A pulsed-laser calibration system for the laser backscatter diagnostics at the Omega laser

A calibration system has been developed that allows a direct determination of the sensitivity of the laser backscatter diagnostics at the Omega laser. A motorized mirror at the target location redirects individual pulses of a mJ-class laser onto the diagnostic to allow the in-situ measurement of the local point response of the backscatter diagnostics. Featuring dual wavelength capability at the 2nd and 3rd harmonic of the Nd:YAG laser, both spectral channels of the backscatter diagnostics can be directly calibrated. In addition, channel cross-talk and polarization sensitivity can be determined. The calibration system has been employed repeatedly over the last two years and has enabled precise backscatter measurements of both stimulated Brillouin scattering and stimulated Raman scattering in gas-filled hohlraum targets that emulate conditions relevant to those in inertial confinement fusion targets

Strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas

In conventional gases and plasmas, it is known that heat fluxes are proportional to temperature gradients, with collisions between particles mediating energy flow from hotter to colder regions and the coefficient of thermal conduction given by Spitzer's theory. However, this theory breaks down in magnetized, turbulent, weakly collisional plasmas, although modifications are difficult to predict from first principles due to the complex, multiscale nature of the problem. Understanding heat transport is important in astrophysical plasmas such as those in galaxy clusters, where observed temperature profiles are explicable only in the presence of a strong suppression of heat conduction compared to Spitzer's theory. To address this problem, we have created a replica of such a system in a laser laboratory experiment. Our data show a reduction of heat transport by two orders of magnitude or more, leading to large temperature variations on small spatial scales (as is seen in cluster plasmas)

High-energy-density radiative and material properties studies using picosecond X-ray spectroscopy

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2018.Advanced experimental and theoretical techniques have been applied to outstanding challenges in high energy density science. By careful selection of laser parameters, target geometries, and spectroscopic diagnostics, it is possible to investigate the intense energy flows that are required to create hot dense matter, the plasma conditions that can be achieved, and plasma-dependent effects on atomic energy levels. The measurements presented in this thesis provide new experimental insight to the creation and measurement of unique high energy density systems and demonstrate their use for sensitive atomic properties studies in extreme conditions. Hot and dense plasma conditions were created by high-intensity laser irradiation of solid foils containing thin buried Al or Al/Fe tracer layers. The material response to intense heating was inferred from picosecond time-resolved intensity measurements of the Al Hea thermal line and broadband x-ray emission. The data show two temporally-resolved x-ray flashes when Fe is present in the layer. Fully explicit, kinetic particle-in-cell and collisional-radiative atomic model predictions reproduce these observations, connecting the two flashes with staged radial energy coupling within the target. The measurements contribute novel data for predicting the behavior of energy density inhomogeneities and understanding the response of high-energy-density systems to intense heating. The instantaneous bulk plasma conditions were inferred using picosecond time-resolved measurements of the Heα spectral line emission from the buried tracer layer. The measured Heα-to-satellite intensity ratio and spectral line width was interpreted using a non-local thermodynamic equilibrium (NLTE) atomic kinetics model to provide the plasma temperature and density as a function of time. Statistical and experimental uncertainties in the measured data are propagated to the inferred plasma conditions within a self-consistent model-dependent framework. The measurements show that high thermal temperatures exceeding 500 eV are achieved at densities within 80% of solid and demonstrate a rigorous approach for future spectroscopic temperature and density measurements essential to hot dense matter studies. Picosecond time-resolved dense plasma line shifts of the 1s2p-1s2 transition in He-like Al ions were measured as a function of the instantaneous plasma conditions. The data show spectral line shifts of 5 eV for electron densities of 1–5x10^23 cm-3 and temperatures near 300 eV. Numerical ion-sphere model calculations demonstrate broad agreement with the measured data over the full range of densities and temperatures studied, providing a new test of dense plasma theories for atomic structure and radiation transport in extreme environments. The hot dense matter systems studied in this work exhibit qualities of both the plasma and solid state. Such material resists theoretical description by the established approaches of solid state or plasma physics, emphasizing the need for experimental data to produce a detailed picture for how the atomic, radiative, and thermodynamic properties of matter are modified in extreme conditions. Contributing data toward these aims is the goal of this thesis

Laser ablation and hydrodynamic coupling in direct-drive inertial-confinement-fusion experiments

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2017.In direct-drive inertial confinement fusion, laser beams are used to ablate a capsule and implode it via the rocket effect. Time-gated images of the x-rays emitted by the capsule were used to experimentally study the hydrodynamic coupling of laser energy to the target. The mass ablation rate, the target trajectory, the laser absorption, and the conduction-zone length were simultaneously measured in spherically symmetric (1-D) implosions. These observables completely constrain the coupling models in simulations. They showed that the long-standard Spitzer-Härm thermal transport model with a time-dependent flux-limiter resulted in a significant underestimate of the mass ablation rate and the length of the conduction zone. Simulations that used models for nonlocal electron thermal transport and for cross-beam energy transfer (CBET) recently developed at the Laboratory for Laser Energetics reproduced all measurements. However, the CBET required a gain modification thatwas not explained by theory. Additional experimentswere conducted to isolate the effect of CBET on hydrodynamic coupling and quantify this modification. Laser beams incident on the equator of the target were turned off and the polar beams were repointed to illuminate the target uniformly (in a polar-drive configuration), nearly suppressing CBET at the poles and increasing its effect at the equator. Angularly resolved mass-ablation-rate and target trajectorymeasurementswere used to compare the hydrodynamic couplingwith andwithout CBET. Results on the pole were used to validate the hydrodynamic coupling without CBET in simulations, and a factor on the CBET gain was determined by matching the measured equatorial trajectories. The gain factor was necessary to reproduce the measurements in all configurations and was found to vary with the laser intensity in polar-drive implosions. This suggests that additional physics is needed in the model to fully capture the effect of CBET

Picosecond Thomson-scattering spectroscopy investigation of thermodynamics in laser-plasma amplifiers

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2019.Ultrafast electron plasma wave dynamics, Thermodynamics, and collisions are fundamental processes in laser-plasma physics that is not well understood. Historically, models have used simple approximations to describe the Thermodynamics in laser-plasma devices or artificially assumed constant plasma conditions. This thesis studies the picosecond ionization and Thermodynamics in laser-produced underdense plasmas using a novel Thomson scattering technique. The unprecedented temporal resolution of the Thomson spectra provided a measurement of collisional electron plasma waves that were modeled to extract the picosecond evolution of the electron temperature and density. This revealed a transition in the plasma-wave dynamics from an initially cold, collisional state to a quasi-stationary, collisionless state. The Thomson-scattering spectra were compared with theoretical calculations of the fluctuation spectrum using either a conventional Bhatnagar Gross Krook (BGK) collision operator or the rigorous Landau collision terms: the BGK model overestimates the electron temperature by 50% in the most-collisional conditions. These picosecond electron temperature and density measurements can be applied to laser-plasma devices that require knowledge of the rapidly evolving plasma conditions, such as a Raman plasma amplifier. These results indicate that the rapidly evolving conditions would result in a strong detuning that would limit the performance of laser-plasma amplifiers