Scientists have taken another stride forward in the quest to develop fusion power - the energy source that powers the Sun and other stars.
By Dr David Whitehouse
BBC News Online science editor
Fusion promises relatively cheap energy - the fuel is seawater - with far fewer pollution problems compared with conventional nuclear power.
The Z machine pulses with energy
But the path to this technology has been long and expensive, with no commercial generation in sight.
Now, US scientists have created a hot, dense plasma that produces thermonuclear neutrons - a step, they say, towards harnessing nuclear fusion energy they say.
The Z machine
To generate fusion power, atoms, usually light atoms like deuterium and tritium - isotopes of hydrogen - have to be forced to combine. This releases vast amounts of energy.
The problem is that forcing these atoms to fuse requires the use of superhot gasses, called plasmas, confined by intense magnetic fields. So far, success has been encouraging but limited.
An alternative method is to compress a pellet by firing powerful laser beams at it from all directions in the hope of forcing its atoms to fuse. Again, there has been encouraging but limited success.
The researchers at the Sandia National Laboratory in America have approached the problem in a slightly different way with their "Z machine".
The pellet before compression...
The Z machine does not confine low-density plasmas in strong magnetic fields, nor does it focus intense laser beams on a target.
Instead, it uses huge pulses of electricity applied with careful timing. The pulses create intense magnetic fields that crush tungsten wires into a foam cylinder to produce X-rays.
The action takes place within a small container, called a hohlraum, the size of a pencil eraser, positioned at the centre of the Z machine, itself 36 metres (120 feet) across.
The X-ray energy, striking the surface of a target capsule embedded in the foam cylinder, produces a shock wave that compresses deuterium within the capsule, fusing enough deuterium to produce neutrons - the signature of fusion.
The scientists say that while they have achieved fusion, an important step, their short-term goal is to achieve ignition, the point at which a fusion reaction becomes self-sustaining.
They envisage a much bigger machine than the Z that could achieve so-called high-yield fusion, which means that more energy is given out from the process than it takes to initiate it.
"Pulsed-power electrical systems have always been energy-rich but power-poor," says Ray Leeper of Sandia.
"That is, we can deliver a lot of energy, but it wasn't clear we could concentrate it on a small enough area to create fusion. Now it seems clear we can do that."
Measurements indicated that the Z machine can produce 10 billion neutrons from a compressed deuterium target, in line with predictions.
The researchers say it is a start but there is a long way to go before any significant amounts of fusion power are produced by the Z machine or its successors.