The Iter fusion reactor has got the go-ahead for completion after the EU agreed additional funds.
The international project, based in Cadarache in the south of France, needed an extra 1.4 billion Euros. The money was approved yesterday.
Iter - the International Thermonuclear Experimental Reactor - is expected to be the first fusion reactor to generate as much energy as it uses. The current record is held by JET, which achieved 70 percent of input power.
But, say the scientists, Iter will produce ten times as much energy than it consumes: "For 50 MW of input power, 500 MW of output power will be produced," they say.
The fuel - a mixture of deuterium and tritium, two isotopes of hydrogen - is heated to above 150 million degrees C, forming a hot plasma. The plasma is contained in a doughnut-shaped vacuum vessel in what's known as the 'tokamak' concept of magnetic confinement.
Iter will be twice the size of the largest tokamak currently operating.
Strong magnetic fields are used to keep the plasma away from the walls; these are produced by superconducting coils surrounding the vessel, and by an electrical current driven through the plasma.
Iter is expected to be completed in 2017.
But, says the team, "Iter is not an end in itself: it is the bridge toward a first plant that will demonstrate the large-scale production of electrical power and tritium fuel self-sufficiency."
The next step will be the Demonstration Power Plant, or Demo. A conceptual design for such a machine could be complete by 2017. This could begin operations in the early 2030s, and, the team hopes, putting fusion power into the grid as early as 2040.