H-mode is the high-confinement regime of tokamak operation, discovered in 1982, in which turbulence is suppressed at the plasma edge and energy confinement roughly doubles.
The regime defines itself against 📝L-mode, the low-confinement state every discharge starts in. On the ASDEX tokamak in Germany in 1982, a team led by Fritz Wagner found that when neutral-beam heating power crossed a threshold, the 📝plasma spontaneously reorganized into a new state that held its heat about twice as long — high confinement, hence "H." The discovery was unplanned, and the regime's reliable appearance on machine after machine has made it one of the most consequential experimental findings in 📝fusion research.
What changes at the transition is the edge. Turbulence collapses in a narrow layer at the plasma boundary, forming an edge 📝transport barrier across which pressure climbs steeply — the 📝pedestal on which the hot core sits. The price is a new class of instability, the 📝edge localized mode, which periodically vents the steepened edge in bursts. H-mode is nonetheless the reference operating scenario for 📝ITER and for most 📝Tokamak power-plant designs, and the standard H98 📝confinement scaling that guides them is fitted to the international H-mode database. 📝Jerry Hughes led the paper projecting SPARC's H-mode access window and pedestal height, summarized in 📝The SPARC Physics Basis (2020).