subject: Scientists clear technical hurdle in fusion research
posted: Mon, 22 May 2006 17:39:21 +0100

Scientists clear technical hurdle in fusion research

Physicists working in the United States believe they have cracked an
important problem facing man-made nuclear fusion, touted as the
cheap, safe, clean and almost limitless energy source of the future.

In fusion, atomic nuclei are fused together to release energy, as
opposed to fission - the technique used for nuclear power and atomic
bombs - where nuclei are split.

In a fusion reactor, particles are rammed together to form a charged
gas called a plasma, contained inside a doughnut-shaped chamber
called a tokamak by powerful magnetic coils.

A consortium of countries signed a deal last year to build the
International Thermonuclear Experimental Reactor (ITER) in southern
France as a test-bed for an eventual commercial design.

But many experts have been shaking their heads at the many challenges
facing the ITER designers.

One of them is a phenomenon called edge localised modes, or ELMs.

These are sudden fluxes or eddies in the outer edge of the plasma
that erode the tokamak's inner wall - a highly expensive metal skin
that absorbs neutrons emitted from the plasma.

Erosion means that the wall has to be replaced more often, which thus
adds hugely to costs.

Eroded particles also have a big impact on the plasma performance,
diminishing the amount of energy it can deliver.

Writing on Sunday in the British journal Nature Physics, a team led
by Todd Evans of General Atomics, California, believes that the
problematic ELMs can be cleverly controlled.

They found that a small resonant magnetic field, derived from special
coils located inside a reactor vessel, creates "chaotic" magnetic
interference on the plasma edge, which stops the fluxes from forming.

The experiments were conducted at the General Atomics' DIII-D
National Fusion Facility, a tokamak in San Diego.

Nuclear fusion is the same process used by the Sun to radiate energy.

In the case of our star, hydrogen atoms are forced together to
produce helium.

On Earth, the fusion would take place in a reactor fuelled by two
isotopes of hydrogen - deuterium and tritium - with helium the waste

Deuterium is present in seawater, which makes it a virtually
limitless resource.

Tritium would be derived from irradiating the plentiful element
lithium in the fusion vessel.

The 10-billion-euro ($16.9 billion) ITER scheme entails building the
largest tokamak in the world at Cadarache, near the southern French
city of Marseille.

The partners are the European Union (EU), the United States, Japan,
Russia, China, India and South Korea.

It is designed to be a test bed of fusion technologies, with a
construction period of about 10 years and an operational lifespan of
20 years.

If ITER works, a prototype commercial reactor will be built, and if
that works, fusion technology will be rolled out across the world.

Other problems facing fusion technology include the challenge of
creating a self-sustaining plasma and efficiently containing the
plasma so that charged particles do not leak out.

In the various tokamaks, no one has achieved a self-sustaining fusion
event for longer than about five seconds, and at the cost of using up
far more energy than was yielded.

A huge jolt of heat, of nearly 100 million Celsius, is needed to kick-
start the process, which then has to be sustained by tiny amounts of
fuel pellets.


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