Spatiotemporal control of two-color terahertz generation

A laser pulse composed of a fundamental and properly phased second harmonic exhibits an asymmetric electric field that can drive a time-dependent current of photoionized electrons. The current produces an ultrashort burst of terahertz (THz) radiation. When driven by a conventional laser pulse, the THz radiation is emitted into a cone with an angle determined by the dispersion of the medium. Here we demonstrate that the programmable-velocity intensity peak of a spatiotemporally structured, two-color laser pulse can be used to control the emission angle, focal spot, and spectrum of the THz radiation. Of particular interest for applications, a structured pulse with a subluminal intensity peak can drive highly focusable, on-axis THz radiation

Ion and Electron Acoustic Bursts during Anti-Parallel Magnetic Reconnection Driven by Lasers

Magnetic reconnection converts magnetic energy into thermal and kinetic energy in plasma. Among numerous candidate mechanisms, ion acoustic instabilities driven by the relative drift between ions and electrons, or equivalently electric current, have been suggested to play a critical role in dissipating magnetic energy in collisionless plasmas. However, their existence and effectiveness during reconnection have not been well understood due to ion Landau damping and difficulties in resolving the Debye length scale in the laboratory. Here we report a sudden onset of ion acoustic bursts measured by collective Thomson scattering in the exhaust of anti-parallel magnetically driven reconnection using high-power lasers. The ion acoustic bursts are followed by electron acoustic bursts with electron heating and bulk acceleration. We reproduce these observations with 1D and 2D particle-in-cell simulations in which electron outflow jet drives ion-acoustic instabilities, forming double layers. These layers induce electron two-stream instabilities that generate electron acoustic bursts and energize electrons. Our results demonstrate the importance of ion and electron acoustic dynamics during reconnection when ion Landau damping is ineffective, a condition applicable to a range of astrophysical plasmas including near-Earth space, stellar flares, and black hole accretion engines

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

Optimization of plasma amplifiers

Plasma amplifiers offer a route to side-step limitations on chirped pulse amplification and generate laser pulses at the power frontier. They compress long pulses by transferring energy to a shorter pulse via the Raman or Brillouin instabilities. We present an extensive kinetic numerical study of the three-dimensional parameter space for the Raman case. Further particle-in-cell simulations find the optimal seed pulse parameters for experimentally relevant constraints. The high-efficiency self-similar behavior is observed only for seeds shorter than the linear Raman growth time. A test case similar to an upcoming experiment at the Laboratory for Laser Energetics is found to maintain good transverse coherence and high-energy efficiency. Effective compression of a 10 kJ , nanosecond-long driver pulse is also demonstrated in a 15-cm-long amplifier

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

Direct measurements of nonlocal heat flux in laser-produced coronal plasmas using Thomson scattering from electron-plasma waves

Thesis (Ph. D.)--University of Rochester. Department of Mechanical Engineering, 2018.Thermal transport is a fundamental process in plasma physics that is not well understood. Historically, local models have been used to describe the heat flux in plasmas but artificially limit the flux at even modest temperature gradients. This Thesis studies electron thermal transport in laser-produced coronal plasmas using a novel Thomson scattering technique. Thomson scattering is sensitive to changes in the electron distribution function caused by heat flux. The experiments show that nonlocal effects must be included in regions where the plasma was not collisional enough for classical theory to be valid. Vlasov-Fokker-Planck simulations self consistently calculated the electron distribution functions used to reproduce the measured Thomson scattering spectra and to determine the heat flux. Measured heat flux values were up to 40% smaller than classical values inferred from the measured plasma conditions in this region. This is the first direct measurement of nonlocal heat flux in plasmas. In the opposite limit, classical theory matched the observed Thomson scattering data. Multigroup nonlocal simulations overestimated the measured heat flux

The multiple-beam two-plasmon-decay instability

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2016.Recent developments in experimental techniques and simulations have led to an improved understanding of the nonlinear evolution of the two-plasmon-decay (TPD) instability relevant to direct-drive inertial confinement fusion (ICF). Experiments on the OMEGA laser used ultraviolet Thomson scattering to observe TPD electron plasma waves driven by multiple laser beams in a variety of experimental configurations. The experiments were modeled in 3-D using a hybrid code (LPSE) that combines a pseudospectral wave solver with a particle tracker to self-consistently calculate Landau damping. Thomson-scattering measurements of several different plasma wavevectors show a highly anisotropic turbulent TPD driven electron-plasma-wave spectrum and are well reproduced by LPSE simulations. Direct comparison between simulated and measured hot-electron spectra indicate that the hybrid-particle model correctly captures the hot-electron generation mechanism associated with the nonlinear evolution of the TPD instability