The repair of the leak at the proton flange NR11 took some time. All
3 seals showed some marks indicating a damage of the flange. The
vacuum chamber was therefore removed to polish the surface of the
flange. The misalignment was improved by loosening the absorber support
before the flange was tightened again.
A leak occurred at a welding seam, most likely due to mechanical
stress. It was rewelded in situ.
Beam bake-out started on Friday. Beam operation was interrupted by a
power glitch on Sunday. It took about 10 hours to recover.
The vacuum pressure at H1 is improving. A factor of
20 is still missing in order to turn on at high currents.
There was one short luminosity fill yesterday afternoon to make sure
that the
machine files are in reasonable state and to check the background
conditions at H1. It was difficult to obtain collisions and good
background conditions. The current of the SL GO horizontal dipole coil
had to be increased substantially. (Possible movement of bridge magnets).
The position of the spin rotators was changed last Tuesday. Polarization
tuning has not been too successful so far. The RF frequencies
were readjusted.
An energy scan may have to be done to retune polarization.
Fabrication of the new absorber 4 with coated plates failed.
There is still a leak after rebrazing
the absorber halves. The coating has bubbles.
A modification of the absorbers, avoiding some of the flange connections
by welding the vacuum chambers, is being designed. F.Willeke proposed
to replace the absorbers during the shutdown.
Switching to electrons (F.Willeke):
It would be very useful to decoupling the switch to electrons from the
shutdown activities. Switching two weeks before the shutdown is too
risky in case there are problems.
The proposal is:
Startup with positrons after the shutdown, i.e. a few days of
beam operation to make sure the machine is working.
(Question: do we need collisions?)
Switch polarity. Circulate electron beam on the same orbit.
Move IR magnets to the electron position.
ZEUS
Need a short luminosity fill before the access on Thursday.
HERMES
Still recommissioning the LPOL.
Schedule
Continue positron beam bake-out until Thursday.
Short luminosity test run tomorrow.
Thursday: maintenance day starting at 06:00hrs.
Warmup north and south GO/GG magnets and regenerate NEG pumps.
The tunnel will be closed as early as possible.
Continue with luminosity operation after the access with high positron
and somewhat reduced proton current.
(No machine studies).