Page 46 - Fundamentals of Magnetic Thermonuclear Reactor Design

28 Fundamentals of Magnetic Thermonuclear Reactor Design

TABLE 2.6 Parameters of ‘Compact’ Stellarators
Facility Commissioning Major/minor Magnetic field on
a
(type ) Country (modification) year radius (m) plasma axis (T)
Heliotron J (T) Japan 1999 1.2/(0.1–0.2) 1.5
HSX (M) USA 1999 1.2/0.15 1.4
H-1 (T) Australia 1998
TJ-II (T) Spain 1997 1.50/(0.10–0.22) 1.2
CAT/CTH (T) USA 1990 (2005) 0.53/0.15 0.5
W7-AS (M) Germany 1988–2002 2.0/0.18 3.0
Y-3M (T) Ukraine 1981 1.00/0.12 2.5
WEGA (C) France– 1975 (2001)–2013 0.72/0.1 0.4–0.9
Germany
HIDRA (C) USA 2015 0.72/0.1 0.4–0.9
L-2M (C) Russia 1975 (1993) 1.00/0.115 1.34
a C, classic stellarator; M, modular stellarator; T, torsatron/heliotron.

Stellarators and tokamaks have similar key systems; although stellarators
have no central solenoid and the rotational transform angles are different (they
are much smaller in tokamaks). Thirty stellarators have been constructed since
the 1970s, of which eight are currently in service (Tables 2.6 and 2.7).
A real breakthrough with stellarator machines was achieved in the past few
decades, when new numerical approaches for the magnetic field topology op-
timisation were proposed. Additionally, a considerable progress in technology
enabled considerable improvements in the magnetic system manufacturing and
assembly precisions, as well as in computational modelling and simulation of
plasma behaviour. The advanced W7-X stellarator is an example of the applica-
tion of the optimisation design philosophy [21].
The introduction of helical coils and the use of non-round (initially ellipti-
cal, later triangular) cross-sections, turned along the toroidal loop, heralded the
move from configurations proposed by Spitzer to state-of-the-art stellarators.
The improvement of the latter follows two main directions:
l Torsatrons–heliotrons that rely on continuous helical coils or systems of
coils to achieve the desired magnetic configuration; currently, the largest
heliotron is the LHD machine in Japan;
l Modular stellarators, in which individual complex coils act as toroidal and helical
coils; currently, the largest modular stellarator is the W7-X machine in Germany.
The torsatron’s important property, from a mechanical view point, is that the
helical coil (the most complex part of the electromagnetic system) can be made
virtually ‘forceless’ and thus reduce the electromagnetic loads.