Page 64 - Fundamentals of Magnetic Thermonuclear Reactor Design

ITER – International Thermonuclear Experimental Reactor Chapter | 3 47

losses in cables/wires, local motions of the conductor components, and the
superconducting-to-resistive state transition;
l Designing high-current connectors for bringing superconductors together
with each other and with current leads;
l Developing current leads for superconducting magnets;
l Studying candidate electrical insulators and developing insulating systems
for the MS normal operation conditions (neutron irradiation, cryogenic tem-
perature range and mechanical compressing loads of up to 600 MPa at a
voltage of 10 kV);
l Modelling full-size MS components for practicing fabrication techniques to
ensure required quality and reliable operation;
l Simulating the MS working conditions (the current density, magnetic field,
mechanical stresses and deformations and electrical voltage); and
l Providing comprehensive computational support for the studies and tests.

3.3.1 Toroidal Field Coils
A toroidal magnetic field around the ITER tokamak torus is produced by 18
coils, encased in sheet non-magnetic steel vessels and distributed uniformly
along the torus. A 400-MN centripetal force acting on each coil is withstood
by arching formed inside the torus by the toroidal coil’s rectilinear parts of
inner ‘legs’ (Fig. 3.6). Their bent part also must withstand the highest pulsed
mechanical loads caused by an overturning moment that results from the pon-
deromotive interaction of coil currents and the discharge current.
The coils are stiff mechanical structures, wound into seven double pancakes
wrapped with case insulation that consists of multiple polyimide/glass fibre
3
alternating tapes. A coil measures 10.7/14.3/0.83 m . A coil cross-section is
shown in Fig. 3.7.
Conductors are embedded in steel radial plates with spiral semicircular
grooves on the upper and lower surfaces (Fig. 3.8), where superconducting
cables encased in round stainless-steel conduits are laid. Individual wires are
760 m long at most. Due to the grooves, the ponderomotive forces acting on
the wires get directly to the plates, preventing the accumulation of mechanical
stresses in the winding and, consequently, allowing the inter-turn insulation to
operate within the range of mechanical stress tolerances.
This is additionally facilitated by the cable’s round-shaped cross-section,
allowing working without stress concentrators. The coil’s ‘two-barrier’ electric
high-voltage insulation (between the turns and at the coil case) provides an extra
advantage of control of fabrication defects by measuring the resistance of the
insulation between a wire and the radial plates [5].
The TF coils, together with the CS and PF coils, make a single mechanical
structure that withstands the ponderomotive forces. It is their inner components,
wedging each other, that make the mentioned arching-shaped strut. Four sup-
port bracket struts withstand in-plane forces acting from the outside of the coils.