Page 43 - Fundamentals of Magnetic Thermonuclear Reactor Design

Facilities With Magnetic Plasma Confinement Chapter | 2 25

FIGURE 2.12 Breakdown area boundary for hydrogen (H) and helium (He).

Very low magnetic stray fields are needed to reduce the breakdown voltage
to reasonable values (∼10 V). In ITER, for example, they are within 2 mT.
The stray fields in the whole breakdown area are formed by the poloidal
coil system as well as by the eddy currents in the vacuum chamber and in
the cryostat. In ITER, these currents reach 1.5–2 MA during the breakdown.
For ITER, the breakdown area corresponds to a vortex electric field E of
0.3 V/m.

2.5.3 Force Loads on Tokamak Components

Tokamak’s key components experience high force loads. Let us make some
2
estimates. In ITER, the vacuum chamber surface area is ∼1000 m . The cham-
ber vacuum pump down causes a force of ∼100 MN, compressing the cham-
ber wall. Tokamak-dominant mechanical loads have an electromagnetic origin
(Fig. 2.13). They act on the TF coils within the plane (the centripetal force F
R
and the tearing force F ) and out of the plane (F ).
Z

The F force tends to overturn a coil. In ITER, F , which pushes every coil

R
to the torus centre, is 365 MH, and the forces tending to tear the coil (F ) and
Z
overturn it (F ) are close to 200 and 40 MN, respectively.

Halo currents occurring at the plasma edge and the vacuum vessel walls
exert the highest forces on the chamber during a plasma current disruption. In
ITER, the vertical component of these forces is close to 50 MN and the horizon-
tal one equals 16 MN.