Fog on demand: Sandia-operated facility makes testing optics more cost-effective, efficient
Fog can play a key role in cloaking military invasions and retreats and the actions of intruders. That’s why physical security experts seek to overcome fog, but field tests of security cameras, sensors, or other equipment are hampered by fog that is either too thick or too ephemeral.
Until now, collecting field test data in foggy environments was time-consuming and costly. “Fog is difficult to work with because it rarely shows up when needed, it never seems to stay around long enough once you’re ready to test, and its density can vary during testing,” says systems engineer Rich Contreras (formerly 6525, now 2134).
That’s why he and others started thinking about developing a controlled fog environment for sensor testing. Sandia has developed a fog chamber — one of the world’s largest — that meets the needs of the military, government agencies, and industry. The chamber is in a tunnel owned by the Air Force Research Laboratory.
“The ultimate goal of this whole endeavor is to defeat fog,” Rich says. “From physical security and force protection aspects, as scientists and engineers who care about national security, we want to be able to make it so that a security force person at a site has the ability to maintain uninterrupted situational awareness.”
Researchers say the chamber will help develop and validate cameras’ and sensors’ abilities to penetrate fog, knowledge that could lead to improved surveillance at sites. The chamber also could be used to answer some fundamental optics questions, which in time could lead to improved security camera lenses and medical imaging equipment, safer aircraft landings, and better vision for drivers in fog.
“People need to see through fog,” says optical scientist Gabe Birch (6524). “So much of the US population is on the coastlines in places where fog exists. If you could discover an inexpensive technique to see better through it, there are a lot of people in industry who would be interested in that.”
Cloud microphysics used to characterize fog, prolong testing
This is not your Halloween party fog machine. Sandia’s fog chamber is 180 feet long, 10 feet tall, and 11 feet wide. The chamber is enclosed by air curtains and rubber baffles to entrap fog closely like real-world fog.
Tunnel walls are covered with a special black paint to reduce reflections and improve data quality, Rich says.
Walk a few steps down the hallway when the chamber’s fog is at full strength and a sense of disorientation washes over an observer as the walls, ceiling, and entranceway disappear and people only a few feet away fade first into dark, obscure silhouettes and then become invisible.
Sandia researchers use cloud microphysics to generate the fog for video analytics, environmental testing, and new sensor development. Currently, the chamber’s fog resembles that found in coastal regions, but output can be customized to produce fog similar to that found in any location, says Crystal Glen (6633), an aerosol scientist. Researchers eventually hope to add smoke and dust to the chamber’s repertoire.
In the atmosphere, fog forms from a seed particle, such as pollen or sea salt, surrounded by layers of water. Seed particles differ based on the fog’s location. Sandia currently uses sodium chloride, or sea salt, as its seed particle to mimic the composition and particle size of coastal fog. By consulting journals or traveling to a region, researchers can measure the droplet size distribution and chemical composition of different fogs worldwide and then alter the seed particles to customize the fog.
The longer the fog’s seed particles hold onto the water layer, the longer they are visible for testing. The length of the test is dependent on the relative humidity in the chamber.
Crystal mixes a solution of sodium chloride and water that produces the desired core particle seed diameter. The pre-mixed solution is then sprayed into the chamber where the relative humidity is above 95 percent. The initial sprayed droplets are roughly 2.3 times their dry diameter.
The deliquescence point, or the amount of relative humidity required for a particle to take on water, happens at 72 percent relative humidity for coastal fog, primarily composed of sodium chloride. The amount of water clinging to the seed particle grows exponentially from there. This process happens naturally in the atmosphere and leads to fog and cloud formation, Crystal says.
To speed up Mother Nature, Crystal checks the rate at which a particle will gain or lose water in relation to the chamber’s relative humidity, termed the hysteresis curve for water interactions with sodium chloride. This information allows the team to target a specific relative humidity and obtain a desirable size for the wet particles, so the droplet size distribution is close to what is found in natural fog, she says. While the fog generated in the chamber is not identical to fog formed in nature, it is physically representative and extremely useful for research involving optical transmission and visibility, she adds.
Typically, gravity causes the fog to settle before the decrease in relative humidity takes effect. The fog density can remain constant for up to 30 minutes, allowing a test to last 10-20 minutes. Adding a 30-second blast of fog particles can prolong the testing, Crystal says, adding that the fog’s density can be controlled by the amount sprayed into the chamber or the particle size.
Optics experts see potential of controlled fog
The layer of water around the fog seed particles either absorbs the photons, the elementary particles and waves that make up light, or causes them to change direction in random ways, so that by the time they reach the cameras being tested, the wavelengths being picked up create a fuzzy image, Gabe says.
Optics researchers refer to fog — and seeing through bodily tissue in medical imaging — as “scattering environments.” Sandia optical engineer David Scrymgeour (1728) likens the photons’ movements in these environments to walking through a sunny, full parking lot and seeing the glints of light bounce in every direction off windshields.
In fog, it’s the scattering of the photons that causes car headlights or a pedestrian’s flashlight beam to illuminate an entire scene, making vision even more difficult, David explains.
In physical security, “the cameras are very sensitive to the sizes of fog particles and how the photons scatter. That’s why it’s so important to know the sizes of particles that we have in the environment, which is something that we in the optical field have not really had before,” Gabe says. “It enables a lot of very interesting testing because you can finally characterize your system’s performance by knowing the scattering that’s happening in the environment.”
Fog chamber tests could lead to security camera improvements
For the military or any agency trying to physically secure a site, not knowing the exact ranges that cameras can penetrate fog in a particular environment makes it difficult to choose the correct cameras and sensors and their placements, Rich says.
Gabe explains: “It’s very difficult to quantitatively compare all those modalities together with the same fog and in the same conditions because you go outside and five minutes later it could be very different.”
Once the chamber’s fog density is set, cameras or sensors mounted at one end of the tunnel are monitored to see how well they detect humans or custom targets, Rich says.
Different types of lighting representing specific sites could be installed in the chamber to see the combined effect of fog and lighting or the desired time of day, he says.
Examples of tests include showing raw data from various cameras, characterizing how different wavelengths and polarization states are influenced by fog, comparing different optical systems in a controlled foggy environment, and resolution testing to see how the optical properties and resolution degrade in a variety of foggy environments, Gabe says.
Fog chamber could provide answers for optics research
The fog chamber also could be used to answer fundamental scientific questions.
“When you look at the huge gap from the visible spectrum all the way up to the far infrared, no one can say we absolutely understand how the polarization states at all these different wavelengths behave as they go through a foggy atmosphere,” Gabe says.
Recent research at Sandia has suggested that the polarization of photons could be exploited to see better through fog or other scattering environments, David says.
Researchers have ideas about how to use optics — for example, a filter on a camera lens — to exploit polarization, he adds, but they need to be tested in a real-world environment, like the fog chamber.
Such testing could inform not only physical security camera design to better handle fog, but also medical imaging, he said. “The physics in scattering events are the same.”
-- Heather Clark
Pavel Bochev receives DOE’s E.O. Lawrence Award
by Neal Singer
Pavel Bochev (1442), a computational mathematician, has received an Ernest Orlando Lawrence Award for his pioneering theoretical and practical advances in numerical methods for partial differential equations.
“This is the most prestigious mid-career honor that the Department of Energy awards,” says Bruce Hendrickson (director 1400, Computing Research).
Lawrence Award recipients in nine categories of science each will receive a medal and a $20,000 honorarium at a ceremony in Washington, D.C., later this year.
In the category “Computer, Information, and Knowledge Sciences,” Pavel’s work was cited for “invention, analysis, and applications of new algorithms, as well as the mathematical models to which they apply.”
Says Pavel, “I am deeply honored to receive this award, which is a testament to the exceptional research opportunities provided by Sandia and DOE. Since joining Sandia I’ve been very fortunate to interact with an outstanding group of researchers who stimulated and supported my work. These interactions, as well as funding from the Advanced Scientific Computing Research Program of DOE’s Office of Science and the ASC program of the National Nuclear Security Administration, helped shape, grow, and mature the research effort leading to this recognition.”
Said Energy Secretary Ernest Moniz, “I congratulate the winners, thank them for their work on behalf of the department and the nation, and look forward to their continued excellent achievement.”
The Lawrence Award was established to honor the memory of Ernest Orlando Lawrence, who invented the cyclotron — an accelerator of subatomic particles — and received a 1939 Nobel Prize in physics for that achievement. Lawrence later played a leading role in establishing the US system of national laboratories.
The most recent prior Sandia recipient of the Lawrence Award is Fellow Jeff Brinker, who received the honor in 2002.
-- Neal Singer
Sandia’s Z machine receives funding aimed at fusion energy
by Neal Singer
A $3.8 million, two-year grant to Sandia and the University of Rochester’s Laboratory for Laser Energetics (LLE) is expected to hasten the day of fusion break-even and eventually high-yield for energy production.
The grant was announced by DOE’s Advanced Research Projects Agency for Energy (ARPA-E).
Previous fusion work at both institutions had been funded by DOE’s National Nuclear Security Administration (NNSA) solely to support the Stockpile Stewardship Program, whose goal is to maintain a safe and reliable nuclear deterrent without nuclear testing.
Break-even means as much energy emerges from a fusion reaction as is put into it; high-yield means that much more energy emerges.
The work to be conducted at both laboratories is expected to advance a promising Sandia energy concept called MagLIF, for Magnetized-Liner Inertial Fusion.
Originally proposed in a 2010 theoretical paper by Sandia researcher Steve Slutz (1684) and colleagues, the concept uses a laser to heat fusion fuel contained in a cylinder, called a liner, as that cylinder itself is compressed by the huge magnetic field of Sandia’s massive Z accelerator. A secondary axial magnetic field embedded in the fuel and cylinder impedes the laser energy from escaping the resultant plasma, which would lower the temperature of the fuel and reduce the fusion output.
The combined heat and pressure, created by the laser preheating and liner imploding over a hundred or so nanoseconds, have been shown to force fuel to fuse in recent experiments on Z. The next step is to force it to fuse more efficiently and, at the same time, allow researchers to learn more about important physical mechanisms at work.
ARPA-E’s bet, and Sandia’s and Rochester’s with it, is that a more efficient coupling of the laser energy to the fusion fuel will increase the number of neutrons produced, the gold standard in judging the efficiency of the fusion reaction.
Smoothing laser beams
As it happens, scientists at the LLE over many years have developed techniques to “smooth” laser beams, a prerequisite for delivering more energy to fusion fuel.
“By smoothing the beam,” says project lead and Sandia senior manager Daniel Sinars (1680), “we eliminate hot spots in the laser beam that waste laser energy and potentially alter the beam path of some of the light. This altered path can disintegrate portions of the liner or other surrounding material. Some of that material then may contaminate the fuel and increase radiation losses, causing the fuel temperature to collapse below that needed for fusion reactions to occur.”
When optimized, the process should allow fusion reactions to occur at 1 to 2 percent of the density and pressure required in traditional inertial confinement fusion (ICF), which has used either laser-created X-ray pulses or direct laser illumination to compress a pea-sized capsule containing fusion fuel.
Says professor and LLE director Robert L. McCrory, “The ARPA-E award will fund research that will benefit from the existing strong collaborative effort between Sandia National Laboratories and LLE.” The two institutions already have traded scientific knowledge and laser components in pursuit of the grand challenge of laboratory-scale fusion. “LLE, with its 60-beam OMEGA and 4-beam high-energy OMEGA-EP lasers, and Sandia, with the world’s largest pulsed-power machine at Z, provide unique capabilities to explore a range of fusion parameters previously unexplored,” he says.
Nuclear fusion joins small atoms like hydrogen, releasing huge amounts of energy in the process. Unlike nuclear fission, which splits large atoms such as uranium, the dream of fusion is that it eventually could provide humanity unlimited energy from sea water and from such abundant elements as lithium with significantly less radioactive hazards than fission energy.
Unlike fission, fusion requires that matter be brought to enormous temperatures such as those found in the center of stars, approximately 50 to 100 million degrees. The challenge of fusion is to create matter at such temperatures at high enough pressures and for long enough times to release significant amounts of energy.
“Creating a high-yield reaction in a MagLIF plasma at Z should demonstrate the promise of the broader field of research we call magneto-inertial fusion — a potentially inexpensive form of fusion,” says Dan. “The overall grant objective is to improve techniques to compress and heat intermediate-density, magnetized plasmas, as well as to provide insights into relevant energy losses and instabilities. We hope that the results of our research will successfully motivate more investment by the Department of Energy and private companies in this field.”
An advantage of laser heating is that ideas involving lasers can be tested on multiple facilities across the country, allowing a much larger number of tests per year than is possible on the unique Z facility.
“It should easily be possible to do more than 200 laser shots a year split among the Z-Beamlet, OMEGA, and OMEGA-EP facilities, in contrast to the two dozen or so integrated MagLIF experiments a year realistically possible on Z,” Dan says.
A new path in fusion research
The LLE’s OMEGA laser, funded and operated as a national user facility with more diagnostics than Z’s Beamlet laser, is expected to greatly speed the work. “OMEGA can fire 12 times per day and can also provide better diagnostic access,” says Jonathan Davies, a research scientist and leader of the effort at LLE. “The ARPA-E project will bring together the resources of Sandia and the LLE to work on the same project with completely different techniques.”
Integrated laser experiments, where 40 of OMEGA’s 60 beams are used to compress the liner as well as heat the magnetized fusion fuel it contains, are also part of the ARPA-E program. “These experiments will allow us to study MagLIF on a much smaller scale and at a faster rate than on Z,” says Davies. “If the small-scale MagLIF experiments are successful and accurately modeled, we will have demonstrated magneto-inertial fusion principles over a very broad range of energy, space, and time scales.”
The collaboration will study fusion in a relatively unexplored intermediate density regime between the lower-than-air density of magnetic confinement fusion, which uses magnetic field to contain fusing plasma, and the greater-than-solid density of ICF, which uses X-rays or direct laser illumination to crush pellets of fusion fuel over times less than a billionth of a second. “With this collaboration, we will apply our expertise to explore a new path in fusion research,” Davies says.
The work will consist of four parallel efforts: achieving fuel pre-heating; determining whether MagLIF can reach fusion conditions on Z and on the OMEGA laser; and validating simulations against experiments.
-- Neal Singer