American scientists have conducted a latest experiment inside a small tokamak reactor, overcoming two key obstacles to achieving stable and powerful fusion reactions: allowing plasma density to exceed the limit by 20% and keeping denser plasma stable. However, the applicability of this technology to larger devices remains to be verified. The relevant paper was published in the journal Nature on April 24th. As a green energy source that does not produce carbon dioxide, nuclear fusion power generation is increasingly receiving attention. The most common way to obtain fusion energy is through the use of tokamak devices. In the tokamak fusion reactor, hydrogen isotopes deuterium and tritium are heated to ultra-high temperatures to generate plasma, and a strong magnetic field confines these charged plasmas in a magnetic cage. But currently, in order for nuclear fusion reactions to operate at the "optimal point" to achieve optimal power generation efficiency, two challenges need to be solved: increasing plasma density and effectively constraining denser plasma. In nuclear fusion reactions, there exists the so-called Greenwald limit. Beyond this limit, if the plasma does not break free from the magnetic field, it cannot increase density, but breaking free from the binding will damage the reactor. Increasing density is crucial for increasing power generation, and experiments have shown that the power generation of Tokamak reactors is directly proportional to the square of fuel density. In the latest experiment, a research team from General Atomics in the United States ran the Tokamak reactor at the DIII-D national fusion facility for 2.2 seconds, with an average plasma density 20% higher than the Greenwald limit. It is crucial that the new experiment is run under conditions where the constraint improvement factor is greater than 1, which means that the plasma is successfully confined in the appropriate position. However, the outer radius of the DIII-D plasma chamber is only 1.6 meters, and it is currently unclear whether this method is suitable for the next generation tokamak device with a radius of 6.2 meters under construction in France - the International Thermonuclear Fusion Experimental Reactor. Because plasma is very complex, small changes in conditions can lead to significant changes in behavior. Researchers have stated that many reactor designs require both high constraints and high density, which is the first experimental implementation of this. This achievement marks an important step towards practical nuclear fusion power plants, but commercial reactors may take many more years to achieve. (Lai Xin She)