Engineers at the University of Rochester's Laboratory for Laser Energetics (LLE) today unveiled the world's most powerful ultraviolet laser, Omega.
The $61 million Omega, completed on time and within budget by LLE staff through funding from the Department of Energy, will play a key role for the next several years in the nation's quest to develop nuclear fusion as a reliable energy source.
The laser makes Rochester home to the world's largest direct-drive fusion effort, where scientists use lasers to directly illuminate, heat and compress a tiny target of hydrogen fuel to fuse hydrogen atoms and release energy.
The new system will allow scientists to study the conditions necessary to ignite and sustain a fusion reaction more closely than previously possible. Results from experiments on Omega will have a significant impact on the National Ignition Facility (NIF), a huge 192-beam laser fusion system planned for later this decade. Scientists at Rochester, Lawrence Livermore, Sandia, and Los Alamos laboratories are designing the NIF, which will be the biggest fusion machine ever built. The Department of Energy has designated Livermore as the preferred site for the NIF and has requested funding for the project in 1996.
"This will allow us to show the efficacy of the direct-drive approach, and to study the physics necessary to ignite fusion reactions and, ultimately, to harness fusion power," says Robert McCrory, director of the laboratory. "Omega will keep open as many options as possible."
The football-field size OMEGA is 25 times more energetic than its predecessor, putting out up to 45 kilojoules of energy in the ultraviolet wavelengths. The 60-beam system, designed to be fired up to once per hour, has passed all of the technical milestones set by the Department of Energy. The system took four and one-half years to design and build.
Omega is the world's most powerful ultraviolet fusion laser, exceeding the present capability of the Nova system at Lawrence Livermore National Laboratory (LLNL) in California. Livermore scientists use Nova for indirect drive experiments, where laser beams are converted to X-rays before hitting a target. Although the new Omega was designed primarily for direct-drive experiments, it can also perform precision indirect-drive experiments that complement the indirect-drive capability of the Nova laser at LLNL. Research with Omega is expected to help physicists understand the physics behind both methods. Since LLE is designated as the National Laser Users' Facility, other scientists from around the country will use the facility to conduct high-energy laser experiments.
"The Omega Upgrade will play a major role in advancing ICF and helping to ensure the success of the NIF," says Michael Campbell, associate director of Livermore. "We at LLNL, and the other laboratories participating in the program, look forward to utilizing this wonderful facility with our Rochester colleagues."
At the Department of Energy, Assistant Secretary for Defense Programs Victor H. Reis stated, "The Omega Upgrade is a first- rate and highly flexible world-class laser that will serve the inertial fusion program and our science-based stockpile stewardship program well for many years. The University of Rochester is a potent and cost-effective team member of Defense Programs. The laser was on-time and on budget. The department is proud of this, the newest, of our facilities."
Tests so far show that Omega's laser beam is one of the best, if not the best, ever produced by a glass laser ("best" means its intensity is distributed evenly across the beam -- the beams are "clean"). This is especially amazing when one considers that after its creation, Omega's beam is amplified, split and filtered many times, traveling more than 500 feet and expanding to 60 beams before reaching the target.
In the target chamber, the beams converge on a target less than a millimeter wide filled with hydrogen isotopes, ablating the target's shell and imploding the thermonuclear fuel of hydrogen isotopes to obtain such high pressures and temperatures (hotter than the inside of the sun itself) that the hydrogen isotopes fuse. All this happens in less than a nanosecond, or a billionth of a second.
"The successful upgrade of Omega is the latest in a long series of remarkable accomplishments by the Laboratory for Laser Energetics," says Thomas Jackson, president of the University. "We're very proud of the key role the laboratory plays in this nation's quest to harness fusion as a reliable source of energy for the future."
LLE is the largest unclassified fusion laboratory in the nation and is an important source of graduate students trained in the field. The laboratory is supported by the Department of Energy, the New York State Energy Research and Development Authority, and the University. The laboratory employs about 220 scientists and staff members and 100 students.
ANATOMY OF A LASER FUSION SHOT
In laser fusion, scientists try to re-create the process that powers the sun and other stars by using laser beams to heat and compress a tiny target of hydrogen to such extreme pressures and temperatures that atoms fuse, releasing energy. Maintaining uniform temperature and pressure is critical. Scientists liken the process to instantly trying to squeeze a balloon down to a tiny size with your hands while keeping it intact; even the slightest aberration will cause the balloon to rupture, ruining the experiment.
For several minutes before every laser shot, huge capacitors beneath the main laser bay store large amounts of electricity. Engineers check and ready diagnostic equipment around the target, along with the computers that are key to controlling the laser beam and analyzing each shot's results.
About once per hour, an engineer commands a computer through a console in the control room above the laser bay, and the capacitors release their huge bank of energy, powering a laser beam that enters the laser bay from below. Beginning as a single beam, the light is amplified, split and filtered several times as it rushes the length of the laser bay, reflects off of mirrors, and then rushes back toward the target -- a tiny sphere less than a millimeter wide containing hydrogen isotopes.
Omega is actually two laser beams in one. The first part of the beam is a "foot pulse" that hits the target for several nanoseconds (billionths of a second), bathing the target in relatively low-intensity light and tailoring the target's temperature, pressure and density for each experiment. Within the tail end of this foot pulse is Omega's main pulse: a foot-long chunk of light, about the size of a football in each of the 60 beams. In less than a nanosecond, the beams converge on the target, burning off the outer shell of the sphere so rapidly and forcefully that the atoms inside the shell are pushed together and fuse.
Scientists compare the process to the force a rocket produces when it takes off from earth. As its fuel tanks ignite, the rocket's exhaust pushes mightily against the earth. Similarly, as the shell's outer sphere is burned away, the remainder is jettisoned inward (scientists call this "imploding"), compressing the fuel and creating temperatures even hotter than found inside the sun. The high temperature and density make it possible for the atoms to fuse.
As the atoms fuse, they give off energy in the form of neutrons, which can be used to generate electricity. For fusion to be useful as an energy source, scientists must learn to control the rate of fusion and develop reactors that will put out more energy than it takes to create the initial reaction.
SOME ROCHESTER CONTRIBUTIONS TO FUSION RESEARCH
Rochester's Laboratory for Laser Energetics (LLE) has made significant contributions to fusion research. Among the major accomplishments:
1995 -- LLE scientists complete the upgrade of the Omega laser, making Omega the most powerful ultraviolet laser in the world. The quality of Omega's laser beam surpasses that of all previous large glass lasers.
1989 -- LLE scientists announce a new method to vary the color (wavelength) of the laser light produced by the OMEGA laser, to create a more uniform illumination pattern on the target pellet. This technology, Smoothing by Spectral Dispersion (SSD), reduced the variations in illumination of a pellet from 30 percent down to just a few percent. Uniform illumination is key to the fusion process; such uniformity on a high-power multi-beam laser system had not previously been demonstrated. SSD has since been implemented on the Nova laser at Lawrence Livermore National Laboratory in California.
1988 -- LLE scientists compress a pellet of liquid deuterium-tritium to more than 200 times its liquid density; at the time this was the highest density ever measured for a fusion fuel pellet. Such high densities are necessary for fusion to occur.
1980 -- LLE scientists develop a way to convert OMEGA's laser light from infrared to ultraviolet light, which is absorbed more efficiently by a pellet of fusion fuel. This method of "blue-light conversion" has been adopted by all high-power solid- state laser inertial confinement fusion programs in the world.
The LLE also has made extensive contributions in other areas, including liquid crystal optics, high-speed switching, X- ray laser technology, spectroscopy, and ultrafast science.
OMEGA UPGRADE FACT SHEET
Energy output: Up to 45 kilojoules in the ultraviolet, and over 60 kilojoules in the infrared.
Peak power: Omega's main pulse packs a walloping 60 terawatts -- nearly 100 times the peak power of the entire U.S. power grid -- into each shot. That's because most of the laser's tremendous energy is unleashed in just a billionth of a second.
Size: 60 beams, covering an area the size of a football field. Omega is the largest laser in a university setting in the nation, and the most powerful ultraviolet laser in the world.
Cost: $61 million.
Components: More than 500,000 parts compose the system, including high-quality mirrors and lenses, optical mounts, amplifiers and filters, 1,200 mini-computers to help aim the laser, and $6 million of neodymium-doped laser glass.
Target chamber: The hydrogen target is housed in an 11-foot aluminum target structure. The structure has ports for several diagnostic instruments as well as for the 60 beams which are reflected by mirrors around the structure into the chamber.
Target: The target itself is a plastic or glass shell less than a millimeter wide, filled with deuterium and tritium, isotopes of hydrogen.
Other features:
Hotter than the sun: Temperatures inside the target chamber can reach up to 50 million degrees -- hotter than even the inside of the sun. The heat generated by a laser shot is why huge lasers such as Omega can be operated only once per hour: The laser's glass, mirrors and lenses need several minutes to cool down after each shot, to prevent overheating.
Dust-free: Since even tiny pieces of dust can damage the laser's optics when the laser shines, the entire laser bay and target chamber is a "clean room." Personnel are allowed inside the rooms only to perform maintenance, and they must wear special clean suits at all times in the laser bay.
Precision aiming: Each of Omega's 60 beams is aimed to hit a specified section of the target within 25 microns (25 millionths of a meter), less than half the width of a human hair.