AC magnetic field applied to a superconductor, London Theory

In summary, the conversation discusses the London theory of superconductors and the application of an AC magnetic field to a superconductor. The AC measurement of a large dipole magnet with superconducting coils is also mentioned, specifically the plot of the AC inductance as the magnet temperature increases. The sudden increase in inductance is attributed to the Meissner effect, where the AC signal is able to penetrate the superconducting coil as it becomes normal. The conversation also mentions the design of the magnet wire to carry a high amount of current.
  • #1
BlindRacoon
4
0
Hey all,

I'm just working through the london theory of superconductors. I've dervied the london penetration depth, the distance for the amplitude to drop by a factor of 1/e... Seems simple...

Now my book talks about applying a an ac magnetic field to a superconductor. How would the field in the superconductor change as a function of time?

I think it should just effectively be a waveform of the same form frequency of the ac magnetic field applied to it with a smaller amplitude?

Any discussion would be greater! Struggling to see any different from this!
 
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  • #2
In http://lss.fnal.gov/archive/tm/TM-0991.pdf is an AC measurement on a large dipole magnet with superconducting coils as it was slowly warming up. These magnets were about 22 feet long, and had 114 turns of a superconducting braided wire made of niobium-3 tin strands.
Look at Fig.5 on page 13. This plot represents the AC inductance of the dipole magnet, measured from 10 Hz to 10,000 Hz. The inductance represents the ability of the magnet volume including the superconducting coil to store magnetic energy. Specifically, the inductance is proportional to the volume integral (1/2μ)∫B·H dV. As the warming magnet temperature reached about 9 kelvin, the inductance rather suddenly increased about 4 milliHenrys at all frequencies. This change in inductance is attributed to the ability of the ac signal to penetrate inside the superconducting coil (Meissner effect) as it became normal. The fact that the ac resistive losses (Fig. 3) did not change implies that the Meissner effect applies down to very low frequencies (probably dc).

The AC signal for this measurement was about 1 amp. The magnet wire was designed to carry about 4500 amps.

See http://en.wikipedia.org/wiki/Meissner_effect
 

1. What is the London Theory?

The London Theory is a model used to explain the behavior of superconductors in the presence of an applied magnetic field. It was proposed by brothers Fritz and Heinz London in 1935.

2. How does the London Theory explain the Meissner effect?

The London Theory states that when a superconductor is exposed to an external magnetic field, the superconducting electrons create a screening current that flows on the surface of the superconductor. This screening current cancels out the external magnetic field within the superconductor, resulting in the expulsion of the magnetic field, known as the Meissner effect.

3. What is the critical magnetic field in the London Theory?

The critical magnetic field, also known as the upper critical field, is the maximum strength of an external magnetic field that a superconductor can withstand before losing its superconductivity. In the London Theory, this critical magnetic field is directly proportional to the temperature of the superconductor.

4. How does the London Theory explain the behavior of type I and type II superconductors?

In the London Theory, type I superconductors are those that have a single critical magnetic field and completely expel all magnetic fields below this threshold. Type II superconductors, on the other hand, have two critical magnetic fields: the lower critical field, where the Meissner effect begins, and the upper critical field, where superconductivity is completely lost.

5. Can the London Theory explain all aspects of superconductivity?

No, the London Theory is limited in its ability to explain all aspects of superconductivity, such as the microscopic mechanisms behind the Meissner effect and the behavior of high-temperature superconductors. However, it remains a useful model for understanding the behavior of conventional superconductors in the presence of a magnetic field.

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