Flaw in the £7bn Iter fusion power plan

By Roger Highfield, Science Editor
Last Updated: 5:01pm GMT 11/03/2008

Scientists have had to devise a magnetic "patch" to fix an explosive flaw in a £7 billion project to harness the nuclear power source that makes the Sun shine.
# £7bn deal to harness Sun's nuclear power

There were headlines around the world two years ago when a treaty launching the international fusion energy research project was signed, marking the latest step in an effort by thousands of scientists inspired by the thought that fusion could solve the world's future energy needs by generating vast amounts of electricity from water, without carbon dioxide emissions and with relatively little nuclear waste.

A violent eruption, called an ELM, 'spins off' energy onto the vessel wall. The pattern is made by plasma particles following the magnetic field pattern
A violent eruption, called an ELM , 'spins off' energy onto the vessel wall. The pattern is made by plasma particles following the magnetic field pattern

The project, which was first mooted as long ago as 1985 and could take 35 years, is known as Iter - International Thermonuclear Experimental Reactor, or "the way" in Latin.

Later this month physicists and engineers will discuss a new proposal to patch up a problem in the design of the fusion reactor, due to begin construction in Cadarache, France, which was first identified in studies five years ago by Alberto Loarte, a European physicist now working at Iter, and Tony Leonard, an American physicist in San Diego.

Iter will mark the essential step to determining whether the use of magnetic fields to confine plasma - a charged soup containing atomic remnants - can generate useful power in the latter half of this century. In essence, temperatures of around 100 million degrees will force particles in the plasma to fuse, releasing energy.

However, there are concerns about unpredictable bursts of energy - known as edge localised modes, or ELMs - akin to the eruptions on the Sun, known as solar flares, which could lead to an immediate shut down of the burning plasma and also reduce the lifetime of materials lining the fusion reactor walls.
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Experiments now suggest that these ELM eruptions occur around once every second and are much bigger than had been realised. Although they last only one millisecond, or thousandth of a second, each eruption has a power of 20 gigawatts - about the expected capacity of China's Three Gorges Dam. The journal Nature was told that this was like "throwing a handful of hand grenades" at the wall of the device every second.

One of the Iter team, Dr David Campbell, Deputy Head of Fusion Science, says the hand grenade comparison is misleading because the energy release is spread over a big area. There "are no circumstances in which the energy released by an ELM could become concentrated in a way which is analogous to the explosive energy in a hand grenade.

"The ELM can neither break components inside the device nor breach the walls of the device. The ELM doesn't "damage" the reactor in the everyday sense of the word. It is grinding away at the surface of the components inside the vessel and that disturbs the operation of the plasma to the point that it could shut off."

Iter already has one feature to deal with ELMs known as "pellet pacemaking", where pellets of frozen hydrogen are fired into the plasma every 25 thousandths of a second to provoke mini-ELMs instead of ones that could trigger a shut down.

Because of the size of ELMs, however, the team is considering a "belt and braces" approach using magnetic coils to help damp down the release of energy.

Norbert Holtkamp, the project's construction leader, will be told to use a complex arrangement of magnets to dampen the effects of the erosive blasts, by in effect poking holes in the reactor's magnetic bottle to bleed out some energy.

The main option - placing 27 magnets in three rings - could still mean that Iter can be completed by its scheduled deadline of 2016.

"We have to convince ourselves that the coils can be manufactured to a standard which will make them essentially 100 per cent reliable inside the Iter device, and that including them doesn't imply design changes to other systems inside the device which would make those other systems more costly or less reliable," says Dr Campbell.

"The extra coils produce a very small magnetic field, about 1 part in 10,000 of the total field from the magnetic bottle confining the plasma," comments Rick Moyer, a plasma physicist at the University of California, San Diego.

"But the structure of this very small perturbation allows it to alter the structure of the field in the edge, allowing just a small additional amount of plasma leakage that allows the plasma to remain below the threshold for the instability."

Dr Moyer concludes: "By adding a little bit of chaotic behaviour to the magnetic field that forms the "bottle" holding the plasma, it's possible to suppress these instabilities without degrading the overall plasma performance."

The aim of Iter is to obtain and study conditions approaching those needed in a fusion power plant. Once it has gone into operation in about a decade the aim is to have it eventually generate 500 MW of fusion power for extended periods, ten times more then the energy input needed to keep the plasma at the right temperature.

Once Iter has provided a full scientific demonstration of the feasibility of fusion in power plant-like conditions, as the first fusion experiment to produce net power, only then would it be followed by a demonstration fusion power station, dubbed Demo. The final step would be to roll out fusion technology across the globe.

http://www.telegraph.co.uk/earth/mai...sciiter111.xml