Fusion energy promises to combine the benefits of renewable resources — clean, carbon-free electric power — with the best qualities of fossil fuels: power day and night, without regard for the vagaries of weather.

The reality is much messier. Fusion power demands heating certain isotopes of hydrogen or other light elements to hundreds of millions of kelvin until they form ionized plasma. The plasma is contained by magnetic fields in a toroidal (doughnut-shaped) chamber until the nuclei fuse and convert mass into energy.

Physicists have struggled to harness fusion for more than six decades. Only in 2006 did an international consortium sign an agreement to start work on ITER, the first reactor designed to ‘ignite’ fusion plasma such that it will be able to sustain its burn and generate more energy than it consumes. ITER has been under construction since 2010 on a site next to the Cadarache nuclear-research facility north of Marseilles, France, but building costs have soared to roughly US$50 billion — 10 times the original figure — and the schedule has slipped by 11 years. Instead of 2016, ITER is expected to start its first burning-plasma experiments in 2027— but only if the ITER team can solve technical challenges. ITER’s plasma chamber follows the tokamak design that has dominated fusion-energy research since the 1970s. Multiple magnetic coils, fuel injectors and the like make tokamaks large and complex.

Even more problematic is the fusion fuel that ITER will ultimately use: a mix of the hydrogen isotopes deuterium and tritium. The mixture has the virtue of igniting at just 100 million kelvin, lower than other potential fuels, but it also produces most of its energy as neutrons, which will damage the reactor walls — and make the reactor radioactive, producing another nuclear-waste-disposal problem.

Given these realities, the prudent course for the world’s funding agencies would be to support research into alternative fusion fuels, such as deuterium–helium-3 or proton–boron-11 — which require higher temperatures to ignite, but produce very few neutrons — as well as alternative reactor designs that would be simpler, cheaper and more in line with the kind of plant that power companies might buy (see page 398).

But that is not happening, because of ITER. The treaty that set up the project requires each of the seven ITER Organization members (the European Union, China, India, Japan, Korea, the Russian Federation and the United States) to contribute a fixed portion to the cost of construction — whatever that happens to be. Overruns have left fusion programmes with little cash for anything but ITER and the research efforts that support it.

“ITER promises to provide insights that will be invaluable in any future power reactor.”

The European Union, responsible for 45.5% of the cost, has been able to keep up by moving money from other projects. But the 9.1% borne by the United States, which historically has been by far the most willing to fund alternative concepts, could not have come at a worse time for the nation. In 2009, as ITER’s costs increased, fusion-programme managers in the US Department of Energy were told by the administration of President Barack Obama that they would have to fulfil their share of ITER from a flat budget. In the ensuing crunch, nearly all the department’s alternative fusion-research programmes have been cancelled.

Congress is furious. This year, the Senate voted to cancel the US contribution to ITER in fiscal year 2015, although the House of Representatives voted to maintain that contribution by boosting the fusion budget. Those contradictory decisions will have to be reconciled in the final budget. But in the meantime, following a congressional mandate in last year’s budget resolution, the energy department has convened a panel of scientists to devise a ten-year strategic plan for fusion-energy research — something the agency has not had for many years.

Both of these activities provide openings for Congress and the energy department to restore some of the funding for alternative fusion research. Academic projects worthy of consideration include a radically simplified design for a fusion power reactor developed by Thomas Jarboe and his group at the University of Washington in Seattle: they believe that it could be built for about one-tenth of the cost of a tokamak. And among the small fusion start-up companies worth considering for a federal small-business grant is Lawrenceville Plasma Physics in Middlesex, New Jersey, which is trying to exploit a configuration known as a dense plasma focus to build an extremely compact reactor that does not emit neutrons.

And ITER? For all its problems, ITER promises to provide scientists with key insight into the physics of burning plasmas — insight that will be invaluable in any future power reactor, whatever its design. On balance, assuming no more major delays or cost surprises, the United States and the other partners should continue their support for ITER — but they must not allow it to drive fusion energy into a dead end.