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W Seka

Publications and source records attributed to W Seka.

At least 19 recordsLinked to original sources
PubMed2005

Intensity limits for propagation of 0.527 microm laser beams through large-scale-length plasmas for inertial confinement fusion.

We have established the intensity limits for propagation of a frequency-doubled (2omega, 527 nm) high intensity interaction beam through an underdense large-scale-length plasma. We observe good beam transmission at laser intensities at or below 2x10(14) W/cm(2) and a strong reduction at intensities up to 10(15) W/cm(2) due to the onset of parametric scattering instabilities. We show that temporal beam smoothing by spectral dispersion allows a factor of 2 higher intensities while keeping the beam spray constant, which establishes frequency-doubled light as an option for ignition and burn in inertial confinement fusion experiments.

Journal Article↗
PubMed2004

Effects of nonuniform illumination on implosion asymmetry in direct-drive inertial confinement fusion.

Target areal density (rhoR) asymmetries in OMEGA direct-drive spherical implosions are studied. The rms variation / for low-mode-number structure is approximately proportional to the rms variation of on-target laser intensity / with an amplification factor of approximately 1/2(C(r)-1), where C(r) is the capsule convergence ratio. This result has critical implications for future work on the National Ignition Facility as well as OMEGA.

Journal Article↗
PubMed2004

Dependence of shell mix on feedthrough in direct drive inertial confinement fusion.

The mixing of cold, high-density shell plasma with the low-density, hot spot plasma by the Rayleigh-Taylor instability in inertial confinement fusion is experimentally shown to correlate with the calculated perturbation feedthrough from the ablation surface to the inner shell surface. A fourfold decrease in the density of shell material in the mix region of direct drive implosions of gas filled spherical plastic shells having predicted convergence ratios approximately 15 was observed when laser imprint levels were reduced and the initial shell was thicker, corresponding to a reduction in the feedthrough rms level by a factor of 6. Shell mix is also shown to limit the spherical compression of the implosion.

Journal Article↗
PubMed2003

Multibeam effects on fast-electron generation from two-plasmon-decay instability.

Experiments with multiple laser beams have been carried out in both spherical and planar geometry to study two-plasmon-decay instability, the predominant source of suprathermal electrons in direct-drive inertial confinement fusion experiments. These electrons are observed using the hard x rays generated through electron-target interactions. The experiments show for the first time that the total overlapped intensity governs the scaling of the suprathermal-electron generation regardless of the number of overlapped beams, in contrast to conventional theories that are based on the single-beam approximation.

Journal Article↗
PubMed2002

Observation of saturation of energy transfer between copropagating beams in a flowing plasma.

Experiments demonstrate energy and power transfer between copropagating, same frequency, beams crossing at a small angle in a plasma with a Mach 1 flow. The process is interpreted as amplification of the low intensity probe beam by the stimulated scatter of the high intensity pump beam. The observed probe amplification increases slowly with pump intensity and decreases with probe intensity, indicative of saturation limiting the energy and power transfer due to ion-wave nonlinearities and localized pump depletion. The results are consistent with numerical modeling including ion-wave nonlinearities.

Journal Article↗
PubMed2002

Multibeam stimulated brillouin scattering from hot, solid-target plasmas.

We report on multibeam, laser-plasma interaction experiments in plasmas relevant to future direct-drive-ignition experiments. Six interaction beams are incident on preformed plasmas containing critical density. Stimulated Brillouin scattering (SBS) shows strong evidence of electromagnetic wave seeding of side- and backscattering. The data are also consistent with shared ion waves driven by the six symmetrically arranged interaction beams. Early SBS quenching is observed and attributed to the hydrodynamics of the plasma.

Journal Article↗
PubMed2002

Shell mix in the compressed core of spherical implosions.

The Rayleigh-Taylor instability in its highly nonlinear, turbulent stage causes atomic-scale mixing of the shell material with the fuel in the compressed core of inertial-confinement fusion targets. The density of shell material mixed into the outer core of direct-drive plastic-shell spherical-target implosions on the 60-beam, OMEGA laser system is estimated to be 3.4(+/-1.2) g/cm(3) from time-resolved x-ray spectroscopy, charged-particle spectroscopy, and core x-ray images. The estimated fuel density, 3.6(+/-1) g/cm(3), accounts for only approximately 50% of the neutron-burn-averaged electron density, n(e)=2.2(+/-0.4)x10(24) cm(-3).

Journal Article↗
PubMed1998

CO2 laser inhibitor of artificial caries-like lesion progression in dental enamel.

Several studies during the last 30 years have demonstrated the potential of laser pre-treatment of enamel or tooth roots to inhibit subsequent acid-induced dissolution or artificial caries-like challenge in the laboratory. The overall objective of ongoing studies in our laboratories is to determine, systematically, the optimum sets of parameters for carbon dioxide laser irradiation that will potentially effectively inhibit dental caries in enamel and tooth roots. The aim of the present study was to examine the roles of wavelength and fluence in the prevention of caries progression in vitro in enamel by means of a pH-cycling model. The hypothesis to be tested was that the highly absorbed 9.3- and 9.6-microm wavelengths would be efficiently converted to heat, creating a temperature sufficiently high to reduce the acid-reactivity of the mineral and inhibit caries-like lesion progression in dental enamel. One hundred and sixty caries-free tooth crowns were cleaned and varnished with acid-resistant varnish, leaving one exposed window of enamel. Twelve groups of 10 enamel samples were irradiated in their individual windows by one of the four wavelengths (9.3, 9.6, 10.3, or 10.6 microm) of a tunable CO2 laser. Energy per pulse was 25, 50, 100, 200, or 250 mJ (25 pulses). Repetition rate was 10 Hz, and beam diameter was 1.6 mm. Fluence conditions of 1 to 12.5 J/cm2 per pulse were produced. All teeth, including 40 non-irradiated controls, were subjected to pH-cycling to produce artificial caries-like lesions. Results were assessed by cross-sectional microhardness testing. Inhibition of caries progression of from 40% to 85% was achieved over the range of laser conditions tested. At 9.3 and 9.6 microm, 25 pulses at absorbed fluences of 1 to 3 J/cm2 produced inhibition on the order of 70% with minimal subsurface temperature elevation (< 1 degree C at 2 mm depth), comparable with inhibition produced in this model with daily fluoride dentifrice treatments. Safety and efficacy studies will be required in animals and humans before these promising laboratory results can be applied in clinical practice.

Absorption↗
PubMed1997

Permanent and transient changes in the reflectance of CO2 laser-irradiated dental hard tissues at lambda = 9.3, 9.6, 10.3, and 10.6 microns and at fluences of 1-20 J/cm2.

BACKGROUND AND OBJECTIVE: Effective use of lasers for preventive dental treatments requires accurate knowledge of the amount and distribution of laser energy deposited during irradiation. At CO2 wavelengths, the reflection losses are considerable and reduce the laser energy absorbed by the tissue surface. STUDY DESIGN/MATERIALS AND METHODS: The specular and diffuse reflectance of enamel and dentin were measured at the 10.6-, 10.3-, 9.6-, and 9.3-microns wavelengths of the CO2 laser. Changes in reflectance during and after laser irradiation were investigated. RESULTS: The low-fluence reflectance (< 1 J/cm2) of calcified dental tissues at CO2 wavelengths varies between 9% and 50%. Permanent and transient changes in the reflectance are induced at higher irradiation intensities. CONCLUSION: These changes resulted in increased energy coupling during irradiation.

Animals↗
PubMed1995

Light deposition in dental hard tissue and simulated thermal response.

Near-IR (approximately 1 micron) lasers are presently used for a variety of intra-oral applications including dental hard tissue ablation, although the light intensity distribution and subsequent heating of the hard tissue are still poorly understood. This paper presents a detailed numerical study of the scattered light intensity distribution along with the corresponding predicted thermal response. The calculations are based on recently published scattering and absorption data for dental hard tissue around 1 micron. Our simulations indicate strongly enhanced energy deposition and concomitant heating near the dentino-enamel junction (DEJ), mainly due to the higher absorption in dentin. We predict from 10 to 20 times higher internal temperatures near the DEJ compared with the surface. For example, for 50-ms pulses of 5-J energy on a 3-mm-diameter spot (approximately 1 kW/cm2 or approximately 50 J/cm2), one can expect internal temperatures near the DEJ in excess of 100 degrees C. Elevated temperatures are predicted to extend far into the dentin, endangering the vitality of the pulp several millimeters below the surface. Our results are compared with published experimental data taken under similar conditions and are found to be in good general agreement. The results of this study do not contradict recently reported ablation of dentin with Nd:YAG laser radiation by contact fiber probes. In this case, the irradiation intensities are 3 to 4 orders of magnitude higher, so plasma formation and plasma shielding of the interior of the tooth are likely.

Absorption↗
PubMed1995

Scanning electron microscope observations of CO2 laser effects on dental enamel.

Studies of the effects of carbon dioxide (CO2) lasers on dental enamel have demonstrated that surface changes can be produced at low fluences (< 10 J/cm2) if wavelengths are used which are efficiently absorbed by the hard tissues. In this study, scanning electron microscopy (SEM) was used to characterize the wavelength dependence of surface changes in dental enamel after exposure to an extensive range of CO2 laser conditions. Bovine and human enamel were irradiated by a tunable, pulsed CO2 laser (9.3, 9.6, 10.3, 10.6 microns), with 5, 25, or 100 pulses, at absorbed fluences of 2, 5, 10, or 20 J/cm2, and pulse widths of 50, 100, 200, 500 microseconds. SEM micrographs revealed evidence of melting, crystal fusion, and exfoliation in a wavelength-dependent manner. Crystal fusion occurred at absorbed fluences as low as 5 J/cm2 per pulse at 9.3, 9.6, and 10.3 microns, in contrast to no crystal fusion at 10.6 microns (< or = 20 J/cm2). Longer pulses at constant fluence conditions decreased the extent of surface melting and crystal fusion. The total number of laser pulses delivered to the tissue did not significantly affect surface changes as long as a minimum of 5 to 10 pulses was used. Within the four easily accessible wavelengths of the CO2 laser, there are dramatic differences in the observed surface changes of dental hard tissue.

Animals↗