabilities dictates that the 15 cm plasma in such a compressor be very low density (\textless \textasciitilde 0.1{\%} crit. [5]) to allow highest performance, which pushes it into a regime where particle trapping and other non-linear effects are also important. Recent and planned simulations and the existing and planned experiments that can benchmark them will be discussed, as will potential applications ranging from radiography to fusion energy. [1] P. Poole in preparation, [2] R. K. Kirkwood et al Nat. Phys. 14 , 80 (2018). [3] R. K. Kirkwood et al Phys. of Plas. 25 056701 (2018). [4] A Colaitis et al Physics of Plasmas 25, 033114 (2018), [5] R. K. Kirkwood et al, APS DPP 2019.   

Collinear Four-Photon Scattering of Mildly Intense Laser Pulses in Tenuous Plasma

Collinear, planar electromagnetic waves of small amplitudes cannot satisfy exact synchronism conditions for four-photon resonance in plasma. However, in tenuous plasma, even a mildly relativistic electron nonlinearity, exceeding a very small threshold, enables the collinear four-photon scattering. The width of the frequency domain in which the exact four-photon resonance is allowed increases with the laser intensities. At yet very small relativistic electron nonlinearity, this width significantly exceeds the tenuous electron plasma frequency. Being far enough from the Raman resonances, the newly found four-photon scattering process becomes insensitive to plasma inhomogeneities, and so may persist even if Raman scattering is suppressed by variations in the electron plasma concentration.   

Creation of Broad Bandwidth using Stimulated Rotational Raman Scattering in the Nike Laser

Broad bandwidth can be used to suppress laser-plasma instabilities generated in applications such as inertial confinement fusion. At the Naval Research Laboratory's Nike Laser Facility, experiments exploring stimulated rotational Raman scattering (SRRS) to enhance bandwidth have been performed. Nike is a krypton-fluoride laser that delivers to planar targets 2kJ of 248 nm radiation spread over 56 beams. While SRRS has been previously observed in Nike [J. Weaver, et al, Appl. Optics, 56, 31, 2017], recent experiments further enhance SRRS production by compressing a single beam by a factor of three and propagating it for 38 meters. Far-field beam image, pulse width, energy, and time-integrated spectrum diagnostics are placed before and after the compression. In addition, a time-resolved spectrometer measures bandwidth post-propagation and imaging cameras resolve the near-field of the compressed beam. Pulse widths range from 0.35 ns to 4 ns and far-field XDLs (times diffraction limit) vary from 15 to 60. Bandwidths of up to \textasciitilde 5 THz (compared to intrinsic 1 THz) have been measured with mild increase in transverse beam size.   

Theory and OSIRIS Particle-in-cell Simulations of Stimulated Raman Scattering in unmagnetized and magnetized Plasmas

We use the particle-in-cell code OSIRIS to study stimulated Raman scattering (SRS) in magnetized and unmagnetized plasmas in a wide range of parameter space. We have previously shown how small magnetic fields can significantly modify the evolution of backward stimulated Raman scattering (SRS) in the kinetic regime ($k\lambda_{De} \approx 0.32$ for backscattered plasma wave) due to the enhanced dissipation of nonlinear electron plasma waves propagating perpendicular to magnetic fields. Driven by the collaboration between UCLA and UCSD, the range of validity of these results is extended, showing that they are valid in a wide range of parameter space, $k\lambda_{De} \approx 0.19 - 0.32$. Furthermore, we have identified a number of areas rich in complex, largely unstudied physics. This includes regimes in which magnetic fields appear to lead to enhanced SRS, which we believe to be related to rescatter appearing to reduce the amount of backscatter and diminished non-linear frequency shift in magnetized plasmas. This also includes frequently observed evidence of competition between forward and backward SRS and backscatter in strongly coupled regimes where $k\lambda_{De} \approx 1$. These observations offer promising opportunities for new avenues of research and future experime   

Simulating Warm Dense Plasmas with a Hybrid Molecular Dynamics - Quantum Hydrodynamics approach

Modeling the slowing-down of charged projectiles in warm dense matter (WDM) is of great interest to the design of ion-beam and intense laser experiments, e.g., in describing fast $\alpha $-heating in ICF. This process is complicated by the electrons being moderately Coulomb-coupled and partially quantum-degenerate, while also dynamically screening the projectile. In this regime, ab initio Molecular Dynamics (MD) simulations are typically employed, and solve for the self-consistent quantum ground-state electron density for the ion configuration at each timestep using Density Functional Theory (DFT). Here, we describe a Quantum Hydrodynamics (QHD) approach, which treats the electrons as a fluid with forces derived from DFT, ensuring accurate equilibrium equation of state properties. We derive the QHD equations from first principles, connect them to the machinery of DFT, and describe the predicted linear response. We developed a parallelized simulation code that combines MD ions and QHD electrons, and simulated a stopping power experiment conducted at the Jupiter Laser Facility. By comparing with a quantum statistical potential MD code, we show favorable results for the QHD-MD approach. We discuss possible extensions into the FLASH code framework.   

Ab Initio Plasmon Dispersion of the Warm Dense Electron Gas

The plasmon dispersion $\omega(q)$ and damping $\gamma(q)$ contain important information on the state of warm dense matter. On the other hand, x-ray Thomson scattering (XRTS) experiments provide accurate data for the dynamic structure factor $S(q,\omega)$ that is directly linked to the plasmon spectrum [Glenzer et al., Phys. Rev. Lett. 98, 065002 (2007)]. However, details of this link depend on the quality of the theoretical model for the dielectric function. Here we present the first ab initio data for the dielectric function that is obtained by quantum Monte Carlo simulations [Dornheim et al. Phys. Rev. Lett. 121, 255001 (2018)]. This allows us to obtain high quality results for $\omega(q)$ and $\gamma(q)$ of the electron component at warm dense matter conditions that differ significantly from previous models. Second, we critically analyze the commonly used weak damping approximation for the dispersion and improve it by performing the analytic continuation of the retarded dielectric function. This yields results that apply at strong damping and large wave numbers as well, which is the basis for a more accurate comparison with XRTS experiments [Hamann et al., submitted for publication].   

Fluctuation Approach to Many-Body Quantum Dynamics

The dynamics of quantum many-body systems following external excitation is of great interest in many areas such as dense plasmas or correlated solids. At present, only the formalism of nonequilibrium Green functions (NEGF) can rigorously describe such processes in more than one dimension. However, NEGF simulations are computationally expensive, among other things, due to their cubic scaling with simulation time $T$. Only recently, linear scaling with $T$ could be achieved within the G1-G2 scheme\footnote{N. Schl\"unzen \textit{et al.}, \textit{Phys.~Rev.~Lett.} {\bf 124}, 076601 (2020)} which could be demonstrated for advanced selfenergies\footnote{J.-P. Joost \textit{et al.}, Phys. Rev. B \textbf{101}, 245101 (2020)}. To further improve the quality of the description and to include three-particle correlations, here a new approach to the NEGF formalism is presented. Instead of a hierarchy for the N-particle Green functions, we consider an approach that is based on fluctuations. While the resulting equations are fully equivalent to the G1-G2 scheme, the new approach has interesting complementary features such as the capability to simulate many-body effects using stochastic methods\footnote{D. Lacroix \textit{et al.}, Phys. Rev. B \textbf{90}, 125112 (2014)}.   

Simulations of DARHT electron-beam heating of thin metal foils for EOS measurements

The LASNEX 2D Lagrangian radiation-hydrodynamics code is used to simulate the expansion of thin electron-beam-heated foils. The energy is deposited using a particle beam source, which is scaled to model the collisional heating process for an electron beam, but neglects bremsstrahlung. The hydrodynamic expansion and time evolution of quantities dependent on the equation of state for the foil material vary with beam energy and energy density. We present the results of simulations using beams of 4 and 19.8 MeV electron energy and various beam widths (\textasciitilde 1 mm) passing through Cu and Al foils. We assume axisymmetric geometry and solve the Navier-Stokes equations with artificial viscosity and electron thermal conduction, plus multi-group radiation diffusion. Previous simulations performed with Cu, using the SESAME EOS table 3336, showed reasonable agreement with DARHT PDV experimental measurements of the Cu foil expansion [1]. This parameter study will require further experimental validation with the ultimate goal of improving EOS tables for warm dense matter conditions. [1] https://doi.org/10.1103/PhysRevE.98.043201   

Creation of Si Warm Dense Matter using an intense proton beam by the OMEGA-EP short pulse laser and simulated with 2D particle-in-cell

The dual OMEGA-EP short-pulse lasers were used to heat a thin Si wafer to the warm dense matter (WDM) regime and measure its temperature. The first beam (10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$, 10 ps) irradiated a CH hemispherical cap, subsequently accelerating energetic protons and ions to heat the Si face-on. The second beam (10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$, 5 ps) irradiated an adjacent Zn wire tip, generating an x-ray backlighting spectrum which passed through the Si hot spot and into a multipurpose spectrometer (MSPEC) to diagnose the instantaneous Si temperature at delays spanning 200 ps. Modeling predicts that the Si remains \textgreater 30 eV and \textgreater 10{\%} solid density during this time window. We present the proton energy spectra measurements from Radiochromic film and a Thomson Parabola which are used as input to simulate the temperature evolution of the Si via the 2D particle-in-cell code LSP. We show the simulation results and compare with temperature measurements from MSPEC.   

PIC simulations characterizing the Radiation Response of a Copper Target Excited by an Ultra-Short Laser Pulse.

The interaction of an intense, ultrashort-pulse laser with a solid conducting, dielectric, or semi-conducting target leads to dramatic modification of its surface and subsequent emission of electromagnetic (EM) radiation. This radiation may be an interesting probe of the laser-target interaction or useful in its own right. We have used LSP [1] particle-in-cell (PIC) simulations to characterize the low frequency (THz and below) radiation response of copper targets illuminated by single high intensity ultra-short laser pulses with intensities up to 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$. Our simulations treat the material permittivity and reflectivity using a realistic collision model [2] based on the binary collision algorithm. This permits a realistic treatment of the target's dynamically changing electromagnetic response and thermal evolution beginning from a room temperature state, which then establishes the initial conditions for the subsequent current evolution and EM emission. This work was supported in part by AFRL under award FA9451-19-C-0011. [1] Welch, D. {\&} Rose, D., Comp. Phys. Comm. \textbf{164}, 183-188 (2004) [2] A.M. Russell and D.W. Schumacher, Physics of Plasmas \textbf{24}, 080702 (2017).   

CCFLY: Modelling the Collisional-Radiative Evolution of a Plasma Driven by an X-Ray Free-Electron Laser

When a solid-density target is isochorically heated by an X-ray Free Electron Laser (XFEL), some portion of the electron energy distribution is highly nonthermal, both during and immediately after the pulse. We have adapted CCFLY, a collisional-radiative code based on the physics of SCFLY\footnote{H.-K. Chung, M. Chen, B.I. Cho, O. Ciricosta, S.M. Vinko, J.S. Wark and R.W. Lee. APIP Conference Proceedings {\bf 1811}, 020001 (2017).}, to model the collisional and radiative interactions between electrons, ions and photons in the system, taking into account the full electron energy distribution function. We present the results of checks on the code that ensure that the non-thermal terms are consistent with the previous purely thermal CCFLY model, as well as presenting initial analyses of the non-thermal nature of the ion charge state populations, electron distribution functions, and the radiative properties of the system in the presence of the intense FEL irradiation.   

Investigation of electron-ion equilibration using time-dependent Stark broadening$^{\mathrm{\ast ,+}}$

Measurement of the electron-ion temperature relaxation has proven to be a difficult quantity to ascertain. We present a different approach to determining the electron-ion relaxation time using spectral line-shape theory. At constant density, the spectral line shape of a Stark broadened line will be driven by the temperature separation between the electrons and ions. We exploit this physics by utilizing time-resolved, x-ray spectroscopy of ultra-short pulse laser heated matter. During this interaction, the laser energy heats the electrons which transfers energy to the ions via collisions. We observe the temporal evolution of the Si 1s-3p transition to explore the possibilities of this technique. Preliminary data and simulations will be presented.   

Background Gas Species and Pressure Dependence of RF Emissions and Lengths of Laser Driven Filament Plasmas

The Air Force Research Lab seeks to understand the mechanism driving RF generation in laser driven plasma filaments. An 800 nm, terawatt class laser is used to propagate a plasma filament with various background gases under a range of pressures to study the RF emission from 1-13 GHz and the length changes of the filament. Air pressure has already been shown to have an inverse relationship with the amplitude of the microwave radiation emitted by the plasma, however this leaves more to be learned by studying individual gas species. Argon, helium, neon, nitrogen, and krypton are used to better understand the contributions of electron-neutral and electron-ion collisions to RF emission. Using a microwave horn with the ranges of 1-13 GHz and an S-band wave guide, the effects of the background gas species and pressure will be quantified and their relation to the RF emission and plasma length will be presented.