At 9:25 AM -0700 10/29/99, Woodrow W. Conover wrote:
>Hello All,
>
>The confusion here may be resulting from differing definitions of a
>"quench". A magnet can come down in at least 2 ways:
>
>1. A voltage spike or transients can exceed the voltage necessary to turn
>on the protection diodes in the magnet. When this happens the voltage across
>the diodes remains high enough to keep the diodes turned on and the magnet
>will continue to discharge.
** More importantly the voltage keeps the main supercon switch resistive,
considered "open" in the switch sense, and dissipating energy. The
protection diodes in parallel across the switch act to limit the voltage
across the supercon switch and therefore protect it.
> This process takes several minutes and is
>accompanied by a high helium boil-off. If this happens a VERY good and VERY
>LUCKY person can adjust a magnet power supply to exactly counter the voltage
>across the diodes and "catch" a partially dis-charged magnet.
** If this voltage across the diodes and supercon switch can be brought to
zero by adjustment of the power supply, then all of the magnet current is
flowing through the supply and none through the switch, and so the switch
cools down to become superconducting and "closed" again.
>
>2. A piece of supercon wire or joint can be penetrated by the magnet field.
>When this happens the superconductor is no longer super conducting and has
>resistance. The current flowing through the resistance generates heat which
>makes more and more of the superconducting wire resistive. This is a
>cascading event causing a complete discharge of the magnet in a matter of
>seconds. This process is accompanied by HUGH helium boil-off and can fill
>the magnet room with a "fog".
>
>Number 1 above is not what I call a "quench" while Number 2 is what I call a
>"quench".
>
>woody@acornnmr.com
>
>
------------------------------------------------
From Alan Olson, Instrument Engineer
National Institutes of Health, NINDS
10 CENTER DR, Room B1D-69, MSC 1060
Bethesda, Maryland 20892-1060
Phone: 301 496 8139, FAX: 301 402 0119
e-mail: awo@helix.nih.gov, olsonc@cvn.net(Alan)
voice mail: 1 800 952 7951 + when it answers: 68139
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