U. Texas Petawatt laser will test fusion, star formation
(Comtex Business Via Thomson Dialog NewsEdge) AUSTIN, Texas, Jun 26, 2006 (Daily Texan, U-WIRE via COMTEX) --The plaza next to Robert Lee Moore Hall buzzed with action as students rushed to their next study session or test, swapping notes or finals war stories with their companions. Two floors below them, an underground chamber buzzed as well, as workers continued construction on the Texas Petawatt Laser. When the laser under RLM fires for the first time in 2007, it will produce one of the brightest lights in the known universe -- for a split second.
When it fires, the laser will produce 1,000 times more power than the entire output of the world's electrical grid, according to Dr. Todd Ditmire, the head of the project and professor of physics at the University. Ditmire and his colleagues will be able to test fusion power, the properties of matter at extreme temperatures and even the formation of new stars.
The laser operates on simple scientific principles. Scientists define power as energy divided by time. By reducing the amount of time the laser fires to a mere 100 femtosecond (or 10 to the -15th power of a second), Ditmire said his team can get the most power out of the laser.
The process starts in a room above the laser where about 20 20,000-volt capacitors gradually suck electricity from the University of Texas's power grid, Ditmire said. These capacitors energize the amplification tubes that will give the laser beam its tremendous power.
The laser begins its trek with little energy, Ditmire said. The electricity must travel through a series of tubes that amplify its beam. Each tube contains an amplifying material, usually glass, that is "excited" by lamps powered by the capacitors. Every time the laser passes through one of these sheets of glass it gains more energy.
However, Ditmire and his team, the High Intensity Laser Science Group, must take many precautions. The laser can only be fired in a "clean room," Ditmire said, as it will produce so much power that it could blow apart any dust, hair or clothing fibers that enter the beam. The team will use several techniques to temper the power of the laser, he said, because if they let the beam get too powerful, it could destroy the very machinery that produces it.
To scale back the laser, the scientists will let the diameter of the beam enlarge with each amplification, said Erhard Gaul, research associate at the Fusion Resource Center. A larger beam means less intensity, and therefore less risk of the laser destroying the instruments around it, Gaul said.
Lasers consist of many different colors, Ditmire said. To further lessen the power of the laser during the amplification process, optical gratings will break the laser beam into pieces of the color spectrum, and each piece will be amplified separately, he said. At the end of the amplification process, the laser will reach about 200 Joules of energy, and another grating will refocus the parts of the laser into a space about the width of a human hair, delivering a petawatt of power (1,000,000,000,000,000 watts) to whatever is placed in the target chamber.
The key to the laser's power is time compression, Gaul said. While the laser doesn't use much energy, all the energy it has is delivered in a matter of femtoseconds, putting the target under enormous stress. "The idea is to see how things react at extremes," Gaul said.
One of the first projects Ditmire's group will test is a small-scale fusion reaction.
Fusion, Ditmire said, is the process that powers the sun. Basically, two hydrogen atoms in the sun combine to form a helium atom. However, a helium atom weighs less than the combined weight of two hydrogen atoms. The missing mass, Ditmire said, becomes energy that bursts forth at the moment of fusion. The sun converts about four million tons of matter into energy every second.
The same process will take place underneath the RLM, on a much smaller scale. A nozzle will spray clusters of deuterium atoms into the target chamber of the laser. The laser will fire and explode the clusters. The deuterium atoms will burst from their clusters at great speed, and some of them will hit each other and fuse together, creating fusion power.
However, the laser won't be powering the campus anytime soon, Gaul said. The amount of energy created by the fusion reactions is nowhere near the amount of energy the scientists store in the capacitors to power the laser. But the tests will help the development of fusion power plants in the future, Gaul said. With the laser, the team of scientists can study the effects of the high levels of radiation produced on various metals and building materials so that the builders of future fusion plants will know the best materials to use in construction.
The birth of stars
Another experiment in the works, Ditmire said, is designed to unlock the secrets of the universe.
How new stars form is one of the questions that vexes astronomers. The leading theory states that supernovae (stars that have run out of hydrogen fuel and exploded) push clouds of gas into one another and that those gas clouds are the building blocks of the next star, Ditmire said. Astronomers have seen tell-tale clumps of gas at the edges of supernova shock waves that indicate the beginnings of star formation. However, the process takes millions of years, and scientists can't observe it firsthand, Ditmire said.
But the laser could shed light on the subject. Ditmire said he hopes to recreate a supernova explosion, on a small scale, in the laboratory.
The scientists will fill the target chamber with a gas at about 20 degrees Celsius, Ditmire said. A pin will be placed in the chamber with deuterium clusters on the tip. The laser will explode the deuterium, heating it to 10 million degrees Celsius. The explosion will push the cold gas surrounding the pin outward in a spherical shock wave that mimics the effects of a supernova. Ditmire's team will then search the shock wave for clumps so that they can study how these clumps form and determine whether the process could produce new stars.
The construction of the petawatt laser will cost about $14 million, said Mikael Martinez, the project's manager. The entire project will be paid for by the National Nuclear Security Administration, an agency within the Department of Energy that oversees the nation's nuclear weapons stockpile. The NNSA's Web site says it is "responsible for enhancing national security through the military application of nuclear energy."
While the fusion research isn't practical for the development of nuclear weapons, Gaul said the laser could help the NNSA with their research on nuclear weapons.
When a metal is heated slowly, it melts, and if the liquid metal gets hot enough, it dissipates into gas. However, the petawatt laser will deliver 10 million degrees of heat to its target in a split second. For a small amount of time, the target will retain that energy without exploding, creating a form of plasma, Ditmire said. Ditmire said the team could study the plasma to shed light on an astronomical phenomena known as brown dwarfs, which contain plasma. However, Ditmire said, the plasma is also produced by nuclear weapons.
The NNSA will have access to all the research done on hot dense matter, Gaul said, and the research could aid them in creating computer simulations of nuclear weapons explosions.
An international treaty prevents the United States from directly testing nuclear weapons, and Ditmire said the research using the petawatt laser will prevent the need for actual detonation of nuclear devices.
"I think it's a very worthy goal, because we can tell the president 'No, don't go set off nukes in Nevada,'" because those nukes could be tested without explosions, Ditmire said.
The laser could also assist the nation's nuclear weapons labs in other ways, Gaul said.
The University has a memorandum of understanding with Sandia National Laboratories, a nuclear laboratory run by the Department of Energy. The memorandum calls for "joint development and implementation of 'strategic program areas that enhance' Sandia's broad missions in national security." The partnership will allow Sandia scientists use of the laser.
"Personally, I don't like seeing the NNSA getting the universities involved in nuclear weapons work. Somehow this condones continued weapons work, and, even worse, this blurs the line between government, big business and higher education," said Scott Kovac, operations and research director of Nuclear Watch New Mexico, a nuclear weapons watchdog group, in an e-mail interview.
Gaul also said the UT laser will serve as a low-cost alternative when larger lasers are built. If one of the NNSA's nuclear facilities builds a $3 billion laser, Gaul said, it would cost about $1 million each time it fires. If they have experiments that could be conducted at UT for much cheaper, the Texas Petawatt Laser will oblige, Gaul said.
But the laser project will be available for all of UT's scientific departments, Gaul said. Already the Department of Mechanical Engineering has plans for extensive materials testing, said Johnathan Brewer, a mechanical engineering graduate student. If anyone brings the petawatt group a good research proposal, the group will gladly set up and run the tests for them, Gaul said.
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