Irving F. Stowers

National Ignition Facility wavefront requirements and optical architecture

With the first four of its eventual 192 beams now executing shots, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the worlds largest and most energetic laser. The optical system performance requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures. These mission requirements have led to a design strategy for achieving high quality focusable energy and power from the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass architecture with a single large amplifier type that provides high gain, high extraction efficiency and high packing density. We have taken a systems engineering approach to the practical implementation of this design that specifies the wavefront parameters of individual optics in order to achieve the desired cumulative performance of the laser beamline. This presentation provides a detailed look at the causes and effects of performance degradation in large laser systems and how NIF has been designed to overcome these effects. We will also present results of spot size performance measurements that have validated many of the early design decisions that have been incorporated in the NIF laser architecture.

INTERFACE TRIBOLOGY VIA NONEQUILIBRIUM MOLECULAR DYNAMICS

By borrowing ideas from control theory, Nonequilibrium Molecular Dynamics incorporates temperature, stress, and heat flux directly into atomistic, time-reversible, deterministic equations of motion. We are applying this technique to studies of surface indentation, surface cutting, friction, ablation, and condensation. Here we describe simulations of the indentation and cutting processes using two-dimensional crystals composed of a few thousand particles.

Molecular dynamics simulation of mechanical deformation of ultra-thin metal and ceramic films

We present an overview of the molecular dynamics computer simulation method as employed in the study of the mechanical properties of surfaces at the manometer scale. The embedded atom method is used to model a clean metal surface and the bond-order model is used to model ceramic surfaces. The computer experiment consists of the indentation and scraping of a hard diamond-like tool into and across the surface. Results are presented for the (111) surface of copper and silver and for the (100) surface of silicon. We explicitly demonstrate in our point indentation simulations that nanoscale plasticity in metals takes place by nondislocation mechanisms, a result suggested by recent nanoindentation experiments. We also observe the surface to accommodate nearly the entire volume of the tip and the annealing out of plastic work as the tip is removed. In our orthogonal cutting simulation, we observe an interesting phenomenon: the system dynamically reorients the gain in front of the tool tip to minimize the work performed on the shear plane (i.e. the shear plane becomes an easy slip plane). Silicon transforms into an amorphous state which then flows plastically.

Achieving and maintaining cleanliness in NIF amplifiers

Cleanliness measurements made on AMPLAB prototype NIF laser amplifiers during assembly, cassette transfer, and amplifier operation are summarized. These measurements include particle counts from surface cleanliness assessments using filter swipe technique and from airborne particle monitoring. Results are compared with similar measurements made on the Beamlet and Nova lasers and in flashlamp test fixtures. Observations of Class 100,000 aerosols after flashlamp firings are discussed. Comparisons are made between typical damage densities on laser amplifier optics from Novette, NOVA, Beamlet, and AMPLAB.

Optical cleanliness specifications and cleanliness verification

Optical cleanliness is important to NIF because it results in beam obscuration and scatter losses which occur in the front- end (containing over 20,000 small optics) and the large- aperture portions of the laser (containing approximately equals 7,300 optics in 192 beamlines). The level of particulate cleanliness necessary for NIF, is similar to the scatter loss due to surface roughness. That is, the scatter loss should not exceed less than or equal to 2.5 X 10-5 per surface. Establishing requirements for optical and structural surface cleanliness needs consideration of both particulate and organic thin-film cleanliness. Both forms of cleanliness may be specified using guidelines specified in Military Standard 1246C and are referred to as cleanliness Levels. This Military Standard is described briefly and displayed in tables and charts. The presence of organic thin-films on structural surfaces is of particular concern if the contaminated surface is near solgel coated optics [solgel coatings provide an antireflection (AR) quality]; or the optic is in a vacuum. In a vacuum, organic contaminant molecules have a much high probability of transporting from their source to a solgel-coated optic and thereby result in the rapid change in the transmission of the antireflection coating. Optical surface cleanliness can be rapidly degraded if a clean optic is exposed to any atmosphere containing an aerosol of small particles. The use of cleanrooms, as described in Federal Standard 209C, minimizes the settling of particulate contaminants and is described using charts and tables. These charts assist in determining the obscuration and scatter loss that can be expected when a clean surface is exposed to various Classes of cleanrooms due to particulate settling.

Cleaning optical surfaces

The advent of very large laser systems with thousands of optical components has necessitated an investigation of methods for cleaning both coated and uncoated optical elements. To perform adequately in laser systems, optical surfaces must be free from both films and particulate matter. Films have undesirable absorption bands and particles scatter light. Furthermore, in high power laser systems contaminants on the surface or trapped between the layers of coatings are known to lower the damage threshold. All surfaces must therefore, be clean when the optical elements are coated and installed and must remain clean for years after installation. Accomplishing this goal of long term cleanliness requires detailed consideration of cleaning methods and all mechanisms of recontamination. Generally, the nature of contaminants are unknown and therefore several cleaning steps may be necessary. Many conventional cleaning methods have proven unsuitable because they damage the precision surfaces they are to clean. Once cleaned, optical elements must be handled in such a manner as to preclude or at least minimize recontamination by airborne particles, aerosols or direct contact transfer. When finally installed the optical housing should not shed particles from fasteners or seals and should not outgas, which could cause an undesirable film to condense on the optical element. This paper discusses many of the techniques currently being used for cleaning optics and several new methods under investigation. It also discusses clean room procedures for reducing recontamination after cleaning.

Simulation of mechanical deformation via nonequilibrium molecular dynamics

We are developing two- and three-dimensional pair-force and embedded-atom simulations of mechanical deformation processes-indentation, machining, and inelastic ballistic-impact collisions-related to current nanometer machining practice. Here we describe these problems and their implementation using both mainframe and parallel-processor computers.

Spacecraft Fabrication and Test Manufacturing Operations and Development Integration Laboratory (MODIL): Establishing a spacecraft fabrication culture

MODILs (Manufacturing Operations and Development Integration Laboratory) were established to mitigate risk and cost escalation of producing SDI systems. The Spacecraft Fabrication and Test MODIL has been established to impact spacecraft producibility and create a spacecraft industry quantity production culture. We describe the background of MODILs, objectives and scope, current organization (including cooperating government agencies), and the current thrusts of Materials and Structures. Test and Assembly, Spacecraft Integration Technology, and Precision Technologies. The current plan is to initiate selected productivity demonstration projects with industry to show benefits of the MODIL approach and prepare comprehensive plans for follow-up activities.