Synchrotron Radiation Measurements at the CERN LHC

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN  BEAMS DEPARTMENT

CERN-BE-2011-018 BI

Synchrotron Radiation Measurements at the CERN LHC S. Bart Pedersen; A. Boccardi; E. Bravin; A. Guerrero; A. Jeff: T. Lefevre; A. Rabiller; F. Roncarolo CERN – Geneva/CH A. S. Fisher SLAC, Menlo Park; CA - USA

01/05/2011

CERN-BE-2011-018

Abstract The CERN LHC is equipped with two systems (one for each beam) designed to image the synchrotron radiation emitted by protons and heavy ions. After their commissioning in 2009, the detectors were extensively used and studied during the 2010 run. This allowed preliminary limits in terms of sensitivity, accuracy and resolution to be established. The upgrade to an intensified video camera capable of gating down to 25ns permitted the acquisition of single bunch profiles even with an LHC proton pilot bunch (~5e9 protons) at 450 GeV or a single lead ion bunch (~1e8 ions) from about 2 TeV. Plans for the optimization and upgrade of the system will be discussed. For the last few months, part of the extracted light is deviated to the novel Longitudinal Density Monitor (LDM), consisting of an avalanche photo-diode detector providing a resolution better that 100 ps. The LDM system description will be complemented with the promissing first measurement results.

Paper presented at DIPAC2011 Conference - Hamburg/DE from 16 to 18 May 2011

Geneva, Switzerland August, 2011

Proceedings of DIPAC2011, Hamburg, Germany

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SYNC CHROTRO ON RADIIATION MEASUR M REMENT TS AT TH HE CERN N LHC F. Roncaroolo*, S. B. Pedersen, P A. Boccardi, E. E Bravin, A. A Guerreroo, A. Jeff, T. Lefevre, A. A Rabiller, CERN, Geneva, G Swiitzerland A. S. Fish her, SLAC, Menlo Park k, Californiaa, U.S.A. Abstract The CERN N LHC is equiipped with two o systems (one for each beam) ddesigned to im mage the synchrotron radiaation emitted by protons an nd heavy io ons. After their commissioninng in 2009, the t detectors were extensiively used and stuudied during the 2010 ru un. This allo owed preliminary llimits in term ms of sensitiv vity, accuracy and resolution to be established d. The upgrad de to an intenssified video cameraa capable of gating g down to 25ns perm mitted the acquisitioon of single bu unch profiles even e with an LHC L proton pilot bbunch (~5e9 protons) p at 450 GeV or a siingle lead ion bunch (~1e8 ionss) from aboutt 2 TeV. Plans for the optimizaation and upgrade of thee system willl be discussed. Foor the last few w months, parrt of the extraacted light is devviated to thee novel Long gitudinal Den nsity Monitor (LD DM), consistin ng of an avalaanche photo-d diode detector provviding a resolution better than 100 ps. The LDM system m description will w be compllemented with h the promising firrst measuremeent results.

ODUCTION INTRO N The two B BSR systems [1] are instaalled about 30 3 m ostats hosting the D3 dipolee and downstream oof the D3 cryo t provide eno an undulator.. The latter has been built to ough synchrotron rradiation (SR) at low beam m energies. Ass the 1.5 TeV, most of the usefu beam energyy reaches 1.2-1 ul SR power starts tto be generateed first by the D3 edge and then by the D3 cenntre. A retracctable extractio on mirror dev viates the light beloow the beam pipe where an optical sy ystem he beam spot o performs the imaging of th on CCD camerras. hown in Fig. 1 and is equip The opticaal system is sh pped with remote ccontrol in ordeer to nt SR sourcess by adjusting  focus on the differen g the gth; optical ddelay line leng  tune thhe light averaage angle and d position att the telescoppe entrance, in order to centre it on n the detectorrs’ sensitive arreas; nd chromatic optical o  adjust tthe neutral an filters that are insttalled in frontt of the detectors, accordin ng as nd energy chan the beam m intensity an nge;  select thhe light comiing from a sh hort section off the D3 maggnet by means of movable sllits. The total S SR power is shared between the Abort Gap Density Monitor GM), the Longitudinal Monitor (AG L meras dedicaated to transv (LDM) and the two cam verse profiles, the Beam Syncchrotron Rad diation Transv verse

RTF) systems. This paper Slow (BSRTS) annd Fast (BSR f with the mainlly contains tthe results obtained so far BSRT TS and LDM M systems. Mo ore informatio on about the AGM M can be foundd for example in [2].

Figu ure 1: Schem matic drawing g of the BSR R telescope systeem sitting beloow the LHC beam b pipe.

TRAN NSVERSE PROFILE E YSTEMS (B MEASURE M EMENT SY BSRT)

BSRTS System D Description a equipped Thee BSRTS systtems (one per LHC beam) are with Proxitronic N Nanocam HF4 S 25N NIR [3] cameras, MCP between the photocath hode and the intenssified via a M camera sensor. Thhey are read out o by 50 Hz electronics. ble: Two operation moddes are possib each acquisition  Continuous or DC mode: m 0 ms of all corresponds to the integrration for 20 circulating buunches;  Gated or Pulssed mode: eacch acquisition corresponds to the integraation over all the time wind dows (gates) programmed iin 20ms. Wh hen the cameraa is in Pulsed mode, the miinimum gate um gate repettition rate is length h is 25 ns annd the maximu ossible to meassure a single 200 Hz. H This meanns that it is po mpled every 55 turns. LHC bunch for a siingle turn, sam The SR power ggenerated by protons and the system efficiency are suchh that there arre no intensity y limitations for prroton beams: a single pilott (~5109 prottons) gives a when meeasured on a signaal well above background even e um of about 30 lead ion singlee turn (89 uss). A minimu for 20 ms (DC bunch hes (~ 3109 Pb82 ions) averaged a light at injection modee) are necessaary to have enough e n frequency in energ gy. This is duee to the shift in nto the infrared off the undulatoor light generatted by ions.

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*[email protected]

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Proceedings of DIPAC2011, Hamburg, Germany

BSRTS Measurements The BSRT TS systems caalibration is do one at first with w a calibration taarget installed d in the BSR R telescope at a an optical distannce equal to the distance between the BSR detector andd the centre of the undulator. This allows measurementt of the optical o system m magnificaation, presently sett at about 0.3. The abso olute and relaative calibration can then be b studied with w beam-b based measurementts aiming at comparing c thee BSRTS with h the Wire Scannerr (WS) monito ors. These mo onitors can be used only below a maximum beam b intensity y (of the ordeer of few 1013 prottons) above which w wire dam mages or quen nches of the downsstream magneets can occur.. The BSRTS S-WS cross-calibrattion yields BSRTS co orrection facctors intended to compensate for aberration n, diffraction and t moment, the t best correcction depth of fieldd errors. At the is believed too be in quadrrature on the measured BS SRTS beam size. A An example of o WS-BSRTS S cross-calibraation is shown in Fig. 2, wherre the two mo onitors have been nce of 24 nom minal used to meassure the horizzontal emittan bunches (~1..11011 p/buncch) at 450 GeV G in one off the LHC rings (B Beam 2).

Simillar results havve been achieeved for Beam m 2 vertical and for f Beam 1 aand Beam 2 at a 450 and 35 500 GeV. A detailled analysis iss in progress to t understand the absolute valuees of the calibbration factorrs that allow the BSRTS system ms to match tthe WS measu urements at th he two beam energ gies. Fig. 3 shows an eexample of bu unch per buncch emittance measu ured by the BSRTS during an LHC physiics fill at 3.5 TeV. The measureement revealeed systematicc differences amon ng bunches thhat were correelated to different settings in thee 4 PS Boosterr rings.

BSRTF Thee BSRTF sysstems consist of a Redlak ke HG-100K camera [4] whose sensor is coup pled via opticaal fibers to a e with h a GM200-3 Photeek MCP125 [55] intensifier equipped gate module. m The maximum accquisition rate of 100 kHz with a minimum ggate size of 3 ns allows a bunch-perbunch h turn-per-turrn mode. Pressently the sysstems are in the co ommissioningg phase, in thee first place to o investigate wheth her the purp rpose- made optical fibeer coupling provides enough inntensification.

ME EASURED S SR POWER COMPA ARED TO SIMULATIONS

Figure 2: B Bunch per bunch horizontal normallized ( emittance ass measured by b BSRTS (average overr 15 nd WS (8 con acquisitions pper bunch) an nsecutive scan ns) in one of the LH HC rings.

Thee simulation code SRW [6] has beeen used to simullate the expeected light att the AGM and a BSRTS detectors, taking into accountt the detecto ors’ spectral respo onse. This has been done for protons and ions as The comparison functiion of the L LHC beam energy. e betweeen the simuulations and measurementts with the 3 AGM M for beam ennergies between 450 and 3500 GeV is show wn in Fig. 4. In particular, the AGM M measured intenssity per charrge for both protons and Pb ions is comp pared to the exxpected visiblee photons per charge.

h emittance ass measured by Figure 3: Buunch per bunch y the on-uniformity among bun BSRTS, shoowing the no nches inherited from m different setttings in the PS booster ring gs.

The measuurement was performed du uring a dediccated machine devvelopment peeriod, in wh hich the firstt 12 bunches weree injected witth normalized d emittances εx,y2 2 mm mrad. The (varying BSRTS meaasured beam sizes s g from 0.8 to o 1.2 d in quadratu mm) have bbeen corrected ure for the same s he correction, WS and BSRT factor 0.65 mm. After th B unch emittance at the 1% level. agree on the bunch per bu

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on intensity per p primary Figurre 4: Synchrootron radiatio nes) and meassured by the charg ge as simulateed (dashed lin of beam energy, AGM M detector (soolid lines) as function f for bo oth protons annd Pb ions. Thee simulation eevidences the shift to higherr energies of the un ndulator peakk from protonss to ions, welll reproduced o evident that at the LHC by th he measuremeents. It is also of dettectable light injecttion energy (4450 GeV) the amount a f ions. emitteed by protons is a factor 104 higher than for

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Proceedings of DIPAC2011, Hamburg, Germany

LONGIT TUDINAL DENSITY Y MONITOR The LDM monitors aree designed to monitor the LHC L beam longittudinal distriibution. Thee system’s large l dynamic raange and high h time resolution allow a characterisatiion of the maiin bunches len ngth as well as a the presence andd extent of th he satellite an nd ghost buncches. Satellites cann develop due to capture/spllitting errors in n the injectors (e.gg. the SPS 20 00 MHz RF can give sateellite bunches sepaarated by 5 ns)) whereas gho osts can arise from capture/splittting errors in n the LHC (tthe 400 MHzz RF system can prroduce ghostss separated by 2.5 ns). The LDM is based on avallanche photod diodes (APD) from id-Quantiquee [7] and Micro Photon Dev vices [8] conneected to a Time Diigital Converteer (TDC) from m Agilent [9]. The detector can resolve singlee photons with h a time resolu ution T also has a resolution of o 50 of the order oof 50 ps, the TDC ps. The APD D presents a short s dead-tim me used to qu uench the avalanchhe (tens of ns) n and theree is also a small s probability (~ ~3%) that at th he end of this dead-time trap pped electrons or hholes will trig gger a new avaalanche produ ucing an after-pulsee. These effeccts, together with w the dark count c rate, althouggh small, aree corrected with w a statisstical algorithm. T The probabilitty of a photton triggering g an avalanche per bunch-crossing must be maintained m below a certain levell (60-70%) otherwise o thee error on these t corrections bbecomes too laarge. This hass an impact on n the maximum coounting rate an nd thus on th he integration time required for aacquiring a prrofile with suffficient resolu ution. me required deepends on wh In fact the inntegration tim hat is m is just to measure the core being observved; if the aim mainly the bun parameters oof a bunch (m nch length) a few seconds are ssufficient; on the other hand d if the populaation of ghosts andd satellites has to be measu ured an integraation of several m minutes may be required. Th he dynamic range r observed in 2010 was of o the order of 105 with h an integration tim me of 500 s.

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such information. T This particularr measuremen nt triggered a camp paign optimizaation in the LH HC and in the injectors.

Figurre 6: LDM m measurement with w sufficientt integration time to t clearly idenntify satellite and a ghost bunches.

CONCLU USIONS AN ND OUTLO OOK Thee LHC BSRT TS systems allo ow monitoring g the bunchper-bunch emittancces by gating a single buncch at a time, at a maximum m freequency of 1 Hz. To reducce statistical errorss it is normallly required to gate on each bunch from 2 to 5 seconds. Thee absolute caliibration is for the moment based d on cross-caalibration with h respect to WS. W During synch hronized meassurements, applying a correection factor to thee BSRTS, the difference between the two o monitors is at thee 1% level. T This was achieved during a dedicated beam m study periodd, during which experts tun ned the WS and BSRTS B to theiir optimal settiings. More stu udies are onon factors and going g to understandd the correctio d to bring the gs during operration. BSRT TS always to ooptimal setting ng commission Thee BSRTF sysstems are bein ned and aim nitoring. at sin ngle turn – singgle bunch mon Thee LDM deteectors are stilll in the com mmissioning prove the LHC operation, phasee, but already served to imp being g the only device able to chaaracterize the longitudinal beam m distribution w with a resoluttion of the ord der of 50 ps and a wide beam inntensity range.

GEMENTS ACKN NOWLEDG S ning and J. Emery for Speecial thanks to B. Dehn nts used in Fig. 2. performing the WS S measuremen

NCES REFEREN Figure 5: Exxample of an LDM measurrement beforee and after the correction for the dead-time and d after-pulse.

Measuremeents t dead-timee and An example of the effeectiveness of the gorithm is sh after-pulse ccorrection alg hown in Fig g. 5. h effects would w Without corrrection, both definitiively ment accuracy. Fig. 6 shows the compromise the measurem result of a measurementt that allowed evidencing g the presence of both satellite and ghost bunches. Att the moment the LDM is the only monito or able to retrrieve

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T. Lefevre et aal., “First Beaam Measuremeents with the [1] T nitors”, Proc. IPAC, Kyoto, L LHC Synchrotrron Light Mon J Japan, (2010). Gap Monitoring and [2] M. M Meddahi ett al., “LHC Abort A yoto, Japan, (2 C Cleaning ”, Prooc. of IPAC, Ky 2010). m, Model: HF4 S 25N NIR. [3] http://www.pro h oxitronic.com HG-100 [4] http://www.red h dlake.com, Model: M 0K. odel: MCP125 [5] http://www.ph h hotek.com, Mo 5. [6] SRW, S O. Chhubar, P. Elleaume, “Acccurate and nchrotron rad e efficient compputation of syn diation in the (1998)). n field regioon,” Proc. of EPAC, near E m. [7] http://www.idq h quantique.com vices.com. [8] http://www.mi h icrophotondev [9] http://www.ag h gilent.com.

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