Summary notes of the LCU meeting on 23/07/2012

Present: RB, HB, RdM, LD, OD, MF, MG, PH, EM, SR, MS, RV, EQ

Report from meetings

• SLM and general information -> MG
  The meeting in two weeks is cancelled (many of us in holidays). The next LCU meeting is foreseen for the 21 August.
  Roderik Bruce introduced  Elena Quaranta who just started as a student to work on collimation. Stefano Redaelli said that another student, Daniele Mirarchi will start on the 1st of August.

High-beta studies -> HB (slides)
Helmut presented an overview of recent high-beta studies for TOTEM and ATLAS/ALFA, covering the two high-beta MDs as well as the recent 90 m recommissioning and operation, see his slides.
The 90 m optics is the same as last year, where it was used successfully with few bunches to study mostly elastic scattering. This year, it was used for the first time with over hundred bunches in both beams and total intensities reaching 1.e12 protons for diffractive physics studies.
This required a full validation of machine protection, followed by a longer 90 m physics fill with stable beams. In this fill, TOTEM was taking data together with CMS. In the first high-beta MD on the 21 June, beams were successfully de-squeezed at flat top from 11 m to 90 m and further on in steps up to 500 m.
The 500 m optics was measured and corrected (except for beam 1 horizontal, where the AC dipole had a fault). The second high-beta MD was used to complete the optics measurements for the corrected machine at a beta* = 500 m in IP1 and IP5, and to continue to de-squeeze in steps of 100 m to the maximum of beta* = 1000 m. This went very well. 1000 m were reached without losses on the first attempt. The next step will be to find and adjust collisions, which maybe non-trivial given the low collisions rates and limitations in corrector strength and aperture. If collisions can be established, the 1000 m optics would become available for special high beta runs in September / October for physics with the prospects to reach the Coulomb interference region in proton proton scattering and obtain a precise, luminosity independent pp cross section measurement.

Intermediate beta* for ALICE -> PH (slides)
Pascal Hermes presented the status of his feasibility studies for an intermediate beta* optics for ALICE.
The beam time for 90 m runs for TOTEM/ALFA reduces the time for high luminosity ATLAS/CMS operation and is therefore limited to few days per year. ALICE is running in proton operation with low luminosity and could potentially always work at a higher beta*. To be compatible with high luminosity operation in the other IPs, this would require a crossing angle, which limits the maximum beta* to about 30 m for 25 ns bunch spacing (see the LCU meeting of the 24/01/2012 ).
Pascal developed a 30 m optics for ALICE. Ideally the phase advance should be 90 degrees in both vertical and horizontal to roman pots to be installed close to Q6. This is not feasible. As a compromise, he uses 120 degrees in both planes which would be fine for ALICE if the would install two roman pots stations separated by several meters.

Pascal checked that the aperture is ok at 30 m (should be repeated for the new smaller beam-pipe). The vertical crossing angle is limited by corrector strength at 7 TeV to 150 murad, which corresponds to 9 sigma separation for the first parasitic crossing at the nominal 3.75mum emittance. This is tight and requires further verification with the beam-beam team.
RV comments that an upper bound to the crossing angle might be given by the need to use the ZDC: this point should be raised and discussed with the Experiment.
The details of how the tune will be compensated should be still worked out.

Last update: 24-July-2012

MG & HB

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