Wednesday, June 11, 2014

Getting Back to Basics, Generating the Quenching Magnetic Field

A question from +Yannick Selles inspired today's post which is a rerun of the answer I posted yesterday on G+ with a few more pictures and an additional book reference.  I blame the evil stomach spirits that attacked for the lack of originality :)  Yannick asked how the magnetic fields required to drive the superconducting samples into their normal states, (quenching), for the H-ray experiment, were generated.  Here are the basics

When an electric current travels through a wire, it creates a magnetic field surrounding the wire.



By wrapping a wire into a coil, (a solenoid), the magnetic fields from each turn of wire align to produce a stronger magnetic field.  The black cylinders you see in the picture of the magnet below are solenoids of copper wire.  The black casing carries water to cool off the wire since it also heats up as current is passed through it.



If you want to make an even larger magnetic field, this is where a material like iron comes in.  Iron is made up of magnetic domains that align when they are placed in a magnetic field.  Consequently, when you wrap your coil around iron, not only do you get the field from the coil, you also get the aligned field of all the magnetic domains in the iron.

Here's a picture of the original coil wrap of my cyclotron magnet.  The wrap was scrapped for a more uniform job later, but it gives you a clear picture of the wrapping of the coil onto the iron pole pieces.



The metal you can just barely see in the center of the black wire cylinders is iron.  The metal painted blue is also iron.  Magnetic fields prefer to stay in iron in much the same way electric currents like to stay in wires.  It' the easiest path for them to move in.  We capitalize on this by surrounding the pole pieces with an iron frame that you can see better here:



In this manner, as much of the field as practically possible is focused between the two cylindrical pole pieces.

Entire books have been written on magnetic circuits including this one, (open access on Google Books), by a historical figure I've been researching, Dr.Vladimir Karapetoff of Cornell University.



Background
Hirsch's theory of hole superconductivity proposes a new BCS-compatible model of Cooper pair formation when superconducting materials phase transition from their normal to their superconducting state[1].  One of the experimentally verifiable predictions of his theory is that when a superconductor rapidly transitions, (quenches), back to its normal state, it will emit x-rays, (colloquially referred to here as H-rays because it's Hirsch's theory).

A superconductor can be rapidly transitioned back to its normal state by placing it in a strong magnetic field.  My experiment will look for H-rays emitted by both a Pb and a YBCO superconductor when it is quenched by a strong magnetic field.

This series of articles chronicles both the experimental lab work and the theory work that’s going into completing the experiment.

References
1.  Hirsch, J. E., “Pair production and ionizing radiation from superconductors”, http://arxiv.org/abs/cond-mat/0508529 


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