Magnetic plasma plucking helps control fusion heat bursts

Researchers in the US have discovered how potentially damaging heat bursts inside the type of nuclear fusion reactor being built for the Iter experiment in Southern France can be controlled. This discovery could prolong the duration of fusion reactions, bringing steady-state fusion a stage nearer, the team hopes.

Tokamak reactors - where the hot plasma that undergoes nuclear fusion is constrained by magnetic fields inside a toroidal vacuum chamber - suffer from an effect known as ELMs (edge-localised modes), where the plasma becomes unstable near the edge of the chamber. This can damage the walls of the chamber, especially at the base where the waste products of fusion are removed.

A team from the US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), working on the DIII-D tokomak at General Atomics’ San Diego facility, discovered some years ago that ELMs could be suppressed with magnetic fields much smaller than those that confine the plasma, which seemed to allow the edge of the plasma to release heat slowly rather than in a damaging burst. A new team, led by Carlos Paz-Soldan of General Atomics, now believes they have discovered how this happens.

Pas-Soldan and his team, along with another team led by Raffi Nazikian of PPPL, have found that the small fields create two effects, rather than one as was previously thought. They used two sets of magnetic coils: one to create the field and the other to act as a pick-up to detect the plasma’s response, like a guitar pick-up detects the vibrations of a plucked string. The teams have reported their findings in Physical Review Letters.

The equipment detected the ripple in the plasma that allowed heat to leak, but also identified exactly what form the ripples took.  The magnetic field seemed to tear in a narrow layer. “The configuration changes suddenly when the plasma is tapped in a certain way, and it is this response that suppresses the ELMs,” Nazikian said in a statement, 

This result could help the teams at Iter stabilise the magnetic field in such a way as to allow long plasma pulses to be sustained, as in previous Tokamaks ELMs have cut short plasma generation.

“The identification of the physical processes that lead to ELM suppression when applying a small 3D magnetic field to the inherently 2D tokamak field provides new confidence that such a technique can be optimised in eliminating ELMs in Iter and future fusion devices,” said Mickey Wade, the DIII-D program director.