The TOTEM experiment at the LHC will operate down to 10 sigma from the beam in the forward region of the CMS experiment. The associated beam loss monitors (BLMs) are crucial to monitor the position of the detectors and to provide a rapid identification of abnormal beam conditions for machine protection purposes. In this paper, the response of the TOTEM BLMs is considered for nominal machine operation and the protection thresholds are defined, withcalculations made of the expected signal fromprotons grazing the TOTEM pot as a function of pot distance from the beam, and the BLM signal from proton collisions at the CMS beam interaction point

Appleby, R B

Hall-Wilton, R

Macina, D

Talanov, V

CERN Document Server

EXPECTED PERFORMANCE OF TOTEM BLMS AT THE LHCR.B. Appleby∗, D. Macina, V. Talanov, CERNR. Hall-Wilton, CERN / University of WisconsinAbstractThe TOTEM experiment at the LHC will operate downto 10σ from the beam in the forward region of the CMS ex-periment. The associated beam loss monitors (BLMs) arecrucial to monitor the position of the detectors and to pro-vide a rapid identification of abnormal beam conditions formachine protection purposes. In this paper, the responseof the TOTEM BLMs is considered for nominal machineoperation and the protection thresholds are defined, withcalculations made of the expected signal from protons graz-ing the TOTEM pot as a function of pot distance from thebeam, and the BLM signal from proton collisions at theCMS beam interaction point.INTRODUCTIONThe roman pots of the TOTEM experiment [1] are in-stalled in the forward region of IR5 to measure the totalcross section and tag diffractive protons coming from theCMS interaction point (IP). The stations, each containingtwo horizontal and four vertical pots, are located at 147mand 220m from the IP, and will be moved into the beamduring stable operation to a distance of closest approach of10 σ. Each pot consists of a Silicon detector, located be-hind a stainless steel window, and thus each pot presentsan aperture restriction with a 5 cm long and 200 μm thickstainless steel target to the beam.The regions of the TOTEM stations are equipped withdedicated beam loss monitors (BLMs), which form partof the machine and detector protection system and moni-tor beam losses and provide protection against downstreammagnet quenches. The BLMs provide active protection,and so are connected to the beam dump system through thebeam interlock system (BIS). A beam dump request is is-sued if the loss in a BLM exceeds a pre-determined thresh-old, chosen to be less than the magnet quench threshold.The standard BLMs are cylindrical ionization chambers,60cm in length with a diameter of 9cm. Their response todifferent particle species is highly non-linear, as shown infigure 1, and in the region of interest for this work (MeV toGeV) the response is dominated by charged hadrons. Theplacement of the dedicated BLMs in the TOTEM pot re-gions was discussed in [3] and were chosen to maximisethe signal from proton interactions with the pot.In this paper, an initial study of the expected signalseen in the BLMs will be performed and normalised tothe rate of expected protons interactions with the pot, fornominal running conditions at a luminosity of 10 34 cm−2s−1. The shower simulations performed are similar to those∗ robert.appleby@cern.chkinetic energy [MeV]-210 -110 1 10 210 310 410 510 610charge per primary [fC/primary]-410-310-210-110110210protonelectronpositrongammaneutronmuon+muon-pion+pion-LHC BLM - longitudinal impact directionFigure 1: The non-linear response of a LHC BLM to thekinetic energy of various incident particles, courtesy ofMarkus Stockner [2].in [3], which computes the shower product distributiondownstream of the pots.The normalization of these showerproducts into the time domain has been performed using amodel of the proton loss rate on the horizontal pots, per-formed with Sixtrack [4], and will be discussed in the nextsection. To compute the ratio of the pot BLM signal tothe flux arising from collisions at the IP, DPMJET [5] andFLUKA [6] simulations were performed to estimate theIP flux and then combined with the pot-induced flux as afunction of pot position. Results are shown for the BLMsat 220m in this paper, with 147m BLM results discussedin [7] and the extension to lower luminosity, special run-ning conditions reserved for later work.PARTICLE RATES INTO THE BLMSIP Proton FluxThe rate of particles into the BLMs from IP protons isperformed using DPMJET for the IP phase space and aFLUKA model of the LSS for the showering, with the par-ticle fluxes recorded on the surface of a 32cm radius cylin-der in the region around the TOTEM pots. The flux is nor-malised to per IP event (with 105 IP events per FLUKArun). The scaling to a rate is performed for nominal lumi-nosity of 1034 cm−2 s−1 and using the total cross section,giving 35 interactions per bunch crossing and a BLM rateproportional to the instantaneous luminosity.Pot-induced FluxThe density of particle flux from the proton collisionswith the pot is estimated using the IHEP version of theMARS code [8] model of the LSS and the pot, with theparticles recorded on the surface of a 32cm radius cylin-der in the region after the pot for a single inelastic pro-Proceedings of PAC09, Vancouver, BC, Canada TH5RFP031InstrumentationT03 - Beam Diagnostics and Instrumentation 3513ton interaction with the pot. The normalisation to a raterequires knowledge of the number of proton interactionswith the pot, which will not be known accurately until theLHC is turned on and the proton impacts on the pot can bemeasured. However, this quantity can be estimated using asimulation of the LHC optics and collimation system.This calculation is done using SixTrack, with the V6.503LHC nominal collision optics and the phase one collima-tion system, and computes the fraction of protons inelasti-cally interacting with the TOTEM pot at variable pot dis-tance from the beam. The interaction point proton phasespace was generated using the double pomeron exchangeevent (DPE) kinematics and cross section, and normalisedto an expected proton loss rate of 82× 106 p s−1 for DPEand single diffractive events. The TOTEM horizontal potsare included as two horizontal collimators, with the gapset as required. The loss distribution of the proton sam-ple is shown in figure 2, which shows the loss fraction ofthe protons as a function of distance around the LHC ring(with 0 corresponding to IP5). The left-hand plot showsthe whole ring, with large spikes corresponding to loss inLSS5, the betatron cleaning section in LSS 7 and the mo-mentum cleaning section in LSS3. These locations are themain regions for proton inelastic interactions. The right-hand plot shows a zoom of IR5. In this plot, the principle(middle) peak corresponds to loss on the TCL collimator(positioned at 10σ), with the two loss peaks either side cor-responding to the 147m (left) and 220m (right) TOTEMroman pot locations. s (from IP5) [m]0 5000 10000 15000 20000 2500000.050.10.150.20.250.30.350.40.45Proton loss fraction s (from IP5) [m]0 50 100 150 200 250 300 350 400 450 50000.050.10.150.20.250.3Proton loss fractionFigure 2: The loss distribution of IP protons for a multi-turn simulation, showing the loss fraction of the protons asa function of distance around the LHC ring for the wholering (left-hand plot) and the LSS5 region (right-hand plot).The numerical loss rate (inelastic interactions) on thehorizontal pots at 10 σ is calculated to be 9.1× 106 p s−1,based on an expected loss rate of the IP proton sample of82× 106 p s−1. It should be stressed that this is an esti-mate with many assumptions and model dependencies andthe true loss rate on a pot will not be known until beammeasurements are made.The additional contribution from the beam halo itself isexpected to be small at pot distances ≥ 10σ, due to thepots being in the shadow of the primary and secondary col-limators. For example, a Sixtrack simulation of 500k haloparticles over 200 turns yielded very few inelastic hits onthe TOTEM pots positioned at 10 σ. Therefore the rate inthe BLM will receive only a minor correction from halohits during collision. However this may differ for the caseof the TOTEM dedicated high-beta optics and its collima-tor settings.SIGNAL IN THE Q6 BLMSFigure 3 shows the flux of neutrons, protons and chargedhadrons arising from proton interactions with the pot as afunction of distance from the IP for the region immediatelyafter the 220m station. The plot is normalised for a potposition of 10σ, using the proton inelastic interaction ratefrom Sixtrack. The shower products reach a peak just afterafter the inelastic interaction, and the charged hadron fluxremains constant in the bare beam pipe region until the be-ginning of the Q6 cryostat at around 224.5m. The chargedhadron flux then peaks again at the start of the cryostat be-fore becoming attenuated by the matter distribution of Q6(dominated by the coils and the yoke). In contrast to thecharged hadrons, the neutron flux remains high in the Q6cryostat. The distribution of flux is in agreement with [3]. s[m]220 222 224 226 228 230 232310410Signal particle flux /cm^2 /s Figure 3: The pot-induced fluxes after the 220m pot stationas a function of distance from IP5, showing the neutronflux (solid line), the proton flux (dashed) and the chargedhadron flux (dotted).Figure 4 shows the fluxes of neutrons, protons andcharged hadrons arising from IP interactions in the samelongitudinal region. This flux shows the same features asthe pot-induced flux - an initial peak and plateau in the barebeam pipe region followed by a second peak at the start ofthe Q6 cryostat. The IP fluxes are more suppressed in thedrift region due to screening by upstream matter and is lesssuppressed in the Q6 cryostat region. This flux is domi-nated by neutrons, both in the beam pipe and the Q6 cryo-stat, and the difference in flux between these two regionsis more pronounced for the IP flux than the pot-inducedflux. The origin of the IP flux suppression in the beam piperegion is screening of the IP source by the distribution ofmatter immediately upstream, and around, 220m, and thisis a feature not seen for the (locally produced) pot-inducedflux.The expected charged hadron flux in a BLM can beestimated by integrating the IP and pot-induced chargedhadron fluxes over the longitudinal and azimuthal accep-tance of the BLM. This has been done in the left plot offigure 5 for the BLM at 221m (solid line) and the BLM at226.5m (dashed line) as a function of pot position, and pre-sented as a (signal to background) ratio in the BLM; henceTH5RFP031 Proceedings of PAC09, Vancouver, BC, Canada3514InstrumentationT03 - Beam Diagnostics and Instrumentation s[m]220 222 224 226 228 230 232310410510610Background particle flux /cm^2 /sFigure 4: The IP fluxes after the 220m pot station as a func-tion of distance from IP5, showing the neutron flux (solidline), the proton flux (dashed) and the charged hadron flux(dotted).unity indicates the point a meaningful signal related to thepot position can be observed. The plot shows the BLM at226.5m does not see flux from the pots over the dominantIP flux rate, while the BLM at 221m sees the pot-inducedshowers when the pots are placed approximately 30 σ fromthe beam, and an increasing signal as the pots are placedcloser.The expected current in the BLMs as a function of potposition is shown in the right plot of figure 5, with the 221m BLM is shown as a solid line and the 226.5 m BLM isshown as the dashed line. The predicted current range iswithin the 2.5 pA to 1 mA acquisition range of the BLMelectronics. The dominance of the IP flux current in the226.5 m BLM means the current is a weak function of potposition and no significant change of current as a functionof pot position is observed. However, the change in currentin the 221m BLM a a function of pot position is significantand readily observable. This is consistent with the large potflux to IP flux ratio observed in this BLM. The experimen-tal measurement of the current in the BLM, when available,can be used to normalise this calculation. The observationthat the 226.5m BLM is dominated by the IP flux means theexperimentally observed rate of IP background events canbe inferred from this BLM (assuming a small halo rate),and hence the true hit rate on the TOTEM pots can be in-ferred from the subtracted signal of the two BLMs. Suchcalculations could be attempted when the first data is avail-able.SUMMARYIn this paper, the expected current in the TOTEM BLMsas a function of pot position has been calculated for nom-inal running conditions at a luminosity of 1034 cm−2 s−1.The work is a first look and will be extended later to specialrunning conditions and lower luminosity. The contribu-tions from IP showers and showers originating on the potsare computed using a showering calculation, with FLUKAfor the IP flux and MARS for a shower from the pots. Ingeneral, it was found that BLMs screened from IP fluxesby distributions of matter immediately upstream and lo-TOTEM pot position (sigma)5 10 15 20 25 30 35 4002468101214 TOTEM BLM S/BTOTEM pot position [sigma]5 10 15 20 25 30 35 40−1010−910−810−710TOTEM BLM current [A]Figure 5: The ratio of pot-induced to IP charged hadronsin the BLMs (left) and the expected current (right) after the220m TOTEM pot station as a function of pot position. The221m BLM is shown as a solid line and the 226.5m BLMis shown as the dashed line, with the 226.5m line close tozero on the left plot.cated at an aperture change are able to see the pot chargedhadron signal, and BLMs with no such environment hasdifficulty seeing signal due to the large number of chargedhadrons from IP. It was shown the BLM at 221m can seethe signal charged hadrons when the 220m pots are approx-imately 30 σ from the beam, although this number shouldbe considered a first estimate with many model-dependentuncertainties.Finally, the BLM response to particle species and en-ergy was used to predict the current in the BLMs as a func-tion of pot position. It was found that the expected cur-rents were within the acquisition range of the BLMs, anda strong change in current with pot position was seen forsome of the BLMs. The experimentally observed currentin the TOTEM BLMs can be used to normalise the simu-lations presented in this paper and refine the understandingof the production of IP particle fluxes and hit rates on thepots.REFERENCES[1] TOTEM Collaboration, CERN-LHCC 2004-002 (2004)[2] M. Stockner, PhD thesis[3] LHC project note 386[4] R. Assmann et al, The proceedings of PAC03[5] S. Roesler, R. Engel, J. Ranft, In Proceedings of the MonteCarlo 2000 Conference, Lisbon, October 23-26 2000, A.Kling, F. Barao, M. Nakagawa, L. Tavora, P. Vaz (editors),Springer-Verlag Berlin, 1033-1038 (2001)R. Engel, Z. Phys. C66 (1995) 203R. Engel and J. Ranft, Phys. Rev. D54 (1996) 4244[6] A. Fasso’, A. Ferrari, J. Ranft and P.R. Sala, CERN 2005-10(2005), INFN/TC 05/11, SLAC-R-773A. Fasso’, A. Ferrari, S. Roesler et al., Computing in HighEnergy and Nuclear Physics 2003 Conference (CHEP2003),La Jolla, CA, USA, March 24-28, 2003[7] R.B. Appleby et al, Under preparation[8] I. Azhgirey and V. Talanov, “The Status of MARS ProgramComplex”, In: Proc. of XVIII Workshop on the charged par-ticles accelerators, Protvino, 2000, vol. 2, p. 184–187Proceedings of PAC09, Vancouver, BC, Canada TH5RFP031InstrumentationT03 - Beam Diagnostics and Instrumentation 3515

Expected Performance of TOTEM BLMS at the LHC

Abstract

Similar works

Full text

Available Versions

CERN Document Server