Page 26 - Fundamentals of Magnetic Thermonuclear Reactor Design

Facilities With Magnetic Plasma Confinement Chapter | 2 9

l Plasma stability, achieved due to a strong TF, which keeps the stability mar-
−1
gin q = µ greater than one:
aB
q = t >1
RB p q=aBtRBp>1

l The plasma equilibrium and required shape, achieved through a system of
poloidal field coils.
l Plasma heating performed by the plasma current and the injection of fast
atoms and use of high-frequency and ultrahigh-frequency (UHF) power (in
the case of a hot plasma also by charged products of the fusion reaction).
After many years of research and experimentation on several tokamak gen-
erations, it is now possible to make a decisive step towards using the energy of
nuclear fusion. The fundamental physical regularities governing hot plasma’s
behaviour in a magnetic field are now clear. Adequate scalings have been pro-
posed. The stability limits to the fusion devices’ operation parameters have been
estimated. Methods of suppressing many types of plasma instabilities have been
found. The scientific and technical progress achieved in this area allowed the
fusion community to realise the International Thermonuclear Experimental Re-
actor (ITER) project.

2.2.2 Stellarators

A stellarator is another type of a promising toroidal magnetic confinement de-
vice, very closely related to the tokamak and only differing from it in how the
confining magnetic field is achieved. Unlike the tokamak, which uses a com-
bination of plasma current and TF coils, the stellarator employs helical coils.
The basic physical concept underlying the stellarator magnet system (MS)
is very complex (see Fig. 2.2A), while the requirement for its implementa-
tion accuracy is more demanding, than for the tokamaks. In addition, at equal
plasma volumes, a stellarator is larger than a tokamak. This ‘initial disparity’
allowed the tokamak to leave the stellarator behind in the race to fusion energy.
However, the tokamak’s advantages are not overwhelming in terms of reactor
design, economics and safety, and the stellarator may become a serious com-
petitor.
Stellarator research involved a search for MS configurations, which are
simpler from the technological point of view. Eventually the TF winding
was abandoned and two stellarator configurations were taken as a basis for
innovations within the MS area. They were the torsatrons/heliotrons using a
combination of a continuous helical and several compensating/auxiliary coils
(Fig. 2.2C) and modular stellarators (Fig. 2.2D). Currently, the world’s largest
devices of these types in operation are the Large Helical Device (LHD) torsa-
tron/heliotron in Japan and the Wendelstein 7-X (W7-X) modular stellarator
in Germany.