Page 150 - Fundamentals of Magnetic Thermonuclear Reactor Design

132 Fundamentals of Magnetic Thermonuclear Reactor Design

To prevent such situations, a protective ‘discharge’ of the winding stored
energy on the resistors, placed outside of the cryostat, is used. To avoid ther-
momechanical damages, the time constant of the protective discharge must be
not more than

2 ∫
T m
τ ≤   T 0 ρ c 2 dT −∆  
t ,
3
2
τ ≤2∫T Tmcρ⋅j dT−∆t,  j ⋅ 
3
0
where Т and Т are the nominal and maximum allowable normal zone op-
0
m
erating temperatures; с/ρ is a strand’s specific heat capacity/electrical re-
sistance ratio in the normal-state; j is the current density in a normally con-
ducting (copper) part of a strand in the moment of the protective discharge
beginning; ∆t is the delay time, elapsed from the normal zone origin to the
protective discharge, needed to detect the normal zone, extinguish the plasma
and initiate the protective switchboards. For the ITER magnet, Т ≤ 150 K,
m
and ∆t = 2 s.
The above conditions determine the minimum necessary cross-section of a
normally conducting material (copper), which is a part of any SC. This mate-
rial shunts the SC, and its cross-section, together with the maximum admissible
voltage on the winding during protective discharge, V , defines the choice of
m
nominal current, I , during the quench. It must satisfy I ≥ 2W/V τ , where W
0
m 3
0
is the energy stored in the protected winding.
5.4.2 Forced-Flow Cooled Superconducting Cables
The tokamak evolution from the T-15-scale machines with a stored energy of
∼0.8 GJ to ITER with a stored energy of 40 GJ has taken the operating currents
and voltages to a new level. The nominal current of superconducting cables has
grown from 6.2 kA in T-15 to 40–70 kA in ITER, while coil maximum operat-
ing voltages increased from 0.5–1 kV to 5–15 kV.
The key requirements for magnets based on forced flow cooling–type
SCs include setting (defining) of normalised (maximum allowable) values
of structural current density, mechanical and electrical strength; presence of
reliable quench detection; and fast-acting protection systems to prevent nor-
mal zone propagation, increase of helium temperature in cooling channels
and explosive-like helium pressure rise in case of loss of the cryostat thermal
insulation.
International experience in the design and operation of electrophysical ma-
chines indicates that superconducting magnets using two- or one-phase helium
forced-flow cooling at 4.5–5.5 K (Figs 5.10 and 5.11) are the best performers in
terms of functional abilities and economic criteria [4].
The relatively inexpensive NbTi alloy is generally used to manufacture
coils producing magnetic fields up to 6–7 T, such as the PFCs and CCs. The
Nb Sn intermetallic SC, which is more costly and is somewhat inferior to
3