22 research outputs found
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Laser irradiation of parylene disks with a 1.06 m laser
Parylene disks supported on glass stalks were irradiated with 1.06 m wavelength laser light pulses focused to flux intensities in the range from 10 to 10 W/cm. According to photodiode measurements the fraction of laser light absorbed, 25 to 50 percent, increased slightly as the laser intensity was increased. However, box calorimeter measurements implied that the fraction absorbed was approximately 30 percent and insensitive to irradiation intensity. Some x-ray spectra are discussed. (MOW
Narrative inquiry into (re)imagining alternative schools: a case study of Kevin Gonzales.
Although there are many alternative schools that strive for the successful education for their students, negative images of alternative schools persist. While some alternative schools are viewed as âidealistic havens,â many are viewed as âdumping grounds,â or âjuvenile detention centers.â Employing narrative inquiry, this article interrogates how a student, Kevin Gonzales, experiences his alternative education and raises questions about the role of alternative schools. Kevin Gonzalesâs story is presented in a literary form of biographical journal to provide a âmetaphoric loftâ that helps us imagine other students like Kevin. This, in turn, provokes us to examine our current educational practice, and to (re)imagine ways in which alternative education can provide the best possible educational experiences for disenfranchised students who are increasingly underserved by the public education system
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Stimulated Raman scattering in large plasmas
Stimulated Raman scattering is of concern to laser fusion since it can create a hot electron environment which can increase the difficulty of achieving high final fuel densities. In earlier experiments with one micron laser light, the energy measured in Raman-scattered light has been insignificant. But these experiments were done with, at most, about 100 joules of laser energy. The Raman instability has a high threshold which also requires a large plasma to be irradiated with a large diameter spot. Only with a long interaction length can the Raman-scattered light wave convectively grow to a large amplitude, and only in recent long pulse, high energy experiments (4000 joules in 2 ns) at the Shiva laser facility have we observed as much as several percent of the laser light to be Raman-scattered. We find that the Raman instability has a much lower intensity threshold for longer laser pulselength and larger laser spot size on a solid target
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Laser-plasma instabilities in large plasmas irradiated at 1. 06. mu. m and the wavelength scaling of the absorption, hot-electron production, ablation pressure for 1. 06-, 0. 53-, and 0. 35-. mu. m light
Plasmas were created by exploding 7000 A thick CH foils at the irradiation conditions: 1.064 ..mu..m, 3 kJ, 2.5 x 10/sup 15/ W/cm/sup 2/, 900 ps FWHM, 400 ..mu..m spot diameter. Ten percent of the laser energy appeared as Raman light and 0.04% as 3..omega../sub 0//2 light. The 3..omega../sub 0//2 light and the 30-70 keV X rays occurred simultaneouly at t=-120/sup +50//sub -//sub 200/ psec and lasted only 300 psec FWHM. The foil was calculated to explode to n/sub c/4 at t=-300 psec. The spectrum and angular distribution of the Raman light were also measured. Time-resolved spectral measurements have been made in experiments with 5320 A laser light in a 600-900 psec FWHM pulse. The scaling of the 3..omega../sub 0//2 light with both the laser spot size and pulse length has been studied
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Optical measurements of the. omega. /sub 0//2 and 3/2. omega. /sub 0/ light from 1. 064. mu. m-irradiated plasmas
The spectrum and absolute level of both the ..omega../sub 0//2 and 3/2..omega../sub 0/ light have been measured at Argus over a wide range of intensities, spot sizes and energies (3 to 500 joules at 700ps). We will also present measurements at Shiva in which the spectrum, time dependence, and angular distribution of the Raman-scattered light have been measured. As much as several percent in some of these experiments. A particularly interesting experiment was performed at Argus in which a 500A thick Formvar (CH) foil was irradiated with 90 joules in 700ps. The spot size was 200 ..mu..m with a diverging f/2 beam. A calorimeter measured 0.1 J/sr in Raman light in the backwards direction. Discrete InAs detectors looking at the amount of Raman light at 1.9 and 2.0 ..mu..m detected about as much in the forward direction as the backward direction
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Brillouin scatter in laser-produced plasmas
The absorption of intense laser light is found to be reduced when targets are irradiated by 1.06 ..mu..m light with long pulse widths (150-400 psec) and large focal spots (100-250 ..mu..m). Estimates of Brillouin scatter which account for the finite heat capacity of the underdense plasma predict this reduction. Spectra of the back reflected light show red shifts indicative of Brillouin scattering
Methods for fabricating arrays of holes using interference lithography
Optical interference lithography offers a robust patterning technology capable of achieving high spatial resolution over extremely large field sizes ( {approx}1 m ). Here, we compare two different approaches for fabricating arrays of holes using interferometric techniques. We show that by applying an image reversal process to standard two-beam interference lithography, arrays of high aspect ratio holes can be generated. This process scales to submicron periods and allows holes as small as 0.1 micron to be patterned. Next, we present an analysis of the multiple-beam approach for patterning holes. We demonstrate that while the formation of higher contrast intensity patterns is possible by interfering four or more beams, the shape and modulation depth of such patterns are inherently sensitive to relative phase variations
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Sources of hot electrons in laser-plasma interaction with emphasis on Raman and turbulence absorption
Heating targets with high power lasers results in a sizable fraction of the absorbed energy going into electrons of temperature much greater than thermal which can pre-heat the pellet core and accelerate fast ion blowoff which results in poor momentum transfer and hence poor compression efficiency. The present emphasis is to build lasers of higher frequency, ..omega../sub 0/, which at the same W/cm/sup 2/ results in more absorption into cooler electrons. Two physical reasons are that the laser can propagate to a higher electron density, n, infinity..omega../sub 0//sup 2/ resulting in more collisional inverse bremsstrahlung absorption proportional to n, and because the hot temperatures from some plasma absorption processes increase as the oscillatory velocity of an electron in the laser electric field v/sub 0//c = eE/(m/sub e/..omega../sub 0/). The heated electron temperatures from other plasma processes (Raman for example approx.(m/sub e//2)v/sup 2//sub phase/ and the higher laser frequency helps by increasing the competing collisional absorption and decreasing the Raman gain
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Energy balance measurements for Shiva
The Lawrence Livermore Laboratory Shiva laser-target interaction facility is designed for experiments at 20 to 30 terawatts. At this power level there will be larger fluxes of neutrons, x-rays, electrons and ions than have been previously measured. The measurements of energy converted into the various reaction products is crucial both in target design and performance analysis of the actual experiment. The total energy absorbed is measured by a box calorimeter surrounding the target except for beam input holes. This measurement prevents the use of other diagnostics, so for normal operation an energy balance module was designed for location on ports on the Shiva target chamber. This module monitors the energy in scattered light at 10640 A and 5320 A or 7118 A. It also contains a faraday cup and plasma and x-ray calorimeters. The distribution of energy in scattered light, plasma and x-rays will be mapped by 58 such modules