During operation, the internal walls of modern particle accelerators are subjected to synchrotron radiation irradiation and/or electron bombardment. Such phenomena do affect surface properties such as the secondary electron yield, (SEY). A low SEY is a key parameter to control and overcome any detrimental effect on the accelerator performance eventually induced by the build-up of an Electron Cloud (EC). In laboratory experiments SEY reduction (called scrubbing) has been studied as a function of dose but the actual kinetic energy dependence has never

been considered as an important parameter. For this reason and given the peculiar behaviour observed for low-energy electrons, we decided to study this dependence accurately. Here we report results of SEY measurements performed bombarding Cu samples obtained from the Large Hadron Collider (LHC) with different doses of electron beams with energy in the range 10–500 eV. Our results demonstrate that

the potentiality of an electron beam to reduce the SEY does not only depend on its dose, but also on its energy. Furthermore, since EC build-up was predicted and observed also in the DAΦNE ring, we report some preliminary measurements on the conditioning of Al samples. An overview of future experiments which we will perform in LNF is then given

Commisso, Mario

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DOI 10.1393/ncc/i2011-10790-4Colloquia: IFA 2010IL NUOVO CIMENTO Vol. 34 C, N. 1 Gennaio-Febbraio 2011Study of the SEY dependence on the electron beams doseand energyM. Commisso(∗)INFN, Laboratori Nazionali di Frascati - Frascati (Rome), Italy(ricevuto il 28 Giugno 2010; approvato il 28 Luglio 2010; pubblicato online l’8 Febbraio 2011)Summary. — During operation, the internal walls of modern particle acceleratorsare subjected to synchrotron radiation irradiation and/or electron bombardment.Such phenomena do affect surface properties such as the secondary electron yield,(SEY). A low SEY is a key parameter to control and overcome any detrimental effecton the accelerator performance eventually induced by the build-up of an ElectronCloud (EC). In laboratory experiments SEY reduction (called scrubbing) has beenstudied as a function of dose but the actual kinetic energy dependence has neverbeen considered as an important parameter. For this reason and given the peculiarbehaviour observed for low-energy electrons, we decided to study this dependenceaccurately. Here we report results of SEY measurements performed bombardingCu samples obtained from the Large Hadron Collider (LHC) with different doses ofelectron beams with energy in the range 10–500 eV. Our results demonstrate thatthe potentiality of an electron beam to reduce the SEY does not only depend onits dose, but also on its energy. Furthermore, since EC build-up was predicted andobserved also in the DAΦNE ring, we report some preliminary measurements onthe conditioning of Al samples. An overview of future experiments which we willperform in LNF is then given.PACS 29.20.-c – Accelerators.PACS 29.27.-a – Beams in particle accelerators.PACS 79.20.Hx – Electron impact: secondary emission.1. – IntroductionIn particle accelerators with intense and positively charged beams and/or vacuumchambers of small transverse dimensions, electrons can be produced by the interaction ofsynchrotron radiation with the walls, by stray beam particles striking chamber walls atgrazing angles, or by ionization of residual gas. These primary electrons may be accel-erated by the Coulomb potential of the circulating beam producing secondary electrons(∗) E-mail: mario.commisso@lnf.infn.itc© Societa` Italiana di Fisica 103104 M. COMMISSOand leading to a formation of an “Electron Cloud” (EC) [1, 2]. These issues have beenaddressed in many international workshops in recent years [3], since EC are importantphenomena occurring at different accelerators.EC build-up and evolution depend strongly on the surface properties of acceleratorwalls such as Secondary Electron Yield (SEY), defined as the number of emitted elec-trons per incident electron. A low SEY is indeed essential for the operation of particleaccelerators, since their design luminosity and performance can only be achieved if theSEY is strongly reduced by surface conditioning during initial operations (or commission-ing). So, a low SEY will ensure the mitigation of the potentially detrimental effects dueto the e-cloud effect. Furthermore, the understanding of the conditioning process mayhelp to make predictions on the conditioning time required to reach accelerator designparameters.Up to now, SEY reduction (called scrubbing) was studied in laboratory experimentsby measuring the electron dose (the number of impinging electrons per unit area onsample surfaces) dependence of the SEY yield. All the available experiments found inthe literature have been performed by bombarding technological metal surfaces withelectron beams of fixed energies [4-9]. They showed that even at low electron exposureof about 10−6 Cmm−2 the SEY starts to decrease, reaching its lowest value after about10−2 Cmm−2 (in the case of Cu samples).Although useful, all those investigations neglected other significant aspects as thedependence of the “scrubbing effect” on the actual energy of primary electrons, evenif theoretical and experimental studies predict that the electrons in the cloud have avery low energy [3], and that such slow electrons can be reflected from the walls withoutinteracting and scrubbing the surface [4]. Our studies proceed in this direction and in thiscontribution we present some experimental results obtained by bombarding the surface ofa representative Cu sample used in the LHC beam screen with doses of electrons havingkinetic energy in the range of 10–500 eV. Our measures show, for the first time in thiscontext, that the potentiality of an electron beam to reduce the SEY (scrubbing effects)does not only depend on its dose, but also on its energy.Finally, since the build-up of EC was also observed for the DAΦNE ring [10, 11], weperform some preliminary measures on a different material, that is the Al representativeof arc vessel of DAΦNE. The results are briefly compared with those existing in theliterature since the conditioning of Al surfaces has been widely investigated [12,13]. Ad-ditional work is ongoing to confirm observations at different energies, by characterizingsample chemistry before, during and after irradiation by means of photoelectron spec-troscopy. These further experiments will be implemented by using two XUV Beam Linesfrom DAΦNE Bending Magnet [14], built at LNF, which will allow us to investigate alsoSEY variations after electron and photon scrubbing for different materials.2. – ExperimentalThe data were acquired with a dedicated experimental apparatus which is describedelsewhere [15]. Briefly, an UHV μ-metal chamber with less than 5mG residual magneticfield at the sample position, and a CTI8 cryo-pump ensured a vacuum better than 10−10Torr after bake-out. The samples studied, mounted on a close-cycle Sumitomo cold-fingermanipulator, were co-laminated Cu for the LHC beam screen, and Al representative of thearc vessel of the DAΦNE ring accelerator. The electron beam was set to be smaller than0.25mm2 in transverse cross-sectional area and stable in current for energies between10 and 500 eV, as confirmed by a line profile and by stability tests done by using aSTUDY OF THE SEY DEPENDENCE ON THE ELECTRON BEAMS DOSE AND ENERGY 105homemade 1mm slot Faraday cup. It has been observed that the beam moves slightly inposition during energy scans, forcing us to manually irradiate with the same doses thesample areas neighbouring the investigated (measuring) spot. Such rastering procedure,although time consuming, ensures that the SEY measurements were done on a uniformlyirradiated area for every bombarding electron energy. To measure low-energy impingingprimary electrons, a negative bias voltage was applied on the sample. Such bias allows usto work at very low primary energy (close to zero eV) while keeping the gun in a regionwhere it is stable and focused.The electron dose is determined by D = Q/A = (I0t)/A, where Q is the total chargeincident per unit area on sample surface, I0 is the impinging beam current (generallyfew nA while dosing the sample) and t is time period during which the sample wasexposed to the beam. The area is determined assuming that the electron beam hits thesurface sample with a circular spot. Unit chosen here for dose are Cmm−2. All SEYmeasurements and electron irradiation have been performed at room temperature andat normal incidence. Uncertainties on the irradiated spot and on the adopted rasteringprocedure doses have to be considered within 20% of their quoted values. The dataacquisition system is a customized LABVIEW program which allows to scan the beamenergy from the lowest to the highest value and to acquire beam and sample current inorder to calculate SEY.3. – Results and discussionFigure 1 shows SEY measurements for LHC-type samples bombarded with differentdoses of electron beams at the energy of 200 eV (top panel), and 10 eV (bottom panel).These curves agree with those found in literature [4,6] and are characterized by a maxi-mum value δMax and the corresponding energy EMax at which it occurs. As reported inthe literature, they depend on surface conditions and roughness. At low primary ener-gies, SEY curves show a value below 1, which is independent of δMax and of the degreeof scrubbing, in agreement with previous experimental and theoretical results [4, 6].Furthermore, in this figure the effect of e− irradiation on SEY curves is evident.It causes a decrease of the maximum value of SEY curves, δMax, and a shift of thecorresponding energy EMax towards lower energy values. In addition, although samplesare conditioned with similar doses, the behaviour of the SEY curves is not the same.To gain insight into this finding we report in fig. 2 the behaviour of δMax as a functionof electron dose, for samples conditioned with electron beams in a wide range of energies.The curve obtained while conditioning the sample with 500 eV agrees well with resultsavailable in the literature, and shows that, for this energy, an electron dose between10−6 Cmm−2 and 10−2 Cmm−2 is necessary to reduce the yield of LHC samples from2.1 to 1.10. Furthermore as we showed in [15], the reduction of δMax vs. the dose is thesame if samples are conditioned at electron energy of 300 eV and 200 eV.When the scrubbing energy is lower than 200 eV, the reduction of δMax with doseproceeds with a slower rate, immediately evident at low doses and this behaviour becomemore evident when the energy of the primary beams becomes very low. In addition thevalue of δMax obtained at the final dose of 10−2 Cmm−2 is different, as indicated by theorange dot line.In the case of conditioning with energies of 20 eV and 10 eV, in order to check theconsistency of δMax at the final dose (10−2 Cmm−2), we irradiated samples with a doseof 10−3 Cmm−2 at 200 eV. This dose, which must be summed to the previous ones,shows that SEY is reduced to 1.10 similarly to the case of 200 eV and 50 eV electron106 M. COMMISSOFig. 1. – SEY measurements for 200 eV (top panel), and 10 eV (bottom panel) impinging electronenergy at normal incidence.bombardment. These measurements have been reproduced on different Cu beam screensamples showing that the conditioning behaviour does not depend on the slightly differentsample initial condition (i.e. δMax on “as received” samples).To the best of our knowledge, these new experimental results are the first of this kindand suggest that the scrubbing efficiency of electrons hitting the accelerator walls de-pends on their actual kinetic energy, being lower than expected at low energy (< 50 eV).Therefore the time required to obtain a low-SEY surface is consequently different es-pecially when low-energy electron beams are considered. This difference in efficiencywith respect to electron energy is consistent with experiments performed in EPA atCERN while conditioning a copper sample with photoelectrons with energies at 100 and820 eV [16].After such evidences we launched two parallel and necessary activities: one theo-retical and one experimental both aimed at a more detailed modelling of the electronenergy distribution in the cloud and on its impact on the commissioning time in modernmachines. The theoretical studies, in progress, will be described in future pubblica-tions [17] and show that by optimising the functioning parameters of each individualSTUDY OF THE SEY DEPENDENCE ON THE ELECTRON BEAMS DOSE AND ENERGY 107Fig. 2. – δMax as a function of dose for different impinging electron energies at normal incidenceon LHC samples.machine it is possible to obtain electrons with energies > 50 eV inside the cloud whichwill enhance scrubbing efficiency and reduce conditioning time. Experimental activitiesaim at measuring the actual energy of electrons involved in the cloud, since it has notbeen measured accurately, but only simulated [3]. For this reason we decided to de-velop the optimized Retarding Field Energy Electrometer, shown in fig. 3. A detaileddescription of the detector with etherodine acquisition technique and of its potentialitieshas been widely given in [18]. Despite some difficulties in running the detector withthe necessary confidence, and some problems related to the gain of the channelplate inuse, we are now obtaining encouraging results and hope to mount two of such workingdetectors in running accelerators, such as DAΦNE and Anka rings [19] to measure theEnergy Distribution Curves (EDC) in different places of the accelerators.Finally, since EC build-up was predicted and also observed in DAΦNE accelera-tor [10, 11] we performed experiments on Al samples, representative of arc vessel of theDAΦNE. For such samples SEY curves (not shown) are very similar to those showed infig. 1, although we remark that their acquisition was more critical than for Cu samples,probably because the inhomogeneous oxide film on the as-received Al samples causedan inhomogeneous charging of the sample during measurements. Al surfaces are morereactive than copper ones, so their SEY might strongly vary from one sample to the otherdepending on surface preparation.Some conditioning results are shown in fig. 4, where we report the behaviour of δMaxas a function of electron dose, for Al samples bombarded with electron beams of 100and 200 eV. For comparison we plot also the behaviour of δMax for LHC-type Cu sampleshown in fig. 2. We can observe that δMax of the as-received Al sample is higher than thoseof LHC samples, consistently with literature results [12,13]. Furthermore δMax reduction108 M. COMMISSOFig. 3. – Photograph of the LNF-Retarding Field Detector built at LNF mounted on a CF 63Conflat flange.vs. dose for Al samples proceeds with a slower rate than for the Cu samples. Thisbehaviour is probably due to the different efficiency of the bombarding beams in removingthe oxide surface layer, which is more stable on Al than on Cu surfaces, as widelydescribed in [12, 13] by means of X-ray (XPS) spectroscopy. The scrubbing efficiency isevident also on Al samples. Moreover the final values of δMax obtained at the dose ofFig. 4. – δMax as a function of dose for different impinging electron energies at normal incidenceon DAΦNE Al samples.STUDY OF THE SEY DEPENDENCE ON THE ELECTRON BEAMS DOSE AND ENERGY 10910−2 Cmm−2 are slightly lower than those reported by several authors for experimentsperformed bombarding similar technical surfaces [12,13] with beams of 100–130 eV, whichshowed that the SEY of samples did not go lower than 1.8. Our low values of final SEYare anyway consistent with the DAΦNE perfomances in terms of positron accumulatedcurrent [11]. Detailed studies are required to understand the observed differences bycharacterizing samples chemistry before and after irradiation by using photoemissionspectroscopy. These investigations will be implemented by using the two XUV beamlinesbuilt in LNF, operating respectively in the 35–200 eV (XUV-Low) and 60–1000 eV (XUV-High) ranges [14].4. – ConclusionWe report experimental results obtained by bombarding Cu samples from LHC withdifferent doses of electron beams in the range of energies 10–500 eV. Our data showclearly that for equal electron doses, it exists a lower scrubbing efficiency for low energy(< 50 eV) electrons compared to the medium energy ones (> 200 eV). The implicationsof these findings are significant and encouraged us to perform other investigations boththereotical and experimental in order to enhance the understanding of the details of thecloud production and its impact on commissioning time in accelerators. Our thereoticalactivities will be the subject of future publications [17], while our experimental studiesare related to the determination of the actual energy of electrons forming the cloud, sinceat present it has not been accurately measured, but only simulated [3]. For this reason weare building a retarding field analyser based on etherodine acquisition technique, whichis subjected to laboratoratory tests.Since the e-cloud was predicted and observed also in the DAΦNE ring [10, 11], wealso performed experiments on Al samples and compared the results with the existingliterature. Further studies are necessary to achieve a deeper understanding of exper-imental data by using the potentiality of the two XUV beamlines built in LNF [14].Furthermore, among the possible multipourpose use of such beamlines, our laboratorywill be the only one in the world able to analyze SEY variation after electron and photonscrubbing on the same samples for different materials. This is a situation which occursin real accelerators, but it has never been studied in a laboratory experiment.∗ ∗ ∗The author would like to thank Dr. R. Cimino, Dr. R. Larciprete for useful dis-cussion and comments, Dr. S. Casalbuoni and Anka ondulator Group and thechniciansfor laboratory supports. Financial support from INFN Group V is acknowledged.REFERENCES[1] Izawa M., Sato Y. and Toyomasu T., Phys. Rev. Lett., 74 (1995) 5044.[2] Ohmi K., Phys. Rev. Lett., 75 (1995) 526.[3] Proceedings of Electron Cloud Conference, ECLOUD’02-ECLOUD’04-ECLOUD’07 andreferences therein, available on JACoW (Joint Accelerator Conference Website) site.[4] Cimino R., Collins I. R., Furman M. A., Pivi M., Ruggiero F., Rumolo G. andZimmermann F., Phys. Rev. Lett., 93 (2004) 014801.[5] Kirby R. E. and King F. K., Nucl. Instrum. Methods A, 469 (2001) 1.[6] Baglin V., Collins I., Henrist B., Hilleret N. and Vorlaufer G., LHC ProjectReport 472, CERN (2001).110 M. COMMISSO[7] Cimino R. and Collins I. R., Appl. Surf. Sci., 235 (2004) 231; Cimino R., Nucl. Instrum.Methods A, 561 (2006) 272.[8] Scheuerlein C. and Taborelli M., J. Vac. Sci. Technol. A, 20 (2002) 93; ScheuerleinC., Taborelli M., Hilleret N., Brown A. and Baker M. A., Appl. Surf. Sci., 202(2002) 57.[9] Nishiwaki M. and Kato S., Vacuum, 84 (2010) 743.[10] Vaccarezza C., Cimino R., Drago A., Zobov M., Sculte D., Zimmermann F.,Rumolo G., Ohmi K. and Pivi M., in Proceedings of the Particle Accelerator Conference(PAC05), p. 779, available on JACoW (Joint Accelerator Conference Website) site.[11] Demma T., Cimino R., Guiducci S., Vaccarezza C. and Zobov M., in Proceedings ofEuropean Particle Accelerator Conference (EPAC08), p. 1604, available on JACoW (JointAccelerator Conference Website) site.[12] Rosenberg R. A., McDowell M. W., Ma Q. and Harkay C., J. Vac. Sci. Technol.A, 21 (2003) 1625.[13] Le Pimpec F., Kirby R. E., King F. K. and Pivi M., J. Vac. Sci. Technol. A, 23 (2005)1610.[14] Balerna et al., LNF Annual Report 2008 and LNF Annual Report 2009.[15] Cimino R., Commisso M., Demma T., Grilli A., Pietropaoli M., Ping L., SciarraV., Baglin V., Barone P. and Bonanno A., Proceedings of European ParticleAccelerator Conference (EPAC08), p. 1592, available on JACoW (Joint AcceleratorConference Website) site.[16] Baglin V., Bojko J., Gro¨bner O., Henrist B., Hilleret N., Scheuerlein C. andTaborelli M., in Proceedings of European Particle Accelerator Conference (EPAC2000),Vienna, p. 217, available on JACoW (Joint Accelerator Conference Website) site; BaglinV., Collins I. R., Gro¨bner O., Gru¨nhagel C., Henrist B., Hilleret N. andJenninger B., in Proceedings of the XI workshop on LEP-SPS performance Chamonix,2001, p. 141, available on CERN website.[17] Commisso M. and Cimino R., in preparation.[18] Commisso M., Demma T., Guiducci S., Ping L., Raco A., Tullio V., Viviani G.,Cimino R. andVilmercati P., in Proceedings of European Particle Accelerator Conference(EPAC08), p. 1086, available on JACoW (Joint Accelerator Conference Website) site.[19] Casalbuoni S., Cimino R., Baglin V. et al., in Proceedings of European ParticleAccelerator Conference (EPAC08), p. 2240, available on JACoW (Joint AcceleratorConference Website) site.

Study of the SEY dependence on the electron beams dose and energy

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Study of the SEY dependence on the electron beams dose and energy

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