Already in 1997, 8 years before the expected commissioning date of the LHC, the design of the detectors for the orbit system has to be completely terminated. The paper describes the BPM design highlighting the chain of arguments that have led to this particular solution

Bovet, Claude

Papis, J P

Schmickler, Hermann

Vos, L

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EUROPEAN  ORGANIZATION  FOR  NUCLEAR  RESEARCHEuropean Laboratory for Particle PhysicsCERN  -  SL DIVISIONCERN-SL-97-69 (BI)LHC BPM designC. Bovet, J.P. Papis, H. Schmickler, L. VosAbstractAlready in 1997, 8 years before the expected commissioning date of the LHC, the design ofthe detectors for the orbit system has to be completely terminated.The paper describes the BPM design highlighting the chain of arguments that have led tothis particular solution.Paper presented at the Third European Workshop on Beam Diagnostics andInstrumentation for Particle Accelerators (DIPAC 97),Frascati, Italy, 12-14 October 1997Geneva, SwitzerlandDecember, 1997LHC BPM designC. Bovet, J.P. Papis, H. Schmickler, L. VosCERN, SL Division, CH-1211 Geneva 23 (Switzerland)Abstract: Already in 1997, 8 years before theexpected commissioning date of the LHC, the designof the detectors for the orbit system has to becompletely terminated.The paper describes the BPM design highlighting thechain of arguments that have led to this particularsolution.1.  SPECIFICATIONS1.1 Reliability: For the LHC orbit system it isforeseen to install a beam position monitor at everysecond lattice quadrupole in the arcs. Each positionmonitor will read both planes. This adds up to about950 monitors at cryogenic temperature. Once themonitors are installed they will be welded to an innerliner of the vacuum system (the so called beamscreen), which is an about 6m long tube inside thevacuum pipe. It will be impossible to exchange acomplete BPM in situ and hence for a repair a wholeshort straight section including the quadrupole wouldhave to be lifted onto the surface. From this it is clearthat the most important design criteria is mechanicaland electrical reliability of the components and inparticular of the electrical connections.1.2 Cryogenic load: About 950 monitors measuringin two planes represent a little less than 4000 coaxialsignal cables from the pickup through the insulationvacuum to the outside world. These cables canrepresent a considerable load to the cryogenic systemand hence they are optimised for minimum heatconduction, but at the same time preservingacceptable high frequency performance for thesignals.1.3 Alignment: The requirements for the precisionof the mechanical offsets to the magnetic quadrupoleaxis are quite relaxed as for most other protonmachines. A tolerance of 500 µm rms has beenspecified, which can easily be met by standardproduction and alignment techniques. Table 1summarises the situation and lists the dominantcontributions to the total expected alignment error of300 µm rms.In order to verify the quality of the alignment and forthe case that at a later stage in the exploitation of theLHC a higher precision is demanded, installationsare foreseen to measure the alignment with thebeams (k-modulation technique) [1]. The precisionby what the offset can be determined this way is ofthe order of 30 µm.Description index ToleranceSupport InstallationAlignment error of BPMsupportA1 ± 200 µmMechanical Tolerances ofSupport itselfA2 ± 100 µmIntroduction of coldmass into cryostatCreation of new fiducialson the cryostatA3 ± 100 µmIntroduction of the BPMinto the cold boreConcentricity, Mechanicalerrors cumulatedA4 ± 50 µmDifference betweenmagnetic and optical axisA5 ± 100 µmAddition of Errors √Σ Ai2 ± 300 µmSpecified transversealignment error± 500 µmTable1: Description of various sources of alignmenterrors of the LHC orbit monitors compared with theacceptable alignment error2. Choice of Button Electrodes 2.1 Expected signal levels: The sensitivity limit ofthe signal processing electronics together with therequirement of obtaining better than 1mm precisionfor trajectory (single pass) measurements dictate theminimum signal to be obtained of the pickups.The original design was based on strip line couplersin order to deliver ample signal for a compensationof cable losses, as the signal treatment electronicswas located up to 700 m far from the pickups. In1996 it became clear that the radiation levels in mostparts of the ring will be so low that the installation ofdistributed electronics will be possible.A button electrode of 24 mm diameter, that is themaximum of which four pieces can reasonably befitted on a beam tube of 44 mm diameter, has abroadband transfer impedance of about 1.5 Ω, to becompared to about 8 Ω for a 20 cm long and 13 mmstrip line in a coupler. Table 2  summarises the rangeof bunch and beam intensities to be covered by theorbit system. More details on LHC filling schemescan be found in [2].Parameter Pilot Nominal UltimateParticles/bunch 5*10 9 1.05*10 11 1.66*10 11No. of bunches 1 2835 2835Beam current 9 µA 0.53 A 0.85 ATable 2: Number of bunches and intensities for threedifferent filling schemes of the LHC.The electrical signal from the minimum bunchintensity of the pilot beam (foreseen forcommissioning and set-up) has to be matched to theinput sensitivity of the analog electronics. A transferimpedance of 1.5 Ω as quoted earlier yields a total ofabout 900 mV peak to peak for this bunch intensityand a bunch length of 0.5 ns (rms). After appropriatelowpass filtering the signal level reduces to about100 mV peak to peak, which gives a security marginof 6...9 dB compared to the sensitivity limit of  thefollowing electronics. The actual choice of thiselectronics has not yet been done, but irrespective ofchoosing a logarithmic amplifier or a normalisingcircuit as in the case of LEP [3] sensitivity limits ofabout 20 mV peak to peak can be achieved.2.2 Control of reflections: The analog electronicsreceiving the button signals should work with ahigher bandwidth than 40 MHz (25 nsec bunchspacing) and will cover the dynamic range in bunchcurrent without switching gains. This requires a tightcontrol of signal reflections. In order to achieve thisthe analog electronics will be mounted at short cabledistance on the outside of the cryostat. A lowpasswith two Zobel networks as shown in figure 1 can beused to assure broad band signal termination.  Such acircuit produces reflections at the level of  0.2%, incase the output load is a perfect broadband 50Ωtermination and the reflections increase to 3.0...3.6%in case the VSWR of the following device (mostlikely a hybrid) is around  1.2.VSWR < 1.2R1104.8R250ZC1 C2 C3 C4L1L2 L3IN OUTFig.1: Low pass filter with broadband inputimpedance matching.2.3 Details of the mechanical design: Figure 2shows a detailed drawing of the button feedthrough.It should be noted that the surface of the buttons isshaped as a segment of a circle in order to be flushwith the vacuum chamber surface.Fig.2: Drawing of the button feedthrough pickup.Four button feedthroughs are mounted as shown infigure 3 onto the vacuum chamber. From the densityof the installation it should be clear that 24 mm is themaximum button diameter that can be fitted onto a44 mm diameter tube. The vacuum tightness isassured by helicoflex washers and the buttonfeedthrough is fixed by screws onto the monitorblock. Although the access in situ to the monitorblock will be very difficult the fixing with screwsgives the possibility of repairing broken buttons.Fortunately the experience from LEP shows thatthese interventions are only needed very rarely.Fig.3: Cross section of an LHC button monitorshowing the four pickup electrodes in position.2.4  Expected linearity:At present there is no prototype monitor available inorder to perform measurements on it. But thegeometry of the LHC button monitors is similar topthe 100 mm diameter LEP circular monitors, whichare used at several locations. By simple scaling of thegeometrical dimensions an estimate of the expectedlinearity for the LHC monitor can be obtained. Afterapplying a 6th order polynomial correction theresidual linearity error within 50% of the aperture isestimated to be below +/- 20 µm.3.  Integration into machine3.1 Cryogenic cables: The cryogenic cables have to fulfil somewhatcontradictory specifications. On one hand theyshould have excellent signal transmission quality forhigh frequencies, which demands good electricalconductors, on the other hand they should conductheat at the least possible level. Fortunately the totalamount of electrical power delivered by the buttonmonitors even in case of the ultimate beam current isvery small, such that a stainless steel cable with thincopper cladding can be used. The electrical losses inthat cable are completely negligible and the totalcryogenic load due to one connection is estimated tobe 58 mW for the ultimate beam current. Moredetails on these calculations can be found in [4].Figure 4 summarises the technical specifications forthe cryogenic cable.Outer conductor stainless steel  ø 3.58  with inside copper lining 100µm Sintered Silicon Oxyde  ø 2.87 mm Inner conductor: stainless steel rod 0.94 mm             with 40µm copper plating Impedance       50±1   •   Length       1300    mm Relative phase between pairs   10   ps VSWR at 200 MHz            0.01   dB Attenuation at 200 MHz (with Cu)  0.148  dB/m  Thermal caracteristics for one cable  1.3 m long Conduction at 5K  with Cu    58   mW Conduction at 5K  without Cu   8   mWFig. 4: Illustration of the cryogenic coaxial cablewith detailed technical specifications.For illustration, figure 5 shows a drawing of the twoadjacent position monitors inside their supportstructure and already assembled to the cold mass ofthe quadrupole.3.2  Time Schedule:Ten sample pieces of vacuum button feedthroughsand cryogenic cables will be ordered from differentfirms by the end of 1997. This should lead toqualification tests by end of October 1998. Twoprototype BPMs for a test string have to be deliveredby march 1999. For the production phase the deliveryof 4000 pieces is scheduled for march 2000 to June2001.Fig. 5: Drawing of two complete beam positionmonitors in their support structure on the cold massof the quadrupole.4.  SummaryButton type monitors have been chosen for the LHCorbit system. The signal amplitudes after are highenough not to limit the choice for the followingsignal treatment electronics. The dynamic range insignal amplitudes for all bunch intensities andposition variations in 50 % of the aperture is smallenough to be covered without gain switching. Thisdemands good control of signal reflections, whichwill be obtained by proper broad band signalmatching and an installation of the analogelectronics very close to the detectors within a totalof 2 m cable length on the surface of the cryostat.Semi rigid stainless steel cables with sintered siliconoxide as insulator will be used to transport thesignals inside the cryostat.5.  References[1] B.Dehning et al., “Beam Position Monitor Offsetdetermination at LEP”, PAC97, Vancouver,CERN SL/97-25 (BI) and references therein.[2] The Large Hadron Collider, Conceptual Design,The LHC Study Group, CERN/AC/95-05(LHC)[3] G.Vismara, “The new front-end Narrow BandElectronics for the LEP BOM system”, EPAC,London, June 1994, CERN SL/94-30 (BI).[4] C.Bovet et al., “Measurements and Modelling ofthe Thermal Dissipation of a cryogenic Cable forLHC BPMs”, CERN, LHC Project Note 79

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