Summary notes of LCU meeting on 21/05/2007

Present: IA, CB, HB, RDM, UD, MG, WH, DK, TP, TR, FS, RT, SW, MA

Follow up of actions

• History of phase advance of TCLIs in TI2, 8  -> TR
  The current TCLI positions have been confirmed and accepted. TR said that no further action is required on our side and that he expects BT to study the performance of the current system.
  TR also commented more generally on recent developments on optics V6.501 relevant for injection. He found good solutions for IR2 and IR8 in which Q5 is kept at its previous V6.500 strength on the side relevant for the injected beam. With this additional constraint, optics V6.500 and V6.501 remain the same as far as injection is concerned and require no rematch of the transfer lines TI2 / TI8.
  TR said that studies of the acceptance for the injected and circulating beams continue.
  MG suggests that a more detailed aperture comparison of V6.501 / V6.500 will be presented and discussed in the next meeting. He also mentioned the issue of out of bucket particles kicked out at injection into the D2, which is currently being looked at.

Report from meetings

• SLM -> MG
  A general ABP group meeting is planned for Friday 1st of June 14:00, with short, about 10' reports from all sections. MG will circulate slides for LCU before the meeting.
• ICC -> MG
  The ICC alternates between more formal meetings with minutes and more informal round table discussions. The last ICC meeting had the more informal discussions on hardware issues for the commissioning and the question of access to control room computers, which already had been discussed in AB. Sector 7/8 which had reached 1.9 Kelvin was switched to a new refrigerator. A recent thunderstorm revealed a problem with switching to backup power. An intervention on the software side also revealed some deficiencies in the robustness of the system. Concerning the triplet cryostat, it appears that a solution has been found and that the repairs can start soon.

Wires in accelerators LHC, RHIC -> UD (pdf file)
UD uses BBTrack for beam-beam simulations (no magnetic errors included). The simulations are of the weak-strong type, in which the strong beams remains unchanged. Off momentum particles are tracked over 300000 turns. He looks for limits of stability and on-set of chaos. He presented tune scans over 0.5 x 0.5 in phase advance difference between 1-5/5-1 (horizontal and vertical). Contrary to previous known results, a dependence of DA on the phase advance at the level of up to 1 sigma is observed. This calls for additional studies including measured errors and with tune scans.
UD also studied wire compensation of parasitic beam-beam effects. He will soon spent time at RHIC and be involved in measurements using a wire which should allow to test and benchmark the simulations.

Study of the 2625 m Atlas optics -> SW (pdf file)
MG recalled that SW is currently working with us for 3 month until end of June on his internship and that he will return in September to start the work for his PhD. MG also introduced MA (Masamitsu Aiba) who is at CERN as fellow and who will work later this year with us on alternative optics solutions in the attempt to conserve the standard integer tunes also in the presence of high beta insertions.
SW showed us the status of his studies for the 2650 m beta* optics for ATLAS. The high beta* is required by the small scattering angles needed to reach the Coulomb region in proton-proton scattering at 7+7 TeV. The first step was to start from the existing Beam 1 solution at beta* = 2650 m and find a solution for Beam 2. He showed that he found a nearly anti-symmetric solution which satisfies all constraints with similar beta functions at the roman pots as for Beam 2. As in previous versions by Andre Verdier, the Q4 polarity is inverted compared to our usual optics and the integer tune reduced by one unit in the vertical plane for the combined Totem + Atlas high beta operation. The tunes for the optical solution presented are not the nominal ones, yet. Re-matching using trim quadrupoles should be applied. In terms of aperture the target of n1=7 is achieved for an emittance of 1 mum: MG suggests to evaluate the maximum emittance that is compatible with the n1 target. SW also looked at injection. He found that the older beta* = 200 m optics by AV has several quadrupoles below 2%. He found that beta* will have to be increased to allow for fractional tunes not too far from the standard tune and at the same time satisfy the lower power supply constraints. The aperture at injection is about n1 = 7 for an emittance of 2 μm, which can be considered as rather safe given that ATLAS ultimately will require an emittance of 1 μm at top energy. SW has started to study the un-squeeze from high beta injection to physics. MG asked to also check that insertion quadrupole strengths fulfill the constraint imposed by the three-lead connection, namely 1/2*I(Beam 2) < I(Beam 1) < 2*I(Beam 2). MG also comments that before putting some efforts on the un-squeeze,  the strategy for how to deal with the different integer part of the tune should be addressed. He also mentions that it would be good to repeat the same study for the high-beta TOTEM optics.
RT suggests to see which residual dispersion would be tolerable at the high beta IPs.
WH reminded us about the difficulty to separate beams sufficiently at the IPs to avoid collisions in high beta optics. This essentially rules out the injection optics with beta*=232 m.

Final (?) squeeze solution for IR8 -> YP (pdf file)
YP studied squeeze solutions for IR8. These studies were triggered by two different motivations: one is to allow for more margin on the triplet strength towards the end of the ramp using a pre-squeeze at constant beta* (10 m); the other to avoid long squeeze times imposed essentially by the behaviour of Q4, which decrease in strength during the squeeze. The solutions shown refer to Beam 2 and it is quoted that Beam 1 is also in good order. It is planned to improve the modeling of the currents needed for the strength change during the squeeze. These data will be transmitted to PO experts (D. Nisbet) and OP (M. Lamont) to be used in the forthcoming hardware commissioning tests of the insertion quadrupoles.
It is likely, that in the long-term the LHC will be operated using a combined ramp and squeeze to improve on the turn around and maximize the time with collision. A solution with separate ramp and squeeze will still be required, at least during commissioning.

AOB

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Last update: 23-May-07

MG & HB

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