Toxic metal dust, which physicists feared could choke their efforts to attain controlled nuclear fusion, may instead be disintegrated by the powerful forces inside fusion reactors.
Experimental reactors called tokamaks are doughnut-shaped devices that contain ionised gas, or plasma, at temperatures of more than 100 million degrees. The latest is a multibillion-dollar project called , set to be completed in southern France in 2019. It is designed to fuse together two heavy isotopes of hydrogen to release vast quantities of energy while producing no carbon emissions.
But doubts have lingered over whether the fusion reactions could poison themselves. That鈥檚 because the plasma bites little pieces out of the beryllium metal walls of a tokamak鈥檚 reactor vessel, generating fine metallic dust.
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The toxic stuff could 鈥渃log up holes in the instruments inside ITER鈥�, says of Imperial College, London. And if it gets into the inner core of hot plasma, it could even choke off the fusion reaction.
Molten clue
Coppins realised that this is not like ordinary industrial or household dust. The dust grains swept up in existing tokamaks are often spherical, implying that they had been molten at some stage. 鈥淚t occurred to me that the dust in this plasma is not solid particles but liquid droplets,鈥� he told 快猫短视频.
Liquid drops should be much less of a problem than solid dust. When one of them reaches the outer layers of the plasma ring, electrons in the plasma will bombard it and build up a strong negative charge. That will produce repulsive forces within the droplet. 鈥淚f it is of sufficiently small size, these forces will lead to total disintegration very swiftly,鈥� says Coppins.
Given the rapid rate of electron bombardment within ITER鈥檚 energetic plasma, he calculates that all but the largest beryllium drops will disintegrate in less than a nanosecond. That means they will be destroyed in the plasma鈥檚 outermost layers before they can reach and pollute the fusion reactions deeper inside.
Some relatively large droplets much bigger than a micrometre across may be able to reach the inner core, but with luck there will be too few of them to pollute the plasma fatally.
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