Behavior of amorphous metal alloy mirrors under ion bombardment

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1 J. Plsm Fusion Res. SERIES, Vol. 8 (2009) Behvior of morphous metl lloy mirrors under ion omrdment Vldimir S. VOITSENYA*, Alexndr S. BAKAI, Alexndr F. BARDAMID 1, Vldimir G. KONOVALOV, Konstntin V. KOVTUN, Din I. NAIDENKOVA, Ivn V. RYZHKOV, Antoly F. SHTAN, Sergei I. SOLODOVCHENKO, Oleg V. TREMBACH nd Andrei A. VASIL EV Ntionl Science Center KIPT, Khrkov, Ukrine 1 Trs Shevchenko Ntionl University, Kiev, Ukrine (Received: 2 Octoer 2008 / Accepted: 28 Jnury 2009) The reflectnce of mirrors fricted from morphous metl lloys nd the inhomogeneities seen on their surfces re investigted s function of ion fluence for exposure to deuterium nd rgon plsm ions. In the cse of deuterium plsms, the uptke of deuterium s function of ion energy ws estimted. The effects of long term sputtering with rgon ions on the reflectnce nd surfce stte of morphous nd crystllized mirrors were studied. Key words: mirrors of morphous lloys, reflectnce, ion omrdment, sputtering, surfce inhomogeneities 1. Introduction One possile reson for the degrdtion of plsm fcing mirrors (first mirrors, FM) in ITER is the development of surfce microrelief due to the impct of chrge exchnge toms (CXA). The use of single crystl mirrors prevents the ppernce of step structure due to long-term CXA omrdment. However, ecuse of the grdul ccumultion of defects in the ner-surfce lyer, such mirror cn lose the dvntge of the single crystl structure with corresponding deteriortion of reflectnce. Mirrors fricted from morphous mterils my hve etter prospects for mintining smooth surfce following long-term sputtering, s they hve only nnoscle order. This ide ws first proposed in pper pulished y the uthors, Ref. [1]. Now, with development of mny different ulk morphous lloys (BAA) [2,3], there is the possiility of providing experimentl verifiction of this ide. In the pper we present results which demonstrte the ility of morphous mirror to mintin their opticl properties even following the removl of ~13.4 m due to sputtering with rgon ions. 2. Smples The experiments were crried out with mirror specimens fricted of morphous mterils prepred y: 1) Liquidmetl Technology Corportion (LTC) in the USA, 2) Institute of Solid Stte Physics, Mterils Science nd Technologies of Ntionl Science Center "Khrkov Institute of Physics nd Technology" (ISSPMT), nd 3) Hhn-Meitner-Institute (HMI) in Berlin, Germny. The elementl composition of the LTC nd ISSPMT smples is - Zr(41.2)Ti(13.8)Cu(12.5)Ni(10)Be(22.5), nd of the HMI smples is - Zr(46.8)Ti(8.2)Cu(7.5) Ni(10)Be(27.5). Due to difference in technologicl processes, in spite of similrity of composition, the mirror smples from LTC nd ISSPMT were found to hve different kinds of inhomogeneities nd little different vlue of reflectnce, s will e shown elow. All smples were disk-shped with thickness ~2.5 mm nd dimeter 22 mm (LTC nd ISSMT smples) or 5 mm (HMI smples). The smples were polished to the mirror qulity with grdully decresing rsive prticles, then wshed in cetone nd stored in ir. Before exposing the smples to deuterium or rgon plsms to see the effect of ion omrdment, the surfce of every smple ws first clened with low energy (50-60 ev) ions from deuterium plsm (typicl fluence ion/cm 2 ) to tke off the orgnic film tht ppers due to wshing in cetone. The reflectnce vlues mesured fter plsm clening were tken s the initil vlues of reflectnce for ll mirror smples. The plsm-exposure experiments were performed in the DSM-2 fcility [4], with prmeters of ECR dischrge plsm: n e ~10 10 cm -3 nd T e 5eV. Smples could e ised to produce more energetic incident ions. 3. Initil surfce chrcteristics nd opticl properties The polishing procedures exposed the existence of numer of inhomogeneities on the surfce of the morphous mterils which could e seen y oth opticl microscope nd y scnning electron microscope (SEM). In Fig.1 SEM photo otined with secondry electrons shows group of inhomogeneities on the surfce of LTC mirror smple. Altogether, these inhomogeneities occupy only smll prt of the smple surfce (1-2 %) y The Jpn Society of Plsm Science nd Nucler Fusion Reserch

They were rised little ove the mtrix surfce, ut did not noticely ffect the reflectnce t norml incidence (mesurements were done in the wvelength rnge 220-650 nm).

1) indicte the predominnce of elements with lower tomic numer in comprison with the mtrix composition; this result ws supported y the results of micronlysis shown in Tle 1. Fig.

1 SEM photos with secondry () nd ckscttered electrons () of inhomogeneities on the LTC smple surfce. Tle 1.

Dt in rckets rtios to the Zr content. Element Nominl composition, from LTC (t. %) Zr 41.2 (100%) Ti 13.8 (33.2%) Cu 12.5 (30%) Ni 10 (24%) Be 22.5 (54,9) Mtrix composition ( t. %) 52.8 (100% ) 17.

2 They were rised little ove the mtrix surfce, ut did not noticely ffect the reflectnce t norml incidence (mesurements were done in the wvelength rnge nm). The SEM photos of such inhomogeneities mde with elsticlly ckscttered electrons (Fig.1) indicte the predominnce of elements with lower tomic numer in comprison with the mtrix composition; this result ws supported y the results of micronlysis shown in Tle 1. Fig.2 SEM photos with secondry () nd ckscttered electrons () of the shllow pits oserved on the ISSPMT smple surfce. Fig.1 SEM photos with secondry () nd ckscttered electrons () of inhomogeneities on the LTC smple surfce. Tle 1. Elementl composition of the LTC mirror mteril: nominl (2 nd column) nd mesured y mens of microproe method in the mtrix nd in the inhomogeneity (without tking into ccount ). Dt in rckets rtios to the Zr content. Element Nominl composition, from LTC (t. %) Zr 41.2 (100%) Ti 13.8 (33.2%) Cu 12.5 (30%) Ni 10 (24%) Be 22.5 (54,9) Mtrix composition ( t. %) 52.8 (100% ) 17.4 (33%) 15.2 (28.8%) 14.6 (27.7%) Inhomogeneity composition ( t. %) 43.0 (100%) 12.0 (28%) 24.3 (56.5%) 20.7 (43%) The irregulrities on the surfces of ISSPMT smples were very different, eing shllow pits (~60 nm in depth) with rther flt lower surfces. Also, they were distriuted more-or-less uniformly over the surfce of smples, Fig.2). In contrst to the LTC smples, these pits were invisile in SEM photos mde with ckscttered electrons (Fig.2), indicting tht the composition of the mteril is uniform, nd thus, these irregulrities re locted only t the surfce. At present we do not hve stisfctory explntion for the ppernce of these pits; possily they re the result of the polishing procedures. Inhomogeneities lso pper on the surfce of the HMI smples, see Fig.3. The mjority of these surfce fetures pper to hve str-like structure ut re not oriented in the plne of surfce nd therefore re only prtly seen. On one occsion, the full structure of such n inhomogeneity ws reveled fter omrdment with rgon ions, Fig.3. Its shpe is very similr to tht of the crystls descried in Ref. [5]. The initil reflectnce, R, of the different morphous mirrors does not differ significntly, ut is strongly dependent on wvelength in the rnge of mesurements, Fig.4. For wvelengths exceeding ~ nm the reflectnce of the BAA mirror smples is close to the reflectnce of tungsten. 4. Experimentl results nd discussion 4.1. Uptke of deuterium In the plsm exposure experiments, smples were exposed to deuterium or rgon ions without mss seprtion of the ion flux. In the cse of deuterium plsms, smll mount of wter nd nitrogen were 1380

Becuse of deuterium sorption, weight gin ws oserved for ion energies elow 1000 ev, even when there ws cler indiction of sputter erosion.

3 It ws found tht mirror smples of ll mterils sor lrge mounts of deuterium when the mirror specimens were omrded with ions of deuterium plsm. As mesured y the lser ltion technique, deuterium ws found t depths much lrger thn the clculted D + ion rnge [7] following exposure of the LTC specimen to fluence of ion/cm 2. Becuse of deuterium sorption, weight gin ws oserved for ion energies elow 1000 ev, even when there ws cler indiction of sputter erosion. The frction of retined deuterium ws estimted for low ion energies ( ev) where the ion sputter erosion ws thought to e negligile. In doing this estimtion, we did not tke into ccount tht in the SHF dischrge there re D + 2 nd D + 3 ions s well s D + (e.g., [8]), therefore the totl tom flux will e somewht lrger thn the ion flux. Fig.5 shows the rise of the portion of trpped deuterium with ion energy over this energy rnge. 25 Fig.3 SEM photos of the polished mirror surfce of HMI smple fter exposure to ions of deuterium plsm with energy 60 ev (), nd fter sputtering with rgon ions of the lyer ~5 m in thickness (). registered y vcuum nlyzer, ut with rgon s the working gs only trces of these components were oserved. The wter-cooled mirror holder ws negtively ised to ccelerte ions either to fixed energy ( ev) or to n energy vrile with time (with frequency 50 Hz) in the rnge ev [4]. Reflectnce, % W Mo AA Kh AK Kh 30 AA L Wvelength, nm AA B Fig.4 The spectrl reflectnce of the morphous mirror smples in comprison with the reflectnce of Mo nd W mirrors [6]. Arevitions ner curves men: AAL LMT smple, AAKh ISSPMT smple, AAB HMI smple, AK Kh crystllized ISSPMT smple. Implnttion, D/ion, % Accelerted voltge, V Fig.5 Energy dependence of the mount of deuterium retined y one of the LTC mirror smples for n incident ion fluence of ion/cm 2. The sputtering yield ws estimted y mesuring the depth of the hole which ppered following omrdment through n 8 mm perture or through nickel metl grid. The yield ws somewht higher thn the sputter yield of pure zirconium - the heviest component of the lloy Effect of ion omrdment on reflectnce The omrdment of smples with kev deuterium ions results in shrp drop of reflectnce even fter quite short exposure times, with the extent of the decrement depending on the wvelength: the shorter the wvelength the deeper the drop. Results of n experiment where LTC mirror ws in series exposed to ions with wide energy distriuted or to low energy ions (60 ev) re shown in Fig.6. Here the reflectnce t three wvelengths is plotted s function of the totl ion fluence (fluence of wide energy distriution ions + fluence of low energy ions). The lue points (dotted curves) were otined fter exposure to ions with wide energy distriution nd the red points (solid curves) fter exposure to the low energy 1381

Reflectnce, % 50 40 30 20 10 650nm 450nm 250nm 4.3. Modifiction of the surfce For the deuterium plsm-exposed LTC specimens (experiments of Fig.

4 Reflectnce, % nm 450nm 250nm 4.3. Modifiction of the surfce For the deuterium plsm-exposed LTC specimens (experiments of Fig.6), the thickness of sputter eroded lyer t the end of the experiment ws estimted to e ~2 m [7]. Most of the surfce re remined smooth, ut the inhomogeneities shown in Fig.1 were trnsformed into shllow pits with depth of m s mesured with n interference microscope, Fig Totl ion fluence, cm -2 Fig.6 Dependence of reflectnce on the totl ion fluence t the indicted wvelengths for LTC smple. The totl fluence is the sum of fluences of wide energy distriution ions nd 60 ev ions. The dt for exposures to wide energy distriution ions dotted curves, nd to 60 ev ions solid curves. ions. The first point for every curve represents the initil reflectnce (fter short clening with 60 ev ions). A short exposure to the wide energy distriution ions resulted in significnt drop of reflectnce (second point on every curve fter the initil point). However, it is seen tht fter n initil trnsient (up to ~ ion/cm 2 ), the long-term exposures to low energy ions (solid lines) leds to grdul increse of reflectnce lmost to the initil level (t fluences ~ ion/cm 2 nd ~ ion/cm 2 ). The followed exposures to the more energetic ions (t fluences ~ ion/cm 2 nd ~ ion/cm 2 ) gin rought reflectnce to low vlues (similr to the level of the first drop). Qulittively similr reflectnce ehvior ws oserved for ISSPMT mirror smples when they were sujected to deuterium plsm ions of ~1 kev energy nd then to ions with 60 ev energy. The ility to restore the reflectnce of BAA mirror smples my indicte tht the deteriortion of reflectnce under energetic ion impct is not due to the development of surfce roughness, ut due to some chemicl processes on the surfce. A similr restortion of reflectnce due to exposure to low energy deuterium ions following high energy ion omrdment ws oserved in similr experiments with Be mirrors [9]. In tht pper it ws hypothesized tht when the eryllium surfce is exposed to deuterium ions in n tmosphere contminted with wter vpor, the norml BeO surfce lyer cn e prtilly trnsformed into the Be(OD) 2 with corresponding decrese in reflectnce. Becuse the current BAA mirror smples contin significnt portion of eryllium (22.5 nd 27.5 %), it would not e surprising to hve similr effect tke plce with these smples. Fig.7 The interferogrm of one of the pits ppered fter LTC smple ws sputtered to the depth ~2 m y deuterium plsm. It is seen tht the pit is out 0.25 m in depth nd hs flt ottom. The fct tht the rte of sputtering of the inhomogeneities is higher thn for the mtrix is in good qulittive greement with the results of micronlysis showing tht the elementl composition inside these inhomogeneities hs lowered portion of zirconium nd incresed portions of copper nd nickel (Tle 1). Higher sputtering yields re expected for the lower mss elements. In ddition, prt of the LTC specimen surfce (~15%) omrded with 300 ev deuterium ions developed lister-like surfce fetures with dimeter from few μm up to ~60 μm, Figs. 8 [10]. Such rtifcts were not found on the surfce of other LTC smples fter exposure to the sme or other ion energies. We thus suggest tht possily the prt of the LTC smples where the listerlike surfce fetures ppered, hd some unique peculirity in the structure of the mteril. These rtifcts strongly differ from the clssicl listers oserved in [11,12] when metl glsses were omrded with high energy He + nd H + ions. Nmely, it ws found [10] tht independently of size, the lister-like surfce fetures re plne or hve the plne pedestl with thin cupol-like roof, hve nnulr groove nd no one ws found to e urst Comprison of the reflectnce ehvior of morphous nd crystllized mirrors To void the prolems which pper to e relted to chemicl processes on the mirror surfce nd to shorten the totl exposure time, experiments directed to checking 1382

the ide of [1] were performed on ISSMT mirror smples with rgon ions. The experiment ws performed with mirror smples fricted of the sme mteril ut with different structure: morphous nd crystlline.

SEM photos of the surfce of ZrTiCuNiBe crystllized () nd morphous () mirror smples (ISSPMT) fter long term omrdment with Ar + ions under similr experimentl conditions.

8 SEM photos with secondry () nd ckscttered electrons () of lister-like surfce fetures on the LTC smple surfce fter omrdment with 300 ev deuterium ions.

9 Results of x-ry diffrction mesurements otined with mirrors fricted of two hlves of the sme ISSMT smple: the un-nneled morphous smple nd the smple nneled t 773 K for 1 hour.

5 the ide of [1] were performed on ISSMT mirror smples with rgon ions. The experiment ws performed with mirror smples fricted of the sme mteril ut with different structure: morphous nd crystlline. Three ISSMT disk smples were cut pproximtely in equl hlves, with one hlf of ech disk left morphous while the second hlf ws nneled t 773 K for 1 hour. Fig.10. SEM photos of the surfce of ZrTiCuNiBe crystllized () nd morphous () mirror smples (ISSPMT) fter long term omrdment with Ar + ions under similr experimentl conditions. The nneled smples developed crystlline structure, s demonstrted in Fig.9. Intensity, rel. units Fig.8 SEM photos with secondry () nd ckscttered electrons () of lister-like surfce fetures on the LTC smple surfce fter omrdment with 300 ev deuterium ions. fter nneling s-cst , degrees Fig.9 Results of x-ry diffrction mesurements otined with mirrors fricted of two hlves of the sme ISSMT smple: the un-nneled morphous smple nd the smple nneled t 773 K for 1 hour. Both mirrors were repetedly exposed to rgon plsms under similr experimentl conditions, with ion energies rnging from ev. After the first series of plsm exposures, the crystllized specimen weight-loss corresponded to n eroded lyer thickness of out 8.9 μm in comprison to 6.8 μm for the morphous specimen. In ddition, the surfce of the crystllized mirror developed surfce roughness tht ws vrile long the specimen surfce. At the sme time, the surfce of morphous specimen continued to e smooth; even the pits (Fig.2) ecme much shllower thn seen initilly. Fig.10, shows the prt of crystllized specimen surfce with high roughness, while Fig.10 is typicl of ny prt of the morphous specimen surfce. Further exposures to 1 kev rgon ions ws crried out with totl sputtering of the crystllized smple y ~21 m nd of the morphous smple ~13.5 m. For oth smples no significnt chnge in the surfce roughness ws oserved, in comprison with tht shown in Fig.10. The reflectnce of crystllized smple significntly decresed fter this second series of exposures ut the morphous mirror mintined its initil reflectnce without ny mesurle chnge. When omrded with 1 kev deuterium ions, weight loss ws oserved for the crystllized mirror, in contrst to its morphous counterprt hlf, which showed no mesurle weight chnge, presumly due to deuterium uptke. A preliminry estimtion, sed on the 1383

6 supposition of equl ion sputtering rtes for oth mirrors, gives deuterium retention rte of out 30% for the morphous mirror. 5. Conclusion All morphous smples sored lrge frctions of deuterium when omrded with deuterium plsm ions, with the frction sored incresing with incresing ion energy in the rnge ev (for LTC smples). On the surfce of one LTC smple, lister-like rtifcts were found which differ strongly from the clssicl listers nd which hve not een oserved for other mterils [10]. Under omrdment with deuterium ions, the reflectnce ehvior of the mirrors is qulittively similr to the ehvior of Be mirror smples, proly due to similr chemicl processes on the surfce, i.e., the prtil trnsformtion of eryllium oxide into eryllium hydroxide. Following long term sputtering with Ar + ions, the morphous mirror smples demonstrte the ility to mintin the good opticl properties in contrst to crystllized mirror smples of the sme mteril, thus supporting the ssumption mde in [1]. Acknowledgments This work ws prtly supported y STCU Grnt #3668. The uthors cknowledge the ssistnce of Dr. J.W. Dvis (University of Toronto) in prepring the English version of the mnuscript. References 1. V.S.Voitseny et l, Rev. Scient. Instrum. 70, 790 (1999). 2. W.L.Johnson. JOM Journl of the Minerls, Metls nd Mterils Society (Mrch 2002) A.Inoue nd A.Tkeuchi. Mteril Science nd Engineer. A , 16 (2004). 4. A.F.Brdmid et l, Surf. Cotings Technol , 365 (1998). 5. N.Wnderk et l, Mter. Sci. Forum , 365 (1999). 6. E.D. Plik (Editor), Hndook of Opticl Constnts of Solids (Acdemic Press, 1985). 7. A.F.Brdmid et l, Physic Script T123, 89 (2006). 8. K.S. Golovnivsky et l, Soviet J. Plsm Phys. 7, 324 (1981). 9. V.S. Voitseny et l, J. Nucl. Mter , 1476 (2004). 10. A.F. Brdmid et l, J. Nucl. Mter. 376, 125 (2008). 11. A.K.Tygi nd R.V.Nndedkr. J. Nucl. Mter. 132, 62 (1985). 12. A.K.Tygi nd R.V.Nndedkr. J. Nucl. Mter. 148, 22 (1987). 1384

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