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Hi Steve

Built a superconducting magnet prototype..Dipole field, destined for use in a 4 Tesla MRI solenoidal field. Requirements for the magnet are operation at about 50-75 hz, up to about 500 amps, and needs about 200-300 volts excitation. Course, it's in liquid helium..

Coupla problems...

First, the power supply has to deliver those kinda numbers. Second, the magnet is a pure inductance...No resistance, very little capacitance, and huge quantities of energy storage (well, pittance actually, in the kilojoules as opposed to a coupla megajoules, but that's a matter of viewpoint).

We're setting up to test this baby soon at up to 50 hz, but since it's a pure inductance, this baby is gonna push the amp into quadrants 2 and 4. The closest to 1 and 3 will be steady state operation, something we don't worry about, as we've already tested that capability using a 10 kiloamp supply.

But in quadrants 2 and 4, the opposing pass elements are pulling the stored energy into the opposite polarity cap banks (dissipating in the pass elements big time), and sometimes little detail is put into the design for use in those quadrants.

We have to make sure the control of the signal amplitude doesn't go the hell in a handbasket in those quadrants, as part of the test regime will be to have the amp provide a DC level of about 250 amps, and superimpose another 200-250 amps 50 hz, forcing the amp to sink and source big time. Damping factor, as you point out, is a dimensionless number, but it gives an indication of control.

And I wanted to know if any of the amp guys here had any novel techniques for testing these quadrants, and the amp's ability to retain control while being jostled with woofer back emf.

The second problem??

The leads from liquid helium to liquid nitrogen are high temp superconductor. From LN2 to room temp, we use a copper feedthrough, about 1 inch in diameter..1 inch and 500 amps..50 hz? Should be easy, but guess what?

Skin effect rears it's ugly head...Because copper conductivity rises about three orders of magnitude when you go cryogenic. Now, can the copper sheet the current actually goes through really support the current??

And youse guys wuz tinkin skin was only fors audio stuffs?

The magnet:
http://www.bnl.gov/magnets/BioMed/BioMed.asp

I got it all over you guys....Because I have the coolest and biggest toys to play with...And he who dies with the most toys wins...

Cheers, John





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