Natural density formation as an H-mode operational limit

The natural H-mode density, i.e. the plasma density evolving in an H-mode discharge without active fuelling, reaches Greenwald fractions in JET typically higher than in ASDEX Upgrade. According to general thinking this reflects device-specific differences as regards recycling-induced fuelling and beam fuelling. This paper presents evidence for a different view, namely that at sufficiently low plasma fuelling rates any fuelling rate dependence of the plasma density vanishes and the plasma particle content is completely determined by the plasma itself. It is shown that this limit, which would impose a lower boundary on the H-mode density, is reached in JET and ASDEX Upgrade natural density discharges, and its scaling is determined. Possible overlapping with existing density limit scalings in next-generation tokamaks is discussed with a view to a narrowing or even vanishing H-mode operation window.