First experimental observation of "Staircase" high confinement mode in tokamak plasma

In magnetically confined nuclear fusion, the achievement of ignition necessitates the tokamak plasma in a high confinement state known as H-mode. A major challenge encountered in the standard H-mode is the abrupt collapse of its ultrathin edge layer. Here, we report a first experimental demonstration that a stable H-mode can be sustained by a staircase plasma profile. The basic physical picture is: by staircasing the plasma profile, the potential free energy stored in one single large edge layer is distributed into multiple mesoscale layers throughout the entire plasma volume. Consequently, the large scale edge collapse is converted into small scale desynchronized fluctuations, effectively reducing the risk of violent relaxation event. This finding uncovers a new benign high confinement regime of the tokamak plasma.