Baglin, V

Henrist, Bernard

Hilleret, Noël

Mercier, E

Scheuerlein, C

CERN Document Server

INGREDIENTS FOR THE UNDERSTANDING AND THESIMULATION OF MULTIPACTINGV. Baglin, B. Henrist, N. Hilleret, E. Mercier, C. Scheuerlein, CERN, Geneva, SwitzerlandAbstractThe resonant multiplication of electrons in acceleratoris known to degrade the performance of devices whereappropriate conditions are fulfilled between, theoscillating electrical field, the dimensions of the vesseland the ability of the surface to multiply electrons. Thislatter property is described by the secondary electron yield(S.E.Y.). Although the S.E.Y. of most pure materials areknown since the beginning of this century, they are oflittle use for the understanding of the multipacting inaccelerators which are constructed using technical metalssuch as stainless steel. For these reasons a dedicatedinstrument was developed in the former ISR vacuumgroup to measure the S.E.Y. of the most commoncompounds encountered in accelerators. The measurementmethod and the results obtained for various materials willbe presented for different realistic surface preparations.Results concerning the energy distribution of the emittedelectron will also be shown as well as recentmeasurements made in an experimental beam line inEPA.1  INTRODUCTIONThe electron multiplication on surfaces exposed to anoscillating electromagnetic field causes the phenomenonof multipacting, which can degrade significantly theperformance of particle accelerators. Such phenomenahave been described in many accelerators, especially inaccelerating cavities and more recently in the SPS wheremeasurable pressure increases have been observed duringthe circulation of LHC type proton beams. In this casethe electric field generated by the passing bunches canaccelerate electrons which at their impact with the vacuumchamber create secondary electrons and stimulate neutralmolecules desorption. Among other parameters, themultiplication of the incident electrons depend on thesecondary electron yield (S.E.Y.) of the bombarded surfacewhich has been measured for most pure metals before19401234. However these data are not applicable to thereal environment of accelerators built out of technicalmaterials e.g. stainless steel, aluminum alloys or coppercovered with their natural oxide and contaminants. In thiscontribution we will present the experimental set up andthe procedure used to study the S.E.Y of technicalmaterials then show some measurement resultsconcerning these. Lastly, possible methods used to limitthe secondary electron yields will be addressed and theireffectiveness discussed in the context of a possible use inan accelerator.2  THE EXPERIMENTAL SET UPAND PROCEDUREThe measurement principle consists in recordingsimultaneously the current at a sample and at a secondarycollector called cage while bombarding the sample byprimary electrons with an energy variable between 60 and3000 eV. A scheme of the set up is shown on Figure 1and a top view of the system on Figure 2.The measurement assembly is mounted in abakeable, all metal U.H.V. system pumped by a 260 l/sturbomolecular pump and equipped with calibrated BayardAlpert gauge and residual gas analyser. A leak valve isused to feed various gases in the system in order tomodify the studied surface using ion bombardment. Thesamples (maximum 13) are mounted on a rotatablesample holder.A very important feature for the measurement ofthe secondary electron yield is to control carefully theelectron dose needed for a measurement. It will be shownSECONDARYELECTRONSELEC TRON  GUN(0-3 kV)SAMPLEECONDARY  ELE CTRONCOLLECTOR (cage)PRIMARYELEC TRONSFigure 1The experimental  set-upFigure 2The experimental systemChamonix X130later in this paper that the S.E.Y. is strongly dependenton the dose of primary electrons. For this reason themeasurements are made using short pulses (typically 30ms) of low primary electron current (some nano amperes).Under these conditions, the total dose of electrons neededto measure the S.E.Y. between 60 and 3000 eV is lessthan 10 nC/mm2.3  THE ORIGIN OF THEDIFFERENCE BETWEEN PUREMETALS AND TECHNICALSURFACESThe difference between pure metals and technical surfacesis illustrated in the Figure 3 which shows the variation ofthe S.E.Y. measured for a copper sample in the asreceived state and after two different in situ treatments: a300 ºC bake-out and an argon glow discharge. In thislatter case the S.E.Y. of the pure material is obtained:maximum yield 1.3 at 600 eV primary energy. Thehighest yield is obtained for the as received sample(greater than 2). A 300 ºC bake out decreases this value to1.8.The S.E.Y. of various technical materials is shown onFigure 4 in the as received state: Aluminum alloys havethe greatest S.E.Y. (greater than 3). For primary energieslower than 300 eV, copper, stainless steel and titaniumare equivalent. A titanium nitride layer5 has asignificantly lower S.E.Y.  for primary energies lowerthan 600 eVThe origin of the S.E.Y. discrepancy betweenpure materials and technical materials is due to thepresence of a surface layer (oxide and contaminants) whichcan be removed by an argon ion bombardment.Furthermore, the S.E.Y. of as received samples issignificantly reduced by baking these materials undervacuum (e.g. to 300ºC), a process leading mainly to theremoval of the water vapor adsorbed on their surface.The two following experiments illustrate theeffect of the oxide layer and of the adsorbed water on theS.E.Y. :The influence of the natural oxide layer is shownon the Figure 5, displaying the evolution of the S.E.Y. ofan aluminum alloy (6061) as a function of the dose ofargon ions impinging on its surface. Starting at an initialvalue of 3, at 300 eV, the S.E.Y. decreases withincreasing bombardment close to the value obtained forpure aluminum: 0.95. The dose of argon ions (3.5 1017ions/cm2) needed to reach this value is equivalent to thesputtering of a 30 nm thick oxide layer, a value close tothe one obtained for a similar layer by Auger analysis6. Ifthe sample is reexposed to air after this treatment, theS.E.Y. is increased to a value higher than 2.5 which issubsequently decreased to the bulk value after the0.511.522.533.50 2E+17 4E+17 6E+17 8E+17 1E+18ARGON ION DOSE (IONS/CM2)SECONDARY ELECTRON YIELDPURE ALUMINIUMnd TREATMENT FTER AIR EXPOSUREINITIAL TREATMENTFigure 5:The S.E.Y. of aluminum alloy 6061as a function of the argon ion dose0.00E+005.00E-011.00E+001.50E+002.00E+002.50E+000 500 1000 1500 2000 2500 3000ENERGY (eV)SECONDARY ELECTRON YIELD  COPPER AS RECEIVEDCOPPER BAKED TO 300 CCOPPER + A.G.D.Figure 3:The S.E.Y. of copper  as a function of the surfacetreatment00.511.522.533.540 500 1000 1500 2000ENERGY (eV)SECONDARY ELECTRON YIELDAluminium 99.5%TitaniumCopper OFHCStainless steelTiNFigure 4:The S.E.Y. of various as received technical materials0.811.21.41.61.822.22.40 500 1000 1500 2000 2500Energy (eV)SECONDARY ELECTRON YIELD Clean sample45 monolayers of water85 monolayers of water200 monolayers of waterFigure 6:The S.E.Y. of copper covered with condensed water131Chamonix Xsputtering of a layer equivalent to a 6 nm thick aluminumoxide.The S.E.Y. of water condensed at liquid nitrogentemperature on a baked copper surface has been measuredfor various water thicknesses. In Figure 6 the variation ofthe S.E.Y. with the primary electron energy is plotted fora clean sample and for coverages corresponding to 45, 85and 200 monolayers of condensed water. The evolution ofthe maximum yield is shown in Figure 7 as a function ofthe number of water monolayers. The S.E.Y. saturates at2.3 for water thickness greater than 160 monolayers. Thisvalue is close to the value measured for unbaked metals ascopper, stainless steel and niobium.4   METHODS TO DECREASE THESECONDARY ELECTRON YIELDThe preceding measurements have shown thestrong influence of an air exposure on the S.E.Y. oftechnical materials. For example the argon ion glowdischarge treatment can reduce drastically the S.E.Y. ofaluminum but the effect is almost completely lost after anair exposure. In the case of accelerator components orvacuum chambers, it is almost excluded to treat thesurfaces in situ with the possible exception of bake- out.Hence the efficiency of all attempts to decrease the S.E.Y.by modifying the surface composition is limited by thesubsequent exposure to air and water vapor unavoidableduring installation. Another possible way to lower theS.E.Y. is to change the surface roughness. This causesindirectly a reduction of the emissivity of the surface asthe solid angle for electrons to escape without furtherinteraction with the vacuum chamber can be significantlydecreased. A third type of process will be described in thelater4.1  Changing the surface compositionThe argon ion glow discharge treatment is a powerful wayto modify surfaces by ion bombardment which has beenalready applied to the 2 km vacuum system of the formerISR. By changing the gas used during this treatment, it ispossible to produce various surface layers loweringpermanently the S.E.Y. even after an exposure to air.This is illustrated on the Figure 8 showing in the case ofniobium the maximum S.E.Y. after various glowdischarge treatments using argon (A.G.D.), argon 10%oxygen  (A+O2 G.D.), nitrogen (N2G.D.). If most of thebeneficial effect of the discharge is lost after an A.G.D., asignificant improvement subsists after a 24 hoursexposure to air after A+O2 G.D. or after N2G.D.. In theseboth cases, the final yield after exposure to air is close to1.4, a value which is also obtained after an in situ bake-out to 300 ºC.In the case of copper, the curves giving theevolution of the S.E.Y. with various treatments are givenon the Figure 9. The permanent reduction of the S.E.Y.obtained after an N2.G.D. is also visible. A mild in situbake out to 100ºC restores most of the effect of the in-situ treatment when the system has been exposed for 24hours to air.Titanium nitride is known7 to produce areduction of the secondary electron which depends largelyon the deposition condition of the films. Figure 11 showsthe S.E.Y. of TiN coatings from various sources showinga large scatter in the maximum yields (between 2.5 and1.5). The best layer5 has a maximum yield significantlylower than the S.E.Y. of any metal in the as receivedstate. After a subsequent 150ºC bake out, the maximumyield is close to 1.4, a higher temperature bake out (300ºC) lowers the yield to 1.2.1.71.81.922.12.22.32.40 20 40 60 80 100 120 140 160 180 200 220NUMBER OF WATER MONOLAYERSSECONDARY ELECTRON YIELDFigure 7:Variation of the maximum S.E.Y. with thecondensed water coverage00.511.522.5MAXIMUM S.E.Y. ASRECEIVEDN2GD AGD AO2 G.D.TREATMENTSN2GD +AIRAGD +AIRAO2GD+AIRBO 300ºCFigure 8:Niobium maximum S.E.Y. after various surface treatments00.511.522.530 500 1000 1500 2000PRIMARY ELECTRON ENERGY (eV)SECONDARY ELECTRON YIELD AS RECEIVED B-ON2GD24h AIRN2GD +24h AIR+BAKED 100CFigure 9:S.E.Y. of copper after various surface treatmentsChamonix X132The activation of a getter layer is another way toeliminate the oxide layer. On the Figure 10 the variationof the S.E.Y. of a Ti Zr getter layer8 is plotted as afunction of the baking temperature. A marked decrease ofthe S.E.Y. is visible when the layer is heated above itsactivation temperature. After a 300 ºC bake out, thehighest yield is less than 1.2.4.2  Changing the surface roughnessAs explained above, the creation of a layer with increasedroughness can permanently decrease the S.E.Y. Thismodification can be produced either by a chemical reactionwith the substrate or by the deposition of a stronglydendritic layer.The first approach was applied to the case ofcopper9 . An in situ bake out to 350ºC at atmosphericpressure for 5 minutes followed by a 6 hours vacuumbake-out creates a surface with a low S.E.Y. (1.05) asshown on the Figure 11. That treatment causes anincrease of surface roughness Figure 12 measured by theBET method10  from 1.4 (as received state) to 6.5 (after airbake).The creation of a strongly dendritic surface on asmall copper sample,as shown on the Figure *(13)produces a surface with a S.E.Y lower than 1 even after0.60.81.01.21.41.61.82.02.20 500 1000 1500 2000 2500 3000 3500PRIMARY ELECTRON ENERGY (eV)S.E.Y.UNBAKED (21nov97)BAKED 24h, 150C measured at 20C (25nov97)BAKED 24h, 200C measured at 20C (28Nov97)BAKED 22h, 300C measured at 200C (3dec97)BAKED 22h, 300C   measured at 20CFigure 10:S.E.Y. of a TiZr getter as a function of the activation temperatureCopper  as receivedCopper after air bakeFigure 12:A copper surface before and after air-baking0.60.70.80.911.11.21.31.40 500 1000 1500 2000 2500 3000primary electron energy in eVC u after oxidation in air at 20°C and 350°C bakeoutCu after oxidation in air at 350°C and 350°C bakeoutFigure 11 :SEY of copper after 24 hours 350°C bakeoutcompared to the SEY of copper after 5 minutes airexposure at 350°C and 6 hours bakeout at 350°Cunder vacuumFigure 13:SEM photograph of  a  textured copper surface133Chamonix Xair exposure. Similar results have been published byA.N.Curren , K.A. Jensen and R.F. Roman11  These methods seem able to produce surfaceswith very low secondary electron yields (lower than 1) butdifficult to apply to large surfaces such as those involvedin an accelerator.4.3  The “dose” effectWhen a surface is exposed to an electron beam,its S.E.Y. decreases12. In Figure 14 the variation of theS.E.Y. is plotted as a function of the electron dose fortwo energies of the primary electrons : 100 and 500 eV.In both cases the S.E.Y. decreases for doses larger than10-6 C/mm2 and stabilizes for doses greater than 2x 10-3C/mm2 at a value close to unity. This effect is permanentwhen the surface is not kept under a moderate vacuum(unbaked system p= 1x10-5 Pa) as it appears on the Figure*(15) where the dotted line indicates a constant lowsecondary electron yield for a subsequent electronbombardment made with the same dose. This dose effectdecreases the S.E.Y. for all energies as can be seen on theFigure 16 displaying the yield as a function of theprimary electron energy before and after a bombardmentwith a dose of 1x 10-3 C/mm2.Although not completely understood and difficultto investigate, as the electron doses involved are too smallfor the usual analysis techniques, this effect has been usedsince very long time in accelerators and is called :processing!As this effect could be a remedy for the electroncloud effect in the LHC, it has been studied in EPA,using a remotely controlable system as shown in figure 2to measure the secondary electron yield of a coppersample. The synchrotron light radiated by the EPAelectron beam ( 194 eV critical energy) travels along atangential photon beam line. The sample is mounted at90 º from the plane of the synchrotron light and thus isnot exposed to direct photons. Three dose experiments0.91.11.31.51.71.92.12.32.51.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01Dose (C/mm2)S.E.Y.  E = 500 eV  E = 100 eVAS RECEIVED COPPERFigure 14:Variation of the S.E.Y. of as received copper with the electron dose0.000.501.001.502.002.503.001.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02Dose (Cb/mm2)Secondary electron yieldINITIAL BOMBARDMENTSECOND BOMBARDMENT (after 16 hours stop, P= 1x10-5 Pa)Ep = 500 eVFigure 15Variation of the S.E.Y. of copper during 2consecutive bombardments0.000.501.001.502.002.500 500 1000 1500 2000 2500 3000Energy (eV)Secondary electron YieldSEY before bombardementSEY after bombardement (1e-3 Cb/mm2)      (500 eV, 1x10-6 A)Figure 16S.E.Y. of copper as a function of the electron energybefore and after electron bombardment ( 10-3 C/mm2)0.00.51.01.52.02.51.E+18 1.E+19 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24Photon dose (ph/m)SEY at 240 eV ample at - 45 Vample at + 100 VSample at + 350 V~ 1 year of nominal LHC operationFigure 17The S.E.Y. measured  at 240 eV against the photon doseCritical energy 194 eV, Sample polarised at + 100 V 0.00.51.01.52.02.50 500 1000 1500 2000 2500 3000Energie (eV)SEYA1 A2A7 3.74E19 ph/mA9 1.6E20 ph/mA12 1.5E21 ph/mA13 3.3E21 ph/mA14 5.5E21 ph/mA17 1.1E22 ph/mA19 1.45E22 ph/m240Figure 18:Variation of the S.E.Y. versus energy  curve as afunction of the photon doseChamonix X134were carried out, using different bias applied to thesample: -45 V ( i.e. the sample was only conditioned bydiffused photons), +100 V and +350 V (i.e. the samplewas bombarded by photoelectrons with approximately 100eV and 350 eV energy). The variation of the S.E.Y.measured at 240 eV is given in the Figure 17 as afunction of the primary photon dose. When the sample ispositively biased, the curves are very similar to thoseobtained in the laboratory and saturate at a value close to1.2. When the sample is only bombarded by diffusedphotons, the effect is less pronounced and seems tosaturate for a S.E.Y. of 1.6. Figure 18 the S.E.Y. as afunction of the primary electron energy evolves during theexperiment : the maximum of the curve is shifted towardslower energies which corresponds to a reduction of thesecondary electron escape depth.4.4  SummaryTo summarize the efficiency of various methods for thereduction of the S.E.Y., the Figure 19 gives the S.E.Y.obtained after treatments applied ex-situ, a subsequentexposure to air and no in-situ bake out. The surfacesconsidered are : as received after chemical cleaning, freonprocessing, nitrogen glow discharge treatment, coatingwith titanium nitride5 and for comparison after an in situelectron bombardment with 1x10-3 C/mm2. The electronbombardment gives the lowest yield, for primary electronswith less than 500 eV, followed by the TiN layer, thenitrogen glow discharge treatment and the freon treatment.5  CONCLUSIONSThe secondary electron emission is a surface dependantphenomenon, more influenced, for technical metals, bythe surface preparation than by the material itself.Insulating layers (e.g. oxides as well as adsorbed water)significantly enhance the emissivity  of surfaces. Varioussurface treatments involving (e.g. ion bombardment) orcoatings (e.g. titanium nitride) can reduce the S.E.Y.although a subsequent air exposure reduces notably theirefficiency. The dose effect (conditioning) is a wellestablished and powerful method to circumvent theproblems related with electron multiplication.Nevertheless, this method can only be applied  under twoconditions:-A significant dose should be applicable to the surface toreach a dose close to 10-3 C/mm2 within a reasonabletime.-The electron multiplication should be mastered in orderto avoid an uncontrolled runaway of the process andpossible damages.REFERENCES                                                1 H. Bruining, J.H. de Boer, Physica, 5, 17, 1938.2 H. Bruining, Philips Tech. Rev. 3, 80, 19383 L. Warnecke, J. Phys. radium, 7,270, 19364 L.R.G. Treloar, D.H. Landon, Proc. Phys. Soc.(London), B50, 625, 19385 Sample provided by. R.E. Kirby   SLAC 19976 A. Grillot, technical note ISR-VA/AG/sm, 12/7/827 E.L. Garwin, F.K. King, R.E. Kirby, O Aita, J.Appl.Phys. 61, 1145, 19878 C. Benvenuti et al., Vacuum, 53, 219, 19999 I. Bojko, J.L. Dorier, N. Hilleret, Ch. ScheuerleinVacuum technical note LHC-VAC/Ch.S  97-19, 199710 V. Baglin Technical note LHC-VAC 97-03, 199711 A.N.Curren , K.A. Jensen and R.F. Roman, NASAreport TP-2967, 199012 M. Lavarec, P. Bocquet, A. Septier C.R. Acad. Sc.Paris, t.288, B277, 19790.50.70.91.11.31.51.71.92.12.32.50 500 1000 1500 2000 2500 3000ENERGY (eV)S.E.YCOPPER AS RECEIVEDFREON TREATMENT  AFTER 20 MN AIREXPOSURETiNN2GD + 24h AIRSEY after bombardement (1e-3 Cb/mm2)         Figure 19:The secondary electron yield of  copper after varioustreatments and exposure to air135Chamonix X

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