Lecture 5 - Superconductor Materials And Losses

Question 1

What are the material requirements for type II superconductors?

Answer 1

They need to have a high Hc2, important for solenoids

They need to have large Jc, important for both solenoids and electricity transport

They need to be easy to form into wires, which is difficult based on the needs for a high Hc2

Question 2

What is the critical current in a type II superconductor?

Answer 2

The point at which the Lorentz force acting on the vortex lines causes them to start moving.

This is hard to pinpoint as thermal fluctuations lead to flux creep. In turn this leads to deviation from the ideal model.

Question 3

Explain the problem with thermal stability.

Answer 3

A small temperature increase in a superconductor is a small perturbation that disturbs the equilibrium of the superconductor. This leads to a drop in Jc, an increase in the penetration depth and heat to be released.
As the superconductor doesn’t dissipate heat well, this behaviour increases the temperature further, leading to a flux avalanche, destroying the superconductor.

Question 4

How do we address the issue of thermal instability in superconductors?

Answer 4

1) cryogenic/cryostatic stabilisation:
Adding a conductive metal in parallel to the superconductor wire to improve cooling and add a shunt for current sharing.

2) adiabatic stabilisation:
Deriving a critical thickness by considering the change in Jc from temperature fluctuations

d=(2/Jc)√(3ρC[Tc-To]/μ)

d: diameter of cylinder
ρ: mass density
C: specific heat capacity

We combine these approaches using fine filaments into a high conductivity matrix of superconductor wires.

Question 5

What losses arise from AC?

Answer 5

Alternating transport current or alternating external field lead to a reversing magnetic field.

This is a dissipative process that gives rise to voltage and as such results in energy loss.

Loss scales with frequency, thickness and from coupling between filaments in a wire matrix.

Question 6

How do we minimise coupling losses?

Answer 6

Twisting individual stands of the conductors inside the matrix

Twisting the entire matrix assembly

Changing the electrical resistivity of the material