Macina, D

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EQUIPMENT OF EXPERIMENTS DIRECTLY INTERFERING  WITH THE BEAM OPERATION D. Macina, CERN, Geneva, Switzerland AbstractAll four experimental insertions will be equipped with movable devices which, in some cases, have to be located very close to the beam. In addition, both ALICE and LHCb use spectrometer magnets which distort the beam trajectories.The location of this experimental equipment and the signals used for its positioning will be presented. The various operation scenarios will be discussed together with the experiments’ proposals for who will be responsible for these operations. INTRODUCTIONBoth ALICE and LHCb use spectrometer magnets which distort the beam trajectories in the vicinity of the interaction point and around the machine and their effect has to be compensated locally with dedicated compensation magnets. In addition the ALICE central detector is completed by a set of zero-degree calorimeters (ZDCs) [1] located far downstream in the machine tunnel. The ZDCs are movable devices and, even though they are not located inside the primary vacuum, a false manoeuvre may damage the vacuum pipe and cause a major machine downtime.  The LHCb detector is equipped with a movable vertex locator (VELO) [2] which has its nominal position during data taking at 5 mm from the beam axis. With a  false manoeuvre, the VELO detector is capable of touching the beam with important consequences for the LHC machine. The same is valid for the TOTEM Roman Pots [3] located in the IR5 which are supposed to go as close as 1 mm to the beam axis. It should be noted that the project of placing Roman Pot in IR1 [4] has been recently submitted to the LHCC and encouraged by the Committee. EXPERIMENTAL SPECTROMETER MAGNETS The ALICE and LHCb dipole magnets have to be compensated placing 2 compensator magnets at about 20 m from the respective IPs. However, it turns out that the compensator magnets fully compensate the effects outside the bump, but create an angle and an offset at the interaction point. The solution adopted to solve this problem is to add a third compensation magnet preferably symmetric to the spectrometer magnet on the other side of the interaction point [5]. The magnet positions and strengths, according to the LHC optics version 6.5, can be found in Table 1. It should be noted that the magnet strengths of the ALICE and LHCb dipoles correspond to the design values. The real values are presently under measurement and the first results confirm the design values reported in Table 1. Table 1: Magnets parameters IP Name L (m) Angle(µrad)³ Bdl (Tm) 2 MBXWT.1L2 -20.490 -77.3 1.8042 MBWMD.1L2 -10.750 +147.3 3.4372 ALICE +9.750 -133.6 3.117 2 MBXWT.1R2 +20.490 +63.6 1.4848 MBXWS1L8 -20.765 -45.7 1.0668 MBXWH.1L8 -5.250 +180.7 4.2168 LHCB +5.250 -180.7 4.2168 MBXWS.1R8 +20.765 +45.7 1.066The controls and safety of all magnets belonging to the experiments is in charge of the PH/DT1 group. The same group is also responsible for the magnet commissioning together with the experiments. However, it has been decided that the magnets will be controlled by the Cern Control Center (CCC). The experiments will log the current values and measure the field via probes located in the magnets: the experiments may ask the operator to change the current values if the measured field is not nominal. The safety issues affecting the beam, and so the operation, are going to be discussed with the Machine Protection Working Group. A special attention should be paid to the dipole magnets since they have to be ramped always together with the compensator magnets and in step with the beam energy. The operational scenarios of these magnets according to physics programme of the experiments can be found in [6]. ALICE ZDC One set of two calorimeters (ZN and ZP) will be installed in the LSS2 on both sides of the interaction point at 115 m from IP2. The ZN is positioned between the two beam pipes of the recombination chamber to intercept the spectator neutrons while the ZP is positioned external to the outgoing beam to collect the spectator protons. Both calorimeters make use of highly radiation hard quartz fibres as active material embedded in a W alloy matrix in the case of the ZN (see Figure 1) and in a brass matrix in the case of the ZP. The Cherenkov light produced by the LHC Project Workshop - ’Chamonix XIV’181Figure 1: The ALICE ZNpassing particles in the quartz fibres is optically guided bythe same fibres to photomultipliers (PMTs). Bothcalorimeters are placed on a movable support table whichwill be lowered by ~ 20 cm during injection to minimizethe absorbed dose and to protect the calorimeters frompossible beam losses at injection. The ZDCs’ transversepositions are fixed and therefore they can only movealong the vertical plane. The ZDC data taking position is at the beam plane which is defined during the commissioning of the ZDC together with the supporttable. According to the actual planning, the ZN will beput in the data taking position only during the heavy ionruns while the ZP is supposed to take data also during thepp runs. Therefore, the support table providesindependent vertical movements for the ZN and ZP. TheZDCs will be located at few mm from the beam pipes.Hence, end switches are needed to avoid collisions withthe beam pipes during the movements of the supporttable. In addition, machine interlocks will disable themovement during injection until beams are ready to collide (the ZDCs are in the garage position duringinjection). Once the interlock is disabled, the ZDCs willbe put into their nominal data taking positions. Then thecrossing angle will be measured detecting the signalsfrom the 4 towers on the ZN and the centroid of thespectator neutrons can be calculated (preliminary studiesindicate that the centroid can be reconstructed with anuncertainty of ~ 0.5-1.4 mm depending on the number ofspectator neutrons). Once the value of the crossing angleis known, the vertical position of the platform can beoptimized until the vertical coordinate of the centroid of the spectator neutrons is zero.Taking into account that:x the ZDCs are located far away from the ALICE experimental cavern in a zone whichhosts mainly machine equipmentsx a false maneuver of the mobile support tablecould have serious consequences for themachine, given the small clearance betweenthe beam pipe and the ZDCs,ALICE proposes that the ZDCs are operated from theCCC on ALICE’s request. This would allow consistentoperation procedures, no change in procedure in case theZN will be used as machine luminosity monitor and a better control of the search procedures like “forgottentools” on the ZDC mobile support table.LHCB VELOThe VErtex LOcator has to provide precisemeasurements of track coordinates close to the interactionregion. For this, the VELO features a series of siliconstations placed along the beam direction. They are placedat a radial distance from the beam which is smaller than the aperture required by the LHC during injection and must therefore be retractable. Figure 2 is a schematic ofthe VELO design. The two main mechanical features of the VELO are the separation into two halves and theencapsulation of the silicon sensors in a secondaryvacuum container. In order to leave enough aperture (~ 3 cm) for the LHC injection and ramping, each station is divided in half; each half station consists of an R and a ĳFigure 2: A schematic of the VELO designmeasuring sensor. The sensors and secondary vacuumbox are retracted using a rectangular bellows during LHCinjection and acceleration. All moving parts are outsidethe secondary vacuum. The secondary vacuum box is designed to limit material before the first measurement.The silicon sensors are operated in a secondary vacuum,with only a thin wall separating them from the primary(LHC) vacuum. An RF-foil, consisting of 250 µmaluminum, surrounds each half of the VELO. In fact,operation within the primary vacuum is precluded due tocontamination by VELO components via out-gassing, andRF-frequency pick-up from the beams leading to large correlated noise in the sensors. Finally, VELO gives a vital input to the L1-trigger to enrich the b-decay contentof the data via a fast and stand-alone pattern recognitionwhich is also the main tool used for the VELO positioning close to the beam.The VELO operation foresees machine interlocks toforbid the VELO positioning during injection and ramping and to forbid unsafe beam operations whenVELO is in the data taking position. As alreadymentioned, VELO has a fast precise standalone trackingsystem and it is able to locate vertices with a precision of~ 10 µm in the transverse plane in a fraction of a second (this is due to the fact that during nominal operationVELO provides input to the L1 trigger whose output rate is 40 kHz). This procedure allows a precise gradualpositioning of the VELO detector into the final positionwhich corresponds, for nominal optics, to ~ 50 ı fromLHC Project Workshop - ’Chamonix XIV’182beam axis. Therefore, LHCb plans to operate VELO fromthe LHCb Control Room.TOTEM ROMAN POTSThree stations of Roman Pots (RP) will be mounted on each side along LSS at IR5. The first station is placed at 148 m from IP5, the second at about 180 m between Q4and Q5 and the third between 216 m and 220 m behindQ5. The second station will not be installed at start-up.Each station is composed of two units, separated by adistance of ~ 4 m, with each unit consisting of two potsthat move vertically and one that moves horizontally (seeFigure 3).Figure 3: RP prototype mounted on the SPS ring. Onlythe vertical unit is present.Like in the VELO case, the TOTEM detectors andelectronics are physically separated from the primary(LHC) vacuum to prevent an unacceptable outgassing and RF-pick up from the beams. They are therefore placed in a pot equipped with a 200 µm window and kept insecondary vacuum. The pots are retracted using bellowsduring LHC injection and acceleration (4 cm from beam axis).The RP operation foresees machine interlocks to forbidthe RP positioning during injection and ramping and toforbid unsafe beam operations when RP is in the datataking position. The RP nominal position during datataking is 10 ı from the beam axis. The initial informationon the beam position is given by the BPMs located closeto the RP stations (TOTEM is presently studying thepossibility of additional BPMs). BLM information is also very useful as demonstrated during the SPS test in 2004.BLMs have been already foreseen at each RP station. Thevertical pots are centered around the beam axis using theon-line data from the detectors located inside the pots andequalizing the data-rate. The distance between the two vertical pots is measured with LVDT sensors with aprecision of few microns. Then the horizontal pot ismoved in using the data rates on its detector. TOTEMwould be in favour of operating the RP from theCMS/TOTEM Control Room.ATLAS ROMAN POTSATLAS has submitted a proposal to install one RPstation on each side of IP1. Each station, located at ~ 240m from IP1, is composed of two units, separated by a distance of ~ 4 m, with each unit consisting of two potsthat move vertically. The project is still at the level ofLetter of Intent and therefore the details of the RP operation have not been discussed yet. However ATLASwould be in favour of operating the RP from the CCC. The RP may not be operational at start-up.CONCLUSIONSEquipment which could interfere with the beamoperation is located in all experimental insertions. Theoperation of the magnets belonging to the experiments isdone from the CCC and the project looks very welladvanced.For what concerns the movable detectors, follow up isneeded for setting up a safe procedure for their operationlike dedicated interlocks and signal exchange between themachine and the experiments. In addition, theresponsibilities for their operation need to be finalized. Itshould be noted that if the interlock system and the signalexchange is very well designed, the choice of  ControlRoom to operate the movable detectors is most probablynot an issue.ACKNOWLEDGEMENTSI would like to thank the following for fruitfuldiscussion in preparation of the talk: M. Bozzo,O. Brüning, M. Deile, K. Eggert, M. Ferro-Luzzi,M. Gallio, P. Grafstrom, W. Herr, L. Leistam,M. Losasso, M .Macri, M. Oriunno, K. Potter, E. Sbrissa,D. Swoboda, J. WenningerREFERENCES[1] ALICE Technical Design Report, “Zero DegreeCalorimeter”, CERN/LHCC 99-5[2] LHCb Technical Design Report, “VELO”,CERN/LHCC 2001-011[3] TOTEM Technical Design Report, “Total CrossSection, Elastic Scattering and Diffraction Dissociation at the LHC”, CERN/LHCC 2004-002[4] ATLAS Letter of Intent, “ATLAS Forward Detectorsfor Luminosity Measurement and Monitoring,CERN/LHCC/2004-010[5] O. Brüning et al.,”A beam separation and collisionscheme for Ip2 and IP8 at the LHC for optics version6.1”, LHC Project Report 367[6] D. Macina, “Desires and constraints of experimentsduring early LHC operation”, these ProceedingsLHC Project Workshop - ’Chamonix XIV’183

Equipement of experiments directly interfering with the beam operation

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