A possible way to prevent super-hot gases in fusion reactors from damaging the containment vessel has been discovered, which could make fusion a more viable energy solution. Fusion reactors generate power by heating hydrogen plasma to 100 million degrees Celsius. This causes hydrogen isotopes to fuse together and release energy. But the blistering plasma has to be contained within a vessel using a donut-shaped magnetic field, created using several powerful superconducting magnets. Over time, the reactor's plasma-containing vessel will inevitably be damaged by instabilities known as "edge-localized modes" (ELMs)1 that occur when hot plasma bursts out of the magnetic field.
Unless these ELMs can be controlled, expensive components need to be replaced regularly. Researchers at General Atomics discovered that by using a separate magnetic coil to induce small perturbations in the reactor's main magnetic field, they found that they could bleed off enough of the plasma particles to prevent the ELMs from bursting out.
The potential solution, tested at an experimental reactor run by General Atomics, could make the next generation of fusion reactors more efficient, saving hundreds of millions of dollars a year. The method could be incorporated into the latest prototype fusion test reactor – the International Tokamak Experimental Reactor (ITER). Uncontrolled ELMs could be expected to damage a part of the ITER reactor called the diverter, which collects and removes helium (a by-product of the fusion reaction). This could have to be replaced every six months to a year says Todd Evans, a plasma physicist with General Atomics. "
Any changes to the ITER must go before an advisory group, notes Bill Spears, a spokesman for the project in Garching, Germany. He adds that there is no consensus on the amount of damage ELMs will cause. Currently, the plan is to only replace the reactor's diverter every two-to-three years, he says.
1. ELMs are MHD (Magnetohydrodynamic) instabilities driven by the steep temperature and density gradients, occurring at the edge of H-Mode plasmas. They are basically short bursts of particles and energy at the edge.
Nuclear fusion plasma problem tackled, NewScientistTech, May 22, 2006
ESOL simulation illustration from European Fusion Development Agreement-JET Bulletin, March 2006