Really Extreme Science
Most powerful laser creates phenomena of universe in physics laboratory
June 15, 2009
Todd Ditmire leads the way into his new laboratory below Robert Lee Moore Hall.
“We’re two floors underground and we’re about to go one more floor down to where we’re under the high bay,” the physics professor says, walking down a flight of stairs. “We’re underneath the plaza in front of Moore, three floors underground.”
He jiggles a key into a lock and swings open the door to a subterranean chamber worthy of a James Bond villain–if the villain were a university professor trying to do world-class research on a limited budget.
And, like the lair of a Bond villain, Ditmire’s lab is the home of a machine of world domination: the Texas Petawatt Laser.
It is the world’s most powerful laser, generating bursts of energy that surpass those that take place in stars.
“For a small area, for a small instance we have the brightest light in the universe,” said Ditmire, director of the Center for High-Intensity Laser Science.
When he says “small,” he means very, very, very small. And “brightest,” means exactly that. He might add powerful, too.
The laser has the power output of more than 2,000 times the output of all power plants in the United States. (A petawatt is one quadrillion watts.) The laser is brighter than sunlight on the surface of the sun, but it only lasts for an instant, a 10th of a trillionth of a second (0.0000000000001 second).
The laser gives University of Texas at Austin scientists and students the capability to experiment with high-energy reactions, simulate the workings of stars and other celestial bodies, and investigate nuclear fusion, the process that powers the sun.
Ditmire, a self-described “laser jock,” uses words like “zap” when describing how the laser works. Zap is a word not used in research papers produced from the laser experiments, but it clearly illustrates what the laser does and demonstrates Ditmire’s enthusiasm.
He worked and conducted research for several years on a previous petawatt laser at Lawrence Livermore National Laboratory in California. That laser was more powerful than the Texas Petawatt, but was taken offline in 1999.
The Texas laser was a beam in Ditmire’s eye even before he formally joined the faculty in 2000. He had been raising money for it before then.
The laser’s cost was $14 million, paid for mostly through the National Nuclear Security Administration, part of the Department of Energy. The project is paid for through 2012, Ditmire said.
How it works
It takes a combination of physics, optics, materials science and other disciplines to make a high-powered laser.
“We’re creating very high powers by amplifying a pulse of light to some reasonable energy and them compressing it down to a very short time duration,” he said. “Power is energy divided by time, right?”
The laser starts with a low-power infrared laser beam that has the energy of a mosquito, Ditmire said.
The beam is broken into different colors of different wavelengths. They are then bounced off amplifiers, which speed them up. At the end, the separated beams come back together and are compressed.
The compression concentrates the energy into a powerful pulse.
It’s like a tube of toothpaste in which the cap has been left off and the toothpaste at the opening has dried. A forceful squeeze on the tube and the toothpaste comes out in a big burst.
Ditmire came up with a couple of changes that make the Texas Petawatt more effective than the one he worked with in California.
The Texas Petawatt uses glass for its amplifiers. The glass has been enhanced so the laser can amplify and maintain a wider width of spectrum of colors than the first petawatt did.
“We don’t have to go to as high an energy to get the same power because our pulse is shorter,” Ditmire said.
To compress the beam, the laser uses pieces of optical equipment called gratings. The two gratings in the Texas Petawatt Laser cost about $500,000 and Ditmire calls them the project’s prized possessions.
“That’s the final step,” he said of the compression through the gratings. “The beam comes in, bounces off these gratings, brings the colors all together and bang, we have a pulse.”
The Texas Petawatt Laser is enclosed in a clean room because if the laser beam collided with a mote of dust, it would lead to problems.
Ditmire describes how an experiment, shooting the laser at a solid such as aluminum, works at such extremes of force and time:
“I zap it with a huge amount of power,” he said. “It goes to very high temperature. And now I’ve got something that’s still at solid density, but now is at the temperature of the center of the sun, 10 million degrees centigrade. It hasn’t had time to react to what’s going on. It will, on the scale of picoseconds, eventually explode.”
By the time all of that happens, the experiment is over. The scientist has collected the information needed and moved on.
“That’s how you do the experiment,” he said. “We observe what happens. We take snapshots of explosions.”
What is left, in the experiment Ditmire describes, is a hot, dense plasma, a state of matter about which little is known.
“We’d like to understand it if we’re going to model the inside of stars or if we want to make fusion work,” he said. “So that’s what we do in the chamber, create these hot dense plasma states and we study their properties: do they conduct electricity, how much pressure do they have at a given temperature?”
The laser can be used to simulate events that happen on much bigger scales because the mathematical equations that describe the events can be applied to the laser’s scale.
The math isn’t exact, but it’s close enough to gather good information. Besides, Ditmire said, the laser is a lot easier to reset for another experiment than the universe.
“The fact is with a supernova they get one shot every 100 years,” he said. “So we get a lot more.”
For Ditmire, the operative word for the laser is extreme. And he likes that.
“The duration of a pulse is an extreme fraction of a minute. The pulses of light are the shortest manmade events. It’s extreme in time, it’s extreme in intensity, it’s extreme in temperature,” the laser jock said. “It all adds up to being pretty sexy.”