Summary notes of the LCU meeting on 30/10/2012

Present: JA, HB, RdM, SF, MG, JJ, YL, EM, SR, MS, RT, RV, PS

General information -> MG

This was a short meeting without general reports from meetings. The only general communication was that the ABP end of year party is scheduled for the 13 December.

Report from MDs : Optics during ramp and IR non-linearities -> RT (slides)

Rogelio reported about the recent optics MDs. The polarities of the skew sextuples in the IRs was checked with beams at injection. The polarity convention is different from what is used in the arcs. Beta-beating was measured at various energies through the ramp (1.3, 2., 2.6 TeV) and changes of beta-beating found to be generally within 10%.

Chromatic coupling was measured and a correction tool developed. IR non-linearities were measured. IR1 is now well understood and corrected. IR5 needs more work including a review of the effects for the experimental magnets.

Detuning with amplitude was measured for the first time using the AC-dipole. The natural detuning with amplitude was found comparable to the amplitude detuning of the octupoles at 100 A. Beam 1 seems to have less detuning with amplitude than Beam 2.

Report from Beta* = 1000 m physics run -> HB (slides)

Helmut reported about the recent high beta run for physics at beta* = 1000 m.
He recalled, that the 1000 m optics was tested, adjusted and commissioned in 3 previous sessions. The main goal was to get to the Coulomb interference region, which is known to be extremely challenging at the high LHC energies. It required both record low beam divergences as well as extremely tight collimator and roman pot settings.
24 hours had been scheduled for the 1000 m physics run. The first fill was used to centre collimators and the TOTEM and ALFA roman pots and to test closing the primary collimators to 2 sigma in both the horizontal and vertical planes. This worked, but resulted in significant losses, in particular on beam 2 in the horizontal plane which had the largest emittance. The fill was lost by a spurious QPS trigger before TOTEM and ALFA could take data and before it was clear if the background conditions so close to the beam would actually allow for data taking. Within the 24 hours there was no time to re-optimize the de-squeeze to minimize emittance blow up.
For the second fill it was decided to keep collimators and roman pots 1sigma further out in the horizontal plane (which is less relevant at 1000m with a non-optimal phase advance from the missing cables).  It took until nearly midnight to get the beams back to 1000 m ready for data taking. Initial backgrounds with the tight settings  (primary collimators at 2sigma in V, roman pots at 3 sigma) were very high, incompatible with good data taking.

The backgrounds reduced by 3 orders of magnitude when the primary collimators were retracted by 0.5 sigma. Fortunately, it turned out that the halo-repopulation was sufficiently slow, so that TOTEM and ALFA could record sufficiently clean data until another halo cleaning at 2 sigma was required. The process of halo cleaning followed by 1-2 hours data taking was repeated 5 times. TOTEM and ALFA recorded over 300k elastic proton-proton collisions.

Another interesting observation in this run was on signs of instabilities in the presence of a high transverse impedance from tight collimator settings. The signs of instabilities were strongest on the non-colliding bunches. About half of the most intense non-colliding beam 1 bunch intensity was lost shortly after closing the primary collimators to their tight (2sigma in V) settings.

See also the CERN bulletin article from the 2 November.

AoB

Stefano reminded us about the the :  Special ColUSM: internal review of "Tevatron hollow e-lens usage at CERN"  on Friday 9 November

and encouraged our active participation.

Last update: 5-November-2012

MG & HB

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