Researchers have been able to experimentally enter, for the first time, a new nuclear fusion regime, with plasma density beyond the conventional limit. While theories had suggested a strategy to circumvent such a limit, making it happen has been a whole other affair.
The breakthrough was performed at the Experimental Advanced Superconducting Tokamak (EAST), nicknamed the “Artificial Sun”, which is China’s cutting-edge fusion reactor. EAST has already broken a variety of records, including the longest sustained ultra-hot plasma. They kept it going for 1,066 seconds – that’s 17 minutes and 46 seconds of plasma well beyond 100 million degrees Celsius.
Nuclear fusion reactors work by reproducing what goes on inside the Sun. Hydrogen atoms merge, turning into helium atoms and releasing a lot of energy. At the core of the Sun, this happens thanks to high temperatures and pressures, so the plasma is very dense. In our fusion reactors, we cannot reach those pressures, so the temperature is 10 times hotter than in our little yellow star.
When it comes to the density, the current designs for reactors known as tokamaks (the ones shaped like a metal donut) place an empirical upper limit. This is known as the Greenwald limit, above which plasma develops instability. The limit is a problem; higher densities mean a higher fusion rate, but instabilities can stop fusion altogether. For this reason, in a tokamak, the density is kept between 0.8 and 1 Greenwald limit.
There have been theoretical pathways proposed to surpass the Greenwald limit; now, EAST has demonstrated them in practice. By tweaking the initial parameters, such as the gas pressure and the heating resonance of the electrons in the plasma – how the particles respond to microwaves – the team was able to get densities with no instabilities at 1.3 to 1.65 times the Greenwald limit.
“The findings suggest a practical and scalable pathway for extending density limits in tokamaks and next-generation burning plasma fusion devices,” study co-lead author Ping Zhu, a professor in the School of Electrical and Electronic Engineering at the University of Science and Technology in China, said in the statement.
The Greenwald limit has been a focus of interest for years. A few years ago, it was breached by over 10 times successfully, but that was in a low magnetic field and with low temperature plasma not capable of nuclear fusion. The EAST results show that there could be a path forward in actual fusion reactors.
The study is published in the journal Science Advances.
