C. E. Capjack

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.

Beam propagation constants for a radial laser array

The beam quality of a radial laser array, quantified in terms of the M(2) propagation constant, is determined as a function of array element configuration. A lower bound on array M(2) is estimated for both phase-locked and nonphase-locked conditions. It is shown that, to achieve near-unity M(2) array, either aperture filling or spatial filtering is required in addition to phase locking. An aperture-filling method suitable for radial arrays of CO(2) slab lasers is presented.

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.

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.

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.

Multichannel slab discharge for CO2 laser excitation

Experimental results on a unique multichannel slab‐type CO2 discharge system are presented. The interdigital discharge geometry incorporates both large‐area and multibeam laser array concepts into a single, compact package. Small signal gain and saturation intensity values indicate that this structure is well suited for use in a CO2 laser.

Heat transport and electron distribution function in laser produced plasmas with hot spots

Using Fokker–Planck and particle-in-cell simulations, the evolution of a single hot spot and multiple hot spot systems have been studied in laser produced plasmas. A practical formula for nonlocal heat flux has been derived as a generalized expression of a nonlocal linear approach [Bychenkov et al., Phys. Rev. Lett. 75, 4405 (1995)] and is tested in simulations. The electron distribution function is studied at different spatial locations with respect to a localized heating source. The electron distribution function displays several non-Maxwellian features which depend on the interplay between the effects of inverse bremsstrahlung heating and nonlocal transport. In particular, significant high-energy tails are found. They may have impact on the behavior of parametric instabilities in nonuniformly heated laser plasma.

Interaction of crossed laser beams with plasmas

The parametric interaction of two crossed collimated laser beams with ion plasma modes has been studied. The underlying process is a density grating that is created by the two laser beams. The Bragg diffraction that is produced enhances forward stimulated Brillouin scattering (SBS) which results in a time dependent energy exchange between the two beams. A diversity of other SBS processes which depend on the symmetry of driving laser beams are also discussed.

Phase-locking phenomena in a radial multislot CO 2 laser array

An investigation of phase-locking phenomena in a multichannel, slab-type electrode, CO2 laser array is presented. External and self-phase-locking percentages of 90% and 60%, respectively, have been demonstrated. Azimuthally distributed radial injection within the central region of this gain geometry has been identified as the principal phase-locking mechanism. Fundamental optical modes are initiated simultaneously within each individual discharge slot via injection from this central core-oscillator region.

Electron distribution function in laser heated plasmas

A new electron distribution function has been found in laser heated homogeneous plasmas by an analytical solution to the kinetic equation and by particle simulations. The basic kinetic model describes inverse bremsstrahlung absorption and electron–electron collisions. The non-Maxwellian distribution function is comprised of a super-Gaussian bulk of slow electrons and a Maxwellian tail of energetic particles. The tails are heated due to electron–electron collisions and energy redistribution between superthermal particles and light absorbing slow electrons from the bulk of the distribution function. A practical fit is proposed to the new electron distribution function. Changes to the linear Landau damping of electron plasma waves are discussed. The first evidence for the existence of non-Maxwellian distribution functions has been found in the interpretation, which includes the new distribution function, of the Thomson scattering spectra in gold plasmas [Glenzer et al., Phys. Rev. Lett. 82, 97 (1999)].