PARIS Physicists have dreamt about it for decades: harnessing the fusion process that powers the sun to make clean, safe and limitless energy. A multinational pact signed Tuesday may bring that dream a step closer to reality.

Seven partners representing half the world's population have agreed to build an experimental fusion reactor in southern France that could revolutionize global energy use for future generations.

Yet it is also just an experiment a bold, long-awaited, $12.8 billion experiment and it will be decades before scientists are even sure it works.

The ITER project by the United States, the European Union, China, India, Russia, Japan and South Korea will attempt to combat global warming by offering an alternative to fossil fuels. Controlling climate change and finding new energy sources are urgent goals for a growing global population.

French President Jacques Chirac hailed Tuesday's agreement as a victory for humanity and for France, which widely exports its nuclear energy expertise and beat out Japan in the bidding to host the reactor. The project's director will be Japanese, and Japan will supply the reactor's most complex parts.

Physicists have been trying for half a century to create fusion, which replicates the sun's power source, produces no greenhouse gases and generates relatively little radioactive waste.

The International Thermonuclear Experimental Reactor project, or ITER, recognizes that no single country can afford the immense investment needed to move the science forward.

It is expected to take eight years to build the reactor in Cadarache in the southern region of Provence. A demonstration power plant may be ready by 2040.