This paper describes experimental studies of the effect of a dipole field on the photoelectron emission and on the photon reflectivities from LHC beam screen material. These studies were performed using synchrotron radiation from the VEPP-2M storage ring at BINP (Novosibirsk). The particular surface roughness and geometry of the prototype LHC beam screen material requires dedicated experimental measurements. The experiments were performed under conditions close to those expected in the LHC. An important result obtained is that a dipole magnetic field attenuates the photoelectron emission from surface by more than two orders of magnitude with the magnetic field aligned parallel to the surface. The measurements of photon reflectivities, forward scattered and diffuse, and the azimuthal distribution of emitted photoelectrons from the same material are reported. These experimental results are important input for the final design of the LHC beam screen

A.A Krasnov

Anashin

Baglin

Brüning

E.E Pyata

Furman

Gröbner

I.R Collins

N.V Fedorov

O Gröbner

O.B Malyshev

R.V Dostovalov

V.V Anashin

Zimmermann

English

Anashin, V V

Collins, I R

Dostovalov, R V

Fedorov, N V

Gröbner, Oswald

Krasnov, A A

Malyshev, O B

Pyata, E E

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsMagnetic and Electric Field Effects on the PhotoelectronEmission from Prototype LHC Beam Screen MaterialV.V. Anashin1, I.R. Collins2, R.V. Dostovalov1, N.V. Fedorov1, O. Gröbner2, A.A. Krasnov1,O.B. Malyshev1 and E. E. Pyata1This paper describes experimental studies of the effect of a dipole field on the photoelectron emission andon the photon reflectivities from LHC beam screen material. These studies were performed usingsynchrotron radiation from the VEPP-2M storage ring at BINP (Novosibirsk). The particular surfaceroughness and geometry of the prototype LHC beam screen material requires dedicated experimentalmeasurements. The experiments were performed under conditions close to those expected in the LHC. Animportant result obtained is that a dipole magnetic field attenuates the photoelectron emission from surfaceby more than two orders of magnitude with the magnetic field aligned parallel to the surface. Themeasurements of photon reflectivities, forward scattered and diffuse, and the azimuthal distribution ofemitted photoelectrons from the same material are reported. These experimental results are important inputfor the final design of the LHC beam screen.1 Members of the CERN-Russia Collaboration for the LHC; Budker Institute of Nuclear Physics, 630090 Novosibirsk, Russia2 CERN, LHC Division 6th European Vacuum Conference (EVC-6)7-12 December 1999, Villeurbanne, FranceGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 373Abstract15 March 2000MAGNETIC AND ELECTRIC FIELD EFFECTS ON THE PHOTOELECTRONEMISSION FROM PROTOTYPE LHC BEAM SCREEN MATERIALV.V. Anashina, I.R. Collinsb, R.V. Dostovalova, N.V. Fedorova, O. Gröbnerb, A.A. Krasnova,O.B. Malysheva and E. E. Pyataa.aBudker Institute of Nuclear Physics, 630090 Novosibirsk, RussiabCERN, CH-1211 Geneva 23, SwitzerlandAbstractThis paper describes experimental studies of the effect of a dipole field on the photoelectron emissionand on the photon reflectivities from LHC beam screen material. These studies were performed usingsynchrotron radiation from the VEPP–2M storage ring at BINP (Novosibirsk). The particular surfaceroughness and geometry of the prototype LHC beam screen material requires dedicated experimentalmeasurements. The experiments were performed under conditions close to those expected in the LHC.An important result obtained is that a dipole magnetic field attenuates the photoelectron emission fromsurface by more than two orders of magnitude with the magnetic field aligned parallel to the surface.The measurements of photon reflectivities, forward scattered and diffuse, and the azimuthaldistribution of emitted photoelectrons from the same material are reported. These experimental resultsare important input for the final design of the LHC beam screen.1 INTRODUCTION The high flux of synchrotron radiation (SR) in the arc of the LHC, about 1017 photons/(s m),causes photoelectron emission from the walls of the beam screen. The emitted low energy electrons areaccelerated in the field of the proton beam and may gain energies sufficient to create secondaryelectrons on the subsequent interaction with the beam screen, resulting in the creation of an electroncloud. The repeated recurrence of this process can result in beam–induced multipacting [1].Theoretical simulations of the electron cloud in the LHC predict significant heat loads on thecryogenic system and electron stimulated gas desorption [2], [3], [4]. Experimental data are required asinput to such simulations in order to estimate the influence of an electron cloud on the LHC operation.A large portion of the LHC beam vacuum system is located inside the dipole magnets. Previous studieshave shown that the presence of the dipole field has a strong influence on the electron emission [5],[6]. In addition the heat loads in the dipole field are predicted to be strongly dependent on theazimuthal distribution of photons, determined by the photon reflectivity [2,3,4]. Experiments wereperformed to investigate the influence of a dipole field on the electron emission of various samples2with different surface roughness [7] and to measure both the photon reflectivities and the effect of adipole field from beam screen material with or without a saw–tooth surface structure [8]. These latterexperiments were performed with SR at grazing incidence in order to simulate closely the geometry inthe LHC. However, here the strength of the dipole field is significantly smaller than that in the LHC(0.3T versus 8.3T) and that the experiment was performed at room temperature and not at cryogenictemperatures. The saw–tooth structure has been proposed as an efficient method of reducing theforward scattered photon reflection in the LHC and hence adsorbing most of the photons on theequatorial plane of the beam screen.The results on the influence of the dipole field on the photoelectron yield, performed with normalincident photons, are presented in section 2. In section 3 the results of the photoelectron yield andphoton reflectivities from beam screen material with or without a saw–tooth surface structure,performed with grazing incident photons, are presented. A brief discussion and conclusions are madein section 4. 2 Experiments performed with normal incident photons2.1 Set-up and sample preparationThe experimental set-up shown in Figure 1 was built on the beamline P2 of VEPP-2M to studythe dependence of the photoelectron emission for photons at normal incidence as a function of atransverse dipole field (up to 0.6 T). The sample was mounted on a sample holder, electrically isolatedfrom the surrounding 46 mm diameter stainless steel chamber, and located in the centre of themagnetic field. An all-metal valve was used to isolate the vacuum of the experimental chamber andthat of the beamline. The distance from the SR source to the collimator is about 11 m. The sample,with vertical and horizontal dimensions of 44 mmu15 mm, respectively is mounted on the sampleholder with its surface perpendicular to the incident SR. The photon flux from VEPP–2M is collimatedresulting in a 35mmu10mm area of the sample to be illuminated, i.e. the edges of the sample are notirradiated.The increased vertical size of the photon beam and the sample was chosen to avoid attenuatingthe lower energy part of the spectrum. The sample was connected via an ammeter to a voltage supply(r300 V). Hence, the potential on the sample could be controlled whilst the electron current from, orto, the sample could be measured. The magnetic field of 0 to 0.6 T was aligned approximately parallelto the sample surface and in the vertical direction. The experimental chamber has a reduced cross-section near the magnet centre to provide a stronger and more uniform magnetic field (see Figure 1).3                   Figure 1: The layout of the installation.Seven different samples were studied: a 316 series stainless steel, four made of copper co–laminated stainless steel (the proposed material for the LHC beam screen), one machined from OFHCcopper and a stainless steel electroplated with gold. The samples were degreased and cleaned withbenzine and alcohol before installation.Table 1: Definition and description of the samples studied.Sample name Sample descriptionSS Stainless steel (316 series) sample made from a rolled sheetCu/SS-1 Copper co–laminated stainless steel with the rolling direction perpendicular to themagnetic field.Cu/SS-2 Copper co–laminated stainless steel with the rolling direction parallel to themagnetic field.Cu/SS-3 Copper co–laminated stainless steel baked in air at 300• C for 5 mins. with therolling direction parallel to the magnetic field.Cu/SS-4 Copper co–laminated stainless steel made with turned-in, long edges, i.e. 5 mmwide strips at the long edges were turned to 10–15q towards the SR with the rollingdirection parallel to the magnetic field.OFHC OFHC copper machined with longitudinal grooves aligned parallel to the magneticfield. The grooves are 1 mm deep and 3 mm wide with a pitch of 3.2 mm.Au/SS Stainless steel electroplated with 6-µm of gold42.2 ResultsIn all experiments, the photoelectron yield was a measured parameter. It was estimated as aratio of the measured output current Imes (i.e. electron flux from or to the sample) to photon flux * .Neither the effect of the beamline collimation nor the sample workfunction is taken into account in theestimation of the photoelectron yield, contrary to that in other studies [5].* emesIphotonelectrons ]/[N ,where e is the electron charge.The photoelectron yield was studied as a function of the different parameters:1. Potential on the sample (or bias) from –300 V to +300 V at fixed dipole field between 0 and  0.6 T;2. Magnetic field from 0 up to 0.6 T at a fixed potential between 0 and –300 V.3. Accumulated photon dose.Table 2: Measured photoelectron yields, N of the various samples. The photon spectrum criticalenergy at which the measurements were made is also indicated.Dipole field efficiency:   T0T60 NN .Sample Critical EnergyEc (eV)Measurements withB = 0 T and U = –300 V:N (e–/J) U = 0 U = -300VSS 259 0.016 0.023 0.028Cu/SS-1 253 0.015 0.010 0.029Cu/SS-2 194 0.014 0.021 0.030Cu/SS-3 112 0.014 0.018 0.015Cu/SS-4 20 0.014 0.020 0.060Cu/SS-4 319 0.018 0.010 0.080OFHC 102 0.008 0.024 0.013Au/SS 356 0.027 0.028 0.042In general, the photoelectron yield was found to increase rapidly for increasing negative bias,corresponding to electrons leaving the sample, and saturates for a bias greater that 100V. It decreasesrapidly with the dipole field when the magnetic field changes from 0 to about 0.1 to 0.2 T, thendecreases slowly, and is practically constant between 0.4 and 0.6 T. Since this suppression at lowfields can be attributed to electrons being constrained to move along the field lines and hence return tothe strip from which they were emitted it is therefore thought unlikely that the yield will change for5higher magnetic fields. The dependence of the photoelectron emission on the photon dose was found incomparing the data without the magnetic field at the beginning of the measurements. After a dose of1022 photons the photoelectron emission yield reduces by about a factor of 2. The main results areshown in Table 2.3 Experiments performed with grazing incident photons3.1 Set-up and sample preparationThe experimental set-up (shown in Figure 2), installed at the end of a second, short SR beam line,was located at a distance of about 1 m from the SR source point where the effect of the verticalcollimation on the photon flux is negligible. Here the dimensions of the SR beam is defined by a set ofhorizontal (Ch) and vertical (Cv) collimators, 2 mm and 10 mm respectively, on entering theexperimental system. The photon flux, through the collimators, was typically 1016 photons/s. Figure 2: Set-up for measurements of the photon reflectivity and azimuthal photoelectrondistribution in a magnetic field.3.2 ResultsThe position of the photon beam with respect to the experimental system could be checked withtwo luminescent screens, (LD1) and (LD2). At the exit of the test system, a calorimeter (CAL) wasinstalled to monitor the SR. This calorimeter was designed in such a way that it could either measure6the total SR power or the total photon flux derived from the photoelectron current produced on thecalorimeter. For the latter measurement the calorimeter was electrically biased with respect to the testchamber. A section of a prototype LHC beam screen, 34 cm in length, was cut into four strips: tworounded and two flat (see Figure 3). In a following experiment strip 1 was replaced with a flat strip ofbeam screen material with a saw–tooth structure on its surface. A dipole magnet with a field of up to0.3 T was installed along the whole length of the vacuum chamber containing the strips. Strips 2 and 4were perpendicular to magnetic field. The rounded strip 3 was opposite to strip 1. The wholeexperimental system could be positioned either aligned straight or inclined at a given grazing anglewith respect to the axis of the photon beam. In the straight position, the SR beam traversed the testsystem and was incident on the end calorimeter, while in the inclined position, the photon beam wasincident at an angle of about 10 mrad along strip 1 only reflected photons are able to reach thecalorimeter.Figure 3: ample configuration for experiments of the photon reflectivity and azimuthalphotoelectron distribution in a magnetic field.During a typical measurement, the SR beam was incident on strip 1. The remaining strips receivedonly reflected photons. Applying a negative bias in turn to each of the four strips while maintaining allothers at the common ground potential, the photoelectron current produced by photons on thecorresponding strip was measured. Assuming that the average photoelectric yield for the directincident photons and for the reflected photons are not too different, it is possible to derive from themeasured azimuthal distribution of the photoelectron signal the azimuthal distribution of the photons.The measurements were performed for a bias voltage from 0 to 290 V and with the magnetic field ofB = 0 to 0.3 T.AStrip 2Strip 4AStrip 1Strip 37Forward scattered reflection is defined as a ratio of measured values on the calorimeter, by currentor power, in the two positions:./;/dirrefWdirrefeWWRIIR  Diffuse photon reflectivity is defined as:YeIIIR 432dif* The yield is defined per adsorbed photon as:  * ¦ e41iiR1eIY ,where the currents Ii from strips were measured at a bias of –300 V.The main results are shown in Table 3. The adsorbed photon flux is that which is not reflected andRdif/(R1+Rdif) gives and indication of the importance of the diffusely reflected SR, with respect to thedirect irradiation, as a source of electrons in the vacuum chamber.Table 3: A summary of the measured reflectivities and photoelectron yields per absorbedphoton. The critical energy at which the measurements were made is also indicated.Forward ScatteredReflectionSampleCriticalEnergyEc (eV)Re (bycurrent)RW (bypower)DiffuseReflectivityRdifPhotonAdsorptionR1 dif1difRRRY(electron/photon)Smoothsurface20 0.67 — 0.04 0.29 0.13 0.0349 0.035 — 0.22 0.74 0.23 0.049Saw–toothsurface 246 0.026 0.03 0.185 0.79 0.19 0.063The photoelectron current on strip 1 is attenuated by a factor of about 200 for the strip withoutsaw–tooth in the dipole field above 0.2 T and about a factor of 5 for the strip with saw–tooth. Thedipole field does not affect the photoelectron current from strips 2 and 4, top and bottom.84 Discussion and ConclusionsThe experiments with SR at normal incidence show that the different materials with the differentsurfaces have about the same photoelectron yield. It is found that the photoelectron current as afunction of dipole field is attenuated by a factor of 30 to 50 reaching a saturation value at 0.4 T.The experiments performed at grazing incidence in a dipole field demonstrate that the electronsare constrained to move along the field lines, resulting in electrons emitted from strip 1 and strip 3 toreturn to the same strip from which they were created. In the case of the LHC these electrons interactweakly with the proton beam, gaining insufficient energy to cause multipacting or contribute asignificant heat load [9]. On the other hand, photoelectrons from strips 2 and 4 (top and bottom) arenot attenuated by the presence of the dipole field and thus they interact with the proton beam andcontribute to the electron cloud.The results presented here are important input to predict the behaviour of the electron cloud in thedipole fields of the LHC and to estimate electron stimulated gas desorption. The forward scatteredreflectivity, as measured by the deposited power and by the photon flux, show significantly differentresults for the smooth surface and for the saw–tooth surface. With respect to the electron-cloud effectin the LHC a low reflectivity is desirable since it will limit the photoelectron production to the regionof the primary impact of the photons in the equitorial plane where electrons are efficiently attenuatedby the strong vertical dipole field [10]. From this point of view, the saw–tooth surface is attractive forthe beam screen of the LHC.References                                               [1] Gröbner, O., CERN LHC Project Report 127 (1997).[2] Zimmermann, F., CERN LHC Project Report 95 (1997).[3] Brüning, O.S., CERN LHC Project Report 158 (1997), Brüning, O.S.,  CERN LHC Project Report190 (1997).[4] Furman, M.A.,  CERN LHC Project Report 180 (1997).[5] Baglin, V., Collins, I.R. and Gröbner, O., CERN LHC Project Report 206 (1998).[6] Anashin, V.V., Malyshev, O.B., Fedorov, N.V., Nazmov, V.P., Goldenberg, B.G., Collins, I.R. and Gröbner, O., CERN LHC Project Report 266 (1999).[7] Anashin, V.V., Dostovalov, R.V., Malyshev, O.B., Krasnov, A.A. and Pyata, E.E., CERN VacuumTechnical Note 99-03 (1999).[8] Anashin, V.V., Malyshev, O.B., Fedorov, N.V.  and Krasnov, A.A., CERN Vacuum TechnicalNote 99–06 (1999).[9] Baglin, V., Collins, I.R., Gómez–Goñi, J., Gröbner, O., Henrist, B., Hilleret, N., Laurent, J-M., Pivi, M., Cimino, R., Anashin, V.V., Dostovalov, R.V., Fedorov, N.V., Malyshev, O.B., Krasnov,A.A. and Pyata, E.E., presented at e+e– Factories’99, KEK in Tsukuba, Japan 21st –24th Sept. 1999.[10] Baglin, V., Bruning, O., Calder, R., Caspers, F., Collins, I.R., Gröbner, O., Hilleret, N., Laurent,J-M., Morvillo, M., Pivi, M. and Ruggiero, F., EPAC-98, June 1998.

Magnetic and Electric Field Effects on the Photoelectron Emission from Prototype LHC Beam Screen Material

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Magnetic and electric field effects on the photoelectron emission from prototype LHC beam screen material

Anashin VV

Collins IR

Dostovalov RV

Fedorov NV

Gröbner O

Krasnov AA

Malyshev OB

Pyata EE

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