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If you remove and later (not too much later) reinstall the probe and you are confident that probe position inside the magnet does not change at all - would you expect the shim set to change? Do eddy currents due to probe movement affect field homogeneity? Thanks. |
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The short answer to both questions is No. However, it is not possible with standard Varian probe mounting hardware to precisely reposition the probe, so you should expect slight changes to the shims even though you think it is in the same place. Bruker probes are much more easily returned to the precise same position as before removal, so long as the upper stack is not disturbed. When moving a probe there could be slight, short term SC shim current changes induced by eddies, but they will equilibrate back to their set values after the eddies stop. RT shim currents will not change from eddies, because they are actively regulated. |
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Having removed/replaced Varian probes 100s of times over the past 25 years (either just to change to a different probe or due to tube breakage), I have found that the minor reshimming required (usually low-order axials only) is no different than that applied after changing samples. I concur with Jerry H's assessment of the fiddly and superficially imprecise feel of the attachment system but, in practice, this seems to maintain precise alignment wrt the shim stack. One caveat (Varian probes) - always ensure that the mounting flange is securely attached to the probe body if routinely swapping probes. |
