4
Wendelstein 7-X
The ASDEX Upgrade tokamak, which has been in
operation since 1991, is investigating all key issues
relating to the tokamak type of a future fusion power
plant. ASDEX stands for “Axially-Symmetric Divertor
Experiment”. It is named after the divertor, a special magnetic
field configuration. It allows the interaction between the hot
fuel and the surrounding walls to be controlled. Its tungsten
wall cladding, which is suitable for use in power plants, and
its flexible, powerful plasma heating makes ASDEX Upgrade
one of the world’s most important tokamaks. It is preparing
for the ITER international large-scale experimental reactor,
which is currently being built in southern France in a worldwide
collaboration. ITER is to show that energy can be derived from
nuclear fusion.
To solve new challenges the ASDEX Upgrade team also
regularly works with partners from industry.
the coils to cool them down to temperatures of approximately
-270 degrees Celsius, close to absolute zero. Due to this low
operating temperature, the coils have been installed in a
cryostat that is made up of a plasma vessel and an outer vessel.
The vacuum generated between the two vessels insulates the
coils from ambient heat.
The plasma can be investigated and heated using the 254 ports.
Three different methods are used for heating the plasma:
microwave heating with a power of ten megawatts, radio
wave heating with four megawatts and neutral beam injection
heating with 20 megawatts.
Five almost identically built modules constitute the
Wendelstein 7-X device. Each module consists of a part of
the plasma vessel, its thermal insulation, ten superconducting
stellarator coils, and four planar coils, as well as the pipe
system for cooling the coils, a segment of the supporting
central ring and the outer vessel. In December 2015,
Wendelstein 7-X was launched after nine years of construction
and over one million assembly hours. It is being further
upgraded step by step.
Tritium
p
p
n
p n
n
p
Hydrogen nuclei fuse in the sun and stars under
enormous gravitational pressure. A fusion
power plant on earth, however, must operate on
another principle. Under terrestrial conditions
it is the two types of hydrogen, deuterium and
tritium, which react most easily. They form a
helium nucleus; a neutron is also released and
huge amounts of usable energy are generated.
In a power plant, one gram of fuel can generate
90,000 kilowatt-hours of energy. This corresponds
to the heat of combustion of eleven
tons of coal.
Companies from all over Europe have worked closely
with the Max Planck Institute for Plasma Physics (IPP)
in developing and manufacturing the Wendelstein 7-X
fusion research device. This process has given rise to
many demanding challenges and provided the participating
companies with valuable technological expertise. The
following pages will describe a few of these achievements.
Wendelstein 7-X is the world’s largest fusion device of the
stellarator type. Its objective is to investigate the suitability
of this type for a power plant. Discharges of up to 30 minutes
will demonstrate its essential property, namely, that of
continuous operation.
A key part of the experiment is the coil system, which
consists of 50 non-planar and 20 planar superconducting
magnetic coils. They create magnetic fields, which confine the
hydrogen plasma at temperatures up to 100 million degrees,
so that it can be investigated with various techniques. In
the magnetic coils a superconducting niobium-titanium
conductor is used rather than a normally conducting copper
material. During the experiment, liquid helium flows through
Neutron
n n
Deuterium
n p
n
n
n
p
Helium
ASDEX Upgrade Nuclear fusion reaction scheme: IPP