2D X-ray radiography of imploding capsules at the national ignition facility.

First measurements of the in-flight shape of imploding inertial confinement fusion (ICF) capsules at the National Ignition Facility (NIF) were obtained by using two-dimensional x-ray radiography. The sequence of area-backlit, time-gated pinhole images is analyzed for implosion velocity, low-mode shape and density asymmetries, and the absolute offset and center-of-mass velocity of the capsule shell. The in-flight shell is often observed to be asymmetric even when the concomitant core self-emission is round. A ∼ 15 µm shell asymmetry amplitude of the Y(40) spherical harmonic mode was observed for standard NIF ICF hohlraums at a shell radius of ∼ 200 µm (capsule at ∼ 5× radial compression). This asymmetry is mitigated by a ∼ 10% increase in the hohlraum length.

Experimental and computational laser tissue welding using a protein patch.

An in vitro study of laser tissue welding mediated with a dye-enhanced protein patch was conducted. Fresh sections of porcine aorta were used for the experiments. Arteriotomies were treated using an indocyanine green dye-enhanced collagen patch activated by an 805-nm continuous-wave fiber-delivered diode laser. Temperature histories of the surface of the weld site were obtained using a hollow glass optical fiber-based two-color infrared thermometer. The experimental effort was complemented by simulations with the LATIS (LAser-TISsue) computer code, which uses coupled Monte Carlo, thermal transport, and mass transport models. Comparison of simulated and experimental thermal data indicated that evaporative cooling clamped the surface temperature of the weld site below 100 °C. For fluences of approximately 200 J/cm2, peak surface temperatures averaged 74°C and acute burst strengths consistently exceeded 0.14×106 dyn/cm (hoop tension). The combination of experimental and simulation results showed that the inclusion of water transport and evaporative losses in the computer code has a significant impact on the thermal distributions and hydration levels throughout the tissue volume. The solid-matrix protein patch provided a means of controllable energy delivery and yielded consistently strong welds. © 1998 Society of Photo-Optical Instrumentation Engineers.

Soft x-ray images of the laser entrance hole of ignition hohlraums.

Hohlraums are employed at the national ignition facility to convert laser energy into a thermal x-radiation drive, which implodes a fusion capsule, thus compressing the fuel. The x-radiation drive is measured with a low spectral resolution, time-resolved x-ray spectrometer, which views the region around the hohlraum's laser entrance hole. This measurement has no spatial resolution. To convert this to the drive inside the hohlraum, the size of the hohlraum's opening ("clear aperture") and fraction of the measured x-radiation, which comes from this opening, must be known. The size of the clear aperture is measured with the time integrated static x-ray imager (SXI). A soft x-ray imaging channel has been added to the SXI to measure the fraction of x-radiation emitted from inside the clear aperture. A multilayer mirror plus filter selects an x-ray band centered at 870 eV, near the peak of the x-ray spectrum of a 300 eV blackbody. Results from this channel and corrections to the x-radiation drive are discussed.

Radiation-driven hydrodynamics of high- hohlraums on the national ignition facility.

The first hohlraum experiments on the National Ignition Facility (NIF) using the initial four laser beams tested radiation temperature limits imposed by plasma filling. For a variety of hohlraum sizes and pulse lengths, the measured x-ray flux shows signatures of filling that coincide with hard x-ray emission from plasma streaming out of the hohlraum. These observations agree with hydrodynamic simulations and with an analytical model that includes hydrodynamic and coronal radiative losses. The modeling predicts radiation temperature limits with full NIF (1.8 MJ), greater, and of longer duration than required for ignition hohlraums.

Photon trapping models for x-ray lasers.

Optimum methods for calculating the effects of photon trapping are discussed. An efficient line-transfer algorithm that can calculate trapping when there are overlapping and interacting lines is described. Escape probability formulas are shown to be appropriate for calculating photon trapping for isolated lines and for the highest-energy line in a group of lines in many situations. Major computational savings are achieved by using cylindrical escape probabilities for recombination x-ray laser schemes. For collisional x-ray laser schemes it is shown that the calculation of line transfer in planar geometry is sufficiently fast that one only obtains substantial savings by exploiting the coarser spatial zoning that is possible when using escape probabilities in regions of steep velocity gradients. The use of escape probabilities is shown to be particularly well suited for single-zone parameter studies.

Laser-tissue interaction modeling with LATIS.

The role of modeling in designing new treatment protocols and instruments is discussed. A computer program for modeling laser-tissue interaction named Latis is described. Interactions are divided into the processes of laser propagation, thermal effects, material effects, and hydrodynamics. Full coupling of the processes is taken into consideration. Applications in photothermal and photomechanical laser-tissue interactions are briefly discussed. A detailed description is given of a particular application of Latis to study the effects of dynamic optical properties on dosimetry in photothermal therapy. Optical properties are functions of tissue damage, as determined by previous measurements. Results are presented for the time variation of the light distribution and damage within the tissue as the optical properties of the tissue are altered. It is found that proper accounting of dynamical optical properties is important for accurate dosimetry modeling.

Investigation of laser tissue welding dynamics via experiment and modeling.

An in vitro study of laser tissue welding mediated with a dye-enhanced protein solder was performed. Freshly harvested sections of porcine aorta were used for the experiments. Arteriotomies approximately 4 mm in length were treated using an 805 nm continuous-wave diode laser coupled to a 1-mm diameter fiber. Temperature histories of the surface of the weld site were obtained using a fiberoptic-based infrared thermometer. The experimental effort was complemented by the LATIS (LAser-TISsue) computer code, which numerically simulates the exposure of tissue to near-infrared radiation using coupled Monte Carlo, thermal transport, and mass transport models. Comparison of the experimental and simulated thermal results shows that the inclusion of water transport and evaporative losses in the model is necessary to determine the thermal distributions and hydration state in the tissue. The hydration state of the weld site was correlated with the acute weld strength.

Investigation of Damage to Multilayer Optics in X-Ray Laser Cavities: W/C, WRe/C, WC/C, Stainless-Steel/C, and Cr3C2/C Mirrors.

In many applications, multilayer mirrors are exposed to damaging fluences of x rays. In x-ray laser cavities intense optical and broad-band x radiation, from the x-ray laser plasma amplifier, can damage multilayer mirrors on time scales of hundreds of picoseconds. We describe experiments using short duration (500 ps) bursts of soft x rays from a laser produced gold plasma to damage multilayer mirrors designed to reflect wavelengths close to 45 Å at normal incidence. The effect of the damaging x-ray flux on normal incidence reflectivity was time resolved for W/C, WRe/C, WC/C, 303-stainless-steel/C, and Cr3C2/C multilayers. The damage thresholds of the different mirrors were compared, and the Cr3C2/C mirrors were found to be the most resilient. The outer layers of the multilayers were observed to expand slowly as x rays were absorbed, and a more rapid expansion then preceded the total loss of reflectivity, at temperatures well below the melting temperature of the mirror components. It is believed that the dominant expansion mechanism is a change in the amorphous carbon layers to a more graphitic structure. The data are fit quite well by a model that assumes expansion of up to 25% in the thickness of the outermost carbon layers, followed by intermixing of the hotter layers. The rapid expansion has been observed to occur in times from 40 to 150 ps and may be the fastest resolution to date of the phenomenon of graphitization. The integrated reflectivity of the mirrors was observed to increase by up to a factor of 2.5 as they damaged; this reflectivity increase may be consistent with a reduction in the layer roughness.