Page 134 - Fundamentals of Magnetic Thermonuclear Reactor Design

118 Fundamentals of Magnetic Thermonuclear Reactor Design

FIGURE 5.1 SC critical magnetic field strength as a function of temperature.

lines are expulsed. The last is due to the presence of persistent surface currents
in the SC. They excite an internal magnetic field whose induction is equal but
the orientation is opposite to the applied magnetic field that cancels one. An SC
is an ideal conductor and a diamagnetic ‘in the one skin.’
Every SC remains superconductive up to a certain (critical) magnetic field
strength, H. The value of the critical field is maximal at absolute zero and
decreases monotonously with increasing temperature (Fig. 5.1). Under the criti-
cal field the SC can carry an electrical current with a density up to a certain lim-
it. The exceeding of this limit destroys the superconducting state of materials.
At critical temperature T , the critical field strength is zero. For ingredients
C
of commercial low-temperature SCs, such as the Nb Sn metallide and the NbTi
3
alloy, critical temperatures are: 9.3 K (Nb), 3.72 K (Sn) and 0.42 K (Ti). Solid
mercury has a critical temperature of 4.15 K.
SCs are classified into type I and type II materials depending on how they
interact with the applied magnetic field. A type I SC includes pure metals, ex-
cepting niobium, vanadium and technetium. No magnetic field can penetrate
inside the type I SC unless H reaches a critical value. If the applied magnetic
field becomes too large, the metals switch abruptly to the resistive state.
A type II SC has two critical magnetic field strengths (Fig. 5.2). No magnet-
HC 1 ic flux penetrates inside the SC, if the field is below H . As the field increases
C 1
HC 2 to above H , the superconductivity is lost, and the sample becomes a ‘normal’
C 2
HC −HC 2 conductor. In the H C 1 − H range, partial field penetration is possible, meaning
1
C 2
that the type II SC takes on a mixed state.
Superconductivity was discovered in 1911 by H. Kamerlingh-Onnes, who
observed that mercury’s resistance disappeared at temperatures below 4.2 K;
however, it took the researchers several decades to develop the fundamental
physical and mathematical model of this phenomenon.
The first attempt to provide the phenomenological description of the
material transition to the superconducting state was based on the classical
electromagnetism theory: in 1935, the London brothers complemented Max-