Search

Inertial Confinement Fusion (ICF) energy hohlraums are composed of a high-Z material filled with foam. Because of the small pore size and transparency, silica aerogels are used in some ICF targets. The traditional synthesis of silica aerogels require sol-gel polymerization of silicon alkoxide followed by supercritical drying. Some constituents in sol-gel polymerization have been found to contribute to leaching of certain metals at the silica/metal interface. Since the hohlraums are composed of metals, possible chemical reactivity at the silica aerogel and metal hohlraum interface was investigated. The hohlraums studied are aluminum lined with either copper or copper/chromium. Upon initial inspection, the aerogel appeared transparent and uniform, however, closer inspection of the copper wall suggested possible leaching. Alternatively the quality of the aerogel in the copper-chromium hohlraum was very poor with the chromium layer of the hohlraum and some copper completely etched. Control experiments were used to determine the cause of the leaching. When copper is in the presence of sol-gel constituents, Cu 2+ ion formed, thus leaching copper from the hohlraum walls. In the presence of chromium, Cr 2 O 7 2- or CrO 4 2- was identified in solution with the Cu 2+ , these anions are believed to form copper chromite under the aerogel synthesis procedures utilized.

Experimental campaigns are being conducted at the 60 beam OMEGA laser at the University of Rochester's Laboratory for Laser Energetics to acquire data to validate hydrodynamic models in the high energy-density regime. This paper describes targets that have been developed and constructed for these experimental campaigns. Targets are 860 {mu}m inner diameter by 2.2 mm length cylinders with 70 {mu}m thick polymer ablator. On the ablator inner surface and located halfway along the axis of the cylinder is a 500 {mu}m wide Al marker band. Band thicknesses in the range 8-16 microns are used. CH foam with densities in the range 30-90 mg/cc fills the inside of the cylinder. While these targets have been fabricated for years, several new improvements and features have recently been developed. Improvements include the use of epoxy instead of polystyrene for the ablator, and the use of electrodeposited Al for the marker band. A critical feature of the target is the surface feature that is placed on the marker band. Experiments are aimed at understanding the hydrodynamic behavior of imploding cylinders as a function of this surface feature. Recent development work has focused on production of engineered surface features on the target marker band. Usingmore » a fast tool servo on a diamond turning lathe, a wide range of specified surface features have been produced. This paper will address improvements to the cylinder targets as well as current development efforts.« less

Laser beams that directly drive a cylindrical implosion are used to create a measurable region of mixed material in a compressible plasma state, for the first time in a convergent geometry. The turbulence driven by the Richtmyer–Meshkov instability by shock passage across a density discontinuity mixes marker material that is radiographically opaque. The width of the mix layer is compared between a system with large surface roughness and an initially smooth system. The experiment is described and results are compared to multi-dimensional numerical simulation, including three-dimensional turbulence calculations. The calculations adequately match the observations provided the measured initial conditions are used.

Hydrodynamic experiments in cylindrical geometry are used to study both mix (compressible, in convergent geometry) and mode coupling (impact of short wavelengths on long). For both types of experiments, knowledge of the initial conditions (the surface roughness spectrum, amplitude versus wavelength, as well as all target metrology) is very important. This paper is a discussion of the techniques and efforts to document and understand our initial conditions and their uncertainties and how well we can control them.

The production of cylindrical targets involves numerous steps. These steps are shared in common with many other types of Inertial Confinement Fusion (ICF) targets but no other single target encompasses such a wide range of fabrication techniques. These targets consist of a large number of individual parts, virtually all fabricated from commercially purchased raw material. As an example, the polystyrene used is synthesized in house from purchased monomer material. This material must be polymerized, purified, characterized and put into solution before it is even first used in the making of a target. Because virtually every manufacturing and assembly process we currently use is involved in the production of these targets, this paper is written as a way documenting the methods used.

Effective membrane structures often rely on the presence of thin discriminating layers that exert selectivity control while maintaining high fluxes. Characterizing these separating layers is often difficult given the scarcity of analytical tools capable of measuring structural detail below sample surfaces. We have shown that Rutherford backscattering spectrometry (RBS) is an effective tool for characterizing asymmetric inorganic membranes. The ability to accurately measure the thickness and composition of thin metal layers using RBS offered a means of analyzing composite structures in which metals are deposited to influence separation or provide strength to the membrane. This analysis of commercially available alumina membranes by RBS quantified the interpenetration depth of deposited gold layers. RBS has also been applied to the measurement of sample porosity in Al 2 O 3 membranes using specialized 16 O resonance experiments. The measured porosity of these asymmetric membranes resulting from the resonance measurements was compared to a graphical analysis of scanning electron microscopy images. The results of this study suggest that RBS may offer advantages in a wide range of membrane materials analysis applications.

At scales appropriate to the National Ignition Facility, calculations indicate double shell capsules reach ignition conditions with room temperature fuel. Los Alamos is laying the groundwork for these double shell ignition targets with a preliminary campaign on Nova. The target design features a 260 μm O.D. inner glass capsule with an I.D. of approximately 200 μm, containing deuterium gas, centered inside a machined polymer shell. The central capsule was supported either with a 30 mg/cc foam or with two polymer films, each less than 0.1 μm in thickness. The materials, machining, assembly, and characterization of the targets are discussed.

Hohlraums are an integral part of indirect drive targets for Inertial Confinement Fusion (ICF) research. Hohlraums are made by an electroforming process that combines elements of micromachining and coating technology. The authors describe how these target elements are made and extensions of the method that allow fabrication of other, more complex target components.

Thin films of chromium carbide have been grown by organometallic chemical vapor deposition (OMCVD) from tetraneopentylchromium (Cr[CH[sub 2]C(CH[sub 3])[sub 3]][sub 4], CrNp[sub 4]). Deposition was performed in a hot wall, low pressure reactor at 520 and 570[degrees]C and a pressure of 1.33 [times] 10[sup [minus]2] Pa (10[sup [minus]4] Torr). No carrier gas was used. The resulting films were characterized by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray diffraction (XRD), scanning electron microscopy (SEM), and elastic recoil detection (ERD). These data show that the films are of high quality, with very low oxygen content, and contain some residual amorphous carbon. Films grown at 570[degrees]C are crystalline. The observed crystalline phase is Cr[sub 7]C[sub 3]. 13 refs., 7 figs., 1 tab.

Hydroxypropyl cellulose (HPC) of degree of substitution three and average molecular weight of 100,000 was processed in a magnetic field of 1.2T. Films of HPC were cast from water, methanol, ethyl methyl ketone, and 1,1,2,2-tetrachloroethane solutions in a magnetic field using static and dynamic casting techniques. The processed films were evaluated for tensile strength, elastic modulus, microhardness, and molecular chain alignment by wide angle x-ray diffraction and polarized infrared. The best alignment and hence the greatest improvement in properties was obtained using a combined magnetic field and flow alignment procedure. These samples showed no increase in the elastic modulus, a 106% increase in the tensile strength, a 21% increase in the microhardness, and were dichroic. © 1994 John Wiley & Sons, Inc.1

A nonplanar X-band cavity amenable to coating with high-temperature superconducting films by conventional physical vapor deposition processes has been designed, fabricated, and tested. The cavity geometry resembles a symmetric clamshell configuration. It consists of two truncated cones joined at their bases. The specific dimensions of the cavity were calculated using the SUPERFISH computer code. Cavities were constructed using a silver-based alloy, Consil 995. The separate cavity sections were coated with Tl-based high-temperature superconducting material using a two-step deposition and annealing process. The unloaded Q values for this coated clamshell cavity are 66,000 and 105,000 at 77 and 20 K, respectively.

Films of Ba-Ca-Cu-O have been RF-magnetron-sputtered onto Consil 995 substrates. A postdeposition anneal in an overpressure of Tl produces the superconducting 1212 and 2212 phases. Varying the annealing procedures changes the electrical properties of the final films dramatically. Dynamic impedance, a novel approach to the electrical characterization of these films on a conductive substrate, is discussed and compared with SEM (scanning electron microscope), XRD (X-ray diffraction) and RBS (Rutherford backscattering) measurements as a function of differing annealing protocols. The improvement in the superconducting responses as a function of annealing procedures correlates well with the improved morphology observed in the SEM photographs. Also, the XRD data show increased c-axis material with the C annealing protocol.

Microwave surface resistance measurements were made on large-area (11.4-cm/sup 2/) Tl-Ba-Ca-Cu-O thick films/ss ( approximately 10 mu m) which were magnetron sputtered onto oriented Ag alloy (Consil 995) substrates. Initial films were prepared by depositing a BaF/sub 2/ buffer layer between the superconducting film and the substrate to control the film/substrate interaction; however, films exhibiting the best microwave properties were prepared without a buffer layer. Surface resistance measurements were made by replacing the end wall of an 18-GHz Cu cavity (TE/sub 011/ mode) with the superconducting film. The best surface resistance values obtained on the unbuffered film were 4 and 14 m Omega at 10 K and 77 K, respectively; the corresponding Cu values are 8 m Omega and 21 m Omega . The dependence of surface resistance on surface magnetic field was measured in a similar way except that an Nb cavity was used instead of a Cu one. Films exhibiting the highest degree of c-axis texturing show the weakest dependence of surface resistance on magnetic field and also exhibit the sharpest high-frequency transition into the superconducting state. These results are important for potential cavity applications of these materials.

Microwave surface resistance (R s ) measurements on large-area (11.4 cm2) Tl-based films deposited onto Ag (Consil 995) substrates have been made at a frequency of 18 GHz as a function of temperature. Deposition onto unoriented Ag substrates yields unoriented films characterized byR s values of 8.2 and 33.6 mΩ at 10.6 and 77 K, respectively. In contrast, similar deposition onto oriented Ag substrates yields oriented films with values of 12.6 and 14.6 mΩ at 11.2 and 77 K, respectively; corresponding Cu values are 9 and 21 mΩ. Additionally, it is found that the dependence ofR s on microwave surface magnetic fieldH s is weaker for the oriented films. These results suggest that the effect of orienting thec-axis in Tl films is twofold: (1) the high-frequency superconducting transition is made considerably sharper, resulting in a lowerR s value at 77 K, and (2) the rate of increase inR s with appliedH s is reduced. Both effects have significant ramifications for the potential application of these materials to high-frequency accelerating cavities.

A multi-step process is used to fabricate Tl2Ba2Ca1Cu2O8 films on (100) LaA103 substrates. Submicron thick precursor films of Ba-Ca-Cu-O are rf magnetron sputter deposited from a single target. Film stoichiometry is measured by ion beam backscattering spectroscopy. Deficiencies of the alkaline earths that are found in the precursor films are then compensated for by the addition of appropriate CaF2 and/or BaF2 films onto the surface of the precursor film. Post deposition annealing of the films is then done in an atmosphere of thallium oxide and oxygen in order to form the superconducting phases. The annealed films are examined using x-ray diffraction (XRD), an ac inductance technique, and critical current in an external magnetic field. XRD shows the c-axis length of the superconducting phase to increase as the overall film stoichiometry approaches 2212. The transition widths measured by inductive coupling weakly correlate with 77 K critical current measurements. Our best critical current results are 1.5*106 amps/ cm2 for a film measured at 4 K in an 8 T magnetic field (parallel to the films' c-axis).

The surface resistance Rs of YBa2Cu3O7 (YBCO) thin films (0.6±0.1 μm) deposited onto 2.5‐cm diam (100) LaAlO3 substrates has been measured at 22 GHz using both Cu and Nb cavities. The surface resistance falls precipitously at the superconducting transition (Tc =90 K) from a normal state value of approximately 2 Ω to a 77 K value of 13.7±1 mΩ, which is 1.6 times lower than Cu. At 4 K the surface resistance is 1±0.1 mΩ, as measured in a Nb superconducting cavity, which is an order of magnitude lower than Cu. The critical current density at 77 K is 4.5×104 A/cm2. Pole figure analyses show the ratio of c‐axis to a‐axis‐oriented material in the film is 2.4:1. YBCO films deposited onto either LaGaO3 or LaAlO3 substrates with varying c/a ratios yield surface resistance values at 77 K that are crudely correlated with Rs. Therefore, the principal effect of orienting the material is to improve the sharpness of the high‐frequency superconducting transition, consistent with the notion that the sharpness is associated...

Surface resistance measurements on Tl‐Ba‐Ca‐Cu‐O thick films (∼15 μm) magnetron sputtered onto BaF2‐buffered, silver‐based (Consil 995) substrates have been made at a microwave frequency of 22 GHz. The relatively large‐area films (∼5 cm2) are characterized by surface resistance values of 6.9±2 mΩ at 11.3 K and 30.2±1 mΩ at 77 K; the corresponding values for Cu are 10 and 22 mΩ, respectively. These results demonstrate that Tl‐Ba‐Ca‐Cu‐O can be deposited onto large‐area, metallic substrates with characteristic surface resistance values lower than Cu at 4 K. Orientation of the film should improve the surface resistance at 77 K, thereby making the fabrication of microwave cavities that are superior to Cu possible.

The purpose of this program is to develop a new process for the fabrication of ceramic matrix composites by chemical vapor infiltration. This period has been devoted in part to the exploration of material systems suitable for MACVI processing. A number of potential processing schemes are possible using combinations of absorbing and transparent material as composite components. This includes the use of an absorbing preform (nicalon fiber) combined with a transparent matrix (silicon nitride). Composites 5 cm in diameter by 1 cm. thick have been fabricated to densities of 65% theoretical. Processing times for these materials are under 20 hours. Higher densities will require additional microwave power now possible with the new reactor. The most effective MACVI scheme will involve the use of a transparent fiber with an absorbing matrix. The hot spot will be initiated by appropriate treatment of the central region of the preform. To this end alumna fibers with pretreatments to control thermal gradients has been explored. Nextel 610 fibers have been effectively pretreated carbon coating resulting in preferential heating in the interior of the preform. Possible matrix materials include siliconized silicon carbide, doped silicon carbide, alumna and zirconia. A patent for MACVI has been issued 10/19/93.

Films of Tl‐Ba‐Ca‐Cu were dc magnetron sputtered from a single multielement target. The films were deposited onto substrates of: (1) magnesium oxide, (2) a silver based alloy (Consil 995), (3) a nickel based alloy (Haynes 230), and (4) buffer layers of barium fluoride or copper oxide on Consil. To form superconducting phases, post‐deposition anneals were made on these films using an alumina crucible with an over pressure of thallium and flowing oxygen. After annealing, the film phases were determined using x‐ray diffraction. The film surface resistances (Rs) were measured at 22 GHz in a TE011 cavity.

Thick (8 to 10 µm) Ba-Ca-Cu-O films have been rf magnetron sputtered onto Ag alloy (Consil 995) substrates. The films were given a post-deposition anneal in an over pressure of Ti in order to form the superconducting phases. Annealing protocols were done which result in predominantly the 1212 and 2212 phases. The substrate orientation was varied to determine its effect on film orientation. Material properties of the films were characterized by x-ray diffraction (XRD), ion beam backscattering spectroscopy, energy dispersive x-ray analysis (EDAX), and scanning electron microscopy (SEM). Electrical characterization of the films was done using dynamic impedance (DI) at 10 kHz and rf surface resistance (Rs) at 18 GHz in a TE011 fundamental mode cavity.

Targets for Inertial Confinement Fusion (ICF) experiments to measure isoelectronic temperatures achieved in open configurations have been manufactured from low density polymethylpentene. The temperature measurements are made by ratioing the intensity of two nearby X-ray lines, in this case from titanium and chromium. This required these metals to be added to the foams during manufacture. Individual targets consisted of machined cylinders of foam 2 mm in diameter by 2 mm long. Because of small scale variability in the foam, it was necessary to individually characterize each target for metal content and density to determine the total mass of plastic and metal present to allow a correct calculation of the temperature to be made. The requested density of the foam was less than 5 mg/cm3 and the metal loading was to be 0.5 wt.% each of titanium and chromium. This means the mass of plastic was roughly 10 μg and the metal mass was roughly 50 ng. This low level of material to be detected required the application and modification of X-ray fluorescence and beta particle transmission methods to characterize the foams.

We present measurements of the performance of a Pockels readout optical modulator (PROM) x‐ray imaging detector using a 30‐kV constant potential bremsstrahlung source. A nickel step wedge was used to measure the spatial resolution and noise of the PROM image. PROMs are unique high‐efficiency x‐ray detectors that image with good spatial resolution over a wide range of x‐ray energy. The PROM is constructed from an optically transparent, active crystal that is sensitive to x‐ray dose. This unique construction allows the use of a thick crystal to increase efficiency for x‐ray detection while avoiding the spatial resolution degradation that usually accompanies thick detectors.

Thin films of the Tl‐based superconductors often have relatively poor properties because of film/substrate interdiffusion which occurs during the anneal. We have therefore investigated the use of BaF2 as a diffusion barrier. TICaBaCuO thin films were deposited by dc magnetron sputtering onto MgO substrates, both with and without an evaporation‐deposited BaF2 buffer layer, and post‐annealed in a Tl over‐pressure. Electrical properties of the films were determined by four‐point probe analysis, and compositions were measured by ion‐backscattering spectroscopy. Structural analysis was performed by X‐ray diffraction and scanning electron microscopy. The BaF2 buffer layers were found to significantly improve the properties of the TICaBaCuO thin films.

Low-temperature chemical vapor deposition of M(allyl)3 (M = Rh, Ir; allyl = η3 -C3H5) in the presence of H• yields thin, crystalline metal films of greater than 97% metal composition. Depositions using H2 result in the formation of materials which are amorphous and contain a significant amount of residual carbon (14%). The composition of these materials does not differ significantly from that obtained from the vacuum thermal deposition of M(allyl)3.

The surfaces of poly(methyl methacrylate) (PMMA) have been modified through a chemical infusion process by treatment of the sample with a solution containing varying amounts of titanium (IV) isopropoxide and poly (vinylpyrrolidone) (PVP). The resulting sample is modified in the outermost 150–325 micron region while the underlying polymeric material is unchanged. The chemical infusion process, a solvent based surface modification technique, is described in detail along with a study correlating treatment conditions with penetration depth. The treated samples were characterized by scanning electron microscopy, optical microscopy, and neutron activation analysis. These samples were evaluated for blood compatibility using two biological assays: (1) an adherence assay in which the adherence of human polymorphonuclear leukocytes to the samples was determined, and (2) a dynamic hemolysis assay using rat blood erythrocytes to determine the hemolytic activity of the samples. Based on the results of these assays, the PMMA samples treated with PVP alone resulted in an improvement in reactivity with the blood cells in a manner comparable to TecoflexR, a commercially available polymer with relatively benign characteristics.

Superconducting films of YBa 2 Cu 3 O 7 deposited onto LaGaO 3 substrates were prepared by e-beam and magnetron sputtering techniques. Surface resistance measurements made at 22 GHz, 86 GHz, and 148 GHz show that these films are superior to those deposited by similar techniques onto SrTiO 3 . Typical surface resistance values measured at 22 GHz and 12 K are ∼2 mω with the lowest value being 0.2 mω, which is only 2–4 times higher than Nb. The surface resistance is proportional to the square of the measuring frequency.

The off-axis forward-scattered radiation from a low energy He-Ne laser, scattered by high density hydrosols, is studied experimentally and theoretically. The validity range of theoretical calculations is determined using four measurements with two optical depths and two detector fields of view. For narrow field of view detectors, experimental and theoretical agreement is good close to the beam axis and decreases with transverse distance; agreement is good at greater distances from the beam axis using an open detector. The small-angle approximation adequately describes multiple scattering close to the beam; calculations are significantly improved by combining the small-angle approximation with diffusion theory.