Producing energy the way
the sun does
Vacuum provides technology for establishing conditions for fusion
Energy production using nuclear fusion is becoming increasingly important in the search for clean alternative sources of
energy. Since as early as the 1950s of the last century, scientists all over the world have been attempting to use nuclear
fusion for peaceful purposes. This process works perfectly in
the sun, after all. However, imitating the extreme conditions
that prevail there is proving a hard nut for physicists and engineers to crack. Basically, it is a matter of fusing together two
hydrogen isotopes at a time to form a new helium nucleus. In
the course of this process, not just helium and a single neutron are formed but also an extraordinarily large amount of
energy. This energy that is released is destined to be used to
generate electricity: Just one gram of fuel can theoretically be
used to generate 90,000 kilowatt hours of energy in a power
plant. This is equivalent to the combustion heat of 11 metric
tons of coal.
The challenge posed by research into fusion is getting the
nuclei to fuse together. Since it is not feasible to replicate the
sun’s own conditions for fusion in a power plant on Earth, this
means that fusion can only take place under high vacuum at
a high plasma temperature of some 100 to 150 million kelvin,
and at in comparison a low particle density of 1020 per cubic
meter. Following the heating up process, the nuclei separate
from the electrons to form the plasma. The charged nuclei are
guided through the reactor with the aid of an extremely
strong magnetic field and kept away from the reactor walls.
When two atomic nuclei approach each other or even collide,
they fuse and release a large quantity of energy. A part of this
energy in turn, ensures that the plasma remains at the same
temperature and retains its state without any additional energy input. A positive energy balance is, of course, necessary
if the aim is to produce energy for generating electricity.
Computer graphic: Cryostat, magnetic coils and plasma vessel at Wendelstein 7-X fusion reactor (Image: IPP)
Electricity can only be generated if surplus energy is produced
during nuclear fusion in a power plant.
Demands placed on vacuum technology in the
fusion reactor
Currently, plants that work on the tokamak and stellarator
principle have proven themselves in fusion research. Both
reactors confine the plasma in a toroidal magnetic field. The
tokamak reactor has a symmetrical design and forms a part of
the confining magnetic field through a strong electric current
flowing through the plasma. Stellarators, on the other hand,
confine the plasma with a magnetic field generated solely by
external coils. This result in a complex asymmetrical reactor
form.
Wendelstein 7-X essentially consists of two interlaced toroidal
vacuum vessels. The external cryostat chamber contains the
isolation vacuum and the cooling technology for the superconducting coils that are essential for generating the magnetic field. The inner chamber, or plasma vessel, is used to generate the actual plasma in a high vacuum environment.
Faced with the high costs involved in research into fusion
technology, separate national research projects are increasingly being consolidated into joint international projects. Two
large research projects with international participation on the
European continent are the ITER tokamak project in Cadarache, France, and the Wendelstein 7-X stellarator project in
Greifswald, Germany. While the goal of the ITER project was
always to generate energy, the Wendelstein 7-X fusion experiment is aiming to achieve a steady combustion period of
about 30 minutes without a focus on producing energy.
Wendelstein 7-X is a key experiment: Its purpose is to
demonstrate the feasibility of stellarator power plants: It is
currently the largest stellarator-based fusion experiment in
the world.
2
Technical data:
Wendelstein 7-X
Large plasma radius
5.5 meters
Small plasma radius
0.53 meters
Magnetic field
3 tesla
Discharge time
Up to 30 minutes, continuous
operation with microwave heating
Plasma heating
20 megawatts
Plasma volume
30 cubic meters
Plasma quantity
5 – 30 milligrams
Composition of the plasma
Hydrogen, deuterium, helium
Plasma temperature
60 million kelvin
An important factor for operating a fusion reactor is the
existence of a strong, reliable, and powerful vacuum system.
All vacuum components therefore had to pass a qualification
procedure at the Max Planck Institute for Plasma Physics (IPP)
to ensure their suitability for use in the fusion experiment.
Strong magnetic fields of up to 3 tesla confine the electrically
charged plasma particles in the inner plasma vessel of Wendelstein 7-X. The magnetic field surrounding the plasma vessel is so intense that it is necessary to install the vacuum
technology on special mounts at a distance of 4 to 9 meters
from the plasma vessel. Even at this distance, the magnetic
field still reaches a strength of 7 millitesla and, from time
to time, even 20 millitesla. These field strengths are almost
1,000 times higher than Earth’s natural magnetic field.
To reach the basic 10-8 hectopascal pressure in the plasma
vessel, it is necessary to pump down not just the 100 cubic
meters vessel volume but also the gas loads emitted from the
roughly 1,300 square meters internal surface. What’s more,
the vacuum pumps used must be capable of reaching the
high compression ratio needed for pumping down the hydrogen, deuterium and helium light gases involved in the fusion
process. Good compatibility between the turbopumps and
the materials used to coat the surfaces of the plasma vessel
is a further requirement.
INFOBOX
Plasma
In physics, plasma is a gaseous state in which free electrons and ionized atoms occur. This state can be attained
at high temperatures (thermal decay) or also, for example,
through strong electric fields (lightning, gas discharge
lamp). At high temperatures (≈ 5.000 kelvin), gases decay
almost completely to form plasma. The plasma is not generated through a phase transition from one phase to
another, such as water from ice, but through a reaction,
causing the decay of a neutral atom into an ion and an
electron. In this process, a balance can be achieved
between neutral atoms and electrically charged ions, and
this is described by the Saha equation.
At even higher temperatures, the atomic nuclei can be fully stripped, which is important in nuclear fusion. As a rule,
plasma behaves like a gas, with electrons and cations or
atomic nuclei as the smallest particles. This property
makes plasma a good conductor of electricity.
Photo of a plasma discharge in a fusion reactor. (Image: IPP)
3
The solution created by Pfeiffer Vacuum
On the basis of the high operating and quality requirements
that the vacuum system was called to satisfy, Pfeiffer Vacuum
successfully qualified as a competent partner for this project.
The necessary vacuum equipment was carefully selected
together with the experts from the Wendelstein 7-X nuclear
fusion experiment and verified for the requirements and
conditions of use:
Turbopumps
Both vacuum chambers use Pfeiffer Vacuum HiPace turbopumps, with a pumping speed rating of 2,000 liters per
second, which meet the particularly stringent requirements of
fusion experiments. On the plasma vessel alone, all installed
turbopumps together provide a pumping speed of some
40,000 liters per second.
Due to the strong magnetic fields that exist around the experiment, magnetic eddy currents are induced on the fast-rotating
turbopump rotors, resulting in a high transfer of heat to the
pumps. The special bearing principle and internal construction
of the HiPace turbopumps ensure that a large quantity of heat
HiPace 2300 turbopump (picture showing example)
Wendelstein 7-X in March 2014, shortly before completion of the main installation phase. (Image: IPP, Beate Kemnitz)
4
can be transferred, and the pumps have therefore proven in
a series of tests that they achieve high thermal operating
reliability in external magnetic fields. The advantage of the
good magnetic field compatibility of these Pfeiffer Vacuum
turbopumps is that they can be installed in close proximity to
the plasma vessel. Due to the shorter pipes needed as a
result, a higher effective pumping speed is achieved in the
plasma vessel.
stringent specifications of the Max Planck Institute for Plasma
Physics. Important factors which are essential for a reliably
functioning vacuum system, and therefore the entire fusion
experiment, include high-precision manufacture, the choice of
suitable materials with optimal magnetic permeability and
low-cobalt content, the use of state-of-the-art manufacturing
technologies such as laser welding, and the necessary quality
certification.
The exceptionally low torques of the hybrid bearing turbopumps, which are designed for maximizing the mechanical
operating reliability, are a further advantage. The torques to
be allowed for when anchoring the pumps are 3 to 4 times
lower than with other types of construction.
Pfeiffer Vacuum delivered several special components in the
DN 400 to DN 250 nominal diameter range for mounting the
turbopumps, and also supplied components for the pipework,
flanges and immersion pipes for visually monitoring the
plasma. All in all, Pfeiffer Vacuum has supplied stainless steel
vacuum components weighing more than 12 metric tons in
total to Greifswald for constructing the fusion experiment.
Measurement technology
Specially developed Pfeiffer Vacuum pressure gauges for use
in harsh conditions are used for the vacuum chambers. The
compatibility of the measuring equipment for use in strong
magnetic fields was successfully evaluated in advance by the
Max Planck Institute. The sturdy cold cathode vacuum gauges
used are designed as passive sensors without any electronic
components on the flange. The electronic evaluation unit is
positioned several meters away in a safe area and is connected to the passive sensors with long cables. The sensors are
also additionally shielded so that they display the pressure
reliably even in the high stray magnetic fields that occur.
Vacuum components
Helium leak detection
The vacuum system was installed on December 10, 2015.
Before the various chambers could be evacuated, the helium
leak-tightness of more than 2,000 welds and additional flange
connections had to be individually tested and assured.
Since some of the welds are difficult to access, the decision
was taken in Greifswald to use the Pfeiffer Vacuum ASM 310
portable leak detector. A decisive factor in this decision was
its excellent reliability in tracing even lowest helium leakage
rates. Due to the lightweight construction and compact size
of the ASM 310, it was easy to transport to the joints to be
inspected and was therefore ideally suited for this type of use.
Hundreds of special vacuum-compatible connecting elements
and piping components are needed for generating the
vacuum, and these must be manufactured according to the
ModulLine IKR 070
5
Special piping components
Helium leak detector ASM 310
Vacuum solutions from a single source
Pfeiffer Vacuum stands for innovative and custom vacuum solutions worldwide,
technological perfection, competent advice and reliable service.
Complete range of products
From a single component to complex systems:
We are the only supplier of vacuum technology that provides a complete product portfolio.
Competence in theory and practice
Are you looking for a
perfect vacuum solution?
Please contact us:
www.pfeiffer-vacuum.com
Pfeiffer Vacuum GmbH
Headquarters · Germany
T +49 6441 802-0
[email protected]
All data subject to change without prior notice. PI0391PDE (January 2016/0)
Benefit from our know-how and our portfolio of training opportunities!
We support you with your plant layout and provide first-class on-site service worldwide.