Los Alamos National Laboratory astrophysicist Didier Saumon is one of the 29 authors of a study recently published in Nature.  Courtesy photo

BY MAIRE O’NEILL
maire@losalamosreporter.com

Los Alamos National Laboratory astrophysicist Didier Saumon is one of a large group of international researchers who have developed an experimental technique to measure the basic properties of matter by using lasers to compress hydrocarbon samples to 100 to 450 million times the Earth’s atmospheric pressure – the highest pressures achieved to date in a controlled laboratory experiment.

The lasers are part of the world’s highest-energy laser system located at National Ignition Facility at Lawrence Livermore National Laboratory (LLNL) in California. They are used to heat and compress a small amount of hydrogen fuel to induce nuclear fusion reactions, imitating the same temperatures and pressures that exist in the cores of stars.

Saumon has been interested in astronomy since he was a teenager and when he was an undergrad student at the University of Montréal in Québec, Canada, he realized he wanted to become an astrophysicist. He went on to earn his master’s degree in astronomy at the University of Illinois-Champaign and transferred to the University of Rochester-New York to complete his PhD in physics.

“After that, I did a postdoctoral fellowship at the University of Arizona in Tucson. That’s where I fell in love with the Southwest which I had never seen before,” Saumon told the Los Alamos Reporter in a phone interview Aug. 7. “After a few years there I got my first real job as a faculty member at Vanderbilt University in Nashville, Tenn.”

Saumon decided he wanted to spend more time focusing on research and had the opportunity to come to LANL.

“I had the opportunity to come back to the Southwest and I loved this area. I’ve been here 18 years now, enjoying myself working as a physicist at the Lab and living on the Pajarito plateau,” he said.

Saumon said his main research interest is the properties of matter such as that found in stars, in giant planets like Jupiter and also in experiments like inertial confinement fusion which he said is a very large national project that tries to achieve the fusion of deuterium and tritium to reproduce the energy source that powers the sun in the laboratory.

“We’re trying to produce this in a lab and my work relates to this in terms of the properties of matter in these really extreme conditions that are necessary to achieve these experiments and also the conditions in stars and very large planets,” he said.

With regard to the study just published in Nature, Saumon said most of the work was actually done by his experimentalist colleagues at Livermore.

“Over the last few years they developed a new experimental technique at their National Ignition Facility which is the most energetic laser in the world and was built as a research facility for inertial confinement fusion. The new experimental technique allows them to accurately measure pressure and density of a compressed sample at a new record of 450 million times the atmospheric pressure which is almost 10 times more than the previous record,” he said.

Saumon said this allows the experimental platform or technique not only to make matter that is very hot – tens of millions of degrees – but has the incredible pressures and also what’s crucial also is that it allows them to make measurements.

“Creating such an extreme state of matter is one thing, but to be able to measure it accurately is the really difficult part because these experiments last only a few billionths of a second so there is very little time to make the measurement,” he noted.

This is very exciting for the general program of inertial confinement fusion, Saumon said, as it allows them to measure the properties of materials that are relevant to that program and refine understanding of such exotic matter.

“In this case they were compressing a small 1 mm ball of plastic with the largest laser in the world to zap it in these extreme conditions and once it’s compressed it’s even. The measurements are made continuously as the plastic ball is compressed up to the maximum compression. So it’s quite a feat that they have accomplished and that’s just the beginning of the productive phase of this new technique. Now they’re going to be able to apply it to all kinds of interesting materials,” he said.

When he saw the team’s data, Saumon realized that they were reaching conditions that correspond to what is found in the interiors of a type of stars called white dwarfs.

“That got me excited because I have been applying my work and doing research on white dwarfs for quite some time, so what this represents to me is the first time that we can reproduce or create in the lab the conditions that are found in the interiors of white dwarf stars. That had never been done before,” he said.

Saumon noted that physicists like him model white dwarf stars and have to know how the materials they are made of behave under those conditions.

“And for white dwarfs that’s usually a theoretical exercise because there’s no data to compare our calculations with in terms of the properties of the materials. We can observe white dwarfs in the sky, that’s what astronomers do, and we have been doing this for a century, so we understand fairly well the surface of white dwarfs, which is all we can see. That’s where the light comes from. But what goes on inside is mostly hidden from sight and based on theory which is informed by the kinds of measurements that can be made with the new experimental technique,” he said.

“It is amazing that you can compress a pellet of plastic that is just a millimeter across and get information and reach conclusions relevant to things that are hundreds of light years away from us. That’s the beauty of physics in particular because physics applies everywhere at all times, basically,” Saumon said. “What we can do in the lab here – what is true in the laboratory is also true out in space as far as we can probe with our telescopes. It is the universality of physics that allows us to make these incredible leaps of thought from our labs to very large and distant in the universe.”

The study that was published recently in Nature had some 29 authors and about half of them are from LLNL.

“These collaborations usually evolve over a period of time. Many of these people have worked together before and have longstanding professional relationships. For example, some more senior people on the team mentor students or postdocs and these people will go on then to their careers elsewhere at other institutions, sometimes in other countries and they keep collaborating. International scientific conferences are a great way to meet other experts in our sometimes arcane fields of study,” Saumon said. “So you develop a wide network that way.”

He noted that one of the other authors, Gilles Fontaine from the University of Montréal, was his undergraduate mentor.

“When I saw the data and the connection with white dwarf stars I contacted him and we worked together on the interpretation of the data in that context. He was a world-renowned authority in the field and I’ve known him since I was an undergrad 35 years ago when we worked together and we kept in contact,” he said.

Saumon said he feels lucky that he achieved his dream but that he didn’t quite know what the dream was early on and whether there was a path to it.

“I wanted to work in astrophysics and I have been able to do that. When I was a teenager and very much involved in amateur astronomy, building telescopes and looking at the sky, I was reading about the activities of other amateur astronomers in North America. That’s how I learned that with their dry air and clear skies, the Southwest and California are the best places to practice this hobby,” he said. “When I got the offer to come and work at the Lab here, I thought, ‘This is it!’ This was my teenage dream – living and working out here and not only having a very satisfying career, but also to pursue my passion in very favorable conditions – living in a small town with dark skies like Los Alamos – is a great perk. My professional dream has been very much realized too so I am very fortunate actually. “