EST-LEA/SW/99-5
LEMIC 35
22nd June 1999
LHC Experiment Machine Interface Committee
(LEMIC)
Minutes of the 35th meeting held on 22nd June 1999
Present : F. Butin, I. Collins, D. Gusewell, M. Hatch, W. Herr, H-J. Hilke, J. Inigo-Golfin, B. Jeanneret, D. Lacarrere, R. Lindner, A. Morsch, R. Ostojic, K.
Potter, N. Siegel, A. Smith, G. Stevenson, T. Taylor, R. Veness, G. Von Holtey, S. Weisz
MINUTES OF THE LAST MEETING
The minutes were accepted without modification.
Experimental insertions in lhc ver.6.1
(R. Ostojic - see transparencies in Annex 1)
The LHC ver. 6.1 lattice corresponds to the introduction of a new corrector scheme in the arcs and insertions, and to a standardisation of the magnet cold masses in the dispersion suppressors and long straight sections. The operating temperature and/or powering of individual magnets has also been modified to increase the flexibility of the insertions. Concerning the experiments, version 6.1 provides warm locations for the TOTEM "Roman pot" stations in IR5. Also, the operating temperature of D1 has been lowered to 1.9° K in IR2/8 to increase the aperture of its beam-screen, in order to maximize the acceptance of the ALICE Zero Degree Calorimeter.
The "Mixed scheme" was adopted for the low-beta triplets: Q1 and Q3, which are 6.3 m long, will be built by KEK while Q2a/b, each 5.5 m long, will come from Fermilab. This solution provides a larger sample to sort the magnets and a reduction of the corrector strengths is expected, with a positive impact on the machine performance. The quadrupole production and integration is simplified, with one magnet length per manufacturer and identical low-beta triplets in the four experimental insertions. A detailed description of the triplet and the associated correctors and their assembly in the cryostat is available (see Annex 1). Since the operating current is different for the two types of quadrupoles, the busbars and leads of the cryofeed boxes had to be modified: it is now compatible with independent powering of the quadrupoles, which is needed for the high beta optics for TOTEM.
Ranko also commented on the modifications concerning the matching sections (quadrupoles Q4 to Q7) of the high luminosity insertions. An important new feature is the provision of a warm section between Q4 and Q5: the space will host TOTEM Roman Pots in IR5, and the same layout will be implemented in IR1. This solution provides a simplification and a standardization of the cryo-feed boxes.
The layouts of IR2/8 are not modified. The interfaces between the machine elements and the Alice ZDC in IR2 need to be clarified: this question was raised at the last LHCC and a detailed description is expected by the end of next year. Bernard Jeanneret also mentioned that the high Ca-Ca luminosity expected in IR2 may require a neutral absorber in front of D2.
status of the lhcb spectrometer magnet
(H-J. Hilke – see transparencies in Annex 2)
The LHCb dipole has to provide a bending strength of 4 Tm and the layout of the experiment shows that there is little space available for this magnet. A supra-conducting "bedstead" geometry (as in the LHCb technical proposal) and a "racetrack" geometry, either cold or warm, were envisaged. The warm "racetrack" option, which is retained, makes it easier to ensure the precise field inversion required to control the systematic effects that might limit the sensitivity of the experiment. The field is less homogeneous, but still acceptable, and the construction is simpler and thus cheaper.
The dipole would have a vertical acceptance of 250 mradian and a horizontal acceptance of 350 mradian. Due to this large aperture, the magnetic field extends from 2 m to 10 m from the interaction point (IP), with a maximum of 1 Tesla at ~5 m from the IP. The power consumption is 5 MW. The vertical field would be reversed about once a day and the possibility to ramp the dipole with the LHC magnets is under study: this would allow the reduction of the radius of the experimental beam pipe and it would also simplify the beam separation at injection.
Hans-Jurgen advocated an asymmetric compensation scheme of the spectrometer dipole such as to free the forward region where an extension of the LHCb detector could be needed. Tom Taylor pointed out that this demands substantially more corrector strength and needs to be fully justified.
report from the beam-beam workshop
(W. Herr – see transparencies in Annex 3)
Beam-beam effects limit the maximum bunch intensity and luminosity. A workshop was held earlier this year at CERN to review the present knowledge in the field and assess what still needs to be studied. The LHC machine presents critical features such as many bunches, crossing angles, little damping and "pacman" bunches due to gaps in the bunch train structure.
Beam-beam effects are non linear and introduce a de-tuning with amplitude. If the tune spread is too large, it will spread over resonances and the particle motion becomes unstable. The tune footprints can be predicted with a simulation that takes into account :
Results presented by Werner clearly indicate that alternating vertical and horizontal crossing plane provides a compensation of the long-range beam-beam tune shifts from the different crossing points. The incidence of the crossing angle on the tune footprint is well understood: there is no blow-up of the beam emittance with a 300 microradian crossing angle (+/- 150 microradian) and this value corresponds to an optimum dynamic aperture.
Beam-beam kicks also affect the closed orbit of the different bunches. The effects can add up, depending on the phase advance between the successive interaction points. This means that the bunches may not all collide head-on, resulting in a decrease of luminosity and beam lifetime. The optimum luminosity will require a precise steering and a bunch to bunch relative luminosity measurement will be highly desirable. Experiments are planned to study the effect of a ~0.2 s beam impact parameter on the beam lifetime.
NEXT LEMIC MEETINGs
The next LEMIC meetings are scheduled for:
24th August
12th October
23rd November
S. Weisz