With the construction of France’s international ITER fusion facility in full swing, the US Department of Energy has highlighted a breakthrough that could help to smooth the path to unlimited clean energy.
Self-sustaining fusion in bagel-shaped tokamak fusion reactors, such as those at ITER, is hindered by ‘sawtooth’ or ‘zigzag’ temperature and pressure fluctuations in the superheated plasma fuel. This can combine with other instabilities to bring the reaction to a grinding halt.
Complex simulations recently produced by a research team at the US Department of Energy (DOE) Princeton Plasma Physics Laboratory (PPPL) have hinted at the nature of magnetic flux pumping – a puzzling mechanism that keeps some plasmas zigzag-free.
Insights gained from the simulations could help engineers and scientists find ways to keep things on the level, according to physicist Isabel Krebs, lead author of a paper describing the mechanism behind magnetic flux pumping, which was published in September and recently highlighted by the DOE Office of Science.
“This mechanism might be of considerable interest for future large-scale fusion experiments such as ITER,” Krebs said.
Stirring things up
The secret appears to be in the plasma mix.
Krebs’ simulations showed that magnetic flux pumping takes place in ‘hybrid scenarios’ where the pressure is sufficiently high and the current at the plasma’s core is flat.
These conditions lead to a quasi-interchange mode that sets off a dynamo effect by generating a massive helical flow of plasma that swirls the central plasma to deform the central magnetic field. This combination generates a negative voltage that keeps the core’s current steady and prevents zigzag instabilities.
The research describes this effect as being similar to the phenomena that maintains the Earth’s magnetic field.
“In the case of the Earth’s magnetic field, the fluid is the liquid part of the Earth’s iron core. In the case of the hybrid tokamak scenario, the fluid is the hot plasma in the center of the tokamak,” the DOE highlight explained.
Krebs said the mechanism is self regulating, with the current staying just below the threshold for the sawtooth instability.
A PPPL media release suggested that ITER operators could use the information gleaned from the simulations to vary the timing of the neutral beam that superheats the plasma fuel to achieve a combination of temperature and pressure that triggers magnetic flux pumping and keeps zigzags at bay.