Bringing Space-Based Optics Down to Earth

LLNL researchers are working to adapt the specialized optics used in orbiting x-ray telescopes to provide high-resolution images of man-made x-ray sources here on Earth, such as those produced at NIF and the Z Pulsed Power Facility, or Z Machine, at Sandia National Laboratories.

As a start, LLNL’s long-time expertise in x-ray optics is being tapped in a multi-institutional effort to develop a new x-ray diagnostic for the Z Machine—like NIF, a key element of the National Nuclear Security Administration’s Stockpile Stewardship Program. The Z Machine concentrates electrical energy and turns it into short pulses of enormous power, which are then used to generate x rays and gamma rays—a technique known as Z pinch. The facility is used for research in inertial confinement fusion and high energy density science.

The new x-ray diagnostic is a high-resolution Wolter optic, which acts as a lens and is capable of providing spatial resolution and sensitivity far superior to existing instruments. It consists of high-precision curved surfaces coated with carefully-designed multilayers to provide a specific, narrow-band x-ray image. Data produced by the Wolter optic are expected to improve the Z Machine’s x-ray source for testing radiation effects on non-nuclear components under extreme conditions.

While the optic is new to the Z Machine, Wolter optics are widely used in space-based telescopes such as the Chandra X-ray Observatory and NuSTAR (Nuclear Spectroscopic Telescope Array) because of their ability to reflect, rather than absorb, high-energy x-ray light from space and focus it onto a detector.

Laboratory researchers designed and calibrated the replicated optic and developed the reflective multilayer coatings on its mirrors. The NASA Marshall Space Flight Center fabricated the high-quality mandrel used to produce the optic, and the Harvard-Smithsonian Center for Astrophysics (CfA) is coating and replicating the multilayer optics.

LLNL astrophysicist Julia Vogel said the Laboratory, as overall project manager, provides the link between NASA Marshall, CfA, and Sandia “to ensure the optic fulfills the requirements needed for successful operation in the Z experiment. NASA and the CfA are the experts in fabricating these mirrors,” she said. “These optics are very, very small compared to the ones for astrophysics, which tend to be much larger. So specialized methods to get the mirrors extremely smooth and deposit the complex coatings are necessary to get as much light reflected as possible. The Z Wolter is actually one of the smallest replicated optics they have fabricated to date.”

Next in the Queue: NIF

Vogel said the first optic should be ready for testing at LLNL and Sandia within the next few weeks; the first shots using the new diagnostic are scheduled for December. In parallel, the LLNL team is working on enhancing the Wolter optic for use in NIF.

“Wolter optics are interesting tools for both Z and NIF,” she said, “but the NIF requirements are very strict; so starting with the optic for Sandia was a natural choice.”

While the Z experiment needed a resolution of about 100 microns over a rather large field of view to image the Z pinch, NIF operators are asking for closer to five-micron resolution. And while Sandia initially wants to image x rays with a wavelength of 17.4 keV (kiloelectronvolts), “NIF experiments are pushing towards higher energies between 20 and 30 keV for starters,” Vogel said.

Compared to other x-ray diagnostics, the Wolter optic “gives better resolution and higher throughput (of photons),” she said, “which is especially valuable for NIF because when we look at the higher energies we tend to have fewer photons. The Wolter can improve over pinholes (current pinhole cameras) because you get more throughput and still have good resolution.”

Along with Vogel, members of the LLNL Wolter optic team are Mike Pivovaroff, Christopher Walton, Bernie Kozioziemski, Perry Bell, Jay Ayers, and Louisa Pickworth.

More Information:

“Turning an X-Ray Eye on Universes, Large and Small,” Science & Technology Review, November, 2015

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