Large-aperture plasma-assisted deposition of inertial confinement fusion laser coatings

Plasma-assisted electron-beam evaporation leads to changes in the crystallinity, density, and stresses of thin films. A dual-source plasma system provides stress control of large-aperture, high-fluence coatings used in vacuum for substrates 1m in aperture.     

Reactively sputtered TiN coatings for tribological applications

The results of tribological studies carried out on TiN sputter-coated ferrous and non-ferrous materials are presented. A disc magnetron system was used to sputter TiN reactively onto samples suitable for standard friction and wear tests. The tests carried out show that thin films of reactively sputtered TiN, typically 5 microm thick, provide good wear protection for ferrous and non-ferrous surfaces subjected to sliding and rolling contacts. Cutting tests with coated cermet tool inserts show that reactive sputtering is also useful for improving the performance of cutting tool inserts. The results obtained suggest that magnetron reactive sputtering can be useful to enhance the wear resistance of finished precision components.     

Reactively sputtered N-doped titanium oxide films as visible-light photocatalyst

Nitrogen-doped titanium oxide (TiO_xN_y) films were prepared by reactive magnetron sputtering of a titanium metal target in gas mixtures of argon, oxygen and nitrogen. Two types of nitrogen species are formed in the films following the fraction of N₂ (F_N2) in the reactive atmosphere. One is substitutional nitrogen in anatase titania phase and the other is nitrogen in TiN phase. In a large range of F_N2 from 0 to 0.57, TiO_xN_y films in anatase structure with about 1.0–1.4 at.% substitutional nitrogen are produced and the films exhibit red shifts to ∼ 500 nm from the absorption edge of ∼ 380 nm of undoped TiO₂. The nitrogen is readily doped in the films by energetic nitrogen ions in the plasma and the films exhibited photocatalytic properties under visible light. When excess nitrogen is supplied as the F_N2 above 0.75, the resulting film contains 20.8 at.% of nitrogen with formation of TiN that makes the film opaque and destroys the photocatalytic activity largely.     

Trident: a versatile high-power Nd:glass laser facility for inertial confinement fusion experiments

The Trident Nd:glass laser system operates as an experimental facility supporting the national Inertial Confinement Fusion program at Los Alamos. The laser has two identical main beam lines with 14-cm-disk final amplifiers. The beams are frequency doubled, expanded to 19.2 cm, and focused on target with a variety of focusing optics. A third beam with 10-cm disk final amplifiers is also frequency doubled and used as a target-shooting or diagnostic beam simultaneously with the other two beams.

The facility provides a flexible combination of energy, pulse-shaping capabilities, and diagnostic tools for laser-target interaction experiments.

     

Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion

The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm x 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1-10 ns up to energies at 1.053 mum of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-mum output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm(2) for 3-ns pulses.     

Time-resolved ten-channel monochromatic imaging of inertial confinement fusion plasmas

We report on what we believe to be the first use of toroidally bent crystals to record two-dimensional, spatially resolved, monochromatic images of laser-produced fusion plasmas combined with a 34-ps fast x-ray framing camera. An array of five toroidal silicon (311) and five toroidal germanium (311) crystals was developed. The imaging properties of the geometries are checked by a ray-tracing program and are compared with experimental results. The total imaging system (crystal and detector) provides an experimentally measured spatial resolution better than 15 mum. Time histories for the hydrogenlike argon emission and the heliumlike argon emission of fusion pellets driven with the GEKKO XII glass laser system are presented.     

Optical coatings for laser fusion applications

Lasers for fusion experiments use thin film dielectric coatings for reflecting, antireflecting and polarizing surface elements. Coatings are most important in neodymium-doped glass lasers. The most important requirements of these coatings are the accuracy of the average value of reflectance and transmission, the uniformity of amplitude and phase front of the reflected or transmitted light and the laser damage threshold. Damage resistance strongly affects the laser's design and performance. The success of advanced lasers for future experiments and for reactor applications requires significant developments in damage-resistant coatings for UV laser radiation.     

Ultrafast picket fence pulse trains to enhance frequency conversion of shaped inertial confinement fusion laser pulses

A high-frequency train of 5-100-ps pulses (picket fence) is proposed to improve significantly the third-harmonic frequency conversion of Nd:glass lasers that are used to generate high-contrast-shaped pulses for inertial confinement fusion (ICF) targets. High conversion efficiency of the low-power foot of a shaped ICF pulse is obtained by use of a low duty cycle, multi-gigahertz train of ~20-ps pulses with high peak power. Even with less than 10% duty cycle, continuous illumination is maintained on the target by a combination of temporal broadening schemes. The picket fence approach is analyzed, and the practical limits are identified as applied to the National Ignition Facility laser. It is found that the higher conversion efficiency allows ~40% more third-harmonic energy to be delivered to the target, potentially enabling the larger drive needed for high-yield ICF target designs. In addition, the frequency conversion efficiency of these short pulses saturates much more readily, which reduces the transfer of fluctuations at the fundamental and thus greatly improves the power stability of the third harmonic.     

RF magnetron sputtered aluminium oxide coatings on iridium

The effects of process parameters on the microstructural morphology of aluminium oxide (Al₂O₃) coatings on Ir have been studied. Al₂O₃ coatings were deposited on Ir-coated isotropic graphite (IG) substrates at substrate temperatures of room temperature (RT)-1073 K, RF power of 200–600 W in an Ar, or Ar+1–10% O₂, sputtering gas atmosphere by RF magnetron sputtering. Al₂O₃ coatings which were deposited at high substrate temperatures and high RF powers in an Ar, or an Ar+O₂, sputtering gas atmosphere were found to contain a dense, fine columnar structure with a -Al₂O₃ phase, low Ar content and a relatively high hardness value of ca. 1050 H _v. Furthermore, high resolution transmission electron microscopy (HRTEM) results revealed the epitaxial growth of Al₂O₃ coatings on Ir-coated IG substrate. It was found that the interface between Al₂O₃ and Ir coatings was sharp and Al₂O₃ coatings remained intact with the Ir-coated IG substrate.     

High band gap oxide optical coatings for 0.25 and 1.06 μm fusion lasers

Optical coatings of the high band gap oxides ZrO₂, Y₄Al₂O₉, Al₂O₃, MgAl₂O₄ and SiO₂ were prepared by reactive sputtering to evaluate their potential for use in fusion lasers operating at wavelengths ranging from the UV to the near IR. The optical properties (refractive index, band gap, absorption and scattering) and materials properties (crystalline structure, grain size, stoichiometry and surface topography) of these oxides are reported and their suitability for use at 0.25 and 1.06 μm is discussed. The deposition rates and the influences of deposition conditions on the optical and materials properties are also reported.     

Reactively sputtered MoO3 films for ammonia sensing

We report the gas-sensing properties of ion-beam sputter deposited MoO₃ thin-films. The change in the DC conductivity was measured in dry N₂ with 10% O₂ in the presence of up to 490 ppm of NH₃, NO, NO₂, C₃H₆, CO and H₂. At ∼440 °C the film was found to be very sensitive to NH₃, with 490 ppm increasing the conductivity by approximately a factor of 70. This was approximately 17 times greater than the response to the other gases. The NH₃ response was strongly affected by the accompanying levels of O₂, NO₂ and H₂O. For example, changing the accompanying O₂ levels from 1% to 20% decreased the NH₃ response by approximately a factor of 20. Similarly, the presence of 100 ppm NO₂ (in 10% O₂) decreased the NH₃ response by approximately a factor of three, and 1% water vapor decreased it by more than a factor of two. The NH₃ response, however, was relatively unaffected by 100 ppm of accompanying NO, C₃H₆, CO or H₂. XPS measurements show that the increased conductivity in the presence of NH₃ was also accompanied by a partial reduction of the surface MoO₃. We observed an increase in the resistance of the films after extended time at elevated temperatures.     

R.F. sputtered thin films for integrated optical components

The attenuation in waveguides operating at optical frequencies has been examined for a range of materials and fabrication techniques. The factors governing the operation of planar and topographically defined waveguides have been discussed in an attempt to determine the effect of the dielectric material indices; some of the limitations of the materials have been reviewed with particular reference to the modifications induced by thin film deposition.The technique of r.f. sputtering has been used to fabricate planar guides from glasses and metal oxides. The difficulties encountered and the results achieved have been presented in some detail.Measurements of the optical indices of the thin films were made by probing with a guided wave; the technique has been explained from first principles.     

Synthesis, structural and mechanical characterization of sputtered tungsten oxide coatings

Tungsten oxide coatings were deposited without substrate bias by DC reactive magnetron sputtering of a tungsten target using oxygen as reactive gas. By tuning the partial pressure of oxygen (p_O2/p_Ar) between 0 and 4, the oxygen content of the films was changed from 0 to 75 at.%. The structure of the films (investigated by X-ray diffraction) depends on their oxygen content. For low oxygen contents, the -W and β-W₃O phases were observed (< 30 at.%), and with the increase of oxygen content (30 at.% < O < 67 at.%) the structure became amorphous. A transition region was obtained for oxygen content between 67 at.% and 75 at.%, and when O > 75 at.%, a nanocrystalline (WO₃) structure was reached.

The hardness and Young's modulus were evaluated by depth sensing indentation. The decrease in hardness followed the four different ranges of chemical compositions accordingly, from ≈ 23 GPa for pure W down to ≈ 7 GPa for WO₃ films. A similar behaviour was observed for the Young's modulus, which ranged from 450 GPa to 150 GPa. The cohesion/adhesion of the films were investigated using a scratch-test apparatus. These coatings displayed a low adhesion (critical load, L_c < 15 N) to the steel substrate because the depositions were carried out intentionally without an adhesion interfacial layer.     

Absorptance behavior of optical coatings for high-average-power laser applications

A modified photothermal deformation technique is used to measure the absorptance behaviors of optical multilayered dielectric coatings for a high-power laser system. The surface thermal-lensing modification uses an enlarged probe beam to facilitate alignment of the laser beam and data acquisition. The coatings, both reflective and transmissive types, are made by a physical vapor-deposition process. Coating absorptances are observed to depend on the laser's exposure time and power density. Time-dependent absorptance defect models are proposed. Also, micrometer-sized sites of high absorptance and an area with physical damage can be found during the spatial scans. It is proposed that absorptance values reported for coatings in high-repetition-rate or cw-laser systems include time- and power-dependent behaviors in addition to other relevant irradiation parameters.