How turbulence sets boundaries for tokamak operation

Various boundaries, such as the transition to high confinement, density limits, and power exhaust requirements, impose limits on the safe and efficient operation of a tokamak. Turbulent transport at the separatrix plays a decisive role in these limitations. To consider the boundaries from a holistic perspective, all operation limits have to be derived using the same turbulence model. This results in an operation phase space in relation to the separatrix electron density and temperature. Here, we discuss the underlying physics, the historical development, and future improvements of this operational space.