Beam-loss monitoring (BLM) [1] is a key element in the LHC machine protection. 4250 nitrogen filled ionization chambers (IC) and 350 secondary emission monitors (SEM) have been manufactured and tested at the Institute for High Energy Physics (IHEP) in Protvino, Russia, following their development at CERN. Signal speed and robustness against aging were the main design criteria. Each monitor is permanently sealed inside a stainless-steel cylinder. The quality of the welding was a critical aspect during production. The SEMs are requested to hold a vacuum of $10^{-7}$ bar. Impurity levels from thermal and radiationinduced desorption should remain in the range of parts per million in the ICs. To avoid radiation aging (up to $2Â·10^{8}$ Gy in 20 years) production of the chambers followed strict UHV requirements. IHEP designed and built the UHV production stand. Due to the required dynamic range of $10^{8}$, the leakage current of the monitors has to stay below 2 pA. Several tests during and after production were performed at IHEP and CERN. A consistently high quality during the whole production period was achieved and the tight production schedule kept at the same time

Chiggiato, P

Dehning, B

Ferioli, G

Grishin, V

Holzer, E B

Jimenez, T M

Koshelev, A

Kramer, Daniel

Larionov, A

Seleznev, V

Sleptsov, M

Sytin, A

Taborelli, M

Wevers, I

CERN Document Server

CERN-AB-2008-05405Sep2008DEVELOPMENT, PRODUCTION AND TESTING OF 4500 BEAM LOSSMONITORSBeam-loss monitoring (BLM) [1] is a key element in the LHC machine protection. 4250 nitrogen filledionization chambers (IC) and 350 secondary emission monitors (SEM) have been manufactured and testedat the Institute for High Energy Physics (IHEP) in Protvino, Russia, following their development at CERN.Signal speed and robustness against aging were the main design criteria. Each monitor is permanentlysealed inside a stainless-steel cylinder. The quality of the welding was a critical aspect during production.The SEMs are requested to hold a vacuum of 10-7 bar. Impurity levels from thermal and radiationinduceddesorption should remain in the range of parts per million in the ICs. To avoid radiation aging (up to 2·108Gy in 20 years) production of the chambers followed strict UHV requirements. IHEP designed and built theUHV production stand. Due to the required dynamic range of 108, the leakage current of the monitors has tostay below 2 pA. Several tests during and after production were performed at IHEP and CERN. Aconsistently high quality during the whole production period was achieved and the tight productionschedule kept at the same time.EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN   -   AB DepartmentGeneva, SwitzerlandAugust 2008AbstractCERN-AB-2008-0541) IHEP, Protvino, RussiaE.B. Holzer,  P. Chiggiato, B. Dehning, G. Ferioli, V. Grishin1), J.M. Jimenez, A. Koshelev1), D. Kramer, A. Larionov1), M. Taborelli, V. Seleznev1), M. Sleptsov1), A. Sytin1), I. WeversCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008DEVELOPMENT, PRODUCTION AND TESTING OF 4500 BEAM LOSSMONITORSE.B. Holzer, P. Chiggiato, B. Dehning, G. Ferioli, J.M. Jimenez, D. Kramer, M. Taborelli, I. Wevers,CERN, Geneva, SwitzerlandV. Grishin, A. Koshelev, A. Larionov, V. Seleznev, M. Sleptsov, A. Sytin,IHEP, Protvino, RussiaAbstractBeam-loss monitoring (BLM) [1] is a key element inthe LHC machine protection. 4250 nitrogen filled ioniza-tion chambers (IC) and 350 secondary emission monitors(SEM) have been manufactured and tested at the Institutefor High Energy Physics (IHEP) in Protvino, Russia, fol-lowing their development at CERN. Signal speed and ro-bustness against aging were the main design criteria. Eachmonitor is permanently sealed inside a stainless-steel cylin-der. The quality of the welding was a critical aspect dur-ing production. The SEMs are requested to hold a vacuumof 10−7 bar. Impurity levels from thermal and radiation-induced desorption should remain in the range of parts permillion in the ICs. To avoid radiation aging (up to 2·108 Gyin 20 years) production of the chambers followed strictUHV requirements. IHEP designed and built the UHV pro-duction stand. Due to the required dynamic range of 10 8,the leakage current of the monitors has to stay below 2 pA.Several tests during and after production were performedat IHEP and CERN. A consistently high quality during thewhole production period was achieved and the tight pro-duction schedule kept at the same time.INTRODUCTIONThe design of the LHC beam loss monitors is basedon the experience gained with the SPS beam loss sys-tem. There, nitrogen filled, sealed, parallel plate ioniza-tion chambers have been in operation for the last 30 years.The monitors installed in the ring have received a radia-tion dose of 0.1 to 1 kGy per year; while the ones closeto the extraction and injection regions have received 0.1 to10 MGy per year. The aging of these monitors was assessedby placing a radioactive source Cs137 on the monitors andreading the induced signal with the installed standard BLMelectronics. Significant gain variations (Fig. 1) were ob-served only at the high radiation areas. The relative sigmaof the 140 regular ring monitors is below 1%, while the42 injection/extraction monitors show a gain varation witha relative sigma of approximately 5%. The estimated ra-diation dose on the monitors in the LHC during 20 yearsof operation is 2 · 108 Gray in the collimation sections and2 · 104 Gray at the other locations. Consequently, no gainvariations are expected on the LHC BLMs for 20 years ofoperation, with the possible exception of the collimationregions.02040608010012014016002468101214161820Frequency distributioncurrent [pA]Extr., inj. BLMsRing BLMsFigure 1: Left: LHC BLM ionization chamber. Right: Gainvariations on installed SPS monitors after 30 years of oper-ation.DESIGN CRITERIA AND SPECIFICATIONSignal speed and robustness against aging were the maindesign criteria for the detectors. Because of the high dy-namic range two types of detectors are used. The standardmonitors are ionization chambers with parallel aluminumelectrode plates separated by 0.5 cm, as shown in Fig. 1.The detectors are about 50 cm long with a diameter of 9 cmand a sensitive volume of 1.5 liter. The chambers are filledwith N2 at 100 mbar overpressure. At locations with veryhigh (potential) loss rates the ionization chambers are com-plemented by secondary emission monitors [2]. They arebased on the same design, but hold only three electrodesmade out of titanium, as its secondary emission coefficientshows better stability as the integrated dose increases. Thechamber is 10 cm long, the pressure inside has to stay be-low 10−7 bar. The sensitivity is about a factor of 7 · 104smaller than in the ionization chamber. Both chambersare operated at 1.5 kV and are equipped with a low passfilter at the high voltage input. To avoid radiation aging(electronegative gases, organic compounds) a strict clean-ing procedure for the chambers is followed (including vac-uum firing of the steel components for the SEM). No or-ganic material is present, neither in the production process(pumping, baking and filling) of the detectors, nor in thedetectors themselves.MONITOR PRODUCTIONAll components for one vacuum stand production batch(with the exception of ceramics components) are cleanedin IHEP [3] shortly before assembly, following the CERNstandard for UHV requirements. The stainless steel com-Figure 2: Layout of the IHEP vacuum production stand.ponents are cleaned in an UHV ultrasonic bath with NGL(NGL 17.40 SP ALU, NGL Cleaning Technology S.A.Nyon, Switzerland) (20 g/l) in distilled water at 65 ◦C dur-ing 30 minutes. After that, they are rinsed in distilledwater at room temperature and again in an UHV ultra-sonic bath during 5 minutes. The last step of the clean-ing procedure is baking at 150 ◦C in air. The aluminumelectrodes are cleaned in the UHV ultrasonic bath withNGL (10 g/l) in distilled water at 65 ◦C during 20 min-utes, rinsed in distilled water at room temperature and in anUHV ultrasonic bath during 10 minutes and finally bakedat 150 ◦C in air. After the cleaning, all components aretransfered to the clean room for assembly with closed pack-ing. Standardized test samples analyzed at CERN periodi-cally helped to check the cleaning performance. The vac-uum production stand (Fig. 2) was designed and built inIHEP. It consists of: a) the ultra-high-vacuum pumping part(pre-vacuum pump, two turbo-molecular pumps - TMP56and TMP170, and an ion molecular pump); b) the vacuumgauges (two PKR261, three APR262, and one Prisma QMS200F2 mass-spectrometer); and c) two manifolds for 18chambers each.The welds have been executed by TIG (tungsten inertgas) arc welding under argon shielding without a filler ma-terial and with 100% penetration of the welds. Several dif-ferent tests were performed at IHEP before, during and af-ter the production to verify the quality of chambers. For ex-ample, quality of the cleaning of the components, record-ing of the vacuum stand data, the dark current measure-ments of feedthroughs, of the “heads” after welding and ofthe monitors after filling and immediately before shippingto CERN. All of these data were written into the equipmentMTF (Manufacturing and Test Folders) database. All weldswere He leak tested. Before heating, the outgasing of thechambers and the stand was measured to estimate the qual-ity of the component cleaning. The same procedure wasrepeated after heating. The temperature of each chamberon the vacuum stand (“Agilent 34970A”) and the pressuremeasurements (“Pfeiffer TPG256A”) were recorded auto-matically during the cycle and displayed online.Ionization ChambersHeating of the chambers started after the pressure hadreached 2 ·10−5 mbar. The temperature of the chamberswas increased at a rate of 50 ◦C per hour and kept at 220 ◦Cfor 6 hours. Then, the heating was switched off and atabout 100 ◦C and a pressure of 5 - 10·10−6 mbar the ionmolecular pump was switched on and kept pumping dur-ing 20 hours to further reduce the pressure to a final valueof 10−8 mbar before filling. Chambers were filled with99.999% nitrogen at 1.1 bar at a temperature of 20 ◦C. Af-ter pinch-off (annealed Cu-tube) and removal of the cham-bers from the vacuum stand, the nitrogen is pumped fromthe stand during one hour (test of the tightness of all pinch-offs remaining on the stand). To control the purity of thenitrogen, the composition and residual gas pressure wasmeasured. Analysis of the residual gas spectrum shows atypical spectrum for this type of vacuum stand and givesevidence of the absence of additional pollution from the ni-trogen gas. After transport of the monitors to CERN bylorry, reception tests were performed at CERNs gammairradiation facility (GIF). For the IC they consisted of a)leakage current measurements and b) measurements of thesignal induced by the radioactive source. 15 monitors wereplaced in a special support at 1.4 m transverse to the flux ofthe radioactive source 137Cs (98 GBq, 4.7 mSv/h), the ge-ometry leading to a difference in flux of +/- 5% between the15 monitors. The measurements are recorded in the MTFequipment database. Fig. 3 summarizes the reception tests.Out of 4259 IC arriving at CERN only 20 did not pass thereception tests.ionisation chamber numberchamber current / (pA)01020304050607080250 500 750 1000 1250 1500 1750 2000 2250 2500Constant   144.8Mean   69.99Sigma  0.9493chamber current / (pA)number of chambers025507510012515017520066 68 70 72 74 76 78Figure 3: Gain of the ICs measured in the GIF faicily,partly corrected for systematic effects.Secondary Emission MonitorsThe SEM monitor [2] requires for its proper functioninga very high vacuum to keep the internal ionization signallevel safely below the secondary electron emission. All thematerials were chosen according to the UHV standards, asthe outgassing needs to be strictly minimized for being ableto achieve the lifetime of 20 years in a very high radiationenvironment. During the design and production, an effortwas made in order to assure the cleanliness of all the sur-faces and the minimization of possible unpumped gas vol-umes or leaks. All the steel components (without threads)were vacuum fired at 950 ◦C and the Ti electrodes at 750 ◦C0 1 4 8 12 1601234567Q SEM/Qbeam [e−/pot]monitor position  measurementsimulationcorr. measurement1 2 3 4 5 6 7024681012mean: 3.81 sigma: 0.75QSEM/Qbeam0 100 200 300 400−0.500.511.522.5offset current [pA]SEM ID numberFigure 5: From left to right: A) Test beam setup with a 20cm Cu target and a container with 16 SEMs on a movable table.B) Response of the SEM detectors for each of the 16 positions in the sample holder. C) Calibration of 250 SEM detectorsin a mixed radiation field corrected for systematic position errors. The simulation result was 3.95 ± 0.17 e-/proton ontarget. D) Leakage current of the 250 tested SEM monitors. The bias voltage was set to 1.5 kV.0501001502002503003504000 5 10 15 20 25 30 35 40 45Time [h]Temperature [°C]1.E-101.E-091.E-081.E-071.E-061.E-051.E-041.E-03Pressure [mbar]18xSEM Manifold SEM pressureVacuumbakeout NEGactivationIon pump startedHe leak testspinchoffFigure 4: Cycle of the vacuum stand for the SEM produc-tion.to outgass the H2 dissolved in the bulk during the produc-tion of the metal. A similar procedure was put in place forthe electrode holders during the brazing with the ceramics.A very high sensitivity outgassing test was performed atCERN to estimate the thermal desorption rate of the de-tector. The results indicated a very slow pumping of theTi, thus the thermal outgassing could be compensated bythis effect. Nevertheless, the radiation induced desorptionhas a potential to degrade the vacuum considerably, so anadditional pumping was needed. A stripe of a high capac-ity Non Evaporable Getter (NEG) ST707 was therefore in-serted in the chamber. The total available surface (170 cm 2)of the NEG has a potential to adsorb the quantity of gasequivalent to one mono-layer covering all the detectors’ in-ner surfaces. The NEG has to be activated by heating atthe temperature of 350 ◦C for 14 hours, so the pumping cy-cle of the SEM is extended compared to the IC (Fig. 4).The He leak testing is performed additionally after the ac-tivation and the final pressure before the pinch-off is below10−10 mbar.A dedicated fixed target setup was installed in the H4secondary beam line to verify the SEM production in amixed radiation field for possible gas contents. A 300 GeVproton beam was shot on a 20 cm copper target, behindwhich a box with 16 SEM detectors was installed on a mov-able table (see Fig. 5). All the chambers were connected inseries to a HV power supply. The signal outputs of theSEMs were connected to a custom designed switch array.The switch was connected by a low noise 10 m long triax-ial Keithley cable. The resulting charge was integrated bythe Keithley 6517A Electrometer and the beam intensitywas measured by an air filled precision ionization chamber(PIC) placed in the beam. There was a systematic offsetfor each row of the detectors observed. The SEMs locatedclosest to the aluminum plate of the table had the highestresponse apparently due to the additional backscattering ofsecondary particles. The mean response per position wascalculated and the difference from the total mean was sub-tracted as a systematic error. The response before and aftercorrection as well as the result of the Geant4 [4] simula-tion are presented on the Fig. 5. Only one chamber had atoo high leakage current (above 2 pA) otherwise it stayedbelow 0.5 pA. The sensitivity of the measurement was es-timated to 4.4 · 10−2 mbar and all the tested SEMs have aresidual pressure below this level.SUMMARY4250 nitrogen filled ionization chambers and 350 sec-ondary emission monitors have been produced at IHEP forthe LHC beam loss monitoring system, following their de-velopment at CERN. The various tests allowed a good con-trol of production and shipping of these delicate devices. Atight production schedule was kept and the very strict toler-ance for cleanliness and leakage current were successfullymet.REFERENCES[1] E. B. Holzer et al., IEEE NSS-MIC ’05, Puerto Rico, CERN-AB-2006-009 BI, 2005. W. Friesenbichler, CERN-THESIS-2002-028, Diplomarbeit, Fachhochschule Wr. Neustadt,2002. C. Zamantzas et al., CERN-AB-2005-082, DIPAC 05,Lyon, France, 2005. C. Zamantzas, PhD Thesis, Brunel Uni-versity, London, UK, 2006. C. Zamantzas et al., IEEE NSS-MIC ’06, San Diego, IEEE Trans. Nucl. Sci. V2 p950, 2006.http://www.cern.ch/blm[2] D.Kramer et al., “Simulations and Measurements of Sec-ondary Electron Emission Beam Loss Monitors for LHC”,Nucl. Phys. B, Proc. Suppl. 172 (2007) 246-249 in these pro-ceedings, 2008.[3] V.Assanov et al., “Facility for vacuum cleaning and filling ofthe ionization chambers (beam loss monitors)”, “EngineeringPhysics” in Russian, No.3 2007.[4] Geant4 - A Simulation Toolkit, S. Agostinelli et al., NuclearInstruments and Methods A 506 (2003) 250-303.

Development, Production and Testing of 4500 Beam Loss Monitors

Development, Production and Testing of 4500 Beam Loss Monitors

Abstract

Similar works

Full text

Available Versions

CERN Document Server