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Modeling of bubble dynamics in relation to medical applications

Description: In various pulsed-laser medical applications, strong stress transients can be generated in advance of vapor bubble formation. To better understand the evolution of stress transients and subsequent formation of vapor bubbles, two-dimensional simulations are presented in channel or cylindrical geometry with the LATIS (LAser TISsue) computer code. Differences with one-dimensional modeling are explored, and simulated experimental conditions for vapor bubble generation are presented and compared with data. 22 refs., 8 figs.
Date: March 12, 1997
Creator: Amendt, P.A.; London, R.A. & Strauss, M.
Partner: UNT Libraries Government Documents Department

Laser Plasma instability reduction by coherence disruption

Description: The saturation levels of stimulated scattering of intense laser light in plasmas and techniques to reduce these levels are of great interest. A simple model is used to highlight the dependence of the reflectivity on the coherence length for the density fluctuations producing the scattering. Sometimes the coherence lengths can be determined nonlinearly. For NIF hohlraum plasmas, a reduction in the coherence lengths might be engineered in several ways. Finally, electron trapping in ion sound waves is briefly examined as a potentially important effect for the saturation of stimulated Brillouin scattering.
Date: April 19, 2006
Creator: Kruer, W l; Amendt, P A; Meezan, N & Suter, L J
Partner: UNT Libraries Government Documents Department

Extended Rayleigh model of bubble evolution with material strength compared to detailed dynamic simulations

Description: The validity of an extended Rayleigh model for laser generated bubbles in soft tissue is examined. This model includes surface tension, viscosity, a realistic water equation of state, material strength and failure, stress wave emission, and linear growth of interface instabilities. It is compared to dynamic simulations using LATIS, which include stress wave propagation, water equation of state, material strength and failure, and viscosity. The model and the simulations are compared using 1-D spherical geometry with bubble in center and a 2-D cylindrical geometry of a laser fiber in water with a bubble formed at the end of the fiber. The model executes over 300x faster on computer than the dynamic simulations.
Date: March 4, 1997
Creator: Glinsky, M.E.; Amendt, P.A.; Bailey, D.S.; London, R.A.; Rubenchik, A.M. & Strauss, M.
Partner: UNT Libraries Government Documents Department

Developing beam phasing on the Nova laser

Description: We are presently adding the capability to irradiate indirectly-driven Nova targets with two rings of illumination inside each end of the hohlraum for studies of time-dependent second Legendre (P2) and time- integrated fourth Legendre (P4) flux asymmetry control. The rings will be formed with specially designed kinoform phase plates (KPPs), which will direct each half of each beam into two separate rings that are nearly uniform azimuthally. The timing and temporal pulse shape of the outer rings will be controlled independently from those of the inner rings, allowing for phasing of the pulse shapes to control time dependent asymmetry. Modifications to the incident beam diagnostics (IBDS) will enable us to verify that acceptable levels of power balance among the contributing segments of each ring have been achieved on each shot. Current techniques for precision beam pointing and timing are expected to be sufficiently accurate for these experiments. We present a design for an affordable retrofit to achieve beam phasing on Nova, results of a simplified demonstration, and calculations highlighting the anticipated benefits.
Date: March 10, 1997
Creator: Ehrlich, R.B.; Amendt, P.A.; Dixit, S.N.; Hammel, B.A.; Kalantar, D.H.; Pennington, D.M. et al.
Partner: UNT Libraries Government Documents Department

Computational modeling of stress transient and bubble evolution in short-pulse laser irradiated melanosome particles

Description: Objective is to study retinal injury by subnanosecond laser pulses absorbed in the retinal pigment epithelium (RPE) cells. The absorption centers in the RPE cell are melanosomes of order 1 {mu}m radius. Each melanosome includes many melanin particles of 10-15 nm radius, which are the local absorbers of the laser light and generate a discrete structure of hot spots. This work use the hydrodynamic code LATIS (LAser-TISsue interaction modeling) and a water equation of state to first simulate the small melanin particle of 15 nm responsible for initiating the hot spot and the pressure field. A average melanosome of 1 {mu}m scale is next simulated. Supersonic shocks and fast vapor bubbles are generated in both cases: the melanin scale and the melanosome scale. The hot spot induces a shock wave pressure than with a uniform deposition of laser energy. It is found that an absorption coefficient of 6000 -8000 cm{sup -1} can explain the enhanced shock wave emitted by the melanosome. An experimental and theoretical effort should be considered to identify the mechanism for generating shock wave enhancement.
Date: March 4, 1997
Creator: Strauss, M.; Amendt, P.A.; London, R.A.; Maitland, D.J.; Glinsky, M.E.; Lin, C.P. et al.
Partner: UNT Libraries Government Documents Department

High convergence implosion symmetry in cylindrical hohlraums

Description: High convergence, hohlraum-driven implosions will require control of time-integrated drive asymmetries to 1% levels for ignition to succeed on the NIF. We review how core imaging provides such asymmetry measurement accuracy for the lowest order asymmetry modes, and describe recent improvements in imaging techniques that should allow detection of higher order asymmetry modes. We also present a simple analytic model explaining how the sensitivity of symmetry control to beam pointing scales as we progress from single ring per side Nova cylindrical hohlraum illumination geometries to NIF-like multiple rings per side Omega hohlraum illumination geometries and ultimately to NIF-scale hohlraums.
Date: September 1, 1999
Creator: Amendt, P A; Bradley, D K; Hammel, B A; Landen, O L; Suter, L J; Turner, R E et al.
Partner: UNT Libraries Government Documents Department

Using Laser Entrance Hole Shields to Increase Coupling Efficiency in Indirect Drive Ignition Targets for the National Ignition Facility (NIF)

Description: Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition targets. The hohlraum originally proposed for NIF coupled {approx}11% of the absorbed laser energy to the capsule as x-rays. We describe here a second generation of hohlraum target which has higher coupling efficiency, {approx}16%. Because the ignition capsule's ability to withstand 3D effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive ICF. The LEH shields are high Z disks placed inside the hohlraum to block the capsule's view of the cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole, (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140 kJ of x-rays to a capsule out of 865 kJ of absorbed laser energy and produce {approx}10 MJ of yield. In the current designs, which are not completely optimized, the addition of the LEH shields saves {approx}95 kJ of energy (about 10%) over hohlraums without LEH shields.
Date: November 3, 2005
Creator: Callahan, D A; Amendt, P A; Dewald, E L; Haan, S W; Hinkel, D E; Izumi, N et al.
Partner: UNT Libraries Government Documents Department

Update on specifications for NIF ignition targets and their rollup into an error budget

Description: Targets intended to produce ignition on NIF are being simulated and the simulations are used to set specifications for target fabrication. Recent design work has focused on designs that assume only 1.0 MJ of laser energy instead of the previous 1.6 MJ. To perform with less laser energy, the hohlraum has been redesigned to be more efficient than previously, and the capsules are slightly smaller. The main-line hohlraum design now has a SiO2 foam fill, a wall of U-Dy-Au, and shields mounted between the capsule and the laser entrance holes. Two capsule designs are being considered. One has a graded doped Be(Cu) ablator, and the other graded doped CH(Ge). Both can perform acceptably with recently demonstrated ice layer quality, and with recently demonstrated outer surface roughness. Smoothness of the internal interfaces may be an issue for the Be(Cu) design, and it may be necessary either to polish partially coated shells or to improve process control so that the internal layers are smoother. Complete tables of specifications are being prepared for both targets, to be completed this fiscal year. All the specifications are being rolled together into an error budget indicating adequate margin for ignition with the new designs.
Date: July 11, 2005
Creator: Haan, S W; Herrmann, M C; Amendt, P A; Callahan, D A; Dittrich, T R; Edwards, M J et al.
Partner: UNT Libraries Government Documents Department

Indirect-Drive Time Dependent Symmetry Diagnosis at NIF-Scale

Description: The scaling to NIF of current techniques used to infer the time-dependent flux asymmetries for indirectly-driven capsules is reviewed. We calculate that the projected accuracy for detecting the lowest mode asymmetries by a variety of techniques now meet the requirements for symmetry tuning for ignition. The scaling to NIF has also motivated the implementation of new, more efficient and hence less perturbative backlighting techniques which have recently provided high quality symmetry data during validation tests at the Omega facility.
Date: October 27, 1999
Creator: Landen, O.L; Bradley, D.K.; Pollaine, S.M.; Amendt, P.A.; Glendinning, S.G.; Suter, L.J. et al.
Partner: UNT Libraries Government Documents Department