Page 69 - Fundamentals of Magnetic Thermonuclear Reactor Design

52 Fundamentals of Magnetic Thermonuclear Reactor Design

A magnetic field at the CS butt ends produces vertical ponderomotive
forces that act on the modules. At the time of the solenoid magnetisation,
these forces are directed towards its central plane, and at the time of plasma
column formation, the current direction at the butt end modules may reverse.
In this case, a separating force of up to 120 MN may arise between the central
modules. An axial pre-compression is applied to the solenoid to restrain this
force and maintain reasonable module-to-module contact. The module surface
is exposed to a pressure of around 24 MPa at room temperature, which goes
down a little with decreasing solenoid temperature. A system of 18 outer ver-
tical rods, encircling the solenoid wall on the outboard side, and nine inner
rods inside the CS are used to achieve a uniform compression. The rods are
connected with the upper and lower rigid flanges. The flanges are divided into
nine sectors, connected with each other and with the TF coil cases by electri-
cally insulated bolts.
The CS leans upon the lower parts of TF coils via its own pre-compression
support structure. The structure’s supports are stiff in relation to vertical forces,
but elastic in the radial direction. For this reason, a mechanism designed to
withstand horizontal dynamic forces and forces resulting from mismatches of
modules, is installed in the CS lower part. Electrical terminals and interlayer
contacts are above and beneath the winding.
Three groups of correction coils are placed above, beneath and on the out-
board side of the TF coils. They are designed to restrain magnetic field perturba-
tions resulting from fabrication errors and deviations from axial symmetry due
to the CS and TF/PF coil configuration.
The magnetic system also includes coils that control certain types of plasma
instabilities (the so-called edge-localised modes or ELM coils) and coils ensur-
ing vertical stability of the plasma column (the so-called VS coils). They are
located inside the VV between the vessel and the blanket shielding modules and
are exposed to heavy cyclic radiation, thermal loads and intensive mechanical
stresses. That is why they use a very durable bronze (CuCrZr) conductor in the
form of a water-cooled tube with a radiation-resistant mineral insulation in a
non-magnetic steel jacket.

3.4 VACUUM VESSEL

The ITER VV is a primary safety barrier. It provides the physical conditions
needed to initiate and maintain a fusion reaction. It must be able to withstand
mechanical stresses during nominal operation and accidents without losing vac-
uum tightness. In addition, the VV acts as a radiation shielding for the TF coils
and helps maintain sanitary conditions in the reactor premises.
The VV consists of the main vessel, three belts of ports for connection with
the cryostat, a support structure bearing the VV weight, fasteners for in-vessel
components and couplings/fittings for the water-cooling system (Fig. 3.11). The
VV total weight is 5125 t.