Reporters learned from the Institute of Plasma Physics of the Chinese Academy of Sciences Hefei Institute of Physical Sciences on the 3rd that the all superconducting Tokamak nuclear fusion experimental device (EAST), known as the "artificial sun", has made important achievements in physical experiments. The research team confirmed the existence of density free zones in Tokamaks through physical experiments based on the self-organization theory of boundary plasma wall interaction. The relevant research results were published on the 1st in the international academic journal "Scientific Progress". For future fusion reactors, fusion power is proportional to the square of fuel density, so high-density operation is an inevitable choice to improve the economic efficiency of fusion energy. The "density limit" is a purely empirical calibration discovered at the end of the 20th century. Tokamaks operating beyond the density limit will cause plasma rupture, and massive energy will be instantly released onto the inner wall of the device, affecting its safe operation. Although the international fusion community has gradually clarified that the physical process that triggers the density limit occurs in the boundary region by improving cross device empirical calibration and achieving ultra density limit operation under specific conditions such as core projectile injection, the physical mechanism behind it is not very clear. Based on this issue, the research team has developed a new theoretical model - the boundary plasma wall interaction self-organization (PWSO) theoretical model. Through this model, the team discovered the critical role of radiation instability caused by boundary impurities in triggering density limits, analyzed the boundaries of radiation instability, revealed the triggering mechanism of density limits, and predicted the density free zone beyond the density limit. In the experiment, the researchers relied on the all metal wall operating environment of the EAST device and used methods such as electron cyclotron resonance heating and pre inflation coordinated start-up to reduce impurity sputtering at the device boundary, delay the arrival of density limit and plasma rupture. The researchers also reduced the physical sputtering caused by tungsten impurities on the target plate by regulating its physical conditions, allowing the plasma to successfully break through the density limit and smoothly enter the predicted density free zone. The experimental results are highly consistent with the PWSO theory prediction, confirming for the first time the existence of the density free zone in Tokamaks. (New Society)