Page 149 - Fundamentals of Magnetic Thermonuclear Reactor Design

Superconducting Magnet Systems Chapter | 5 131

FIGURE 5.9 Critical current density as a function of temperature: В = 12 T; ε = −0.25%.

at 4.2 K) generally has SCF of within 100 µm dia. For high-temperature SCs
with an operating temperature of above 40 K, this constraint is less relevant.
The ‘flux jump’ can be prevented using dynamic stabilisation, that is, fast heat
removal from areas of excessive heat generation combined with the simultaneous
reduction of heat generation intensity. To achieve this, the rate of heat diffusion
in a metal composite of superconducting strands or cables should be greater than
the electromagnetic diffusion rate. Today, the adiabatic and dynamic stabilisation
methods used in the design of SCs and MSes usually complement one another.

5.4 WINDING SUPERCONDUCTORS

5.4.1 Normal Phase Effect
A ‘normal zone’ is part of an SC that has lost its superconductivity as a result
of exceeding any of the SC’s critical parameters (temperature, current density
or magnetic field). A normal zone leads to the appearance of electrical resis-
tance and, as a result, local heating of the corresponding part of the SC. Also,
the neighbouring regions are heated resulting in normal zone propagation.
The propagation of an irreversibly developing normal zone may cause an
emergency situation. The winding SC in normal state is unable to carry a current
with a designed density. SC burn-out is unavoidable; moreover, the adjacent ele-
ments of the winding are likely to get damaged too.