There are still very many difficulties but perhaps in a few decades we could have commercial fusion reactors in cities providing cheap pollution-free power

Dr Alan Sykes

Fusion is the form of nuclear energy that powers the stars. Although, it has many advantages over conventional nuclear power, it has been technically difficult to develop.

The best approach appears to be to confine a superhot gas, called a plasma, in a magnetic field. Some success has been achieved this way using huge experimental fusion reactors.

But now, according to United Kingdom Atomic Energy Authority (UKAEA) scientists, making smaller versions of the same equipment may be technically easier, cheaper and swifter to develop. The most recent experiments show promise, they claim.

Leaner and swifter

"I believe that if our experiments are successful, and they are promising, we could be designing the forerunner of the first commercial fusion reactor," said UKAEA's Dr Alan Sykes, as he showed BBC News Online around his laboratory at Culham, near Oxford.

Dr Alan Sykes in the Mast control room

Called Mast (Mega Amp Spherical Tokamak), the new equipment could be the design breakthrough needed to make fusion power a reality - at long last.

It is a leaner version of a prototype fusion reactor that has already solved many technical problems.

"Building Mast is like building a fighter aircraft when you have already built an airliner. It could be faster and more efficient at reaching our goal of significant fusion power," said Dr Rob Akers, of the UKAEA.

Star power

Few would argue that fusion power holds great promise.

It is the energy that allows the Sun to shine. But taming the power that lights up the Universe is not proving easy. For almost 50 years, scientists have been trying to harness star power in the laboratory.

As nuclei fuse, a vast amount of energy is released

To make nuclear fusion happen atoms must first be broken down into electrons and atomic nuclei. This produces an electrically charged gas called a plasma. The bare nuclei must then be forced together so that they merge. Because like charges repel, this is difficult.

At the heart of our Sun, fusion takes place at a temperature of 15 million degrees and a pressure of 100,000 atmospheres.

Because it is not possible to reproduce these conditions on Earth, terrestrial fusion reactors must operate at lower pressures and higher temperatures - about 100 million degrees.

There is also the major problem of confining the plasma.

'Naughty child'

"A plasma is a form of gas that has a great deal of free energy that is just looking for a way out," explained Dr Akers. "You could say that plasmas are like naughty children."

Mast is smaller and leaner than previous tokamaks

The best way to control the plasma is to "bottle" it, corralling the electrically charged gas in powerful magnetic fields.

So far, the most successful magnetic bottle is a tokamak, a doughnut-shaped device invented by the Russians. In a tokamak, two magnetic fields are combined to confine the plasma.

The world's largest tokamak is called Jet, the Joint European Torus. It is also at Culham.

Using the Jet, scientists have heated plasma to 300 million degrees - more than is needed to achieve fusion ignition. But magnetic confinement is easier if the prototype reactor is small.

Smaller is better

"That is where Mast comes in," said UKEA's Dr Chris Warwick. "Mast keeps the plasma in a tighter configuration that is more energy efficient."

A plasma dances inside the larger Jet tokamak

Controlling the eddies and whirls of the writhing plasma so that it can burst into life as a miniature Sun has been a formidable, and so far only partially met, engineering challenge.

"If we follow the Mast idea and not the Jet one, we could imagine a string of medium-scale fusion reactors instead of a few very big ones," said Dr Sykes.

"There are still very many difficulties but perhaps in a few decades we could have commercial fusion reactors in cities providing cheap pollution-free power," he added.