Scientists produce tokamak plasma that’s stable at 10x Greenwald limit. Future reactor-scale tokamaks will likely need to operate near or above the Greenwald limit.
Physicists from the University of Wisconsin–Madison have developed a tokamak plasma that is stable at ten times the Greenwald limit. Their achievement has implications for tokamak fusion reactors, but scientists caution that their plasma is not directly comparable to that in a fusion reactor.
The Greenwald limit was identified by Martin Greenwald almost 40 years ago as the density limit above which tokamak plasmas become unstable. Only a handful of devices have operated above this limit, and only by a factor of two.
Tokamaks are leading contenders for nuclear fusion reactors that generate power like the Sun. The new study published in Physical Review Letters is part of tokamak experiments in the Madison Symmetric Torus (MST) with a density up to an unprecedented level of about ten times this limit.
The radial profile of the toroidal current flattens around twice the limit without the edge collapse routinely observed in other experiments. MST has operated for many years as one of the leading programs studying the reversed field pinch, a toroidal configuration closely related to the tokamak.
Researchers looked into plasma density and tried to destabilize the plasma by puffing in more gas. They set the power supply to provide whatever voltage was needed to maintain a steady current in each plasma. The achieved plasma density was measured with interferometers viewing the plasma along 11 different lines of sight.
According to the University of Wisconsin, the Greenwald limit is just the ratio of the plasma density to the product of the plasma current and plasma size. Since the limit was defined, only a handful of devices have operated above it, and by at most a factor of two.
Future tokamaks will likely need to operate near or above the Greenwald limit. The researcher also underlined that these results are unlikely to be directly applicable to fusion reactors, such as ITER and others that are being built in the hopes of being the first net-positive energy production tokamaks.
Source: https://interestingengineering.com/energy/tokamak-plasma-stable-10x-greenwald