L-H transition is the abrupt, spontaneous switch of a tokamak plasma from the low-confinement to the high-confinement regime that occurs when heating power crosses a threshold.
The transition is the doorway between 📝L-mode and 📝H-mode, first observed on the ASDEX tokamak in 1982. It is startlingly fast: within about a millisecond, turbulence at the 📝plasma edge collapses, a 📝transport barrier forms there, and the whole discharge steps up into a state that holds its heat roughly twice as long. The prevailing explanation is that sheared E×B flows at the edge, fed by the steepening gradients themselves, suddenly grow strong enough to tear the turbulence apart — but four decades on, no first-principles theory predicts the threshold from scratch.
In its absence, the field leans on empirical scalings fitted to the multi-machine international database, in which the threshold power rises with plasma density, magnetic field, and machine surface area. The threshold is a design constraint of the first order: a 📝Tokamak intended to run in H-mode must budget enough heating power to punch through it, and access to the regime shaped the heating systems of ITER and of the machines that follow it.