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FROM THE JOBSITE
FROM THE JOBSITE
ArmaflexCryogenic Systems in the
nuclear fusion reactorWendelstein7-X
Mankind has long dreamed of harnessing the sun’s energy for use on
Earth. Stars like our sun generate energy through the fusion of atomic
nuclei. Nuclear fusion promises infinite generation of electricity with
minimal fuel input andwithout the riskof catastrophic incidents.Unlike
nuclear fission, nuclear fusion produces only very small quantities of
radioactivity. From the point of viewof physics, nuclear fusion is possi-
ble. The scientific and technical feasibility of generating energy in this
wayhasbeen the subject of research for over 50years.
In nuclear fusion, hydrogen atoms are fused into helium atoms. At a
pressure of around 2 bar hydrogen gas is heated to temperatures of
100 to150milliondegreesCelsius, theelectronsare separated from the
atomic nuclei and the gas becomes an electrically conductive plasma.
Currently, themost promising concepts for reactors are tokamaks and
stellarators. Theworld’s largest stellarator isnowbeingbuilt at theMax
Planck Institute for Plasma Physics in Greifswald (Germany): Wendel-
stein7-X allows themagnetic confinement of fusionplasma in continu-
ousoperation. Thenewbuildingwasbegun in1997andopened inApril
2000. At the end of 2003, the first main components – a non-planar
superconductingmagneticcoil and the first sectionof theplasmavessel
were delivered. In May 2014, preparations for the operation of the
devicebegan.
Cryogenics for superconductingmagnetic coils
Stellarators create themagnetic fieldneeded to confine theplasma via
energized coils arrangedoutside theplasma vessel. In the caseofWen-
delstein7-X thesecoilsare superconducting, i.e. once supplied theelec-
tric current can flow indefinitelywithout electrical resistance and thus
permanently maintain the magnetic field. To this end the coils are
cooledwith liquidhelium. Thesuperconductingcoilsand thesteel struc-
tures which support them have to be thermally insulated both against
theirenvironmentandagainst thehotplasma. Following theprincipleof
a thermos flask (although in this case the coldmaterial is inside) they
are cooled in a cryostat: the coils are located in a vacuum chamber,
which is formedby theplasmavessel on theone sideand theouter ves-
sel of thedeviceon theother side. Cryogenic shields surround the coils
and– themselvescooled–keep residualheat radiationaway from them.
Access to the plasma through this vacuum vessel and between the
superconducting coils – for heating, cooling pipes or diagnostics – is
enabled by 254 approximately 1.8 m long, thermally insulated ports.
The stellarator is supplied by the helium cryosystem, water cooling
devices, vacuumpumps and systems for providingelectrical energy.
Despite thermal insulation, 5kWheatoutputhave tobedissipateddur-
ing the experiments to cool themagnets and their enclosure (around
425 tonnes of material) to superconductivity temperature and then
keep them cool. This permanent cooling is needed due to the residual
thermal conductivity of the insulationmaterials used. At temperatures
close to absolute zero, cooling canno longer be achievedby a standard
refrigeration system, but requires liquid heliumwhich boils at 4.22 K
(−268.93 °C). This cooling systemmust beexceedinglygas-tight inorder
toprevent heliumdiffusing into the insulatingvacuumof the stellarator
and the insulationdeteriorating.
Relief pipe insulatedwithArmaflexCryogenic Systems
To insulate the relief pipe for the helium the project management
installedArmacell’s innovative system for extremely low temperatures.
Armaflex Cryogenic Systems are specially developed for applications at
temperatures ranging from -180 to +125 °C. The multi-layer systems
ensure exceptional thermal insulation, reduce the risk of corrosion
under insulation (CUI) and aremuch easier to install than rigid foams.
The coreof the insulation system isArmaflex LTD, a speciallydeveloped
polymer, which prevents thermal tension. Armaflex Cryogenic Systems
maintain their flexibility even at extremely low temperatures. This flex-
ibility ensures that vibrations and impact are absorbed and the risk of
cracking as a result of extreme temperature cycles is significantly
reduced. A key advantageof the cryogenic foams is that they neednei-
ther additional expansion jointsnor vapour barriers.
Special insulation construction
On the relief pipeof theWendelstein7-X stellarator a special insulation
construction was used. To better withstand possible vibrations of the
pipework, a layer of Armaflex LTD anti-abrasive foil was applied both
directly to thepipeworkand to the firstArmaflex LTD insulation layer. It
gives the insulation greater surface strength and acts as an additional
vapour barrier. Then a further layer of Armaflex LTD and a third insula-
tion layer of NH/Armaflex, the halogen-free elastomeric insulation
materialmadebyArmacell,wereapplied.
During the installation thegreatest carewas taken toensurepreciseand
neat workmanship: each layer was applied step-by-step and then
inspected by a project engineer. For the elbows with various radii the
shapes neededwere plotted, printed on paper and cut out to serve as
templates for the sheet material. The employees of the Max Planck
Institutewho carriedout theworkwere trainedbyArmacell in the use
of theproducts, especially inadhesion, beforehand.
Wendelstein7-X is set togo
Commissioningof thenuclear fusiondevice isunderway. After themain
installation had been completed, the plasma vessel was closed in the
middleofMarch. The first plasma is scheduled for this year.
(HeikoKind)
developed by Armacell for applications at
extremely low temperatures
