In plasma physics and magnetic confinement fusion, the high-confinement mode (H-mode) is a phenomenon and operating regime of enhanced confinement in toroidal plasma such as tokamaks. When the applied heating power is raised above some threshold, the plasma transitions from the low-confinement mode (L-mode) to the H-mode where the energy confinement time approximately doubles in magnitude. The H-mode was discovered by Friedrich Wagner and team in 1982 during neutral-beam heating experiments on ASDEX.[1] It has since been reproduced in all major toroidal confinement devices, and is foreseen to be the standard operational scenario of ITER.
In H-mode, the change in confinement is the most apparent at the edge of the plasma where the pressure gradient increases rapidly due to the increase in edge density, leading to the formation of a pedestal-like structure in the radial profile of the plasma parameters. It also typically features a type of magnetohydrodynamic instability called the edge-localized modes (ELMs), which appear as periodic bursts of particle and heat flux and can potentially cause excessive heating of plasma-facing components.
The physical origin of H-mode is currently unclear. The improved confinement is believed to be related to significantly reduced plasma turbulence at the edge. A possible explanation concerns increased flow shear which suppresses turbulent transport at the plasma edge.