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SPG Mitteilungen Nr. 40
Progress in Physics (35)
A snowflake in a million degree plasma
Y. Martin, B. Labit, H. Reimerdes, W. Vijvers
CRPP, EPFL, Association EURATOM – Confédération Suisse, CH-1015 Lausanne
Introduction
Fusion is the energy that powers the stars. Harnessing fusion
on Earth would offer the world almost inexhaustible,
environmentally clean and safe energy, see box 1. Research
and development performed during a few decades already
brought significant results. In the domain of magnetic confinement,
for instance, 16MW of fusion power have been
produced in the JET tokamak [1] in 1997. Since then an
international collaboration was set to build the next step
device, ITER 1 , the first fusion reactor that would deliver
500 MW of fusion power, 10 times the power injected into
the device, see box 2. Recently, the European fusion scientists,
under the banner of European Fusion Development
Agreement 2 (EFDA), developed and published a roadmap 3
that describes the steps and the challenges to achieve the
production of electricity from fusion before 2050, see box
3. The steps consist in completing the construction of ITER,
operating ITER, designing, building and operating DEMO, a
prototype reactor that would provide the electrical network
1 ITER: www.iter.org
2 EFDA: www.efda.org
3 Fusion roadmap: http://www.efda.org/efda/activities/the-road-tofusion-electricity/
with several hundreds of MW. The roadmap is divided into
8 missions, including the 'heat exhaust' issue: if one extrapolates
from the present devices to a reactor grade device,
the heat flux density produced by particles escaping
from the plasma would reach levels that may exceed the
material capabilities. To mitigate the heat flux impact several
strategies are proposed. One of these strategies aims
at the exploration of the so-called snowflake configuration.
The scientists of the CRPP-EPFL were the first to realise a
snowflake configuration in a tokamak, thanks to the high
shaping capability of the TCV tokamak but especially to
the hard work of these dedicated scientists. It should be
emphasized that this work was subsequently awarded the
R&D100 prize in 2012 4 . This paper presents the achieved
snowflake configuration and its advantages with respect to
heat exhaust.
Plasma configuration
The TCV tokamak is equipped with 16 independently driven
poloidal field coils that are used for shaping the plasma.
Depending on the combination of coil currents, the plas-
4 R&D magazine: http://www.rdmag.com/award-winners/2012/08/highperformance-tokamak-exhaust
Fusion
The fusion of hydrogen isotopes, deuterium and tritium,
is the easiest reaction that could be implemented
on Earth, because of its higher cross section at a lower
temperature than other possible reactions.
Temperatures of about 100 MºC must be achieved
so that fusion power becomes exploitable. Before
reaching these temperatures, a gas turns to plasma
wherein particles are ionised.
Magnetic fields then provide a good way to guide the
ionised particles.
The most attractive magnetic confinement device so
far is the tokamak, a toroidal device wherein a poloidal
field, induced by a toroidal current, is superimposed to
a toroidal magnetic field to form the magnetic structure
that guides and maintains the plasma away from the
vacuum vessel walls. In addition poloidal field coils are
used to shape the plasma.
To reach the required temperatures, the plasma is heated
by either energetic neutral particles or by microwaves
delivering their power in the plasma through
different possible resonant schemes.
The CRPP-EPFL tokamak, called TCV 1 for 'Tokamak
à Configuration Variable' has a high shaping capability
and an Electron Cyclotron Heating (ECH) system of 4.5
MW.
Figure: The CRPP-EPFL Tokamak à Configuration Variable (TCV).
Cyan: vacuum vessel; green: main toroidal field coils; orange:
poloidal field coils (for plasma current induction and plasma shaping);
yellow: microwave launchers; pink: plasma. TCV diameter:
3.3m
1 http://crpp.epfl.ch/research_TCV
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