I spent this morning investigating SFT Hall probe mystery. The main
manifestation of the problem
is significant change of the probe reading during the ring fill. It
depends on the ring current in a rather
funny way, changing the reading by as much as 10 G. During the data
taking the probe is more stable,
although some beam dependence still exists.
The investigation showed that the problem is the probe itself. Most
likely, the reason is radiation damage.
The MFT probe, which is located much farther from the beam, shows the
same symptoms, but an order of
magnitude smaller. Holding field probe, located very close to the beam,
is basically destroyed.
Our resonances are wide enough, so when the target switching happens
during the data taking phase, the
magnetic field is set correctly. But if the switching happens during the
ring fill, the resonance is likely to be missed,
which lead to effectively reduced polarization. We can try to forbid
switching durin the fill (which is not easy!), but
the situation grows worse as the radiation damage accumulates, and I
don't expect that we'll last very long.
Another possible solution - rely on the power supply current only
hardly look adequate because of the hysteresys
in the SFT magnet (it is BIG magnet, and we can't drive it over the
whole hysteresys cicle).
It is my strong believe that we should replace SFT probe... perhaps
MFT probe as well while we at it. It will take
about 3 days, and new probes would last for the rest of the run.
Genya
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