Lawrence Livermore National Laboratory
On the verge of creating a mini fusion explosion
Lawrence Livermore National Laboratory
A NIF hohlraum. The hohlraum cylinder, which contains the NIF fusion fuel capsule, is just a few millimeters wide, about the size of a pencil eraser, with beam entrance holes at either end. The fuel capsule is the size of a small pea.
The interior of the NIF target chamber. The service module carrying technicians can be seen on the left. The target positioner, which holds the target, is on the right.
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The eureka moment — the solution to a scientific problem that comes when the researcher was looking for something else — makes for interesting stories. More often the moment is the result of moving closer to the solution slowly, steadfastly day after day. That moment, or more precisely, a few picoseconds or trillionths of a second, is about to occur for Lawrence Livermore National Laboratory (LLNL) scientists. Mines Research Professor George Gilmer is one of those scientists. Gilmer actually wears two hats: professor in the Division of Engineering and physicist with the National Ignition Facility (NIF) at LLNL —he splits his time evenly between the institutions. The NIF laser and target area building at Livermore is a structure so large three football fields could fit inside. The complex experiments performed there required scientists and engineers to develop materials that could withstand NIF’s extremely high energies and make improvements in pulsed-power electronics, innovative control systems and advanced manufacturing capabilities. The NIF project is on the verge of creating a miniature fusion explosion — something that’s never been done before. It will be the first time scientists create conditions akin to those in the sun. The hitch so far has been in attaining ignition. Ignition is the reaction that occurs when a fusion event between two atoms in a group emits energy and/or particles that may cause other atoms to react. In this way the reaction spreads throughout the entire group. The process Gilmer and his colleagues are involved in takes a “target,” a pea-sized spherical shell containing deuterium and tritium (isotopes of hydrogen) enclosed in a gold cylinder. Then 192 pulse lasers in two-foot-diameter tubes, all aligned and controlled by a single laser, zap the target. The laser beams impinge on the gold cylinder and produce x-rays that strip the outer shell of the pellet and heat it to millions of degrees. The fusion of deuterium and tritium forms helium. If it works, the energy released by this fusion heats up neighboring atoms enough for the process to self-propagate and produce an explosion. Gilmer’s research is in materials science modeling. The same skills he developed for 30 years at Bell Labs for AT&T is now helping the scientists at Livermore working on this multi-billion dollar project prepare for ignition by ensuring the loaded shell retains its spherical symmetry. “The frozen deuterium and tritium form a neat little shell of uniform thickness inside the spherical shell. When the lasers ablate and compress the shell, it should end up about a thousand times smaller in volume and located in the center. If it’s off center, one region will bulge out, it becomes unstable and you don’t get the compression you need. A very important part I’ve been working on is understanding how to get uniform layers of these frozen gases on the inside of the shell,” Gilmer said. Numerical computer models aim to predict whether there will be enough compression for ignition to occur. NIF employs one of the most sophisticated computer control systems in government or private industry. LLNL also has some of the fastest computers in the world with more than a petaflop of available computing power to perform calculations. So far, the team’s major accomplishments have been getting the 192 laser beams to fire and getting symmetrical compression of the fuel capsules. The next step toward fusion, anticipated in summer 2010, is to begin experiments approaching ignition with capsules containing fuel. Applications for the research, which is funded by the U.S. Department of Energy, include basic science research such as understanding cosmic processes, large-scale clean energy production, improving nuclear reactor efficiency and nuclear weaponry. When the facility is operational, Uwe Greife, Mines physics professor, is planning to carry out astrophysical experiments. “We have close ties with Livermore in many areas but none more pressing or exciting than the campaign for ignition. This is undoubtedly the greatest experiment underway in the world. Achieving ignition will be a true scientific milestone on the road to clean energy,” said John Poate, vice president for Research and Technology Transfer at Mines.
Colorado School of Mines