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Friday, 6 April, 2001, 11:59 GMT 12:59 UK
Super laser advances fusion research
University of Rochester
A burst of energy at the focus of the laser beams
By BBC News Online science editor Dr David Whitehouse

Scientists have taken a step forward in their attempt to harness fusion energy, the power behind the Sun.

One of the world's most powerful lasers, called the Omega, has imploded a super-cooled pellet of solid hydrogen as part of a research programme to find ways of compressing the element to a critical point where nuclear reactions will occur.

The Omega is a testing platform for technologies to explore fusion at the United States National Ignition Facility (NIF) under construction at Lawrence Livermore National Laboratory in California. It will be completed in 2003.

The super-cooled target tests are the latest in a series of experiments aimed at creating sustainable fusion, which could generate near-limitless power using water as fuel.

Very hot, very small

Nuclear fusion occurs when atoms are compressed so much that they fuse together and release vast amounts of nuclear energy.

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The National Ignition Facility will be completed in 2003
One way to do this is to heat gas to tens of millions of degrees and confine it in a magnetic cage. This approach is being investigated by several countries. Success is not expected for decades, if at all.

Another approach is to crush hydrogen pellets by firing laser beams at them from all directions. To ignite fusion power, scientists have to pack the biggest punch they can in the smallest space possible.

One of the limits to the process is the amount of material being fused. By chilling material to low temperatures, more can be squeezed by the laser.

Just testing

To find out how to do this, researchers at the University of Rochester's Laboratory for Laser Energetics (LLE) filled a hollow plastic ball with solid hydrogen under immense pressure and chilled it to minus 250 deg C.

The pellets are some of the smoothest, most perfectly round objects in the world. Each pellet must be almost perfect so that the energy from the laser is most efficiently used to start the implosion.

The tiny pellet was mounted inside a target chamber. As the laser beam struck it, the pellet was stripped of its protective housing in a split-second and blasted with more energy than 100 times the peak power output of the entire US power grid.

The lasers crush the pellet from 60 directions at once, vaporising the plastic shell and sending a shock wave into the frozen ice inside. The hydrogen atoms are superheated, causing them to undergo momentary fusion.

Bigger target

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 27 million deg C, twice as hot as the core of the Sun.

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

Eventually, the ultra-cold approach will be used at the NIF to create a fusion reaction that generates more power than it consumes.

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 50 million deg C.

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