# Publications

## Wakefields in a cluster plasma

PHYSICAL REVIEW ACCELERATORS AND BEAMS **22** (2019) ARTN 113501

## Identification of Phase Transitions and Metastability in Dynamically Compressed Antimony Using Ultrafast X-Ray Diffraction.

Physical review letters **122** (2019) 255704-

Ultrafast x-ray diffraction at the LCLS x-ray free electron laser has been used to resolve the structural behavior of antimony under shock compression to 59 GPa. Antimony is seen to transform to the incommensurate, host-guest phase Sb-II at ∼11 GPa, which forms on nanosecond timescales with ordered guest-atom chains. The high-pressure bcc phase Sb-III is observed above ∼15 GPa, some 8 GPa lower than in static compression studies, and mixed Sb-III/liquid diffraction are obtained between 38 and 59 GPa. An additional phase which does not exist under static compression, Sb-I^{'}, is also observed between 8 and 12 GPa, beyond the normal stability field of Sb-I, and resembles Sb-I with a resolved Peierls distortion. The incommensurate Sb-II high-pressure phase can be recovered metastably on release to ambient pressure, where it is stable for more than 10 ns.

## Retrieving fields from proton radiography without source profiles.

Physical review. E **100** (2019) 033208-

Proton radiography is a technique in high-energy density science to diagnose magnetic and/or electric fields in a plasma by firing a proton beam and detecting its modulated intensity profile on a screen. Current approaches to retrieve the integrated field from the modulated intensity profile require the unmodulated beam intensity profile before the interaction, which is rarely available experimentally due to shot-to-shot variability. In this paper, we present a statistical method to retrieve the integrated field without needing to know the exact source profile. We apply our method to experimental data, showing the robustness of our approach. Our proposed technique allows for the retrieval not only of the path-integrated fields, but also of the statistical properties of the fields.

## Free Electron Relativistic Correction Factors to Collisional Excitation and Ionisation Rates in a Plasma

High Energy Density Physics Elsevier BV (2019) 100716

## Energy absorption in the laser-QED regime.

Scientific reports **9** (2019) 8956-

A theoretical and numerical investigation of non-ponderomotive absorption at laser intensities relevant to quantum electrodynamics is presented. It is predicted that there is a regime change in the dependence of fast electron energy on incident laser energy that coincides with the onset of pair production via the Breit-Wheeler process. This prediction is numerically verified via an extensive campaign of QED-inclusive particle-in-cell simulations. The dramatic nature of the power law shift leads to the conclusion that this process is a candidate for an unambiguous signature that future experiments on multi-petawatt laser facilities have truly entered the QED regime.

## Kinetic simulations of fusion ignition with hot-spot ablator mix.

Physical review. E **100** (2019) 033206-

Inertial confinement fusion fuel suffers increased x-ray radiation losses when carbon from the capsule ablator mixes into the hot-spot. Here, we present one- and two-dimensional ion Vlasov-Fokker-Planck simulations that resolve hot-spot self-heating in the presence of a localized spike of carbon mix, totalling 1.9% of the hot-spot mass. The mix region cools and contracts over tens of picoseconds, increasing its α particle stopping power and radiative losses. This makes a localized mix region more severe than an equal amount of uniformly distributed mix. There is also a purely kinetic effect that reduces fusion reactivity by several percent, since faster ions in the tail of the distribution are absorbed by the mix region. Radiative cooling and contraction of the spike induces fluid motion, causing neutron spectrum broadening. This artificially increases the inferred experimental ion temperatures and gives line of sight variations.

## Inverse problem instabilities in large-scale modeling of matter in extreme conditions

Physics of Plasmas **26** (2019)

© 2019 Author(s). Our understanding of physical systems often depends on our ability to match complex computational modeling with the measured experimental outcomes. However, simulations with large parameter spaces suffer from inverse problem instabilities, where similar simulated outputs can map back to very different sets of input parameters. While of fundamental importance, such instabilities are seldom resolved due to the intractably large number of simulations required to comprehensively explore parameter space. Here, we show how Bayesian inference can be used to address inverse problem instabilities in the interpretation of x-ray emission spectroscopy and inelastic x-ray scattering diagnostics. We find that the extraction of information from measurements on the basis of agreement with simulations alone is unreliable and leads to a significant underestimation of uncertainties. We describe how to statistically quantify the effect of unstable inverse models and describe an approach to experimental design that mitigates its impact.

## Molecular dynamics simulations of grain interactions in shock-compressed highly textured columnar nanocrystals

PHYSICAL REVIEW MATERIALS **3** (2019) ARTN 083602

## Observation of He-like Satellite Lines of the H-like Potassium K XIX Emission

ASTROPHYSICAL JOURNAL **881** (2019) ARTN 92

## Orbital Angular Momentum Coupling in Elastic Photon-Photon Scattering.

Physical review letters **123** (2019) 113604-

In this Letter, we investigate the effect of orbital angular momentum (OAM) on elastic photon-photon scattering in a vacuum for the first time. We define exact solutions to the vacuum electromagnetic wave equation which carry OAM. Using those, the expected coupling between three initial waves is derived in the framework of an effective field theory based on the Euler-Heisenberg Lagrangian and shows that OAM adds a signature to the generated photons thereby greatly improving the signal-to-noise ratio. This forms the basis for a proposed high-power laser experiment utilizing quantum optics techniques to filter the generated photons based on their OAM state.

## Thomson scattering cross section in a magnetized, high-density plasma.

Physical review. E **99** (2019) 063204-

We calculate the Thomson scattering cross section in a nonrelativistic, magnetized, high-density plasma-in a regime where collective excitations can be described by magnetohydrodynamics. We show that, in addition to cyclotron resonances and an elastic peak, the cross section exhibits two pairs of peaks associated with slow and fast magnetosonic waves; by contrast, the cross section arising in pure hydrodynamics possesses just a single pair of Brillouin peaks. Both the position and the width of these magnetosonic-wave peaks depend on the ambient magnetic field and temperature, as well as transport and thermodynamic coefficients, and so can therefore serve as a diagnostic tool for plasma properties that are otherwise challenging to measure.

## First demonstration of ARC-accelerated proton beams at the National Ignition Facility

Physics of Plasmas **26** (2019)

© 2019 Author(s). New short-pulse kilojoule, Petawatt-class lasers, which have recently come online and are coupled to large-scale, many-beam long-pulse facilities, undoubtedly serve as very exciting tools to capture transformational science opportunities in high energy density physics. These short-pulse lasers also happen to reside in a unique laser regime: very high-energy (kilojoule), relatively long (multi-picosecond) pulse-lengths, and large (10s of micron) focal spots, where their use in driving energetic particle beams is largely unexplored. Proton acceleration via Target Normal Sheath Acceleration (TNSA) using the Advanced Radiographic Capability (ARC) short-pulse laser at the National Ignition Facility in the Lawrence Livermore National Laboratory is demonstrated for the first time, and protons of up to 18 MeV are measured using laser irradiation of >1 ps pulse-lengths and quasi-relativistic (∼10 18 W/cm 2 ) intensities. This is indicative of a super-ponderomotive electron acceleration mechanism that sustains acceleration over long (multi-picosecond) time-scales and allows for proton energies to be achieved far beyond what the well-established scalings of proton acceleration via TNSA would predict at these modest intensities. Furthermore, the characteristics of the ARC laser (large ∼100 μm diameter focal spot, flat spatial profile, multi-picosecond, relatively low prepulse) provide acceleration conditions that allow for the investigation of 1D-like particle acceleration. A high flux ∼ 50 J of laser-accelerated protons is experimentally demonstrated. A new capability in multi-picosecond particle-in-cell simulation is applied to model the data, corroborating the high proton energies and elucidating the physics of multi-picosecond particle acceleration.

## Laboratory measurements of geometrical effects in the x-ray emission of optically thick lines for ICF diagnostics

PHYSICS OF PLASMAS **26** (2019) ARTN 063302

## Laboratory study of stationary accretion shock relevant to astrophysical systems.

Scientific reports **9** (2019) 8157-

Accretion processes play a crucial role in a wide variety of astrophysical systems. Of particular interest are magnetic cataclysmic variables, where, plasma flow is directed along the star's magnetic field lines onto its poles. A stationary shock is formed, several hundred kilometres above the stellar surface; a distance far too small to be resolved with today's telescopes. Here, we report the results of an analogous laboratory experiment which recreates this astrophysical system. The dynamics of the laboratory system are strongly influenced by the interplay of material, thermal, magnetic and radiative effects, allowing a steady shock to form at a constant distance from a stationary obstacle. Our results demonstrate that a significant amount of plasma is ejected in the lateral direction; a phenomenon that is under-estimated in typical magnetohydrodynamic simulations and often neglected in astrophysical models. This changes the properties of the post-shock region considerably and has important implications for many astrophysical studies.

## Radiation transfer in cylindrical, toroidal and hemi-ellipsoidal plasmas

Journal of Quantitative Spectroscopy and Radiative Transfer Elsevier BV (2019)

## Field reconstruction from proton radiography of intense laser driven magnetic reconnection

PHYSICS OF PLASMAS **26** (2019) ARTN 083109

## Enhanced fluorescence from x-ray line coincidence pumping of K-pumped Cl and Mg-pumped Ge plasmas

X-Ray Lasers and Coherent X-Ray Sources: Development and Applications XIII SPIE (2019)

## Ab-initio simulations and measurements of the free-free opacity in Aluminum

Physical Review E: Statistical, Nonlinear, and Soft Matter Physics American Physical Society (0)

The free-free opacity in dense systems is a property that both tests our fundamental understanding of correlated many-body systems, and is needed to understand the radiative properties of high energy-density plasmas. Despite its importance, predictive calculations of the free-free opacity remain challenging even in the condensed matter phase for simple metals. Here we show how the free-free opacity can be modelled at finite-temperatures via time-dependent density functional theory, and illustrate the importance of including local field corrections, core polarization and self-energy corrections. Our calculations for ground-state Al are shown to agree well with experimental opacity measurements performed on the Artemis laser facility across a wide range of x-ray to ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, and find good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV.

## The blind implosion-maker: Automated inertial confinement fusion experiment design

PHYSICS OF PLASMAS **26** (2019) ARTN 062706

## Observing thermal Schwinger pair production

Physical Review A American Physical Society (APS) **99** (2019) 052120