You have fast ones, slow ones and very sloooow ones. This year, a few months
after being recharged we were not able to find any lock signal on our 400
MHz system any more. It turned out the thing was slowly quenching. We lost
roughly 1/2 of the fieldstrength in the course of a week. When the magnet
was more or less stabilizing at this level, service engineers have stopped
it and recharged it.
We have been working since then without problems. Our magnet is ~ 15 years
old.
Hein Schouwenaars
Molecular Spectroscopy Discipline
Analytical Sciences
Dow Benelux
Terneuzen
The Netherlands
-----Original Message-----
From: Woodrow W. Conover [mailto:woody@acornnmr.com]
Sent: vrijdag 29 oktober 1999 18:25
To: ammrl@wwitch.unl.edu
Cc: Rainer Haessner
Subject: RE: information about quench
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. 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.
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
-----Original Message-----
From: Rainer Haessner [mailto:Rainer.Haessner@bigfoot.de]
Sent: Thursday, October 28, 1999 12:37 PM
To: ammrl@wwitch.unl.edu
Subject: Re: information about quench
> This guy doesn't know what he is talking about
when he says a
quench
> happens over 5~15 mins. No, a quench happens over a few
seconds.
I am very sorry. What's this?
Did you ever experienced a quench live? Did you ever saw the
the really
impressive work,
how a service engineer "caches" a quench during the first 15
seconds.
Really!
He lowers the output voltage of his power supply and
sometimes this
stabilizes
the magnet. Of course during the charge procedure only. And
during the
mentioned
15 seconds the current went down by about 12 amperes (the
maximum
current of
the given magnet was 170 amperes).
I am no specialist. But I understodd, that one underlying
principle is,
to try to
remove all of the energy with the help of the internal
diodes. And
that's why the
speed of the quench is a function of the electric properties
of these
diodes
(the correct german word would be "Flussspannung", but I
found no
appropriate
translation). To my information the largest problem is the
inverse
charge of
the Z0 coil during the quench process.
But anyway, maybe there is a real magnet specialist here to
give a
fundamental
explanation.
Rainer Haessner