H-mode
An operating regime in which turbulence at the plasma edge suddenly dies down and heat confinement roughly doubles. Discovered on Germany's ASDEX tokamak in 1982.
Until then, tokamaks suffered from confinement getting worse the harder they were heated (the state now called L-mode). Fritz Wagner and colleagues reported that above a certain heating power a thin insulating layer forms at the plasma edge and confinement abruptly improves[1].
It was not predicted by theory; it simply happened during an experiment. It proved highly reproducible, and today's tokamak reactor designs, ITER included, assume operation in H-mode.
Its weakness is the ELM: pressure built up in the edge layer collapses periodically, sending bursts of heat into the wall. Controlling ELMs remains a major research problem.
Related terms
- TokamakA doughnut-shaped machine that confines plasma by combining the magnetic field of external coils with the field produced by a current driven through the plasma itself. The dominant approach to magnetic confinement.
- Magnetic confinementHolding a hot plasma away from the vessel walls in a cage of magnetic field, using the fact that charged particles spiral along magnetic field lines.
- PlasmaA gas in which electrons have been stripped from their atoms, so that electrons and nuclei move around separately. Often called the fourth state of matter, after solid, liquid and gas.