Posted on: 2nd February 2005

“At t=1.18 s, the density suddenly increases without modifications from the external controls. The gas valve closes, but nevertheless the density continues to rise and exceeds the value obtained during the plateau of the Ohmic phase. From bolometric measurements and from the intensity of OVI and FeXVI radiation (O and Fe are intrinsic impurities), it can be excluded that the density rise is caused by an enhanced impurity influx. All three signals, normalised with respect to the plasma density, decrease at the transition into the H regime.The increase in density is caused by a sudden improvement in particle confinement.”

from F. Wagner et al.: “Regime of Improved Confinement and High Beta in Neutral-Beam-Heated Divertor Discharges of the ASDEX Tokamak”, Physical Review Letters Vol. 49 (1982) page 1408

The H-mode was unexpectedly discovered in the ASDEX Tokamak at Max Planck Institute for Plasma Physics, Garching, Germany, on 4th February 1982, during intense plasma heating experiments in the new “divertor” configuration. The phenomenon was then confirmed by many other magnetic fusion experiments, including JET in 1986. (To learn about the current JET divertor see this Focus On article).

A transport barrier that builds up at the very edge of the plasma is responsible for the H-mode phenomenon. The barrier is due to suppression of plasma turbulences at the edge, but the detailed mechanism causing the suppression is still unclear and challenges many plasma physicists specialised in plasma theory and computer modelling.

The H-mode is characterised by an improvement of plasma confinement by a factor of about two, which enhances our prospects of mastering fusion power at an industrial scale. Nowadays the H-mode is considered to be a “standard scenario” for most magnetic fusion experiments. Indeed the future ITER device, which has been designed to release significant fusion power, is assumed to operate in the H-mode.

More information about the H-mode can be found, for example, in the on-line article Overview of Tokamak Results by B. Unterberg and U. Samm from Forschungszentrum Jülich, Germany.

Picture of the Month

picture of ASDEX

This picture shows the ASDEX tokamak where the H-mode was observed for the first time. The insert picture shows a plasma in ASDEX (click to enlarge). At present, the Max Planck Institute for Plasma Physics in Garching operates a larger tokamak, ASDEX Upgrade, and is building a major superconductive stellarator Wendelstein 7-X in its new branch in Greifswald. The Max Planck Institute for Plasma Physics is an EFDA Associate and its experts are also significantly involved with JET work, both on-site and remotely.