Lawrence Livermore National Laboratory

An Optical Revolution for X-Ray Imaging

Space Program Innovation: One Small Satellite at a Time

NIF Experiments Close In on Burning Plasma

May 8, 2019

The following is an edited excerpt from an article by Arnie Heller in the April 2019 issue of Science & Technology Review.

Photo of Big-Foot ShotThis colorized image shows the ICF program’s first layered DT fusion implosion using the big-foot pulse strategy in a subscale HDC ablator. The big-foot platform uses a shortened three-shock pulse and a thinner DT ice layer that puts the fuel and the diamond ablator on a higher adiabat (resistance to compression) than previous designs. Credit: Don Jedlovec

The 192-beam National Ignition Facility (NIF), the world’s largest and most energetic laser, supports the National Nuclear Security Administration’s Stockpile Stewardship Program and is a powerhouse for a broad range of scientific research.

For decades, scientists have pursued inertial confinement fusion (ICF) as a means for achieving ignition—a fusion process by which self-heating generates more fusion heating than is lost to all cooling processes in the target’s fusion fuel. Recent experiments at NIF have shown that the “burning plasma” stage needed to reach this challenging goal may be within reach.

In ICF experiments, a deuterium-tritium (DT) fuel capsule is seated inside a gold or depleted uranium hohlraum, a cylindrically shaped device with open ends. NIF’s laser light striking the hohlraum walls generates a bath of x rays that causes the capsule to implode, heating and compressing the DT fuel into a central hot spot. Fusion reactions within the hotspot produce an alpha particle (helium nucleus) and a neutron.

The number of neutrons generated characterizes the extent of the fusion process. For ignition to occur, enough alpha particles must be present to generate the heat needed to initiate further fusion reactions in the hot spot, creating a thermal runaway effect.

Graphic Showing Neutron ScatteringData analysis of scattered neutrons in the hot spot of an ICF experiment yield images similar to this one. This image shows data from the January 2018 ICF experiment that produced a record 2.0×1016 neutrons and 55 kilojoules of energy. Color gradient indicates the level of neutron scattering.

As part of a successful experimental campaign that took place over several months in 2017 and 2018, scientists at NIF successively produced record numbers of fusion neutrons. One of the experiments, conducted in August 2017, delivered 1.9×1016 neutrons—the highest yield to that point—and generated about 53 kilojoules (kJ) of energy (nearly double the previous performance). A subsequent shot in January 2018 further increased the output, producing 2.0×1016 neutrons and 55 kJ of energy.

The shots used an experimental platform aimed at controlling implosion asymmetries and hydrodynamic instabilities, thereby increasing the transfer of energy from the implosion to the DT fuel. Performance of the fuel capsules has improved to such a degree that scientists may be able to create a reaction in which the alpha energy deposited in the hot spot is the dominant source of heating the implosion—known as the “burning plasma” stage.

“The people in magnetic fusion research have been working towards this achievement for 50 years,” says Omar Hurricane, chief scientist for Livermore’s ICF program. “Our focus has been on systematically identifying limiting factors and then exploring ways to overcome them. We are making important progress.”

To read the full story, go to Science & Technology Review.

For more information, go to "Approaching a burning plasma on the NIF" in Physics of Plasmas.

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An Optical Revolution for X-Ray Imaging

May 8, 2019

The following is an edited excerpt from an article by Lauren Casonhua in the April 2019 issue of Science & Technology Review.

In a research project spearheaded by Livermore astrophysicist Julia Vogel, Livermore scientists, in collaboration with the Harvard-Smithsonian Center for Astrophysics, NASA Marshall Space Flight Center, and Sandia National Laboratories, have designed, built, and characterized an optical instrument intended specifically for imaging pulsed-power x-ray sources.

Julia Vogel Examines the Wolter x-ray MicroscopeLivermore scientist Julia Vogel examines the Wolter x-ray microscope developed for pulsed-power x-ray sources.

Known as a Wolter x-ray microscope, the optic significantly improves image resolution and throughput compared to conventional imaging systems. The new instrument has been fitted to Sandia’s Z machine—the world’s strongest pulsed-power facility—to help researchers gain a better understanding of the x-ray sources under observation during high energy density experiments at the Z machine and NIF.

Wolter optics were first developed by the German physicist Hans Wolter in the mid-20th century. Characterized by two conical grazing incidence mirrors, the optics reflect x rays at shallow angles, allowing the image of an object under observation to be focused as a sharp image onto a detector.

Originally designed for implementation in x-ray microscopes, the optics were ultimately realized in a more accessible telescope design for applications in astronomy (see “Bringing Space-Based Optics Down to Earth”). However, recent advances in fabrication techniques have allowed the optic to be adapted for microscopes as Wolter initially intended.

Wolter’s innovation—joining together two mirror surfaces with different curvatures—makes the optic’s field of view considerably wider and also suppresses several intrinsic optical effects that degrade the image quality, allowing for a larger margin of error when pointing at an object.

“The Wolter setup provides a sharp image even if the source is not located right on the system’s optical axis,” Vogel says.

Although the geometric design of the Wolter optic largely determines its spatial resolution and field of view, spectral response and throughput (the number of photons that pass through it) can be enhanced by depositing multilayer coatings onto the mirrors’ surfaces.

“The resolution of traditional pinholes can be improved by making them smaller, but that means fewer photons can pass through the aperture,” says Vogel. “Wolter optics decouple these two quantities.”

Bernie Kozioziemski Sets Up the Wolter Microscope Calibration SystemLaboratory physicist Bernie Kozioziemski sets up the Livermore-developed calibration system for testing the performance of the Wolter microscope. During tests, a charge-coupled-device camera records two-dimensional images, while a spectrometer measures the energy and number of x rays produced. Credit: Randy Wong

Vogel and the team eventually plan to integrate the Wolter optic into NIF, where the imaging specifications are much more stringent than for the Z machine—better than 5 micrometers. “A challenge at NIF is detecting hard x rays greater than 50 kiloelectronvolts,” says Louisa Pickworth, leader of the NIF X-ray Measurement and Diagnostic Science group. “Hard x rays tend not to interact with detectors—so many photons must be collected to sense them. We can improve detectors by either making them more sensitive or by channeling more photons their way with more efficient imaging systems.”

The first reflective optics developed for NIF (a Kirkpatrick–Baez microscope) can achieve a spatial resolution better than 5 micrometers, but this system collects fewer photons than a Wolter system would, limiting the highest energies it can view to approximately 13 kiloelectronvolts.

With a Wolter microscope, NIF could achieve the large collection efficiency and spatial resolution needed to image hard x rays. “The ability to look at hard x rays would open experimental schemes that are inaccessible today,” Pickworth says. “This capability could also be coupled with more advanced detector technology under development.”

The team’s efforts for adapting the Wolter microscope to NIF include further fine-tuning the optic fabrication method and the multilayer coating. Pickworth emphasizes the importance of the optic for NIF in a way that only a scientist can. She says, “It will be a transformative technology.”

To read the full story, go to Science & Technology Review.

—Lauren Casonhua

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Space Program Innovation: One Small Satellite at a Time

May 8, 2019

The following is an excerpt from the cover story by Dan Linehan in the April 2019 issue of Science & Technology Review.

Cover of April 2019 Edition of S&TR

The population of human-made satellites orbiting Earth has skyrocketed over the past 60 years. Launches nearly doubled from 2016 to 2017, and a significant contributor to this growth has been the development and implementation of small satellites that are easier and less expensive to build and more cost efficient to launch than conventional ones.

Today, the hottest destination for these spacecraft is low-Earth orbit (LEO)—in the range of a few hundred kilometers above the planet’s surface.

To read the full story, go to Science & Technology Review.

To read about NIF’s connection to CubeSats, go to “NIF and the Rise of LLNL’s Nanosatellites (Part 1)”.

—Dan Linehan

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