Page 48 - Fundamentals of Magnetic Thermonuclear Reactor Design

30 Fundamentals of Magnetic Thermonuclear Reactor Design

along the spiral line in the plasma periphery at a small angle to the magnetic
field lines. This reduces the heat load on the divertor plates significantly.
The nTτ plasma parameter obtained in stellarators is lower than that
E
achieved in tokamaks. After the launch of the W7-X machine in final configu-
ration, stellarators are expected to catch up with tokamaks’ progress in this re-
spect.
When compared with tokamaks, stellarators have two key advantages: the
stationary plasma operation and the ‘no current in the plasma’, and therefore
no current-driven instabilities and disruptions. Stellarators are not subject to
restrictions relating to the Greenwald limit, an operational limit for the plasma
density inherent in tokamaks. However, stellarators should use more complex
configurations and replacement procedures for in-vessel components including
blankets and divertor targets. Unlike tokamaks, stellarators are yet to prove their
ability to confine the D–T plasma in a steady state and to control impurities.
Frameworks for the MFR system and parameter optimisations include set-
ting of concrete functional targets for each machine type. These targets drive the
activities under the HELIOSCOPE LHD-type heliotron project (the NIFS Fu-
sion Engineering Research Project, Japan) [24], the HELIAS modular (Plasma
Physics Institure, Germany) [25] and compact (mainly in the United States [26])
stellarator-type research programmes.

2.6.2 Research Facilities

Currently, the world’s largest stellarators are the LHD machine in Japan and
the W7-X facility in Germany. The LHD developed in the National Institute of
Fusion Studies (NIFS) in Japan had been, for quite a while, the world’s largest
torsatron/heliotron-type operating fusion device (Fig. 2.14). Fig. 2.15 presents
the launch and operation of the W7-X modular stellarator [21].
Large stellarators, designed to operate in stationary conditions, rely on su-
perconducting magnetic systems. In smaller research stellarators, copper coils
are used.
In most cases, a stellarator’s vacuum vessel is geometrically similar to the
last closed magnetic surface confining the plasma. In the heliotron, it also acts
as a support for the helical coils.
The first wall element functions are much like those in the tokamaks. They
should remove heat and protect the in-vessel components and the diagnostics
equipment against the plasma. In the W7-X stellarator, there are more than 2500
2
of them, and they cover an area of 265 m .
The proper stellarator functioning is strongly affected by the MS deviations
from the ideal shape, caused by fabrication/assembly imperfections, such as
tolerances and inaccuracies in the superconductor windings, coil case casting,
welding operations, matching of adjacent parts, or assembly with positioning
deviations. In addition, friction coefficients, bolt tightening, material properties,
and so on, may vary from module to module. All these may result not only in
additional mechanical stresses in the MS and cryostat, but also cause magnetic