EST-LEA/SW/2000-02
LEMIC 39
4th May 2000
LHC Experiment Machine Interface Committee
(LEMIC)
Minutes of the 39th meeting held on 28th March 2000
Present: F. Butin, A. Faugier, D. Gusewell, M. Huhtinen, B. Jeanneret, C. Lasseur, D. Lacarrère,
P. Lefèvre, P. Lepeule, J. Osborne, R. Ostojic, K. Potter, M. Price, J-P. Quesnel, H. Rammer, E. Sabin, F. Sanchez-Galan, M. Silari, G. Stevenson, E. Tsesmelis, T. Taylor,
B. Turner, G. Von Holtey, W. Witzeling, S. Weisz
Excused: I. Collins, H. Foeth, W. Kienzle, J. May
Bernard Jeanneret recalled that, within the mechanical tests to be performed on the prototype of the LHC-b vertex detector, special attention should be given to the vibration of the cooling system. The minutes were accepted.
(W.C. Turner – see transparencies in Annex 1)
The TAS and TAN will absorb about 50% of the collision energies at point 1 and 5:
The TAS has important interfaces with the forward shielding of ATLAS and CMS. These are reviewed in the "TAS functional specification" document and the "TAS interface specification" that is in preparation. Bill Turner recalled the recent history of the TAS design and presented the schedule leading to the delivery to CERN by the end of 2002.
The TAS and TAN absorber will be instrumented to provide a means of optimising the luminosity, and monitoring the beam crossing angle and the transverse position of the collision point. Thin gap ionisation chambers, with Ar + 1% N2 at 4 bar, are presently foreseen. They are located in a very high radiation environment and a first prototype will be tested in a high intensity beam at the SPS this summer. The system should allow for bunch to bunch measurements: tests in a 25 ns bunched beam are expected in 2001.
Pierre Lefèvre asked about the status of the inner triplet internal absorbers: these are implemented by Fermilab, responsible for the integration of the low-beta quadrupole, and will be presented in another Engineering Design Report. Georg Von Holtey asked how the luminosity will be monitored at IP8: Berkeley proposes a solution for IP 1 and 5 only, but similar techniques could be implemented at point 8 by the BI group in a simplified TAN-like absorber. Emmanuel Tsesmelis also mentioned that tests in a high radiation environment can be carried out in other labs and that CMS is currently using the PSI facility for that purpose.
(S. Weisz – see transparencies in Annex 2)
The members of the TAS panel are representatives of experiments (ATLAS & CMS) and of the LHC machine (magnet, survey, radioprotection, vacuum and integration groups) with the task to review all interface and handling issues concerning the TAS. The aim is to provide all input necessary for the TAS Interface Specification, currently in preparation at Berkeley.
The TAS will absorb about 250W at nominal luminosity and its residual activity is a major concern. It can reach 50mSv in front of the TAS (1 day cool down after 30 days at peak luminosity) and such a high value has strong consequences on the implementation of the various equipment around the TAS:
Besides, the TAS absorbers are local aperture limitations and good alignment is crucial. The design proposed for the ATLAS and CMS forward shielding integrate all these requirements. They also take account of the opening scenarios of the experiments, and this imposes another set of constraints, essentially on the vacuum equipment in front of the TAS. In conclusion, the design concept proposed by LBL in the "TAS functional specifications" are confirmed. However, the remote-handling device for the activated flanges in front of the TAS is still a critical issue.
Sylvain Weisz also commented the situation at point 2 and 8 where no TAS/TAN absorbers are foreseen:
This has to be compared to a power deposition expected of the order of 30W in Q1, for a high luminosity insertion, and to a quench limit of 5-10mW/cm3 for peak power deposition.
The conclusion is that there is indeed no need for TAN/TAS absorbers in insertions 2 and 8, provided that the experiments do not revise their request on peak luminosity. Bernard Jeanneret pointed out that the quench limit usually taken may not be valid for D2, as this magnet is cooled in a static helium bath at 4.5K.
(F. Butin – see transparencies in Annex 3)
Francois Butin presented the design of the ATLAS forward shielding: its functional specification is available, all envelopes are defined and agreed, static mechanical calculations have been made (seismic calculations still to be performed), and installation scenarios are investigated. The TAS itself would come in a "cradle" that will slide into the fixed part of the forward shielding. The study of this support and the installation of a lead plug to shield the TAS front face during intervention will be done at LBL. ATLAS will provide the cradle and the system to handle it for installation and maintenance.
However, the ATLAS group is still concerned with the position of the TAS alignment fiducials. They would prefer to have them located at the periphery of the Æ 3.0m support tube: this avoids the installation of a platform to access the fiducial marks, which has a financial impact and complicates the handling during the closing-up of the detector. Tom Taylor reacted to these arguments, pointing out that the optimisation of the luminosity is in the balance and that funding concerns are equally shared by the machine and the experiments. The fact that the TAS could only be aligned during a shutdown period, when the external shielding is removed, would be a serious drawback. Mike Price admitted that ATLAS should make all efforts to ease the TAS survey process.
(E. Tsesmelis – see transparencies in Annex 4)
The TAS will be the main source of background in the experimental area. The CMS forward shielding is required to reduce the hit rates in the outer muon stations to an acceptable level and to protect the front-end electronics in the cavern. This forward shielding is also an integral part of the personnel shielding during operation, leading to firm limits on the allowed radiation levels.
The performances of the forward shielding were derived in the CMS Muon Project TDR. The present design is more realistic and the shielding efficiency has unfortunately decreased by almost a factor two compared to the TDR situation. All efforts are being made to restore the initial performance, filling all gaps with iron, enlarging the beam pipe radius and generalising the use of boron-loaded concrete.
The first Engineering Design Review for the forward shielding was held end of November 1999. The interface to the TAS absorbers was scrutinised: a few points still need to be clarified, such as the consistency of reference frames used by CMS and by LBL, the location and tolerance (envelope) of holes for the support and alignment of the absorbers, the provision and handling of a temporary shield on the TAS front end during maintenance of the vacuum equipment.
A second EDR for the forward shielding is expected in May 2000 and the approval of the detailed drawings is expected before the end of 2000. The "blockhouses" and fixed iron noses are part of the LHC machine project and the moving components (rotating shielding, hinges + control) are the responsibility of CMS. However, both parts will be built as one at Protvino: assembly tests and pre-survey would also be done in Russia (without concrete) before shipping to CERN. All components of the CMS forward shielding should be delivered to CERN no later than May 2003.
Finally, a short animation of the opening of the CMS forward shielding and of the retrieval of the HF calorimeter in order to access the central tracker was presented. Emile Sabin prepared this animation using the ROBCAD program.
The next LEMIC meeting is scheduled for Tuesday the 9th of May.
S. Weisz