Finding a Fusion Method That Holds Water
Physicist Richard D. Petrasso has a nifty parlor trick. It challenges the performer with the same difficulty the United States Department of Energy's proposed $1.1-billion giant-laser facility must overcome to develop one form of hydrogen fusion power.
Fill a glass of water to the brim. Cover the mouth with a paper towel. Invert the glass while holding the towel in place. Carefully let go of the paper towel. If you do it right, the water stays in the glass. Sea-level air pressure is more than enough to support the water's weight.
Don't try this at home unless you're standing in the shower. Dr. Petrasso, who pursues fusion research at the Massachusetts Institute of Technology, does it to show how tough this kind of trick really is. Where an interface requires a lighter fluid (in this case, air) to support a denser one (water), instabilities easily develop. The two fluids begin to intermix. That destroys the interface and splat goes the water all over the floor.
This is an old story. British physicist Lord Rayleigh first elucidated it in 1880-- hence the term Rayleigh instability. But when Petrasso (successfully) performed his trick in Madison, Wis., for a recent meeting of the Council for the Advancement of Science Writing, he explained that Rayleigh's instabilities are impeding laser fusion. That includes the 500-trillion-watt laser facility the Energy Department wants for its Lawrence Livermore National Laboratory in California.
Here's the problem: Laser- fusion researchers in the US, Europe, and Japan typically work with tiny pellets of hydrogen fuel a millimeter or less in diameter and encased in a thin glass shell. The fuel is a mixture of deuterium and tritium - doubly and triply heavy hydrogen. Laser energy vaporizes the outer part of a fuel capsule. The vapor squeezes the fuel to high densities and temperatures where fusion can take place.
There you have it - a lighter fluid (vapor) pushing on a denser fluid (the hydrogen fuel). Rayleigh instabilities ruin the fusion reaction. The unresolved issue is whether or not researchers can restrain the instabilities long enough for useful fusion to take place. If they can, a single tiny fuel pellet could release as much energy as burning 3.5 gallons of oil. The National Ignition Facility proposed for the Lawrence Livermore laboratory would aim to ignite fusion reactions that produce more energy than the laser consumes.
The Energy Department has other agendas in proposing this facility. It would secure Lawrence Livermore's future at a time when the role of national laboratories generally is uncertain. It also would provide for continuing research in thermonuclear weapons. Fuel-pellet fusion is essentially a micro hydrogen-bomb explosion. The Defense Department, along with Lawrence Livermore, has been lobbying to keep such research going. They argue it is needed to maintain existing weapons stockpiles properly and ensure that the country has the expertise to cope with unexpected future weapons developments elsewhere.
There is no assurance the facility will materialize. A few weeks ago, Energy Secretary Hazel O'Leary announced approval only for design studies. But civil fusion-power research sweetens the military flavor of the project. To support this, the Energy Department has dropped much of the traditional secrecy that has cloaked Lawrence Livermore's fusion-power work. This is enabling closer cooperation with laser-fusion laboratories in Europe and Japan. Such cooperation means that the world now can move ahead more rationally with both main approaches to controlled fusion - laser fusion and fusion using magnetic forces to control the hydrogen fuel.
As Petrasso notes, both approaches ``are difficult ... but both deserve to be ... pursued vigorously.'' Unless the laser-fusion workers can overcome Rayleigh instabilities, however, they could wind up - metaphorically speaking - with water all over the floor. @QUOTE = Rayleigh's instability, explained by the 19th-century physicist, has come back to challenge today's scientists working on laser-fusion technology.