Biomedical subjects

W Rozmus

Publications and source records attributed to W Rozmus.

At least 19 recordsLinked to original sources

PubMed2004

Self-organization of a plasma due to 3D evolution of the Weibel instability.

The nonlinear evolution of the thermal Weibel instability is studied by using three-dimensional particle-in-cell simulations. After a fast saturation due to a reduction in the temperature anisotropy, the instability evolves to a quasistationary state which includes a single mode long wavelength helical magnetic field and a finite degree of temperature anisotropy. The nonlinear stability of this state is explained by periodic variations of the temperature anisotropy axis. At long time scales the magnetic field, wave number, and temperature anisotropy slowly evolve to the decreasing magnitudes.

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PubMed2004

Kinetic susceptibility and transport theory of collisional plasmas.

A system of nonlocal electron transport equations for electrostatic perturbations in (omega,k) space in a high-Z plasma is derived from the Fokker-Planck equation for arbitrary relations between the time, space, and collisionality scales. The closed scheme for obtaining the longitudinal plasma susceptibility epsilon(omega,k) in the entire (omega,k) plane is proposed. Regions in the (omega,k) plane have been mapped for problems such as the relaxation of the local temperature enhancement with a time-dependent heat conductivity. The electron dielectric permittivity has been calculated over the entire range of parameters, including the transition region between Vlasov and Fokker-Planck equation solutions.

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PubMed2004

Effect of nonlocal transport on heat-wave propagation.

We present the first direct measurements of spatially and temporally resolved temperature and density profiles produced by nonlocal transport in a laser plasma. Absolutely calibrated measurements have been performed by Rayleigh scattering and by resolving the ion-acoustic wave spectra across the plasma volume with Thomson scattering. We find that the electron temperature and density profiles disagree with flux-limited models, but are consistent with nonlocal transport modeling.

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PubMed2004

Analysis of cellular structure by light scattering measurements in a new cytometer design based on a liquid-core waveguide.

The results of applying a novel microfluidic optical cytometer to generate and observe the light scattered from biological cells over a wide range of angles are presented. This cytometer incorporates a waveguide that increases the intensity of the scattered light to the extent that an inexpensive digital camera can be used to detect the light over a large solid angle. This device was applied to yeast cells and latex beads and experimental data were compared with the results of a finite difference time-domain (FDTD) method of simulation. The simulated scattering patterns were calculated from reported values of optical parameters and are in good qualitative agreement with experiment. It is demonstrated that this system could be used to acquire information on the microstructure and potentially the nanostructure of cells.

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PubMed2003

Theoretical model of x-ray scattering as a dense matter probe.

We present analytical expressions for the dynamic structure factor, or form factor S(k,omega), which is the quantity describing the x-ray cross section from a dense plasma or a simple liquid. Our results, based on the random phase approximation for the treatment on the charged particle coupling, can be applied to describe scattering from either weakly coupled classical plasmas or degenerate electron liquids. Our form factor correctly reproduces the Compton energy down-shift and the known Fermi-Dirac electron velocity distribution for S(k,omega) in the case of a cold degenerate plasma. The usual concept of scattering parameter is also reinterpreted for the degenerate case in order to include the effect of the Thomas-Fermi screening. The results shown in this work can be applied to interpreting x-ray scattering in warm dense plasmas occurring in inertial confinement fusion experiments or for the modeling of solid density matter found in the interior of planets.

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PubMed2002

Anomalous absorption of high-energy green laser light in high- z plasmas.

We observe strong anomalous absorption of green laser light in mm-scale high-temperature gold plasmas. Both the laser light absorption and the resulting increase of the electron temperature, which was measured independently with Thomson scattering, have been successfully modeled by including enhanced collisions due to heat-flux driven ion acoustic fluctuations. Calculations that include only inverse bremsstrahlung significantly underestimate the experimental laser absorption and the electron temperature.

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PubMed2001

Nonlinear propagation of a randomized laser beam through an expanding plasma.

We present simulations of the interaction of a random phase plate laser beam with an underdense, expanding plasma for conditions typical of recent LULI experiments. We use a new code that describes the paraxial propagation of the laser, accounting for the nonlinear evolution of the plasma in an isothermal fluid description with weakly collisional electrons. The transmitted light, in excellent agreement with experiment, is shown to be strongly redshifted as a result of self-phase modulation due to self-focusing.

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PubMed2000

Enhanced forward scattering in the case of two crossed laser beams interacting with a plasma

The nonlinear enhancement of large-angle forward scattering of two identical laser beams propagating in a preformed plasma has been observed experimentally. The spectral analysis of the forward-scattered light shows two components, one which is unshifted with respect to the initial laser light frequency, and the other which is redshifted by a few angstroms. The redshifted component is found to be strongly enhanced in the case of crossed beam interaction in comparison with that of one beam illumination. Two-dimensional numerical simulations show that this enhancement is due to large-angle forward stimulated Brillouin scattering in which each beam serves as seed for the forward scattering of the other.

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PubMed2000

Resonant instability of laser filaments in a plasma

The stability of nonlinear laser light filaments in a homogeneous isothermal plasma with respect to coupled electromagnetic and density perturbations is examined. In addition to the previously known modulational instability of a trapped electromagnetic mode, a new fast growing resonant instability is found. It corresponds to the growth of an excited eigenmode in the waveguide formed by the filament density depletion, the associated density response being supersonic and transversally localized. The evolution of the instability is illustrated by numerical simulations in two and three spatial dimensions.

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PubMed1996