When the whole inner surface of a vacuum chamber is coated with a non-evaporable getter film, very low static and dynamic pressures are expected after activation. In an accelerator environment this could result in a longer beam lifetime, in a lower risk of pressure bumps, and in a lower level of bremsstrahlung radiation due to the beam-gas interactions. To substantiate these favourable characteristics a Ti-Zr-V coated stainless steel chamber has been tested on a dedicated beamline at the ESRF. It is shown that a large reduction of the synchrotron radiation-induced desorption occurs after activation

Chiggiato, P

Kersevan, Borut P

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN - ESTCERN EST/99-004 (SM)Synchrotron Radiation-Induced Desorption from aNEG-Coated Vacuum ChamberP. Chiggiato, R. KersevanAbstractWhen the whole inner surface of a vacuum chamber is coated with a non-evaporablegetter film, very low static and dynamic pressures are expected after activation. In anaccelerator environment this could result in a longer beam lifetime, in a lower risk ofpressure bumps, and in a lower level of bremsstrahlung radiation due to the beam-gasinteractions. To substantiate these favourable characteristics a Ti-Zr-V coated stainlesssteel chamber has been tested on a dedicated beamline at the ESRF. It is shown that alarge reduction of the synchrotron radiation-induced desorption occurs after activation.Paper presented at the 6th European Vacuum ConferenceUniversity Lyon 1, Campus La Doua/IPNL, Lyon, France7-10 December 1999To be published in "Vacuum"Geneva, SwitzerlandDecember 1999                                            ESRF, BP 220, F 38043 Grenoble- 2 -1. INTRODUCTIONVacuum systems of particle accelerators have to be designed so as to maintain apressure in the beam pipe low enough to ensure that the beam lifetime is always longer than thatdefined in the design and induced radioactivity is always lower than that allowed by the personalsafety standards.Recently a new pumping solution for the vacuum system of particle accelerators hasbeen developed and studied at CERN [1,2]. It consists in coating the vacuum chamber ex-situwith a µm thick non-evaporable getter (NEG) film. The activation of the film is then performedduring the in-situ bake-out of the vacuum system. Many elements and alloys have been tested(essentially those of the 3rd and 4th group of the periodic table) but the Ti-Zr-V combination is byfar the one with the lowest activation temperature (about 180 °C for a 24 hours long bake-out)[3].The potentialities of this solution in reducing synchrotron induced desorption seemedclear from the beginning.NEG coatings offer two advantages compared to the standard approach. First of all thewhole inner surface of the chamber can be coated therefore allowing a large increase of thepumping speed per meter of chamber after activation. On the other hand during activation thesurface oxide and contaminants are dissolved into the bulk therefore giving rise to a very cleansurface which should have a lower photon desorption yield as has already been proved forelectron stimulated desorption [4].To substantiate these potential benefits a Ti-Zr-V coated chamber has been tested in adevoted synchrotron light line at the European Synchrotron Radiation Facility (ESRF).2. EXPERIMENTAL SET-UPThe chamber fabricated for the experiment was a 316LN, 60 mm diameter, 2 m longtube provided with a double wall for water-cooling. The chamber had been cleaned by applyingthe CERN standard procedure [5] and then vacuum-fired at 950 °C for 2 hours.The chamber was coated using the cylindrical magnetron configuration. The axialcathode was made by twisting together three 1-mm diameter wires of Ti, Zr and V (about99.8 % purity). Pure Ar at a pressure of about 10-2 mbar has been used to generate the plasma.The cathode potential was –500 V during the deposition with the chamber grounded. Themagnetic field, which was generated by an external solenoid, had an intensity of about150 Gauss. The sputtering rate was of the order of 1 Å s-1, and the final thickness of the coatingwas about 2 µm.Before its installation in the ESRF line, the chamber had been stored under dry nitrogenfor some days after venting.At the ESRF, the D31 beamline has been used for the experiment [6]. The mainparameters of the ESRF storage ring and of the synchrotron light provided to the D31 beamlineare summarised in table 1. A schematic view of the line is shown in Figure 1.An 8 mm high, 60 mm wide stainless steel diaphragm was interposed between theexperimental chamber and the rest of the beamline vacuum system. The diaphragm, which wascoated on the side facing the chamber, was designed to prevent large photon backscattering fromthe experimental chamber. A water-cooled flange, which was also coated, closes the chamber atthe opposite side.- 3 -Pressures in the system were measured by Penning and Bayard-Alpert gauges on bothsides of a 19.1 l/s (N2-equivalent) conductance aperture (4 mm high, 54 large, on a 2 mm thickcopper disc).Pumping was provided by a 400 l s-1 ion pump and by a 2000 l s-1 Ti sublimation pumplocated between the line insulation gate valve and the aperture.An indication of the residual gas composition can be obtained by means of a quadrupolegas analyser (not calibrated) mounted between the conductance and the chamber.  A shutter anda gate valve were installed between the experimental line and the front-end on one side and thestorage ring on the other.The incident angle of the synchrotron light was usually set at approximately 25 mrad,but it could be changed remotely during the experiment by means of a stepped motor from 0 to50 mrad. Before the measurement all the line components had been baked at about 300 °C.During bake-out the coated chamber was heated only at 100 °C to desorb water vapour withoutactivating the chamber therefore preventing pumping of gas released by the rest of the system.After cooling at room temperature the total pressures were of the order of 10-11 mbar onthe pumps’ side of the system and of the order of 10-10 mbar on the chamber’s side. As usual, H2was the leading gas.The activation of the coating was performed by heating the chamber at 250 °C for about20 hours while keeping the rest of the system at room temperature. After activation, the pressurerecorded on the chamber’s side dropped by a factor of 10, becoming lower than the pressurerecorded on the pumps’ side, where the effect of activation is only small.During bombardment the chamber and the end flange were water-cooled. The samechamber and end flange could also be cooled with liquid nitrogen transported from a dewar,placed outside the main ring, by mean of a dedicated cryogenic line.3. EXPERIMENTAL RESULTSThe desorption yields of the unactivated coating were measured by bombarding thechamber for a short time (about one hour) in order to prevent unduly radiation cleaning of thewall of the chamber. The total desorption yield at the very beginning is about 10-2 molecules perphoton. Measurements performed in the past on bare stainless steel and copper gave values [7]of the same order.The higher dose data can be fitted, as usual, by a power function of the dose. Theexponential coefficient of the power law is equal to –1, which is in the upper range of thecoefficients normally obtained for standard materials (in general between -0.6 and -1)[7]. Thelargest signal intensity corresponds to that of mass 2, followed by mass 28 (3 times lower), 15,16 and 44 (each one 10 times lower).When the chamber is rotated to the straight position so that only the end flange isbombarded by photons, no changes in the total desorption flux and in the mass spectrometerpeaks were recorded.After activation a sharp decrease (about a factor 380) of the total desorption occurred asshown in Figure 2 and, as a consequence, a value of the desorption yield of about 7 x 10-6molecules per photon is obtained after only some minutes of bombardment. Such a low valuecan also be obtained with standard materials but only after months of bombardment [7].The total desorption yield after activation can again be well fitted by a power functionof the dose but with a lower exponential factor, namely -0.38. The bombardment is continued upto a total dose of 1.2 x 105 mA h. At this final value of dose, the total desorption yield is in thehigh 10-7 molecules per photon range.- 4 -The intensities of the quadrupole gas analyser peaks underwent a similar decrease afteractivation, but the ratios of the intensities before and after activation were unequal for thedifferent peaks. CO and CO2 peak intensities decreased by a factor 400, the mass 2 peak signalcorresponding to H2 was reduced 300 times while CH4, C2H6 and C3H8 typical peaks suffered adecrease by a factor 200.Bombarding the chamber in the straight position at a dose of 5 x 103 mA h resulted in a33 % decrease of the total desorbed flux. H2, CO and CO2 peaks experienced a 36 %, 15 % and a20 % reduction of their intensities. On the other hand the intensities of the CH4 peaks increasedby a factor 3.6.At the end of the measurement the chamber was cooled down to liquid nitrogen duringbombardment. The cooling was effective in reducing the total degassing by a factor two. TheCO peak intensity remained nearly constant while that of H2 was reduced by a factor two. Theratio between the H2 and the CH4 peaks, which during all the experiment were lower than 10,became as large as 40 during the liquid nitrogen cooling.4. DISCUSSIONThe decrease of the measured desorption yield of a NEG surface after activation isalways the result of two different causes. On the one hand the intrinsic desorption yield of thesurface decreases because the surface oxide layer is dissolved into the bulk during activationleaving a much cleaner surface. On the other hand molecules leaving the NEG surface have aprobability to be repumped by the surface that depends not only on the sticking probability ofthe NEG surface but also on the geometry of the system.A Monte-Carlo simulation of the D31 set-up allows the two contributions to bedisentangled if the sticking probabilities of the NEG surface can be measured independently.This independent measurement can be done only if a constant source of gas is available in thechamber. This unfortunately can be hardly carried out without perturbing the chamberenvironment and so the total gas-load during bombardment. Therefore sticking probabilityvalues obtained with a dedicated laboratory system were assumed in the simulation.It has been measured [3] that for a fully activated Ti-Zr-V surface the stickingprobabilities are in the of 10-1 for CO and 10-3 for H2 range and that even for a partially activatedsurface the CO sticking probability is always about a factor 100 larger than that of H2.As a consequence the Monte-Carlo simulation shows that CO degassing should drop bya factor larger than 2000 after activation while a reduction by a factor ranging between 5 and 50should be expected for H2 by the effect of the wall pumping alone (see Figure 3). The globalreductions have to be much larger than those calculated above due to the reduction of theintrinsic desorption.Because the measured reduction for CO is only 400 we are forced to assume that afterthe activation most of the CO molecules are generated outside the coated chamber and that thosecoming from the chamber contribute such a low amount to the gas load that they can beneglected.It can be shown that, if the desorption yields of the inactive Ti-Zr-V surface and of thestainless steel vacuum system walls are similar, about one hundredth of the total number ofphotons hitting outside the chamber gives rise to the measured reduction factors for CO.The same assumption can also be applied to H2 simply by taking into account that thecapture probability of the chamber diaphragm aperture is about 0.5 for this gas instead of about1 for CO.- 5 -Because CH4 cannot be pumped by NEG’s, its reduction factor can be ascribed only tothe reduction of the intrinsic desorption. The experimental result indicates that this is at least afactor 200.The presence of an external source of gas is substantiated by the results obtained duringthe straight direction bombardment and the liquid nitrogen cooling. Monte-Carlo simulationshows that molecules generated at the end flange have a much lower probability to escape fromthe chamber than those generated uniformly along the tube wall. For instance for a stickingprobability of 10-2 the probability to escape is more than 20 times higher in the latter case.Therefore it can be settled that by turning the chamber to the straight position while bombardinga huge decrease of the RGA signals should be recorded except for those corresponding to CH4which is not pumped. In spite of that the experimental results show only a partial decrease of thesignal. On the contrary the CH4 peaks increased probably because the accumulated dose of theend flange is lower (unfortunately by an unknown amount) than that received by the chamber.When the chamber is cooled with liquid nitrogen a decrease by about a factor 10 isexpected for the H2 peak if this gas comes entirely from the chamber because, as it is wellknown, its sticking probability increases by about a factor 10 on the cold getter surface [8].But the results show that a reduction of only a factor two is obtained, which could beexplained by the increase of the capture probability of the diaphragm from 0.5 to 1 for the gasgenerated outside the chamber.Other less direct evidence for the dominance of an external source after activation isgiven in Figure 2 where if the dose scale of the activated-case portion of the plot is compressedby about a factor 100, the slopes of the curve before and after activation are quite similar.5. CONCLUSIONSA reduction of the total degassing produced by synchrotron radiation bombardment byat least a factor 380 has been measured for a Ti-Zr-V coated stainless steel chamber afteractivation. This has been obtained in a very short time compared to that needed for theconditioning of standard materials.Some evidences exist that a large part of the remaining desorption after activation couldbe due to a small fraction of the photon flux striking outside the chamber. New measurementsare needed to consolidate this conclusions.An aluminium coated chamber will soon be tested to verify if the Ti-Zr-V filmperformances are affected by a different substrate material. In parallel 5 m long chambers will becoated and mounted in the ESRF main ring to test them in a real accelerator environment and tocollect other data that cannot be obtained in a beam line, like for example the effect of thecoating resistance on the electron beam wall impedance.- 6 -REFERENCES[1] C. Benvenuti, P. Chiggiato, F. Cicoira, and Y. L’Aminot, J. Vac. Sci. Technol. 1998; A16: 148[2] C. Benvenuti, J. M. Cazeneuve, P. Chiggiato, F. Cicoira, A. Escudeiro Santana, V. Johanek,V. Rouzinov, and J. Fraxedas, Vacuum 1999; 53: 219[3] C. Benvenuti, P. Chiggiato, P. Costa Pinto, A. Escudeiro Santana, T. D. Hedley, A. Mongelluzzo,V. Ruzinov and I. Wevers, Proceeding of EVC-6, Lyon 1999[4] P. Chiggiato, J. M. Cazaneuve and V. Johanek, CERN-EST-SM-DA/98-03 note[5] A. G. Mathewson, CERN-LEP-VA/87-63 note and R.J. Reid, Proceeding of the CERNACCELERATOR SCHOOL on VACUUM TECHNOLOGY, edited by S. Turner,Geneva: CERN99-05 Yellow Report, 1999, p.143[6] R. Kersevan, Proceedings of EPAC-98, Stockholm 1998 can be found at the URL:http://www.cern.ch/accelconf[7] C.L. Foerster, Synchrotron Radiation News 1998;11(5)  and O. Gröbner, Proceeding of the CERNACCELERATOR SCHOOL on VACUUM TECHNOLOGY, edited by S. Turner Geneva: CERN99-05 Yellow Report, 1999,  p.130-131[8] P. A. Redhead, J. P. Hobson and E. V. Kornelsen, The Physical Basis of Ultrahigh Vacuum, NewYork: American Institute of Physic, 1993, p. 401- 7 -Table 1 - Fundamental parameters of the ESRF main ring and of the D31 beamlineSynch. Radiation Critical Energy 20.5 keVAngular Acceptance 4.234 mradPhoton Flux (E>10 eV) 2.94x1015 photons s-1 mA-1e- Beam Energy 6 GeVTypical e- Beam Current 185 mA- 8 -Figure 1 - Schematic view of the D31 beamline vacuum systemSIPSIPSPGate ValveTur bomolecular  pumping groupPGPGBAGPGBAGQMAFront-End Slits  and AbsorberConductanceDiaphragmPG : Penning  Gauge BAG : Bayar d-Alpert Gauge QMA : Quadrupole Masss Analyser SIP : Sputter  Ion Pump SP : Sublimation PumpWater or liquid nitrogen  cooling circuitNEG-coated chamber- 9 -Figure 2 - Total molecular desorption yield K (N2 equivalent) of the Ti-Zr-V coated stainless steelchamber as a function of the accumulated dose before and after activation10-710-610-510-410-310-210-1100 101 102 103 104 105 106Effective desorption yield  [molecules photon-1]Dose [mA h]Activation 20 hours at 250° C= 0,38 D-1K= 4,2 x 10-5  D-0,38KDose [photons m-1]1019 1020 1021 1022 1023 1024- 10 -Figure 3 - Pressure reduction factor as a function of the coating sticking probability obtained by Monte-Carlo simulation assuming a constant gas load before and after activation.10010110210310410510610-5 10-4 10-3 10-2 10-1 100Reduction factorSticking probability

Synchrotron Radiation-Induced Desorption from a NEG-Coated Vacuum Chamber

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