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Showing posts from July, 2012

Electromagnet Impedance

The data taken last week showed a linear dependence between the voltage measured in the pick-up coil when the superconductor is levitated and the frequency of the current driving the levitating electromagnet.

While reading an article on a susceptometer for superconductors, I came across the graph shown below that shows the decrease in the magnetic field of a solenoid driven at 5 V rms as frequency is increase.  A solenoid is an inductor with an impedance that is linearly dependent on the frequency of the current flowing through it.  The drop in the magnetic field is a result of of the impedance of the solenoid increasing with increasing frequency and reducing the current trough the coil.

I'd like to see if the linear increase in the voltage required to attain levitation is just a result of the increasing impedance of the electromagnet.  My first task was to determine a relationshiop between the pick-up coil voltage and the voltage driving the electromagnet.  To do that, I attached…

Linear Trend Develops Between Frequency and Required Field Strength for Levitation

I was greeted by a hissing liquid nitrogen Dewar when I got to the lab today.  Apparently it's normal Dewar venting, but our last two didn't do this, so it was a bit interesting.  I was able to take more data today and there's a linear trend evolving.  As the frequency of the current driving the electromagnet is increased,the amplitude of the current has to be increased to achieve levitation of the superocnductor.  A picture of theresulting graph is shown below.  The x axis is frequency.  The y axis is the peak voltage detected on the pick-up coil wrapped around the electromagnet.

Here's a picture of the modified pick-up coil.

It's just three windings around the top of the magnet.

Liquid Nitrogen, Water, Conductivity, and the new Data Acq System

There's all kinds of stuff going on today as I try to get the apparatus ready to go to measure the amplitude of the magnetic field required for levitation at each frequency.

First, the levitation detector on the sample holder.
Building the levitation detector on the sample holder and the liquid nitrogen reservoir didn't work out nearly as well as I'd hoped that it might.  First you ask, what's a levitation detector?  Up until now, I've been determining the moment of levitation after the experiment by making videos that watch the superconductor as the AC magnetic field is increased.  This was fine until I wanted to know what the magnitude of the voltage seen on the pick-up coil used to detect the levitation magnetic field was at the exact moment the superconductor first levitated.  There were a number of problems with doing this with cameras, the least of which was actually needing two cameras.

I came up with a system of attaching a piece of aluminum foil to the boom…

A Horse Trough a Quonset Hut and Data Acquisition

It gets hot in Las Cruces, and sometimes there's nothing to do for it but take a dip.  That's how I found myself sitting in our horse trough with Junior this weekend cogitating about the new data acquisition system.  I had a glass of McAllen, and Junior had her boat, (at 17 months, she's much too young for McAllen), and a good time was had by all.  The horse trough is left over from when we lived in a Quonset hut.  The building didn't come with a bathtub, but a short trip to the local feed store a few blocks away and we were set.  It was kind of cush, the trough sat near the back door about five paces from the train tracks, so you could sit there and watch the trains go by, (see the video below).

That was before our landlord decided the space would make a better art gallery.

I digress though!  Back to the data acquisition system and its requirements.  I've come across two articles about the effect of alternating current magnetic fields on superconductors that see…

Superconductor Trapped Field and its AC Attenuation: Weekend Reading

I'll be doing some reading this weekend trying to get some more information behind the experimental results I've seen this week.  The rough observations this week were:

1.  Levitation force seems to go down for the same peak magnetic field as frequency increases.
2.  The levitation force can be increased at a given frequency by increasing the peak magnetic field.

I haven't found much research that measures the levitation force between an AC electromagnet and a superconductor.  I have found some interesting papers on the attenuation of magnetic flux trapped within a superconductor when an AC magnetic field is applied to the superconductor.  Two of the more interesting papers I've found are

Ueda, H., Itoh, M., & Ishiyama, a. (2003). Trapped field characteristic of HTS bulk in AC external magnetic field. IEEE Transactions on Appiled Superconductivity, 13(2), 2283-2286. doi:10.1109/TASC.2003.813075


Ogawa, J., Iwamoto, M., Yamagishi, K., Tsukamoto, O., Murakami, M., …

Levitation Frequency Characterization: Train of Lab Work Notes

Great news!  The bridged output mode on the Peavey amplifier I'm using here works!  That means I can apply roughly twice as much power to the levitation force providing electromagnet.  The bridge mode appeared not to be working two days ago.  A little reading of the manual provided the reason.  When in bridge mode, the speaker outputs don't like to be applied to any kind of ground.  I was trying to measure the output of the amplifier using a grounded oscilloscope.  Apparently this is what was causing the amplifier to shut itself off in bridged mode.  After removing the oscilloscope from the speaker outputs today, everything is working great.  Instead of hooking the scope directly to the electromagnet inputs, I attached it to  the pick-up coil.  As I mentioned a few days ago, this a better measure of the power available for levitation anyway.

I don't have time to write up everything in a classy fashion at this point, but I would still like to post on what's going on ar…

Measuring Magnetic Field vs. Frequency Using a Transformer

Alternating current power can be reflected by a reactive load, (like an electromagnet). Ham radio operators are familiar with this concept and measure the amount of reflected power as the standing wave ratio, (SWR). As the frequency driving a ham radio antenna is changed away from the antenna's resonant frequency, the SWR increases, less power is driven into the antenna and more power is reflected back into the radio's amplifier.

On Friday, I observed that the levitation height of the superconductor decreased when the frequency of the current driving the levitating electromagnet was increased. This is the expected result based on the experience of the other teams that tried to replicate Podkletnov's experiment. However, the same behavior would have resulted if power from the amplifier was being reflected without entering the electromagnet.  The amount of reflected power is usually measured with a directional wattmeter, or with an SWR meter.  I don't have either of …