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Fusion boffins crack shreddy eddy plasma puzzle

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Physicists in the US are patting themselves on the back today as they prepare to announce that they've got a grip on a knotty problem troubling anyone designing working nuclear fusion powerplants - which could solve pretty much all of the human race's problems, but have proved very difficult to actually achieve.

The idea of a fusion plant is to generate energy not by splitting rare heavy atoms as in today's nuclear powerplants, but by fusing together much more common light ones. This is the process which powers the Sun, the ultimate source of all other energy used by humanity, and it is potentially much more powerful than fission. Fusion powerplants would also enjoy a practically infinite fuel supply, and would produce much less in the way of troublesome wastes.

A fusion-powered humanity would be so rich in energy that pretty much all its problems would be solved: as much fresh water as anyone required could be made from the sea, deserts could become green, the cheapest way to produce petrol would be to make it out of carbon-bearing waste such as sewage or landfill, etc etc.

Unfortunately the only practical application of fusion so far has been in hydrogen bombs, where the terrific heat and pressure required to kick off a fusion reaction is created by using a fission bomb as the trigger.

For decades, top physicists and engineers have sought to create a sustained, contained reaction to generate heat and run a power plant. The usual means of doing this is to confine a superhot plasma of appropriate light nuclei under massive heat and pressure inside a doughnut-shaped vessel using magnetic fields to suspend the plasma and prevent it destroying the structure. Sadly, so far these doughnut "tokamaks" have always required more power to run than they can generate.

Another throw of the dice is planned, however, with a worldwide international collaborative project now building the multibillion-pound ITER tokamak at Cadarache in France. If ITER is successful it will generate more power than it consumes and a new era will dawn for the human race.

For that to happen, however, various issue with plasma containment must be solved - and solved now, so that the knowledge can be incorporated into ITER's design.

One such problem is the interplay between turbulence on the surface of the bottled-up plasma and flows which build up as heat increases, eventually causing the turbulent eddies to be suppressed. This is important as turbulence dissipates energy and could rob ITER of success if not properly understood and controlled.

However, boffins in the US have been studying this effect using the smaller DIII-D tokamak in San Diego. The problem was that, until work began in San Diego, nobody really understood how and when turbulence ceased as surface flow built up. But Dr Lothar Schmitz and his crew are pleased to report that their method of using microwave radar guns - not dissimilar to police speed guns aimed into the torus using focusing mirrors - has given them a good handle on what's going on.

"We found that the turbulent eddies on the surface of the plasma produced surface flows that eventually grow large enough to shred the eddies, turning off the turbulence," says Schmitz. "Much like the population of predators and prey find a balance in the wild, we find that the plasma flow and the plasma turbulence reach an equilibrium in the tokamak plasma."

According to a statement (pdf) highlighting the research issued by the American Physical Society:

The reduction in turbulence improves the thermal insulation provided by the magnetic field so that much less power is required to achieve temperatures required for fusion (100 million degrees). Until [these] new measurements were obtained, researchers were not able to observe the very rapid change in edge turbulence which occurs in less than a millisecond over a zone less than 1cm thick ...

Now that they've seen the process up close, Dr. Schmitz hopes to use the improved understanding to figure out ways to make it easier to achieve and maintain high thermal insulation in future fusion experiments such as the ITER experiment now under construction in France.

The so-called "H-mode" where turbulence ceases and a tokamak becomes much more efficient was discovered as long ago as the 1980s, but working out how to make it happen - and keep happening, sometimes a tokamak will flick in and out of H-mode hundreds of times a second - has been difficult.

Schmitz and his crew intend to present full details of their research at a physics conference next week. ®

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