Lawrence Livermore National Laboratory



April 2019

Repairing NIF Optics Damage Shows LLNL’s Advanced Manufacturing Potential


Line VISAR Commissioned at Sandia's Z Machine

Photo of Z MachineA joint LLNL/Sandia National Laboratories team commissioned a Line VISAR diagnostic at Z machine, Sandia’s pulsed power facility. Credit: Randy Montoya, Sandia National Laboratories

A joint LLNL/Sandia National Laboratories team has commissioned a line-imaging Velocity Interferometer System for Any Reflector (Line VISAR) at Sandia’s Z Machine. Line VISAR is used to measure the velocities of shocks and surface motions when enormous pressures are applied to target materials in high energy density science experiments. Such data inform fundamental studies of materials at extreme conditions and provide insight into the hydrodynamics of inertial confinement fusion (ICF) targets.

Invented in the 1970s, VISAR was developed by Sandia scientists and has since become a standard measurement tool in many areas where dynamic pressure loading is applied to materials. Driven by the needs of National Ignition Facility experiments, LLNL developed expertise in the technology and implemented Line VISAR nearly 20 years ago. The technology has now come full circle; scientists from LLNL, Sandia, and other partners will use Z Line VISAR on Z Machine.

“The Z Machine gives us a different approach to exploring ICF,” said Kumar Raman, a physicist at LLNL who is part of a joint LLNL-Sandia team conducting Z Machine experiments for the Sierra ICF campaign.

The power of Line VISAR is the ability to measure the velocity continuously along a “line” in space as a function of time. In Line VISAR, light reflected from the target passes through interferometers and forms a fringe pattern, which is projected on the slit of a streak camera. The fringe pattern moves in proportion to the velocity of the reflecting surface because of Doppler shifts that are imparted to the reflected light beam. In this way, the spatial and temporal arrangement of the fringe pattern in the output streak record reveals how different parts of the target move during the experiment.

“For validating numerical models, the time history of the current is the most fundamental input on a pulsed power facility,” said Clayton Myers, a physicist and Z principal experimenter at Sandia. “Z Line VISAR is the most capable diagnostic for collecting that data.”

At the Z Machine, a series of large capacitors deliver up to 26 million amps to a centimeter-sized target over sub-microsecond timescales. The current flows axially along the outer surface of the cylindrical target (the “z”-direction), generating magnetic pressure that drives the target radially inward, creating a “z-pinch” that heats and compresses the target fuel. The performance of the pinch is strongly affected by current loss that can occur in the power feed or within the target itself due to a variety of mechanisms that are not fully understood.

“The magnetic field varies inversely with radius and is proportional to the current delivery, so the target motion can involve complicated radial gradients. By allowing us to resolve these gradients, we expect Line VISAR will give us new and detailed insights into how the Z Machine couples current to our targets,” explained Raman.

Designing for a Different Environment

While the diagnostic was modeled closely to the NIF Line VISAR, the operational environments of NIF and the Z Machine are very different. To achieve the precise alignment of NIF’s line-of-sight, the requirements for vibrational, thermal, and seismic stability are demanding. The NIF facility is a very clean environment, designed to minimize extraneous debris that can erode laser and optics performance.

In contrast, when the Z Machine fires a shot, the entire facility shakes. The final optics assembly is a sacrificial component. Mounted directly on top of the target, it is destroyed during the violence of a Z Machine shot and replaced for each subsequent shot. In addition, as part of the Z Machine’s capacitor and current-delivery design, the enclosing high bay is exposed to open cavities of transformer oil and cooling water pools that would degrade the instrument performance if not accounted for in the engineering design.

Photo of the final optics assembly before and afterThe Z Line VISAR final optics assembly, which is destroyed in the shot. The images show the before (left) and after (right) of a Z shot. Credit: Michael Jones/Sandia National Laboratories

In designing the Z Line VISAR, the team had to protect sensitive optical components from shock loading; adequately shield electrical equipment from the electromagnetic pulse that initiates each shot; and protect the optical design and associated beam path from temperature and humidity fluctuations as well as airborne hydrocarbons.

Meeting these requirements within the available space at Z Machine was also a challenge. As a result, the 50-meter transport beam snakes its way to the interferometer room. “The engineering team deserves a great deal of credit for preserving the physics requirement along this very complicated transport line,” said Raman.

The effort required specialized parts and expertise in many areas. Custom-made optics housings allow proper positional tolerancing while still preserving critical wavefront performance. The sealed beam tube was designed for constant operation with clean dry air, controlled temperature and humidity flowing at a slight positive pressure. An Ion Beam Sputtering-type coating (hardened coating) is used for the optics to reduce the effects of varying humidity.

The team fabricated a prototype transport mount and tested it at LLNL’s Site 300 shaker table to evaluate performance with shock loads similar to those expected at the Sandia facility. This took multiple iterations, but the effort paid off with a robust design.

Z line-VISAR beam transportThe beam transport line (in green) at the Z Machine. Members of the joint LLNL-Sandia team are standing below it to the right. Credit: Michael Jones/Sandia National Laboratories

Most of the subassemblies were built and evaluated at LLNL and then installed at Sandia. “For the subassemblies that were installed earliest, we expected some degradation because of the environment,” said Phil Datte, the LLNL project lead. “But we didn’t see any. The design team did an excellent job of mitigating and managing the harsh environment.”

The entire effort required close collaboration between the LLNL and Sandia teams. In December, they came together for a commissioning shot to compare data from the existing point velocimetry systems at Z Machine and the new Z Line VISAR.

“The first shot was an unqualified success,” said Myers. “It returned data that was consistent with what we expected and with which we can begin to do science.”

Another commissioning shot will be fired in the next few months. In summer 2019, the scientists expect to conduct regular experiments with the Z Line VISAR diagnostic. “We’ll compare data from those experiments with our numerical models, which will make them more robust,” said Myers. “We expect this data to provide new insights into how Z delivers current to ICF targets.”

Photo of the Z Line VISAR LLNL teamMembers of the LLNL Z Line VISAR Engineering Design Team (from left): Justin Galbraith, Peter Cardinale, Nan Wong, Jeremy Lusk, Bill Thompson, Phil Datte, Jose Hernandez, Thomas McCarville, Michael Crosley, Michael Vitalich, A. Martinez, Gene Frieders, Gene Vergel de Dios, William Lew, and Simon Cohen. Credit: Randy Wong/LLNL

—Patricia Koning

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Repairing NIF Optics Damage Shows LLNL’s Advanced Manufacturing Potential

The following is an edited excerpt from an article by Allan Chen in the March 2019 issue of Science & Technology Review:

Photo of Optics Mitigation Facility operator Constantine Karkazis inspecting the facility’s CO2 laser processing hardwareOptics Mitigation Facility operator Constantine Karkazis inspects the facility’s CO2 laser processing hardware. Credit: Bruno Van Wonterghem

NIF’s laser optics damage mitigation program is one example of how the private sector is benefiting from Lawrence Livermore National Laboratory’s Advanced Manufacturing Laboratory (AML).

AML is a 1,300-square-meter facility where Livermore scientists and engineers are working side by side with partners in the private sector and academia to create new materials and technologies. AML is currently home to more than half a dozen partnerships, and more are in the pipeline.

One partnership involves NIF, where large fluxes of energy pass through the optics of the 192 beams comprising the world’s highest energy laser. With repeated shots, tiny pits in NIF’s optical glass can enlarge to become damage sites that compromise performance.

“The Laboratory has made a substantial effort to mitigate damage precursors and initiated damage sites on NIF’s large optics,” says Ibo Matthews, a group leader in Livermore’s Materials Science Division. “The damage mitigation process we developed uses carbon dioxide lasers to repair damage on the surfaces of silica optics, smoothing their imperfections. We realized that this process could be used for the laser polishing of glass, even the localized repair of NIF optics.”

Schematic showing the NIF laser damage repair processSchematic showing the NIF laser damage repair process, which uses a scanned, pulsed, and focused CO2 laser spot to evaporate a conical pit: (Left) cross-section view and (Right) angle view from top.

The enabling research was funded primarily by the Laboratory Directed Research and Development Program.

At a conference in 2014, a presentation about the technology by Matthews, Materials Science Division staff scientist Nan Shen, and their team attracted the attention of Edmund Optics, which quickly entered into talks with Livermore.

The U.S.-based company eventually established a Cooperative Research and Development Agreement (CRADA) to work with the Lab at AML. The partnership’s goal is to extend Livermore’s technology into a commercial system capable of polishing industrial lenses and mirrors to the same high surface quality demanded by NIF.

Photo of NIF method to mitigate optics laser damage using laser ablationNIF researchers have developed a method to mitigate optics laser damage using laser ablation.

Advanced technology is reshaping and transforming manufacturing the world over. Signs of this transformation are everywhere: factory automation, machine learning, additive manufacturing, robotics, and cloud-based process management, to name only a few trends.

Livermore contributes to this renaissance through its research and development (R&D) in fields such as advanced manufacturing, industry partnerships that use the Laboratory’s high-performance computing to improve industrial processes, and commercializing new manufacturing technologies.

R&D at the AML aims to further Livermore’s national security missions while enabling partners to release new products and services into the marketplace, a process called spin-in/spin-out technology development.

AML’s facilities reflect Livermore’s process expertise across a broad spectrum of materials and scales, including direct ink writing, powder bed fusion, electrophoretic deposition, projection microstereolithography, and laser-based processes such as two-photon lithography and selective laser melting.

With these capabilities, partners on the applications track can develop new materials and components for nearly any sector — transportation, defense, energy, or biomedicine, for example. With some advanced manufacturing processes promising shorter development times, the qualification and certification track is tailored to accelerate the commercial acceptance of new materials, processes, and components.

For more information, go to str.lln.gov.

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