When you design a machine, says David Campbell, Assistant Deputy Director General for Fusion Science and Technology, you "try to squeeze out as much operational capability as you can". In ITER's case this meant adding to the four "reference" or "design scenarios" a set of alternatives called "assessed scenarios", which would offer the fusion community "interesting things to explore."
As early as 2001, the ITER designers concluded that in order to reach a tenfold amplification of the input energy—the famous Q > 10, the plasma current intensity had to be set at 15 MA. JET, as a comparison has operated at currents up to 7 MA, and JT-60 has a capability of 5 MA. But the designers also made room for alternative scenarios with a 17 MA plasma current.
"Getting Q = 10 for 400 seconds is what the machine is aiming for", David Campbell says. "But we haven't excluded a higher Q, which we could get if we can increase the current intensity".
Increasing the current in the plasma—which improves plasma energy confinement time, one of the essential parameters for achieving fusion—is also an interesting option to compensate for any shortfalls such as energy confinement time at 15 MA not being as good as expected, or impurities being higher than predicted.
So "pushing" to 17 MA for 100 seconds is a possible extension to the tokamak's capabilities which some fusion experts during the recent Design Review have given a high priority.
Of course, as David Campbell says, "there's a number of issues". Operating at 17 MA will bring ITER's plasma closer to the "stability boundary" where, "if you're not careful", the plasma current can suddenly disappear, and the magnetic energy exert tremendous forces on the machine structure. "We have designed ITER so that the structures can resist the forces at 15 MA and it is possible that, at 17 MA, the forces will be too high. In any case, the regulator, the French Agence de Surete Nucleaire, will want us to demonstrate that disruptions at 17 MA do not damage the machine. So we'll 'shake' it a bit first at lower current to see what happens..."
After having deliberately run "disruptive events" in the first years of operation, and if Q = 10 is achieved in the 5 or 6 following years, ITER's operators might then try to go to 17 MA but without having a 100% cast-iron guarantee of getting there. At 17 MA, plasma in ITER could reach the threshold of the ignition regime. No one in the fusion community wants to pass by this opportunity to explore or even to glimpse this fascinating new territory. return to Newsline #40