Summary notes of the LCU meeting on 16/12/2008

Present: MA, RA, GB, CB, HB, RC, SF, MG, WH, JJ, JM, JN, SP, FR, FS, TW, SW, SY, FZ, AR, GV

Report from meetings

• SLM -> MG
  Adriana Rossi and Bernard Holzer have joined our group. AR works in the collimation team and BH mostly on white paper activities.
  The 2009 schedule for the injectors was modified and now includes an ion run for the injectors.
  IT has stopped FORTRAN support. Our group and the RF group still need FORTRAN support and the NAG compiler. A request for FORTRAN will be prepared from our department.
  BOINC LHC@home supplied lots of free CPU. However, the quality of the support is degrading with time and make the service pretty useless. Alternative solutions are being looked for.
  Several of us will likely need to follow refresh courses on "Habilitation électrique" to maintain permissions to access accelerator tunnels. A general email was already circulated and our training officer contacted to see when and to whom this applies.

Recent results for ion collimation studies -> JJ  (pdf)

JJ reported about recent progress for ions collimation, namely cryogenic and magnetic collimators. The subject was also discussed in the Phase II collimation meeting on 4 Nov.

A recap of the recent activities is made. The change in geometry in the LHC ring following the installation of the cryogenic collimators next to Q8 and Q10 was correctly implemented and the optics mismatch (about 20%) cured. The net gain produced by the cryogenic collimators was confirmed even under these new conditions (the improvement was already showed by Giulia Bellodi for the previous version of the optics, with a non-corrected geometry and optics).
It is claimed that cryogenic collimators might also useful for alleviating the BFPP effect. In this case they should be installed in all three experimental insertions IR1/2/5. In the case of IR1/5 the compatibility of such collimators with FP420 should be carefully studied.
The other topic is the use of magnetic collimators. they were already considered by P. Bryant in 1993, but the idea was not pursued further. H. Braun proposed them more recently. For these studies, ICOSIM was improved and extended by Nina Holden. A peak field of 2 T at the secondary collimator surface is generated. the field goes to zero on a few millimetre scale. Two cases were considered, namely one horizontal magnetic collimator or one skew magnetic collimator. In both cases the reduction of the beam losses in the dispersion suppression of IR7 is of a factor of 2. This sort of proof-of-principle study shows that the gain introduced by the use of magnetic collimators is certainly not comparable with the one obtained with cryogenic collimators, which remain the most promising solution.
SF comments that the option of changing the geometry of the dog-leg in order to increase the dispersion should be re-considered and this is the most efficient solution and it entails similar layout problems as introducing cryogenic collimators. RA replies that there is no space to envisage such a situation.

Update on the new optics for reduced off-momentum beta-beating   -> MA (pdf)

As previously discussed (meetings on 26/8, 6/10 and 4/11 in 2008) a phase advance of pi/2 between IP1 and 5 reduces the off momentum beta beating in IR7. In previous solutions, such a condition was achieved only in the horizontal plane, thus implying that the vertical off-momentum beta-beating was minimized in IR3.
By changing slightly the phase advance in the arcs (2-3, 3-4, 6-7, 7-8), MA was able to find a solution with pi/2 phase advance between IP1 and 5 in both the horizontal and vertical plane. The neighbouring insertions were re-matched to the slightly modified arcs. The remaining arcs were not modified in order to avoid re-computing the optics and the squeeze sequence for IR1 and 5.
Advantages are small off-momentum beta-beating in the betatron collimation insertion in both planes (which is the required condition as found out in the framework of the Phase I upgrade studies), equalization of beam1/2 phase advances around the ring, good aperture and zero dispersion in IR7 LSS. The changes of IR2/8 are small and in any case affect the non-injected beam.
Some detailed cross checks will be performed such as aperture, quadrupole strength (avoiding too weak quadrupoles). The squeeze sequences for IR2 and 8 will be adjusted to the new phase advance.
The optical solution will be considered finalized upon completion of the tracking studies and the evaluation of the loss maps for collimation performance.

Recent results for Phase 1 upgrade optics studies -> JM    (pdf)
JM reported about further work on insertion optics for the LHC upgrade with a modified larger aperture triplet. As reported previously by him (confirming the findings of SF presented at the LIUWG meeting in May 2008), a stable rematch for 25 cm beta* appears to require modifications in the matching section.
To better understand the matching stability for the existing matching section, JM made a rather complete study of the parameter space for the new triplet imposing beta* = 35 cm and scanning the length of Q1, Q2, Q3 and the strength of Q1 and Q2. The strength of Q3 is used as matching parameter to adapt the triplet to the current layout of the matching section. This approach was applied to the right side of the insertion, only. Stable regions in the parameter space were identified and a selection of the key parameters (length and strength) of the triplet quadrupoles proposed.
However, this is only the first step towards the definition of the solution for the Phase I upgrade optics. Indeed, the left side of the insertion should be checked too, together with the usual details (aperture, tracking, collimation performance). It is already clear that the vertical phase advance is lower than the nominal one (0.25 less in each IP).
JM's presentation was followed by a long discussion, triggered by the observation made by SP that the TDR should be ready by summer 2009 and an optical solution is not available to date, in which several of us expressed their concern on the tight constraints and various currently not well understood issues for the upgrade. MG will contact R. Ostojic to discuss the situation.

AOB

Merry Christmas. See you next year.

Last update: 05-January-09

MG & HB

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