152 research outputs found
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Measurements of direct drive laser imprint in thin foils by XUV radiography using an X-ray laser backlighter
In direct drive inertial confinement fusion, the residual speckle pattern remaining after beam smoothing plays an important role in the seeding of instabilities at the ablation front. We have used an x-ray laser as an XUV backlighter to characterize the imprinted modulation in thin foils for smoothing by random phase plate and spectral dispersion at both 0.35 pm and 0.53 pm irradiation, and induced spatial incoherence at 0.53 pm irradiation. We also demonstrate measurements of the modulation due to a single mode optical imprint generated by a narrow slit interference pattern, and modification of the imprint with a superposed smooth irradiation to study time dependence of the imprinting process. 8 refs., 10 figs
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XUV radiography measurements of direct drive imprint in thin aluminum foils using a Ge x-ray laser on Vulcan
One key aspect for high gain direct drive inertial confinement fusion is the imprint of perturbations in the outer surface of a capsule due to nonuniformities in the direct laser illumination of the capsule. Direct drive implosions are achieved by uniformly irradiating the outside surface of a hollow spherical capsule that contains a layer of fusionable D-T on its inner surface. The intensity of laser irradiation is down with a low intensity ``foot`` at 10{sup 13} W/cm{sup 2} for several nanoseconds before it builds up to more than 10{sup 15} W/cm{sup 2} during the main drive portion of the pulse. Laser ablation of the capsule surface produces a high pressure that accelerates the capsule shell radially inward in a spherical implosion. During this acceleration, perturbations due to surface roughness and due to imprint from spatial nonuniformities in the laser irradiation undergo Rayleigh-Taylor growth, potentially severely degrading performance. Our interest is in studying the imprint process and subsequent Rayleigh-Taylor growth of perturbations in a foil target that is irradiated by a low intensity laser speckle pattern. Previous experiments have been done to study laser imprint with an x-ray laser backlighter at the Nova laser using 0.35 micrometer laser irradiation of a 3 micrometer Si foil. In these experiments we irradiated a 2 micrometer thick Al foil with 0.53 micrometer laser light at 2-8 {times} 10{sup 12} W/cm{sup 2} using the Vulcan laser. We used a Ge x-ray laser as an XUV backlighter to measure the modulation in optical depth of the foil on a CCD during the initial imprint phase and after Rayleigh-Taylor growth with different laser smoothing schemes. 4 refs., 6 figs
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XUV probing of laser imprint in a thin foil using an x-ray laser backlighter
For direct drive ICF, a capsule is imploded by directly illuminating the surface with laser light. Beam smoothing and uniformity of illumination affect the seeding of instabilities at the ablation front. We have developed a technique for studying the imprint of a laser beam on a thin foil using an x-ray laser as an XUV backlighter. We use multilayer XUV optics to relay the x-ray laser onto the directly driven foil, and then to image the foil modulation onto a CCD camera. This technique allows us to measure small fractional variations in the foil thickness. We have measured the modulation due to imprint from a low intensity 0.35 pm drive beam incident on a 3 {mu}m Si foil using an yttrium x-ray laser on Nova. We present results from a similar technique to measure the imprinted modulation due to a low intensity 0.53 {mu}m drive beam incident on a 2 {mu}m Al foil using a germanium x-ray laser at the Vulcan facility
Laser plasma diagnostics of dense plasmas
The authors describe several experiments on Nova that use laser-produced plasmas to generate x-rays capable of backlighting dense, cold plasmas (p {approximately} 1--3 gm/cm{sup 3}, kT {approximately} 5--10 eV, and areal density {rho}{ell}{approximately} 0.01--0.05 g/cm{sup 2}). The x-rays used vary over a wide range of h{nu}, from 80 eV (X-ray laser) to 9 keV. This allows probing of plasmas relevant to many hydrodynamic experiments. Typical diagnostics are 100 ps pinhole framing cameras for a long pulse backlighter and a time-integrated CCD camera for a short pulse backlighter
Deformation Substructures and Their Transitions in Laser Shock–Compressed Copper-Aluminum Alloys
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X-ray laser measurements of direct drive imprint on vulcan
High gain direct drive inertial confinement fusion requires very uniform irradiation of a hollow spherical shell with a layer of fusionable deuterium and tritium on its inner surface. The intensity of laser irradiation builds up in several nanoseconds from an initial `foot` at {approximately}10{sup 13} W/cm{sup 2} to more than 10{sup 15} W/cm{sup 2} during the main drive pulse. Laser ablation of the capsule surface produces a high pressure, accelerating the shell radially inward, and resulting in Rayleigh-Taylor (R-T) growth of surface perturbations originating from both the initial surface roughness of the capsule and from imprint of spatial non- uniformities in the laser drive intensity early in the laser pulse. The uniformity of illumination on a direct drive implosion capsule is determined on a large scale by the multi-beam irradiation geometry, and on a small scale by beam smoothing techniques. By using a large number of beams (such as the 60 beams of the Omega laser or 48 beam clusters for the NIF), large scale non-uniformities due to the overlap of the laser focal spots are adequately reduced. Random phase plates (RPPs) are introduced to smooth the individual beam focal spots. The spatial intensity variations of the individual beam speckle patterns may be smoothed by spectral dispersion (SD) with induced spatial incoherence (ISI) or by using partially coherent light. We performed experiments to study the imprint under conditions simulating the low intensity foot of the pulse on an ignition target, such as designed for the NIF. We used a 0.53 micrometer laser wavelength, and considered the imprint in thin Al foils due to both a broadband distribution of modes such as those in smoothed speckle patterns, and a single mode optical intensity variation. We characterized the laser imprint using a Ge x-ray laser and multilayer imaging optics, as described previously. In this paper we summarize and compare the multiple and single mode imprinting results. 9 refs., 7 figs
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Measurements of laser imprint by XUV radiography using an x-ray laser
We have developed a technique for studying the imprint of a laser beam on a thin foil using an x-ray laser as an XUV backlighter and XUV multilayer optics. This technique allows us to measure small fractional variations in the foil thickness due to hydrodynamics imprinted by direct laser irradiation. We present results of imprinted modulation and growth due to a low intensity 0.53 {mu}m drive beam incident on a 2 {mu}m Al foil using a germanium x-ray laser at the Vulcan facility. We present measurements of the modulation due to static RPP, SSD smoothed, and ISI smoothed speckle patterns at 0.53 {mu}m irradiation
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X-ray framing cameras for > 5 keV imaging
Recent and proposed improvements in spatial resolution, temporal resolution, contrast, and detection efficiency for x-ray framing cameras are discussed in light of present and future laser-plasma diagnostic needs. In particular, improvements in image contrast above hard x-ray background levels is demonstrated by using high aspect ratio tapered pinholes
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