Dear Rainer,
First you said the magnet started drifting by 100 Hz/hour - I assume you mean
here it drifted *down* in frequency first? And only sometime afterwards it
started drifting upwards? Or is that its current speed drifting up? Indeed that
may be its z0 coil discharging (a current built up in opposite direction to the
main coil) faster than the main coil’s natural drift.
Anyway, it could be that the magnet is experiencing an overpressure in its
helium vessel which should be vented as soon as possible: could you try to open
one by one both helium turrets (maximum attention - magnet may be about to quench!!)
and go in with e.g. an L-tube (the one used to precool a magnet with liquid nitrogen)
to check for any air ice?
If the tube “sinks in” a bit slowly it may indicate air ice -
pull back, warm up the tube with a heat gun and repeat. In case the magnet/helium
vessel is/was under pressure, it may vent quite violently suddenly - be prepared
for a sudden helium outrush (may look like a mini quench)…
If possible, have liquid helium at hand (you have a helium liquefier, right?)
to refill quickly in case it loses a lot of helium.
After getting rid of the ice, go search for a (likely present) cracked O-ring
or check vale (stuck open)…
Good luck!
Bert
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