During heavy ion operation in several particle accelerators world-wide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion induced gas desorption scales with the electronic energy loss (dE{sub e}/dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering

Bender, M

Bieniosek, F M

Covo, M K

Kollmus, H

Kramer, A

Kwan, J

Mahner, E

Molvik, A W

Prost, L

Seidl, P A

Westenskow, G

UNT Digital Library

UCRL-JRNL-225817Electronic Desorption of gas frommetalsA. W. Molvik, H. Kollmus, E. Mahner, M. Kireeff Covo,M. Bender, F. M. Bieniosek, A. Kramer, J. Kwan, L.Prost, P. A. Seidl, G. WestenskowNovember 3, 2006Physical Review LettersDisclaimer   This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  1Electronic desorption of gas from metalsA.W. Molvik,1,2 H. Kollmus,3 E. Mahner,4 M. Kireeff Covo,1,2 M. Bender,3 F. M.Bieniosek,1,5 A. Krämer,3 J. Kwan,1,5 L. Prost,6,5 P. A. Seidl,1,5 G. Westenskow,1,21Heavy-Ion Fusion Science Virtual National Laboratory, Berkeley, CA 94720, USA2Lawrence Livermore National Laboratory, Livermore, CA 94550, USA3GSI, Planckstr. 1, 64291 Darmstadt, Germany4CERN, 1211 Geneva 23, Switzerland5Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA6Now at FermiLab, Batavia, IL 60510-0500, USA  AbstractDuring heavy ion operation in several particle accelerators world-wide, dynamic pressure risesof orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamberwalls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriouslylimit the ion beam lifetime and intensity of the accelerator. From dedicated test standexperiments we have discovered that heavy-ion induced gas desorption scales with theelectronic energy loss (dEe/dx) of the ions slowing down in matter; but it varies only little withthe ion impact angle, unlike electronic sputtering.PACS Numbers: 41.75.Ak, 34.50.Dy, 79.20.Rf, 68.45.Da, 07.30.KfEnergetic ions incident on matter sputter target material and also desorb gas from the targetsurface. The sputter and desorption yields (number of sputtered or desorbed particles per ionimpact) are known to be linked to the energy loss of the projectile inside the target. Two energyloss regimes, nuclear and electronic, have been known for decades. An example for potassiumions impacting onto stainless steel, calculated with the SRIM code [1], is shown in Fig. 1. Herefor low projectile energies the nuclear energy loss dominates and for higher energies theelectronic energy loss dominates the total energy loss. Sputter and desorption yieldmeasurements from ~1 µm thick targets in the regime of electronic energy loss have shown thatboth scale with the electronic energy loss (dEe/dx)n  to the power of n = 1-3. This was observedfor targets of frozen gases [2-5], for sputtering from micron-thick coatings of protein [6,7], andfor desorption of nitrogen from a conductor (carbon) by 6-13 MeV/u ions (u ≡ nucleon mass)[8,9]. Electronic sputtering and desorption from ~1 µm thick targets is found to vary with theion impact angle from normal, θ, as 1/cosm(θ) to the first or higher power of m [5,10].Our work was motivated by the copious gas desorption that results from lost heavy ionsstriking particle accelerator vacuum chambers leading to dynamic pressure rises which limit thebeam intensity in a number of heavy-ion accelerators [11]. Related work dates back more than30 years, when a vacuum instability in the Intersecting Storage Rings (ISR) at CERN wasidentified above a critical beam current [12]. Recent requirements for orders of magnitudeincrease in beam intensity have motivated our search for further understanding and mitigationmechanisms. In preparation for the heavy ion program of the Large Hadron Collider at CERN,beam-loss induced molecular desorption was intensively studied in ultra-high vacuum chambersat CERN’s Heavy Ion Accelerator (LINAC 3) [13] and the Super Proton Synchrotron (SPS)2[14]. Large effective desorption yields of up to 2 × 104 molecules/Pb53+ ion (4.2 MeV/u) and 3.7× 104 molecules/In49+ ion (158 GeV/u) were measured for ions impacting under various angles (θ= 0° (perpendicular), 84.8°, 89.2° at LINAC 3, and 88.3° at the SPS) onto stainless steelsamples which were chemically cleaned, 950°C vacuum fired, and in situ baked at 300° C.Similar yields of 1.1 – 2.7 × 104 molecules/ion were measured at the GSI Darmstadt SIS 18for 8.9 MeV/u U28+ ions impacting near grazing incidence onto baked stainless steel vacuumchambers [15]. In a dedicated test stand (GSI-HLI) yields were 300 for 1.4 MeV/u Cr7+ and1900 for 1.4 MeV/u Pb27+ perpendicularly impacting on stainless steel. In the High-CurrentExperiment (HCX) at LBNL, gas desorption coefficients of 9000 – 14000 molecules/ion werealso measured for low energy 0.025 MeV/u K+ ions incident at grazing angles (θ = 80° - 88°)onto chemically-cleaned stainless steel baked at 220° C in a high-vacuum chamber [16].Enhanced desorption by energetic ions may impact other areas. In magnetic-fusiontokamaks, wall desorption is dominated by physical and chemical sputtering from low energyion bombardment [17]. However some instabilities carry energetic ions to the wall [18],resulting in the more copious electronic desorption. In astrophysics, cosmic ray desorption ofgas condensed on dust surfaces can affect gas densities in nebulae, and increase the temperaturerange over which the normally non-reactive, H2 and CO on iron dust may form CH4 and otherprebiotic molecules [19], as are observed in the Orion nebula [20].All previous molecular desorption yield measurements by heavy-ion beams used fixed ionenergy at each facility. Here, molecular desorption from accelerator-type stainless steel isinvestigated for a range of ion energies  at GSI Darmstadt and at Lawrence Livermore NationalLaboratory (LLNL), plus a range of angles of incidence at LLNL. We found the desorption yieldscales with the electronic energy loss (dEe/dx)n, with n between the first and second power, as inmany electronic sputtering studies. However, the desorption dependence on θ is significantlyless than 1/cos(θ), contrasting with previous studies.In order to study the link between molecular desorption and the electronic energy loss of theprojectiles in the target, we have measured desorption yields in two energy regimes:(1) A high-energy regime well above the peak of the electronic energy loss, the so-calledBragg-Peak, where dEe/dx decreases with increasing energy. This beam was available atthe SIS18 of GSI Darmstadt [21]. A low-energy regime (70 to 1000 keV with K+ ions)for which nuclear slowing dominates below and electronic energy loss dominates above250 keV, as shown in Fig. 1. The opposite slopes and nearly constant sum provide thepossibility that the energy scaling of desorption will unambiguously link to onecomponent. The low-energy beam was available at the STS-500 Ion Source Test Stand[22] at LLNL which provided 70 – 400 keV K+ ions. The High-Current Experiment(HCX) at Lawrence Berkeley National Laboratory (LBNL) delivered K+ ions at ~1000keV [16,23].The measurement technique and experimental setups have been described in detail elsewhere[13,16,24]. Briefly, we determine desorption coefficients by measuring the pressure rise Δp in aknown volume V, or with pumping speed S, from which we calculate the number of desorbedmolecules. A Faraday cup for the low-energy range, or a current transformer for the high-energyrange, measures the beam current transmitted through the entrance aperture, which we integrateto determine the total number of ions N or ion flux N& incident on a stainless steel target. The3ion charge state is known from the accelerator. The effective ion-induced desorption yield ηeff(molecules/ion) is given byTkNVpbeff ⋅⋅⋅Δ=ηfor a single shot of ions (low-energy range) or byTkNSpbeff ⋅⋅⋅Δ= &ηfor repetitive ion bombardment (high-energy range), where  kb is the Boltzmann constant, and Tis the temperature. The data shown here are all derived from pressure rises measured with iongauges calibrated for nitrogen. With “effective“ desorption yield we point out that all measuredvalues from one experimental setup can be compared, since the same systematic errors are validfor all data points. The comparison of desorption yields from different test stands (differentconductances, volumes, and vacuum instrumentation like pumps, gauges, and residual gasanalyzers), can only be done with extreme care and detailed knowledge, and is not attemptedhere.The effective ion-induced desorption yield has been measured for K+ ions as a function ofthe impact angle in the energy range of 68 – 972 keV, as shown in Fig. 2. The experimentalsetup has been described in Ref.16.In this low-energy range experiment the desorption yield increased with rising projectileenergy. We measured that grazing incidence of 89° yields more desorbed molecules than the lessgrazing incidence of 81.5°, but only by the surprisingly small factor of ~1.5 as one can see fromFig. 2. If ion-induced desorption were to scale with (1/cos(θ))m, with m ≥ 1, as in electronicdesorption and sputtering from thick layers, the increase over this range of angles would be afactor of  ≥ 8.5.A similar angular dependence was previously reported from CERN measurements using 4.2MeV/u lead ions incident onto stainless steel, i.e., desorption coefficients between 84.8° and89.2° varied only by a factor of ~2.4 for Pb53+ and a factor of ~4.5 for Pb27+ rather than a valueof 6.5 expected from a 1/cos(θ) variation [13]. The measured desorption coefficients vary as afunction of ion impact as follows: for ions onto stainless steel, we obtain m=0.12±0.06 forPb53+, and m = 0.31±0.26 for Pb27+, based on data in Table III of Ref. 13, whereas for electronicsputtering by ions onto insulators one finds 1≤m<2 [5,10].We have not corrected for ion reflection near grazing incidence. The SRIM code [1] predictsthat ~70% of 1 MeV (0.025 MeV/u) potassium [16] and ~30% of 4.2 MeV/u lead ions willreflect, then hit a downstream surface causing additional desorption. A correction for reflectionswould make desorption even more independent of angle of incidence.In order to analyze our data as a function of beam energy and therefore as a function of thetotal energy loss of the potassium ions in stainless steel, we averaged desorption yields over anarrow range of angles 84.0° ≤ θ ≤ 85.5°, as plotted in Fig. 3. The choice of angles has only asmall effect on the energy scaling as the measured desorption yields for 68 – 393 keV potassiumions do not significantly change for impact angles between 81.5° and 85.5° (see Fig. 2); althoughthe data for 972 keV ions show a greater change. The investigated potassium energy regime isbelow the Bragg-Peak and one can clearly see from Fig. 3 that the ion-induced desorption yield4is increasing with increasing projectile energy following the trend of the electronic energy lossonly.The effective ion-induced desorption yield has been measured in the high energy regimeusing U73+ heavy ions with energies of 15, 40, and 100 MeV/u bombarding a stainless steel targetunder perpendicular incidence (θ = 0°). Further details of this work are available elsewhere [24].A summary of the results obtained is displayed in Fig. 4. Here, we are well above the Bragg-Peak and one can observe that desorption yields again follow the trend of the electronic energyloss, both are decreasing for increasing projectile energy. Both low and high ion-energydesorption yields are plotted in Fig. 5 versus the calculated electronic energy loss dEe/dx.The slopes of the dashed lines in Fig. 5 indicate the power n of the ion-induced desorptionscaling with (dEe/dx)n. Both the low energy K+ and the high energy U73+ results scale roughlywith n=2. Yields of 103 – 104 molecules/K+ ion and 102 – 103 molecules/U73+ ion have beenmeasured in two different experiments with both showing a dependence of the moleculardesorption yield on the electronic energy loss dEe/dx.The measured dependence on dEe/dx limits which models of desorption can be applicable.Elastic thermal spike models yield n=2; shock wave models n=1.5, and excitation model n=1[25]. With desorption from adsorbed gas layers on the surface of metal, one expects rapidelectron cooling by conduction electrons, with inelastic energy absorption by the insulatingsurface gas layers, for which inelastic thermal spike modes, that allow coupling betweenelectrons and the lattice, may prove more appropriate [25].Surface properties of stainless steel play a crucial role for the desorption coefficients [13,26]. The influence of various surface preparation techniques like vacuum firing at 950°C, glowdischarge cleaning with Ar-O2 and He-O2, in situ baking, chemical etching, electropolishing, andcoatings with noble metals (Au, Ag, Pd) or nonevaporable getters (TiZrV) had been previouslystudied at CERN and found to affect desorption significantly [13, 26]. Our results for ionsincident on metals show a scaling of desorption with the first to second power of electronicenergy loss, similar to that seen for ions incident on frozen gases. However, we observesignificantly less yield variation with the angle of incidence, than the 1/cos(θ) seen previouslyfor sputter measurements [5,10]. It is important to mention that all electronic energy losscalculations, using the SRIM code [1], were performed for clean stainless steel targets, nottaking into account the natural oxide layer or any surface layer contamination like carbon forexample. But exactly this surface layer contamination is predominantly desorbed (H2, CH4, CO,CO2) under ion bombardment and scales – as shown – with the electronic energy loss of theprojectile in the target.It is apparent from this work that ion-induced desorption of surface adsorbates is moderatedby the energy given to electrons in the target material, but further work is required to determinethe details of the mechanism(s). The identification of electronic desorption as the mechanism forgas desorption from metals by energetic heavy ions is a significant step towards understandingion-induced desorption in accelerators, and more effective mitigation. The data presented hereplace severe constraints on detailed models of the underlying physical mechanisms and thesources of gas.Acknowledgments5AWM thanks T. Schenkel for introducing him to the concept and literature of electronicsputtering, E. Bringa and S. Kucheyev for additional discussions, W. van Breugel and A. G. G.M. Tielens for astrophysical implications, and B. G. Logan for support. Experimentaloperations at LLNL were ably supported by G. Freeze and R. Hall, and at LBNL by R. Hipple,W. Strelo, R. Beggs, T. Katayanagi, C. Rogers, and E. Romero II. This work performed underthe auspices of the U.S Department of Energy by University of California, Lawrence Livermoreand Lawrence Berkeley National Laboratories under contracts No. W-7405-Eng-48 and DE-AC02-05CH11231. HK MB AK and EM would like to acknowledge the support from the GSIaccelerator crew providing an excellent beam and E. Hedlund and L. Westerberg for their supportduring beam time.References1. J. F. Ziegler, http://www.srim.org/.2. R. E. Johnson, Rev. Mod. Phys. 68, 305 (1996).3. W. L. Brown et al., Phys. Rev. Lett. 45, 1632 (1980).4. W. L. Brown et al., Nucl. Instrum. Methods B 14, 392 (1986).5. R. E. Johnson et al., Surf. Sci. 179, 187 (1987).6. A. Hedin, et al, Phys. Rev. B 35, 7377 (1987).7. K. Wien, Radiat. Eff. and Defects in Solids 109, 137 (1989).8. M. Caron, et al., Nucl. Instrum. Methods B 179, 167 (2001).9. M. Caron, H. Rothard, A. Clouvas, Surf. Sci. 528, 103 (2003).10. E. M. Bringa, R. E. Johnson, Nucl. Instrum. Methods B 180, 99 (2001).11. S. Y. Zhang, “ICFA Workshop on Beam Induced Pressure Rise in Rings,” http://www.c-ad.bnl.gov/icfa  (2003).12. O. Gröbner and R. S. Calder, Proc. IEEE  Trans. Nucl. Sci. 20, 760 (1973).13. E. Mahner, et al, Phys. Rev. ST AB 6, 013201 (2003).14. E. Mahner et al., Phys. Rev. ST AB 7, 103202 (2004).15. E. Mustafin et al., Nucl. Instrum. Methods A 510, 199, (2003).16. A. W. Molvik et al., Phys. Rev. ST AB 7,  093202 (2004).17. G. F. Matthews, J. Nucl. Mater. 337-339, 1 (2005).18. W. W. Heidbrink, G. Sadler, Nucl. Fusion 34, 535 (1994).19. M. Kress, A. G. G. M. Tielens, Meteoritics and Planetary Sciences 36, 75, (2001).20. G. J. White et al., Astron. Astrophys. 407, 589 (2003).21. P. Spiller and G. Franchetti, Nucl. Instrum. Methods A 561, 305 (2006).22. J. W. Kwan et al., Nucl. Instrum. Methods A 544, 134 (2005).23. L. R. Prost et al., Phys. Rev. ST AB 8, 020101 (2005).24. H. Kollmus, et al., Proc. of the 2006 European Particle Accelerator Conference, currentlyavailable at http://oraweb.cern.ch/pls/epac06/toc.htm as paper TUPCH174. Subsequentlyavailable at JACOW Website.625. M. Toulemonde, et al., Nucl. Instrum. Methods B 212, 346 (2003).26. E. Mahner, et al, PRST-AB 8, 053201 (2005).List of figures & captionsFigure 1. Potassium ion energy loss in stainless steel calculated using the SRIM code, thenuclear and electronic energy losses are displayed.7Figure 2. Effective desorption yields of potassium ions bombarding stainless steel at variousenergies and impact angles. For normal incidence, θ  = 0°. Random variations of ±300 areindicated by shot-to-shot scatter.8Figure 3. Nuclear and electronic energy loss for potassium ions in stainless steel calculated withthe SRIM code [1], the upper line is the sum of both. The ion desorption yields (forcomparison multiplied by a factor of 10-4) measured for impact angles (θ = 84.0 - 85.5°). Thedesorption yield error bars (±300) take into account the observed fluctuations in ion beamcurrent and pressure rise which were similar for all experiments.9Figure 4. Desorption yields measured for 15, 40, and 100 MeV/u U73+ ions impactingperpendicularly onto stainless steel. For comparison, the evolution of the electronic energy losscalculated with SRIM [1] is also shown.10Figure 5. Measured ion induced desorption yields versus electronic energy loss of thecorresponding ion (potassium and uranium); square points: data for K+ ions with energiesbetween 70 and 1000 keV impacting (θ = 84.0 - 85.5°) onto stainless steel; diamond points: SIS18 data for U73+ ions with energies 15, 40, and 100 MeV/u impacting perpendicularly (θ = 0°)onto stainless steel. The dashed curves indicate the power law (n value) for the electroniccomponent of dE/dx.

Electronic Desorption of gas from metals

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Electronic Desorption of gas from metals

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