W. Rozmus

Fast Ignitor Concept with Light Ions

A short-laser-pulse driven ion flux is examined as a fast ignitor candidate for inertial confinement fusion. Ion ranges in a hot precompressed fuel are studied. The ion energy and the corresponding intensity of a short laser pulse are estimated for the optimum ion range and ion energy density flux. It is shown that a lightion beam triggered by a few-hundreds-kJ laser at intensities of ≳1021 W/cm2 is relevant to the fast ignitor scenario.

Effect of laser-focusing conditions on propagation and monoenergetic electron production in laser-wakefield accelerators

The effect of laser-focusing conditions on the evolution of relativistic plasma waves in laser-wakefield accelerators is studied both experimentally and with particle-in-cell simulations. For short focal-length (w_{0}<lambda_{p}) interactions, beam breakup prevents stable propagation of the pulse. High field gradients lead to nonlocalized phase injection of electrons, and thus broad energy spread beams. However, for long focal-length geometries (w_{0}>lambda_{p}), a single optical filament can capture the majority of the laser energy and self-guide over distances comparable to the dephasing length, even for these short pulses (ctau approximately lambda_{p}). This allows the wakefield to evolve to the correct shape for the production of the monoenergetic electron bunches, as measured in the experiment.

Nonlinear evolution of stimulated Raman scattering in homogeneous plasmas

A model for stimulated Raman scattering (SRS) in a homogeneous plasma has been designed to account for the presence of stimulated Brillouin scattering (SBS) and the nonlinear coupling between Langmuir and ion waves described by Zakharov equations. The nonlinear evolution of electron plasma waves also includes an effective damping resulting from electron diffusion in localized Langmuir fields produced during simultaneous SRS and SBS evolutions. Numerical results based on this model show two distinct SRS behaviors. Close to ncr/4 the Langmuir collapse dominates nonlinear evolution of the instability. At lower densities low level SRS is observed for a relatively long time after which SRS is terminated as a result of ion fluctuations produced by SBS. In addition, the anomalous absorption of backscattered SRS radiation by ion fluctuations produced by the collapse is proposed as a mechanism that may explain some recent experimental observations showing a gap in the SRS spectrum.

A model of ultrashort laser pulse absorption in solid targets

A model for ultrashort laser pulse absorption and solid target heating has been developed. It combines a description of laser light absorption in the skin layer with a simple model of plasma heating. Heat wave propagation into the cold target material is the only loss mechanism balancing energy deposition due to absorption. An absorption coefficient is derived from the plasma conductivity and includes a description of physical processes responsible for collisional and collisionless skin layer absorption mechanisms. Comparison with recent femtosecond laser pulse interaction experiment data show good agreement over a wide range of pulse intensities. For laser intensities above 1016 W/cm2 plasma hydrodynamical expansion, which is neglected in our model contributes to a discrepancy between the calculated absorption and experimental data.

3-D simulation of light scattering from biological cells and cell differentiation

A 3-D code for solving the set of Maxwell equations with the finite-difference time-domain method is developed for simulating the propagation and scattering of light in biological cells under realistic conditions. The numerical techniques employed in this code include the Yee algorithm, absorbing boundary conditions, the total field/scattered field formulation, the discrete Fourier transformation, and the near-to-far field transform using the equivalent electric and magnetic currents. The code is capable of simulating light scattering from any real cells with complex internal structure at all angles, including backward scattering. The features of the scattered light patterns in different situations are studied in detail with the objective of optimizing the performance of cell diagnostics employing cytometry. A strategy for determining the optimal angle for measuring side scattered light is suggested. It is shown that cells with slight differences in their intrastructure can be distinguished with two-parameter cytometry by measuring the side scattered light at optimal angles.

Space and time behavior of parametric instabilities for a finite pump wave duration in a bounded plasma

The space and time behavior of the decay waves is computed analytically in the regime of standard parametric decay. The plasma is assumed to be homogeneous and bounded. The pump wave has a finite pulse duration. The propagation of the pump wave is taken into account, its depletion is ignored. The parametric growth is solved in terms of fluctuating initial and boundary conditions corresponding to thermal noise at equilibrium. Fluctuating source terms, representing noise emission, are accordingly retained in the coupled mode equations. The initial stage of parametric growth is investigated in detail; the time from which the asymptotic concept of absolute or convective instability applies is computed. The connection between the Manley–Rowe and flux conservation relations is discussed.

Enhanced inverse bremsstrahlung heating rates in a strong laser field

Test particle studies of electron scattering on ions in an oscillatory electromagnetic field have shown that standard theoretical assumptions of small angle collisions and phase independent orbits are incorrect for electron trajectories with drift velocities smaller than quiver velocity amplitude. This leads to significant enhancement of the electron energy gain and the inverse bremsstrahlung heating rate in strong laser fields. Nonlinear processes such as Coulomb focusing and correlated collisions of electrons being brought back to the same ion by the oscillatory field are responsible for large angle, head-on scattering processes. The statistical importance of these trajectories has been examined for mono-energetic beam-like, Maxwellian and highly anisotropic electron distribution functions. A new scaling of the inverse bremsstrahlung heating rate with drift velocity and laser intensity is discussed.

Nonlocal electron transport in laser heated plasmas

Nonlocal theory of an electron transport in laser-produced plasmas with the large ion charge and arbitrary ratio of the characteristic spatial scale length to the electron mean free path has been developed for small potential perturbations. Closure relations have been derived from the solution to the electron Fokker–Planck equation which includes inverse bremsstrahlung heating and ponderomotive effects. All electron transport coefficients and their dependence on the laser intensity have been found. An expression for the electron heat flux includes laser field and plasma flow contributions. Identification of these different sources is necessary for the unique definition of the thermal transport coefficient which is independent of the particular application. A complete derivation of the potential part of the ponderomotive force in the presence of inverse bremsstrahlung heating has been presented.

Measurements of light scattering in an integrated microfluidic waveguide cytometer.

An integrated microfluidic planar optical waveguide system for measuring light scattered from a single scatterer is described. This system is used to obtain 2D side-scatter patterns from single polystyrene microbeads in a fluidic flow. Vertical fringes in the 2D scatter patterns are used to infer the location of the 90-deg scatter (polar angle). The 2D scatter patterns are shown to be symmetrical about the azimuth angle at 90 deg. Wide-angle comparisons between the experimental scatter patterns and Mie theory simulations are shown to be in good agreement. A method based on the Fourier transform analysis of the experimental and Mie simulation scatter patterns is developed for size differentiation.

Saturation of stimulated Raman scattering by Langmuir and ion‐acoustic wave coupling

Studies of stimulated Raman scattering (SRS) based on the one‐dimensional Zakharov–Maxwell equations for homogeneous, finite plasmas are presented for a wide range of parameters relevant to current laser–plasma interaction experiments. It was found that the primary mechanism responsible for saturation of SRS is the parametric decay instability (PDI) of the resonantly driven Langmuir wave. The different spatiotemporal evolution of SRS (convective) and PDI (absolute) leads to new physics in SRS nonlinear evolution, including disruption of PDI cascade, localization of Langmuir fields, and burstlike behavior of SRS reflectivity. The asymptotic saturation levels are well approximated by a simple scaling law, which is proportional to the PDI threshold and depends on the SRS convective gain factor.