Tuesday, January 13, 2015

Quench Detector Modifications: Lab Book 2015_01_13

If you're new to the experiment, scroll to the bottom for background that will catch you up with what's going on.

Liquid Nitrogen Fill
The liquid nitrogen Dewar has been filled.  It’s initial and final weights were 146 and 170 pounds, respectively.  I’m guessing I must have read the initial weight incorrectly, but we may be paying too much for ’45 liters’ of nitrogen which might be closer to 13.
Total Liters

I’m doing the same measurement as yesterday with the sample immersed in liquid nitrogen.  There seems to be no change. 

I have however invented a reasonably good vibration detector for the vacuum pump across the room, see video here.
I’m going to try a new configuration of the two coils next.  Pancake coils will be constructed with one placed on each of the opposing flat faces of the sample.
Checking resistance of lead sample
Resistance of leads was 0.0544 ohms, the resistance with the lead block between the leads was too close to be able to tell the difference.

Back to work on the quench detector
With the large melt growth YBCO sample and a a set of solenoidal, (not pancake), coils, the normal state results are quite different.  The x axis of the plot below is the output voltage that drives the primary coil.  The y axis of the plot is the signal from the pickup coil on the opposite face of the superconductor.  The signal generator is set at a frequency of 5000 Hz.

It should be noted at this point that the slope of the major axis of the above loop is very dependent on the orientation of the top coil. Care was taken to ensure that the top coil didn’t move significantly during the experiment.

imag0153 is the room temp result.  0155 is completely cooled.  0156 is with half of the liquid nitrogen evaporated and the upper coil once again suspended freely by the wires.  In other words, in the last picture there is no possibility of the liquid changing the orientation of the upper coil.  


Turning on the signal after cooling gives roughly the same results.  The first photo is room temp.  In the second photo is with the sample has been cooled to its superconducting state

We need more data.  It doesn't seem to make sense that the pickup signal became larger after the liquid nitrogen was added.  First off, this did not happen with the other setup from yesterday.  Secondly, with the superconductor expelling field from its interior, I thought the amount of flux through the pickup coil would have been reduced.  There is a possible explanation.  The liquid nitrogen should have also reduced the resistance of the primary coil allowing it to pull more current from the oscillator supply.  The increased current would lead to a larger magnetic field.  Tomorrow the current will be measured with the coil at room temperature and at the temperature of liquid nitrogen.

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.

The lab book entries in this series detail the preparation and execution of this experiment… mostly.  I also have a few theory projects involving special relativity and quantum field theory.  Occasionally, they appear in these pages.

*Call for Input*
If you have any ideas, questions, or comments, they're very welcome!

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

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