The capacitor bank of the Dresden High Magnetic Field Laboratory. |
On
June 22, 2011, the Helmholtz-Zentrum Dresden-Rossendorf set a new world
record for magnetic fields with 91.4 Tesla. To reach this record,
Sergei Zherlitsyn and his colleagues at the High Magnetic Field
Laboratory Dresden (HLD) developed a coil weighing about 200 kilograms
in which electric current create the giant magnetic field – for a period
of a few milliseconds. The coil survived the experiment unscathed.
“With
this record, we’re not really that interested in reaching top field
values, but instead in using it for research in materials science,”
explains Joachim Wosnitza, the HLD’s director.
According
to Wosnitza, the scientists are proud of being the first user lab
worldwide to make such high magnetic fields available for research. The
more powerful a magnetic field is, the more precisely the scientists can
examine those substances which are used for innovative electronic
components or for so-called superconductors which conduct electricity
without any resistance.
Such
high magnetic fields are generated by passing an electric current
through a copper coil. But the magnetic field also influences the
electric current because it tries to push the electric current out of
the coil. The stronger the current flows, the more powerful these forces
are.
“At
25 Tesla, the copper would be torn apart,” Joachim Wosnitza says,
describing a potential scenario of the conflict between the magnetic
field and the metal. In comparison: A standard commercial refrigerator
magnet has 0.05 Tesla. In order to examine as closely as possible the
electric charge in the materials of tomorrow, researchers need higher
magnetic fields, up to 90 or 100 Tesla.
“At
100 Tesla, though, the Lorentz force inside the copper would generate a
pressure which equals 40,000 times the air pressure at sea level,”
calculates Wosnitza.
These
forces would tear copper apart like an explosion. That is why
researchers use specific copper alloys which can withstand ten thousand
times the atmospheric pressure. They then add a corset made from a
special fiber that is typically used for bulletproof vests and which
holds the alloy together from the outside. The HZDR technicians wind six
of these special wires with corsets into a coil that has a hollow space
of 16 mm at its center. This permits the generation of 50 Tesla within
this special coil when a brief but powerful electric pulse flashes
through the copper — a process that is over after a mere 0.02 seconds.
Magnetic coil. |
But
that’s still far away from the world record of 89 Tesla which the U.S.
held in Los Alamos for several years. This is why technicians put a
second coil consisting of twelve layers of copper wire around the first
one. This wire can only withstand 2,500 times the atmospheric pressure.
But protected by a plastic corset, a current pulse lasting only a fifth
of a second suffices to create a 40 tesla magnetic field inside the
coil. Together with the 50 Tesla of the inner coil, this adds up to the
world record of more than 90 Tesla. Covered by a steel jacket, this
double coil has a height of 55 centimeters and a diameter of 32
centimeters; thus, resembling a fairly large water bucket. For several
weeks, the HZDR technicians worked on the coil which not only set the
world record, but which will also permit many future studies of new
materials in the record magnetic field.
For
such experiments, researchers are flocking to Dresden not only from
Regensburg, Garching, and Karlsruhe, but also from all over Europe. Even
Japanese and US American scientists are already making reservations at
the HZDR so that they can analyze their materials here.
The
breakthrough has generated such interest, Wosnitza reports, that the
existing five rooms equipped with similar coils can no longer handle the
crowds of researchers so an additional six of these “pulse cells” will
be built by 2015.
