Year of Physics
Article of the month - February 2005
Discovery of the high confinement mode (H-mode) in plasma discharges
in the ASDEX Tokamak, first published in Physical Review Letters in
1982 |
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"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 |
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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
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.
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