Page 376 - Fundamentals of Magnetic Thermonuclear Reactor Design

354 Fundamentals of Magnetic Thermonuclear Reactor Design

The total moments and forces acting on the TFC system are zero. There
are no net torque and toroidal force in machines with symmetric poloidal field
configurations.

12.2.2 Poloidal Field Coils and Central Solenoid
Poloidal field coils (PFCs) are arranged either symmetrically or asymmetrically
relative to the tokamak equator depending on the desired plasma shape. Con-
tinuous body ponderomotive forces acting on a PFC arise from the coil current
interaction with the poloidal fields due to all of the PFCs, the central solenoid
(CS) and the plasma current. These forces may be reduced to radial and vertical
force components. They are symmetric relative to the tokamak vertical axis.
The in-plane radial forces tend to ‘burst up’ the coil, although in some configu-
rations the radial forces may ‘compress’ the coil. The radial forces are generally
taken up by the coil’s circumferential strength. The vertical forces are accom-
modated by the PFS supports. Forces acting in between the supports result in a
torsional bending of the coils.
Additional load on the PFCs is associated with the toroidal field ripple.
Forces acting on the PFCs reproduce themselves cyclically with every dis-
charge and should, therefore, be taken into consideration in static and cyclic
structural assessment.
Axial and radial forces acting on the CS are similar in nature. A ‘remagne-
tisation’ technique is employed to duplicate the variations of the magnetic field
generated by the CS and changing the CS current from +I max to −I max during
the operating cycle. The ponderomotive forces reach their peak twice during a
cycle, which should be accounted for in the fatigue analysis [4].

12.2.3 General Algorithm for Design and Computation

Spatial distribution of ponderomotive forces is generally analysed quantitative-
ly using special codes (see Chapter 4). Apart from these forces, the following
factors are accounted for in the structural assessment:
l Dead weight of structural components.
l Thermomechanical loads, which result from the heating of coil cables by
electrical current (in ‘warm’ magnets) or cool-down/warm-up (in supercon-
ducting magnets).
l Stresses due to fabrication/assembly processes. A typical example of these
are the postwinding residual stresses and deformations, which have a mean-
ingful effect on the magnet system’s resistivity to cyclic loads.
l Seismic loads.

Mechanical loads develop at all stages of the machine manufacture and op-
eration, which typically include assembly, tightening up of bolted parts, cre-
ation of pre-stressed structural components, feeding of current to the toroidal