University of Rochester

Testing of Super-Cooled Fusion Targets Under Way

February 27, 2001

A super-cooled laser target comprised of solid hydrogen has been imploded by researchers at the University of Rochester. The experiments were done with the most powerful ultraviolet laser in the world, Omega, which is a testing platform for technologies to explore fusion at the National Ignition Facility (NIF) under construction at Lawrence Livermore National Laboratory in California. The super-cooled target tests are the latest in a series of experiments aimed at creating sustainable fusion, which could generate near-limitless power from water.

To pack the biggest punch in the smallest space, scientists at the University's Laboratory for Laser Energetics (LLE) filled a hollow plastic ball thinner than the lead in a pencil with fusion fuel under immense pressure and chilled it to minus 424 degrees Fahrenheit. The tiny pellet is then mounted inside the target chamber, and in a split second it is stripped of its protective housing and blasted with more energy than 100 times the peak power of the entire U.S. power grid. In less than a billionth of a second, the laser sends the temperature in the pellet from just a few degrees above absolute zero to nearly 50 million degrees Fahrenheit-twice as hot as the core of the sun.

"So far we're just testing the system," says David Harding, senior scientist at LLE, "but we have fired on several frozen targets and the results are looking good."

The ultra-cold approach will be used at the NIF to create a fusion reaction that generates more power than it consumes for the first time on Earth. Fusion occurs when atoms are compressed so hard they fuse together and release incredible amounts of energy-nuclear weapon detonations are essentially uncontrolled fusion and fision reactions. One of the key limits to fusion is the amount of material being fused. By chilling material to such low temperatures, more can be squeezed in beneath the laser's crosshairs.

In the 1980s, University of Rochester scientists were the first to design a smoothing method that focuses the laser more uniformly than any high-power laser had ever done before, and they developed a way to shift Omega's laser light into the ultraviolet to make its energy strike the target more efficiently. This method has since been adopted by all high-power solid-state laser fusion programs in the world as researchers strive to close the gap between fusion theory and reality.

The whole process starts with the pellets-some of the smoothest, most perfectly round objects in the world. Each pellet must be perfect so that the energy from the laser is most efficiently transferred to the pellet to effect the implosion. To create these millimeter-wide objects, researchers heat a hydrogen and hydrocarbon gas and let it condense on a tiny polymer ball, much like moisture from the air condenses on the sides of a cold soda bottle. When the condensed gas has cooled and formed a shell around the ball, they are both carefully heated again, forcing the polymer ball to essentially melt inside the shell and permeate out as a gas, leaving just the hollow shell.

Harding says that just as the semiconductor industry has been able to lower the cost of chips, the fabrication of pellets could be made cheaper and easier through mass production.

Harding takes the pellets, four of them at a time, into a room bristling with stainless steel canisters and liquid nitrogen tanks. There he places the pellets into a pressure vessel located inside a vacuum vessel and fills the pressure vessel with deuterium-tritium gas. Deuterium and tritium are actually just regular hydrogen atoms with one and two extra neutrons, respectively. It's those neutrons that catalyze the fusion reaction, so Harding wants to increase the number of them in the pellet.

Over the course of a week, the gas is slowly pressurized up to 20,000 atmospheres-18 times the pressure of the deepest part of the ocean. The pressure has to be just enough to force the gas through the plastic shell of the pellet without crushing it. When the pressures inside and outside the pellet are both a steady 20,000 atmospheres, Harding triggers the pressure vessel to move the pellets-still under constant pressure-to a cooling chamber. There the tricky task of cooling the pellets from room temperature to minus 424 degrees Fahrenheit takes place.

"This was the hardest part of the whole procedure," says Harding. "If we don't cool it at exactly the right rate the pellet would either explode or implode."

As material cools, it contracts, as anyone trying to get the collapsed lid off a Tupperware bowl of once-hot soup knows. Likewise, ss the pellet is cooled, the pressure around it must decrease in step or the pellet will buckle. Over the course of several more days, researchers watch as their instruments cool and depressurize the pellets at the perfect rate until, finally, the pressure inside the cooling chamber is below one atmosphere-room pressure.

With the pellet safely kept in the chilled vacuum vessel, the scientists push the contraption down the hall and through a series of airlocks to a low-ceiling room that looks like a basement packed with electronic gear. Above a nondescript spot in the room the ceiling is cut away, revealing the bottom of the Omega targeting chamber. The 11-foot chamber is the focal point for the football-field-sized laser.

The tiny pellet is lifted into the chamber and positioned with an accuracy of a hundredth of a millimeter, all while kept within a shroud that keeps it chilled at exactly minus 424 degrees Fahrenheit. When the laser begins its firing sequence, the shroud is whisked off the pellet at five meters per second, and a billionth of a second later, more than 100 times the total power used by all the businesses and homes in the United States is focused on the one-millimeter pellet. The lasers actually crush the pellet from 60 directions at once, vaporizing the plastic shell and sending it as a kind of shock wave into the frozen ice inside, heating the atoms and causing them to undergo momentary fusion. If the laser could keep the pressure on longer, the fusion reaction could eventually generate more power than the laser expended to create it.

Keeping the pressure on long enough is the goal of the NIF, which will focus 192 laser beams on a frozen target and heat it to more than 100 million degrees Fahrenheit.




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