Caspers F

Di Girolamo B

Jones RM

Kroyer T

Roncarolo F

English

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CHARACTERIZATION OF THE ATLAS ROMAN POTS BEAMCOUPLING IMPEDANCE AND MECHANICSFederico Roncarolo∗, Roger M. Jones, U. Manchester and Cockcroft Institute, Daresbury, UKBeniamino Di Girolamo, Fritz Caspers, Tom Kroyer, CERN, Geneva, SwitzerlandAbstractAt the LHC, four Roman Pot (RP) type detectors willbe installed on both sides of the ATLAS experiment withthe aim of measuring elastic scattering at very small an-gles and determining the absolute luminosity at the interac-tion point. During dedicated LHC runs, the detectors willbe positioned at about 1 mm from the nominal beam or-bit. Numerical simulations and laboratory measurementswere carried out to characterize the RP impact on the to-tal LHC beam coupling impedance. The measurement re-sults assess the effectiveness of RF-absorbing ferrite platesthat have been mounted in convenient locations in order todamp high Q resonances of the RP structure. In addition,we review the RP mechanics emphasizing the accuracy andreproducibility of the positioning system.INTRODUCTIONRoman Pot (RP) experiments consists of devices de-signed to host particle detectors with the possibility toapproach locations very close to the circulating beam.At LHC, both sides of the ATLAS experiment will beequipped with RP detectors at about 240 m from the in-teraction point, with the aim of measuring elastic cross-sections and the collider luminosity [1]. The measurementand mechanical concept is shown in Fig. 1. An experi-mental unit will consist of 2 RP that will independently ap-proach the beam from the bottom and the top, at the samelocation. Two units 4 m apart will be installed on each side,giving a total of 8 RP.The mechanical design of a single RP is very similar tothe one adopted for the TOTEM experiment [2] with fewimportant differences reported in [1]. A 3D view of the RPunit outside (i.e. what will be seen by the beam) is shownin Fig. 2.Due to the small distance from the beam during data tak-ing and in order to guarantee accurate measurement results,two key issues in the RP operation are: i) the characteriza-tion of the beam-coupling impedance related to the wake-fields induced by the beam on the surroundings ii) the ac-curacy and reproducibility of the positioning system.BEAM COUPLING IMPEDANCEThe installation of the ATLAS RP units will contributeto the total LHC coupling impedance budget. The RP de-sign must aim at minimizing the longitudinal and trans-verse impedances in order to avoid the generation of beam∗ federico.roncarolo@cern.chFigure 1: Roman pot detector concept.Figure 2: 3D view of a RP unit, side seen from the beam.instabilities and power exchange between the particles andthe RP walls. In general the impact of any accelerator ele-ment on the impedance is determined by:• a resistive wall effect, enhanced as the beam ap-proaches the surrounding materials and when lowelectrical conductivity materials are used;• a geometrical effect, generated by any abrupt changeof the beam pipe cross section and the presence ofcavity-like structure (as for RP experiments).In addition, transverse impedances are very much re-lates to single an multi-bunch instabilities while longitu-dinal impedances characterize the electromagnetic powerexchange between the beam and the surroundings.Transverse Resistive Wall ImpedanceAt the LHC the transverse resistive wall impedance willbe dominated [3] by the collimator system and the contri-bution of the ATLAS RP positioned in data taking positionat 1 mm from the beam was estimated 2 order of magni-tude smaller over all the frequency band of interest. ThisProceedings of EPAC08, Genoa, Italy TUPP06305 Beam Dynamics and Electromagnetic Fields D04 Instabilities - Processes, Impedances, Countermeasures1685is explained by the limited portion of near-beam material(46 mm per unit) compared to the collimators (several me-ters) and the better conductivity of stainless steel (RP) withrespect to graphite (collimators).Longitudinal Geometric ImpedanceThe cavity-like nature of the RP devices can have a sig-nificant impact in terms of narrow-band resonances in thebeam coupling impedance, in a frequency band of interestdefined by the LHC beam spectrum. Here we present re-sults of laboratory measurements and numerical simulationof the longitudinal impedance up to 3 GHz.Both simulations and experiments are based on thewell know coaxial wire method [4] which consists ofa thin wire that is stretched inside the Device UnderTest (DUT) along the reference beam trajectory and pow-ered with an RF source. The simulated (or measured)scattering parameter S21 of the resulting loaded trans-mission line, compared to the same quantity evaluatedfor a reference vessel (smooth beam pipe with sameDUT initial diameter) allows determining the longitudi-nal impedance according to the well known ’log formula’ZL(ω) = −2Zcln[SDUT21 (ω)/SREF21 (ω)], where Zc is thecharacteristic line impedance.Numerical simulations (here performed with AnsoftHFSS [5]) are based on the finite element method. Thevolume meshing and related calculations can be memoryand time intensive (in terms of accuracy and needed CPUtime) for complicated or large geometries. In addition, sim-ulating the real structure in terms of material properties andmechanical tolerances can be problematic.On the other hand, laboratory measurements can be diffi-cult to perform due non-ideal RF matching between the RFsource and the DUT and other issues like wire alignmentand sag.The comparison between simulations and measurementsin terms of |S21| is shown in Fig. 3. The agreement interms of resonance frequencies and amplitude is better than30 MHz and 5 dB respectively when looking at the 4 mainmodes from 1 to 3 GHz. Remanent differences are not sur-prising, given the simulation and measurement issues men-tioned above. In addition, such an agreement validates themeasurements results presented in the next paragraph. Themeasurements were performed for various RP operationalsettings, from a ’retracted’ position at 36 mm from the wireto ’data taking’ at 1 mm.The same set of measurements were repeated aftermounting on the pot walls a number of ferrite tiles. Thisis a commonly used method for absorbing the electromag-netic power associated to the high Q resonances with a highresistivity material.The comparison between measurements before and af-ter mounting the ferrite tiles is shown in Fig. 4, in termsof real and imaginary part of the longitudinal impedanceZL/n = ZL/(f/f0), where f0 is the particles revolution fre-quency. The plots refer to a ’retracted’ position, but sim-f [GHz]0.5 1 1.5 2 2.5 3| [dB]21|S-35-30-25-20-15-10-50MeasurementSimulationPots at +- 36mmFigure 3: Modulus of the transmission scattering param-eter S21 as measured and simulated with HFSS, for a RPretracted position at 36 mm from the wire (i.e. form thebeam)ilar results exist for pots close to the beam. In all studiedcases, after installing the ferrite, the resonances Q factorsare attenuated and the longitudinal impedance ZL/n resultsbelow 10 mΩ at each considered frequency. This is consid-ered a small effect w.r.t. the total LHC impedance.MECHANICS AND POSITIONINGSYSTEMThe RP unit will allow independent moving of the topand bottom pot to the nominal position via a high precisionroller screw moved by a step motor. The pots are mountedon bellows that allow an excursion of 58 mm for the posi-tioning around the circulating beam. The two bellows arepart of the main vacuum chamber that is directly connectedto one of the two LHC vacuum beam pipes. A compensa-tion system [1] has been developed for balancing the forcesacting on the pots, namely the pressure from the LHC pri-mary vacuum and the gravity force due to the movable partsweight. Such a system allows a smooth operation of themotors by means of an over compensating force that al-lows to compensate the contrasting vacuum force, gravityand the residual torque of the motors, to achieve a moderateauto-retractive action.For each unit, a very precise tuning of the movement interms of relative position between the two pots and withrespect to the beam will be insured by the overlap detectorsinside the pots themselves. However, for a safe operationof the experiments and protection of the LHC, a reliableand reproducible system is required.For this reason a series of tests in the laboratory are on-going with a RP unit prototype assembled on a dummyLHC beam pipe. The system is equipped with the fullmotor system and a 10−4 vacuum in the main chamber isachievable.The tests presented here aimed at assessing the position-TUPP063 Proceedings of EPAC08, Genoa, Italy05 Beam Dynamics and Electromagnetic Fields1686D04 Instabilities - Processes, Impedances, Countermeasuresf [GHz]0.5 1 1.5 2 2.5 3]Ω/n) [mLRe(Z024681012 Without FerriteWith FerritePots at +- 36mmf [GHz]0.5 1 1.5 2 2.5 3]Ω/n) [mLIm(Z-5-4-3-2-1012345Without FerriteWith FerritePots at +- 36mmFigure 4: Real and imaginary part of the longitudinalimpedance as measured for RP unit in retracted position,before and after mounting ferrite tiles.ing reproducibility by using position sensors mounted onthe top and bottom flanges, in locations where the repro-ducibility is expected to be worse than at the detectors level,mainly for mechanical play reasons.Several measurements were performed, on the bottomand top flanges an the preliminary results are summarizedin Table 1. Every time the pots position was readout atthe “zero” position, 500 steps and 1000 steps (correspond-ing to an excursion of 5 mm), with and without vacuum.The error in columns 3 and 5 gives a measure of the re-producibility. The maximum error is for the top pot and isΔ = 30 μm. For the measurements at atmospheric pres-sure the reproducibility is better than 10 μm for the bot-tom pot. For the top pot it has a maximum value of 30 μmwhich is attributed to the uncertainty on the sensor angularposition and to the out-of-tolerance roughness of the sur-face on which the sensor was fixed. The reproducibilityobtained with vacuum in place results on average better forboth pots. In this second set of measurements the absolute’zero’ position moved, due to the intervention of the com-pensation system mentioned above.Table 1: Reproducibility for top and bottom flanges. Forconvenience the value is in mm and the error in μm.No vacuum VacuumSteps y [mm] σy [μm] y [mm] σy (μm)Bottom 0 0.018 5 0.012 3500 2.478 8 2.461 171000 4.939 1 4.908 2Top 0 0.027 3 -0.045 8500 2.451 22 2.326 71000 4.949 30 4.771 2CONCLUSIONSThe ATLAS Roman Pot units that will be installed atabout 240 m from the interaction point, must guarantee asafe operation with regard to the wake-field induced beaminstabilities and the LHC machine protection. The addedbeam coupling impedance has been characterized in termsof transverse resistive wall impedance and longitudinal nar-row band impedance. In both cases the results indicate rela-tively small values w.r.t. the total LHC budget. For the nar-row band longitudinal impedance this is particularly trueafter mounting ferrite tiles, which have been added to thefinal RP design. Preliminary laboratory measurements ex-hibit an excellent reliability and reproducibility of the RPpositioning system.AcknowledgementsThe authors are grateful to P. Grafstrom, E. Metral andA. Grudiev for all the help and support related to this work.REFERENCES[1] ATLAS collaboration, ”ATLAS Forward Detectors forMeasurement of Elastic Scattering and Luminosity”,CERN/LHCC/2008-004.[2] TOTEM Collaboration, ”TOTEM Technical Design Report”,CERN-LHCC-2004-002.[3] E. Metral et al., ”Transverse impedance of LHC collimators”,Proceedings of PAC07.[4] V.G. Vaccaro, ”Coupling Impedance Measurements”,INFN/TC-94/023 (1994).[5] Ansoft Corporation, URL: http://www.ansoft.com/ .Proceedings of EPAC08, Genoa, Italy TUPP06305 Beam Dynamics and Electromagnetic Fields D04 Instabilities - Processes, Impedances, Countermeasures1687

ATLAS Forward Detectors for Measurement of Elastic Scattering and Luminosity”,

Coupling Impedance Measurements”,

Transverse impedance of LHC collimators”,

URL: http://www.ansoft.com/ .

Characterization of the ATLAS Roman Pots Beam Coupling Impedance and Mechanics

Characterization of the ATLAS Roman Pots Beam Coupling Impedance and Mechanics

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