Hello:
I would like to thank all the people who read and responded to my posting and I want to post my summary back to the group.
Most people think that the main magnetic field should not interact with the cryoshims, but the real world is not perfect. A drop in the main field causes inhomogeneity and it has to be corrected by adjusting the room temperature shims. It seems to me that a magnet with higher drift rate can have higher change in room temperature Z1 shim. One user reported that the room temperature Z1 shim for a 600 MHz actively-shielded magnet with a drift rate of 8 Hz/hour changed from 0 to 32767 in less than 3 years. On the other hand, the room temperature Z1 shim for our 600 MHz unshielded magnet (drift rate < 1 Hz/hour) did not change very much during the past 13 years.
Most people also think that the change of the room temperature Z1 shim is due to the drift of the Z1 cryoshim. I compared all the 8 cryoshims and found that some of the corresponding room temperature shims move in the negative direction while others move in the positive direction. Based on the information from other people, I came up with the following assumption and your comment is appreciated.
For negative cryoshim current, if the cryoshim current decreases faster than the main field current (relative relation), the room temperature shim will move in the negative direction. If the cryoshim current decreases slower than the main field current (relative relation), the room temperature shim will move in the positive direction. For positive cryoshim current, the room temperature shim will move in the opposite direction compared with the negative cryoshim current.
Charles Fry reported that there is a kit from Agilent that boosted the current of the room temperature shim so that they can keep the spectrometer shimmed, without having to touch the cryoshim immediately.
Thanks,
Weixing Zhang
-----Original Message-----
From: Zhang, Weixing [mailto:Weixing.Zhang_at_STJUDE.ORG]
Sent: Monday, January 03, 2011 5:34 PM
To: ammrl_at_ammrl.org
Subject: AMMRL: Does magnet drift affect the room temperature Z1 shim?. .
Hello:
Our Active-shilded magnet has been drifting at 4.5 Hz/hour during the past 10 years.
The Z1 shim has been changing from about -20000 to about the limit value -32767.
Is the Z1 shim change related to the magnet drift?
Did you notice this kind of relation with your magnets?
Thanks,
Weixing Zhang
St. Jude Children's Research Hospital
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Received on Wed Jan 05 2011 - 07:04:26 MST