Minutes of the 9th LHC Insertions Upgrade Working Group held on 7th February 2008
Present: V. Baglin, A. Ballarino, S. Chemli, R. Denz, S. Fartoukh, P. Fessia, M. Giovannozzi, H. Mainaud-Durand, K.H. Meß, D. Nisbet, R. Ostojic, H. Prin, E. Todesco, R. Van Weelderen, E. Wildner, F. Zimmermann
Excused: O. Brüning, J.-P. Koutchouk
Invited: P. Bestmann, M. Mauri, Y. Muttoni, J.-P. Quesnel, R. Veness
1. News and approval of the minutes of the last meeting
The minutes of the last meeting are approved without any comments.
2. Alignment of the new triplets (H. Mainaud-Durand, ppt file).
Hélène summarized the alignment strategy which was followed to position the current inner triplet quadrupoles into the LHC tunnel and the main difficulties encountered in practice during its implementation. She then listed a list of possible modifications to be envisaged in order to improve the situation for the new inner triplets. This strategy can be split into 5 consecutive steps:
a) the fiducialisation step F0 performed onto the surface which essentially consists in determining the mechanical axis of the magnet w.r.t. the external world (i.e. the survey targets). While, in practice, a measurement accuracy of 0.1 mm is reached after fiducialisation, transverse displacements up to 0.3 mm were measured in the tunnel for the cold mass w.r.t. its vacuum vessel. Furthermore longitudinal displacements up to 6 mm was also shown to be possible during transportation. For the next triplet generation, Hélène then insisted to review the MQX supporting system (presently quite similar to the one used for the MB's) in order to improve its stability both in radial, vertical and longitudinal. She also suggested to implement dedicated markers onto the quadrupole cold masses. These markers should be easily accessible in order to monitor accurately any possible displacements of the cold mass w.r.t. to its cryostat which could take place after the fiducialisation step (as the pins D9/D10/D11 which were used for the main dipole magnets).
b) the alignment of the inner triplet w.r.t. to the main elements of the corresponding arc and LSS (step F1), first using the geodetic network and then performing a smoothing procedure onto a displacement curves obtained. The positioning specifications have been met for this step, both in planimetry and altimetry (0.1 mm r.m.s.), but sometimes with difficulties due a tricky access to the fiducials hidden by the monitoring system equipments. In addition, due to the fact that the longitudinal dimensions of the beam tubes did not always meet their technical specifications, and due to the tight tolerances imposed for the longitudinal extension/compression of the PIM's (+/- 6mm), some MQX supporting jacks are already running out of range and longitudinal misalignments up 1.6 cm have be recorded for some low-beta quadrupoles as for the case of Q2.R5. A strict quality assurance plan has then to be set up to follow up the geometry of the different components which will equip the new inner triplets, in particular the length of the cold bore tubes and of the cold masses. The dynamic range of the jacks has to be increased, in particular longitudinally. The repartition and the redundancy of the survey targets has also to be reviewed in order to improved their accessibility once the quadrupole are installed into the LHC tunnel. Finally, the integration of all equipment volume would need to be done as soon as possible in the 3D layout of the area in order to avoid last minute interfaces for accessibility to the survey targets.
c-d-e) the alignment of the experiment w.r.t. one triplet (step F2), the alignment of one triplet w.r.t. to the other one (step F3), the alignment of the low-beta quadrupoles w.r.t. to each others (step F4). For this purpose, stretched wires are running from the left to the right of the four LHC experiments (Wire Positioning system WPS) to monitor any radial displacements of the inner triplet (within 0.1-0.2 mm r.m.s.). Each cryostat is equipped with several hydrostatic leveling sensors (HLS) for a monitoring of its altimetry (within 0.1 mm r.m.s.), including possible roll angle errors. The electronics of these two systems is located in the UPS galleries and the resistance to radiation of the sensors has been successfully validated. With some possible further improvements, these alignment systems will certainly be kept for the new inner triplet.
3. A first summary of the LIUWG discussions and organization of the Project (R. Ostojic, ppt file).
Ranko made a summary of the eight LIUWG meetings which took place since the creation of the working group. He then said a few words concerning the present organization of the LIU project.
Ranko first reminded the mandate of the working group (see also mandate), the various boundary conditions imposed on the phase I project, and the different milestones of the project, in particular the writing up of a conceptual design report with a deadline fixed around June-July 2008 (see also minutes of LIUWG#1). He then reviewed the different topics treated so far, insisting on the main conclusions drawn. Concerning the cold vacuum (see minutes of the LIUWG#2 and LIUWG#3), actively cooled beam-screens are still needed for the large aperture quadrupoles of the new inner triplet. They must be thicker than presently (2mm vs 0.6 mm) with, however, an operational temperature range which may eventually be increased from 5-20K to 40-60K. Concerning the optics and the triplet layout (see minutes of the LIUWG#2 and LIUWG#3), severe aperture restrictions observed in the LSS magnets limit the maximum possible aperture of the triplet itself. Out of the four triplet options which were proposed, only two are not strictly incompatible with the phase I boundary conditions, but still assuming slight modifications in the LSS (such as rotating the b.s. in Q4/D2/Q5 and/or displacing Q4/D2 and Q5 towards the arcs). Concerning the constraints imposed on the dimensions of the new inner triplet (see minutes of the LIUWG#3), it was stressed that the outer diameter and the length of the new vacuum vessel should not exceed that of the main dipoles due to access and transport limitations to points 1 and 5. Concerning the collimation related issues (see minutes of the LIUWG#4), larger triplet aperture offers in principle the possibility of opening the collimator jaws, leading to a net reduction of the collimator impedance. This however has a negative impact on the local collimation efficiency in the dispersion suppressors of IR3 and IR7. Furthermore, the large chromatic aberrations driven by the reduction of beta*, in particular the off-momentum beta-beating induced in IR3 and IR7, will certainly corrupt the collimation hierarchy if a suitable correction is not found. This shows that the modification foreseen for Phase I are not strictly local but are liable to affect the performance of other LHC sub-systems. Concerning possible cryogenic limitations (see minutes of the LIUWG#5), it was reminded that the cooling capacity of one inner triplet is limited to 400W @ 1.9 K (and possibly less for the triplet in 5L which uses the same refrigerator as the one needed to cool down the RF cavities of Point 4). This capacity will not be sufficient to cope with the ultimate intensity if the electron cloud related heat load is as high as presently expected. Finally due to the coupling of the cryogeny between the arcs and the LSS's, it was stressed that the triplet replacement in points 4 and 5 requires at present the warming-up of 4 LHC sectors with all the technical risks, delays and costs that this implies. Concerning the D1 magnets, the latter should be modified or changed to match the triplet aperture. The option of rebuilding a new normal-conducting D1 seems to be the most appropriate option (see minutes of the LIUWG#6). This issue will be discussed again at the next meeting and a cold version of D1 will also be presented. Concerning the powering and the protection of the inner triplet (see minutes of the LIUWG#7), the quench protection must be considered from the beginning as an integral part of the string design, with, in particular, energy extraction boxes included. The favored powering scheme is one single 13kA PC powering in series Q1/Q2/Q3 with 600 A bipolar trim for each magnet. Finally, concerning the energy deposited by the debris produced at the IP (see minutes of the LIUWG#8), the protection of the Q2 and Q3 magnets seems to be ensured by the new dimensions of the cold bore and beam-screens (~6 mm in total) while an additional dedicated liner is needed in Q1. The total power of 300 W deposited in the TAS requires a cooling system. The total power lost in the triplet is about 380 W (for a lumi of 2.5 ×1034 cm-2s-1), that is 10 W/m in average but with peaks up to 30 W/m. Nevertheless, in the present configuration, only 10% to 30% of this power is lost in the b.s/absorber. Said in other words, about 300 W will have to be extracted at 1.9 K which is already very closed to the cooling capacity of the inner triplet
The working group will continue to meet every 2 or 4 weeks. In parallel a matricial project structure has been set up with a certain number of work-packages, most of the work-package holders already identified, and a project team interacting "vertically" with one or several work-packages (see Ranko's slides for more details) . This project team will have the responsibility to identify the priorities, ensure the coherence and coordinate the work between different work-packages, inform the management on the resource needs and detailed planning, and prepare the LIUWG meetings.
4. A.O.B. and follow up of actions.
Next meeting scheduled for 21 February 2008: Options for D1 (D. Tommasini).
S. Fartoukh and R. Ostojic