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Re: persistent currents



It is also critical to know how to ramp down the magnet in a reasonable
amount of time - most people use an energy absorber of some type.
http://www.americanmagnetics.com/magnets.html#power

Critical current density of superconducting wire depends on magnetic field
and temperature, so it not meaningful to discuss a value in the absence of
these parameters. Also, the area needs to be defined as well - area of the
entire wire or area of the superconductor in the wire - since most magnet
wires are multifilamentary with much normal stabilizer.

Larry Woolf; General Atomics; 3550 General Atomics Court, San Diego, CA
92121; Phone:858-455-4475; FAX:858-455-4268; http://www.sci-ed-ga.org

-----Original Message-----
From: John S. Denker
Sent: Friday, January 11, 2002 11:45 AM
Subject: persistent currents


You can buy superconducting wire that has a
critical current density in excess of 50,000
amps per square cm. Jc in excess of 300,000
A/cm^2 has been claimed for small samples.

The tricky part is that you can't change the current
very fast, because of all that inductance. If you
try to change it quickly, especially when the thing is
almost fully charged up (or should I say fluxed up?),
you'll just drive everything normal and a whole lot
of B^2 energy will be converted to heat all at once.
Wheeee!!!

So you need a controller that synthesizes a nice slow
linear ramp up to the desired current. A microprocessor
plus D-to-A is the only game in town. If somebody
tries to sell you an analog controller, tell him
you'll call back later. Much later.