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Surface deformation due to high fluence helium irradiation
Vacuum/volume 37/numbers 1/2/pages 41 to 45/1987
0042-207X/8753.00+.00 Pergamon Journals Ltd
Printed in Great Britain
Surface deformation due to high fluence helium irradiation G Mezey, F Pfiszti, L Pog6ny, M Fried, A Manuaba, G Vizkelethy, Cs Hajdu and E Kbtai, Central Research Institute for Physics, H- 1525 Budapest 114, PO Box 49, Hungary
During the past ten years a new branch of materials science has been developed. This field is called plasmasurface interaction (PSI). PSI itself is a broad field of increasing importance and includes both in situ impurity observations in different machines (i.e. deposition and erosion probes) and model experiments to understand the basic processes such as physical and chemical sputtering and recycling. This paper makes an attempt to summarize the results concerning surface deformations (blistering, exfoliation, flaking) and special pattern formation accompanying them. Mainly of the effects due to high energy bombardment (0.8-3.52 MeV) are reviewed. A theoretical explanation based on a stress model for the pattern formation is also discussed.
1. Introduction
Extremely high fluences of particles (D, T, He, electrons, fast neutrons, medium and heavy mass ions) and photons will hit the first wall components of a fusion type reactor (CTR machine). Approximately 60 elementary processes take place on the walls. Of course, their relative importance in a given machine will depend upon the operation regime and the location of the first wall component inside the machine. For example, limiters, divertor plates or shields are mainly subjected to erosion type processes. The common feature of the elementary processes is that they tend to contaminate the plasma by impurity production and recycling; introducing cold working gas atoms into the plasma. These processes either tend to prevent ignition in the existing big tokamaks (TFTR, JET, JT-60) or diminish the future reactor efficiency. In the past ten years a new branch of materials science which is called plasma surface interaction (PSI) has been developed. PSI studies have concentrated on the above mentioned problems. As the building of the first demonstrator type reactor machine approaches, the importance of this research is increasing 1,2. Roughly one can divide the PSI studies into two groups: - - i n situ observations in present-day machines (erosion and deposition probes, sample transfer systems, surface analysis stations, etc.) 3 7. --model experiments for both the understanding of the mechanism of elementary processes and for seeking new classes of materials together with an optimum first wall structure (for example the honeycomb structure) to minimize the erosion and recycling and improve the tritium inventory. Most of the PSI studies should be carried out in close cooperation with plasma physicists and construction engineers. But for model experiments a relative independence is necessary for better understanding of the physics of elementary processes since
a firm basis will help a lot in 'tailoring' the first wall components in the future controlled thermonuclear reactors. This paper tries to outline experiments concerning a narrow field of PSI, namely surface deformations induced by high fluence helium implantation. Some experiments which are reviewed here will, at first sight, be peculiar as the targets were not chosen among the 'candidate' first wall materials of fusion reactors but experiments made on these materials could contribute to a better understanding of the evolution of surface deformations and their contribution to surface erosion and plasma contamination. The model experiments have an obvious danger because equally an over or underestimation of the real physical situation could take place since in tokamaks more than one process affects the first wall simultaneously. So the relevance of results to the practical situation (tokamak conditions) should always be investigated.
2. Helium generation rate and its effect on surface deformation
In the plasma, deuterium and tritium will react producing 14.1 MeV neutrons and 3.52 MeV alpha particles. The neutrons will leave the plasma and go directly to the wall of the vacuum vessel, while the alpha particle trajectories will be guided by the magnetic field and they will mostly be trapped in the plasma. A small fraction of the alpha particles, typically a few per cent, will be created at such location of the plasma and with such direction of motion (loss cones) that they will drift with nearly their full energy to the vessel wall. The other part of the alpha particles will transfer their energy to plasma ions and electrons. The thermalized alpha particles will arrive at the wall presumably with an energy corresponding to the temperature of the plasma boundary. If the mean energy of the plasma boundary stays below some 100 eV, the He particles hitting the vessel wall with this energy will 41
G Mezey et al. Surface deformation due to high fluence helium irradiation
mainly contribute to wall sputtering. However, the He particles arriving at the wall with their full energy can cause heavy surface modifications like blistering, exfoliation, flaking and spongy structures. In order to estimate this damage the fluence, the current density, the energy and angular distribution of He particles have to be known. The behaviour of the particles produced in a magnetically confined fusion plasma has been studied extensively for tokamak reactors 9 ,4 For an estimate of wall damage in a fusion reactor it was assumed I s (i) the He ions go to the vessel wall in the same way as the hydrogen plasma, (ii) the fractional burn up of tritium and deuterium is about 5%, (iii) the recycling can be neglected and (iv) the He flux is uniformly distributed over the first wall. This gives a composition for the wall bombardment of about 5% He 47.5% T and 47.5 D. The total He flux can be estimated from today's fusion reactor design studies. Here, generally, an average neutron load to the vessel walls of 1.3 to 2 M W m 2 is taken. This corresponds to an average 14.1 MeV neutron current of about 0.6-1 × 10 TM n m 2 s 1, which also gives the average number of alpha particles going to the first wall. Recycling may increase the He flux by more than a factor of 2. This estimation suggests that blistering will appear in the first 2-10 days of the reactor operation. The available simulation techniques have already demonstrated that the helium generation (or introduction) rate has a decisive effect on the cavity structure and hence on the whole microstructural evolution of the reactor wall. For example, the weakening of grain boundaries is very sensitive to the He generation rate via the effect of He generation rate on cavity structure, particularly on the width of the cavity-denuded zone from which He diffuses to the boundaries. For the simulation of radiation effects, where the He generation or introduction rate is an important parameter, it is obviously essential that He is introduced continuously (rather than by some pre-implantation procedure). In the characterization and evaluation of a simulation technique the ratio between helium generation and displacement damage rate (the appm to dpa ratio) is often taken to be the most important single parameter. On the other hand the particular importance of this parameter has been questioned on theoretical as well as experimental grounds. It has been argued that, for many damage accumulation processes the individual rates are more important than their ratio. It appears that the above mentioned grain boundary weakening (by preferential nucleation and growth of He bubbles) is mainly governed by the He generation rate and less by displacement damage s . 3. Surface deformation due to He bombardment The first consequence of the high fluence of helium implantation into the first wall is the appearance of bubbles near to the projected range 16 18. O n increasing the helium f l u e n c e ~ e p e n d ing on the experimental conditions (target composition, bombarding flux, energy, target temperature)--three basic types of surface deformation will develop. (i) Blistering. This formation dominates in the low energy region (10 keV-1 MeV depending upon the target material and temperature). (ii) Exfoliation is a large formation covering almost the whole implanted spot. (iii) Flaking takes place when almost the total area of the implanted spot leaves the surface with practically uniform thickness. 42
In the model experiments monoenergetic helium beams with perpendicular angle of incidence are the most frequently used for implantation. Two fundamental theoretical models have been developed to explain blister formation: the gas pressure model and the stress model. The first direct demonstration of the presence of gas in bubbles, hence in blisters was done by Kaminsky ~8, who used a mass spectrometer to detect bursts of He gas released from ruptured blisters, This was subsequently confirmed by several authors2O 23 Blistering was initially attributed to a sudden coalescence of bubbles within the range distribution of the incident ions. This coalescence would lead to a high pressure exerted on the surface 24'25. The idea was developed in greater detail. Erents and McCracken 2~, for example, assumed that all the implanted gas was collected in the coalescence process. They found that the pressure inside a blister was directly proportional to the total dose of implanted helium and to the average depth of the void in the solid. Their calculations suggested that a proportionality between the blister radius and the energy of the incident ions should be expected. Their own experiments indicated that the expected relationship was approximately correct. The experimental evidence on which the gas pressure model was based were: (i) the observation of gas bursts and their correlation with the number of pits observed on the surface e7 {ii) the spherical symmetry of blisters 2<2s, (iii) the exfoliations (the extremely large blisters) due to the lateral coalescence of small blisters with high density 2<3°'4° and (iv) the fact that, at implantation energies exceeding 100 keV, the thickness of the blister covers ('Deckeldicke') was the same as the mean range of the implanted inert gas 3~. In the middle of the seventies as an alternative, the stress model was developed 32'33. The main observations on which this model was based were the following, (v) the 'deckeldicke' was 2 3 times larger than the calculated mean range of implanted ions in amorphous materials. It remains unclear, however, why this effect was observed mainly for energies below 15 keV 33-~4. The observed thickness of the blister covers were interpreted as an indication that blisters did not break at the depths of the maximum implanted ion concentration but rather that the implanted ions produced large stresses in the implanted layer. This was interpreted as support to the idea that the major driving force for blister formation was stress release and not the gas pressure. Furthermore, (vi) large stresses were measured in the implanted surfaces 3s 3~ and the blister diameters were proportional to (deckeldicke) 32 ref 38. Thus both models are able to explain part of the experimental observations. Probably the final theoretical description will be a kind of in-between. Some attempts have already been made in this direction 3°. 4. Surface deformation due to MeV energy helium implantation Some investigations of this phenomenon on "non candidate" materials resulted in a better understanding of this complicated process. For example, gold proved to be useful to describe the inner morphology of exfoliations (large size of blisters). Gold was chosen because of its good thermal conductivity, hence problems of high enerygy bombardment (E~> 1 MeV) such as beam heating of the target could be minimized and room temperature irradiation conditions could be easily fulfilled even for the increased beam power. Secondly the gold has better secondary
G Mezey et al:
Surface deformation due to high fluence helium irradiation 5
39 1 2;
~
6
7
; I
1
.9
8 l
///11
Figure 1. Schematic drawing of SEM observations after mechanically opening up a gold exfoliation of size 0.8 mm: (l) the edge of exfoliation, border zone under formation, (2) quasi-circular dip on the bottom and the missing material on the skin, (3) secondary blister on the bottom, (4) lamellae border zone, (5) secondary blister on the skin, (6) surface scratch and its print on the bottom, (7) radiation hardened part of the cover and crack due to the radiation hardening, (8) slip on the cover, (9) soft, thick layer of the cover, (10) thin, hardened part of the cover, (11) inner volume of exfoliation, (12) the bottom of the exfoliation.
electron emission properties than the candidate materials, so more details of the surface morphology can easily be observed. Figure 1 shows a summary of observations on gold exfoliation induced by 3.52 MeV helium bombardment. The SEM micrographs confirm that the larger formation is due to lateral coalescence of blisters 4°. Also on gold, a transition energy between blistering and exfoliation was found at 2 MeV 41. To generalize the observations for other materials together with exclusion of properties specific to gold further experiments are necessary. The transition energy for inconel and stainless steel was below 1 MeV 42. But the investigation of different types ofinconels showed extremely different behaviour under helium irradiation. While INCO-625 exhibited exfoliation, EP-753, with nominal composition of 19 Cr, 1.5 Mn, 33 Fe, 41 Ni, 0.5 Nb, 5 Mo, flaked immediately the critical dose was reached (Figure 2(a), (b)). Peculiar phenomena may accompany the surface deformations.
For example, wave pattern formation was observed on the flaked surface of several types of metallic glasses and silicon 43-46. The waves consisted of elevations of asymmetric triangular cross section (Figure 3). One side of these elevations slopes up to a certain height with a small angle and its surface is rather smooth whereas the other descends steeply and not so smoothly, and it is
Figure 2(a). Exfoliation on INCO 625 after 1 MeV 4He + bombardment.
Figure 3. Pattern formation on Metglass 2826A after flaking induced by 2 MeV4He + formation.
Figure 2(b). Flaking on inconel type EP-753 after 1 MeV implantation.
43
G Mezey et al: Surface deformation due to high fluence helium irradiation
often extended in a zigzag line. Generally the waves regularly extend for a relatively long distance (for example, in one case some hundreds of wave lines 1 mm in length followed each other in a parallel manner). This surprising regularity is, however, broken in some places by imperfections. The surface at the central region of the flaked spot is rough in every case and, going outwards, the waves appear gradually. The region of wave appearance is still uncertain. Two directions compete with each other and, as a result, an interference-like structure is developed. Later on, one of these directions gradually disappears and a region of pure waves is formed. The pattern formation was also observed when more than one generation of flaking took place on the implanted spot. Three further observations seem to be important: (i) the border of the last zone covered by waves is often given by the frame of the flaked layer and on the broken surface of the remaining part of the flaked layer the waves continue, (ii) the wavelength varied between 0.9 and 1.8/~m and its mean value did not change significantly although the sample composition was rather different, (iii) in an experiment where two metallic glass strips were put together and the He implantation was performed along the edges of the ribbons, the waves crossed continuously the border line between the two stripes. No pattern formation was observed on the polycrystalline modifications of the metallic glasses, which were obtained by annealing. Suprisingly a similar pattern was observed after flaking on single crystal silicon wafers of (111) orientation by 2 MeV helium irradiation. The pattern followed the border of the flaked spot 46. To explain these observations a model based on mechanical stress was proposed. The idea was to consider the implanted zone as a thin sheet that tries to swell due to the large amount of implanted helium. The surrounding material will, of course, try to prevent the lateral expansion, so huge stresses are developed within the sheet ¢6. The detailed formalism is in publication4v. Exceeding a critical stress value the layer becomes bumpy and the new surface after flaking bears the mark of it. This process could take place only in the case when the lateral stress is not able to relax in another way, for example, by forming blisters. The model suggests that no pattern will be observed in the case of polycrystalline materials where the different orientation grains and the presence of grain boundaries smear out the periodicity. For amorphous materials and single crystals, however, the initial homogeneity allows a wave pattern to form before flaking. A further prediction of the model is that the wavelength of the pattern is proportional to the range straggling of the implanted specimen. Further experiments are necessary to clarify what classes of materials and experimental conditions produce such effects. The observation of such 'side effects' may, however, bring us closer to a better understanding of the evolution of surface deformations. In a reactor a broad spectrum of alpha particles will hit the first walls mostly at oblique incidence with a distribution between 0 and 20" to the surface. A way to approach this situation is to simulate the multiple energy bombardment, i.e. to make implantation where the helium beam energy varies between zero to several MeV-s quasi-simultaneously by transmitting a monoenergetic helium beam through a continuously tilted absorber foil4s. This method seems to have vital importance for checking theoretical considerations suggesting that blistering and flaking will not take place on the first wall due to the sputtering and channel formation (the coalesence of bubbles perpendicular to the surface) of the low energy part of the helium distribution. On a system, where single crystal silicon was covered by 44
aluminium foil, the quasi-simultaneous multiple energy irradiation was applied. The He distribution was monitored by proton backscattering during implantation. It was found that the He concentration gradually increases up to 80 at% in silicon while in aluminium the peak concentration was only 30 at%. The accelerated He re-emission in aluminium may be explained by the formation of an interconnected channel network which intercepts the surface. This is confirmed by the porous structure observed by SEM. On the other hand, flaking also took place according to SEM micrographs. To interpret these results one can either assume that flaking and channel formation simultaneously took place which means that the 30 at% is the critical concentration for both phenomena or at the moment of flaking, the channels have already formed but they have not reached the surface. The second version may be supported by the presence of surface oxide that could cause depletion of helium in the surface layer or the beam could accelerate the out-diffusion4'. Figure 4 summarizes the evolution of surface deformations under different experimental conditions49.
"7
T
fo,iation I I Porous structure]
¢) :%
ii i Potter2~orsmulla!ed~d~eform~Qtion IChonne[ formotion I 1 ~
I
II
Bubble coalescence ]
11
Bubble formation and stress build up
1"
[ He bombardment I
Experimental conditions (more than one dimensions)
Figure 4. The evolution of surface deformations. The thick arrows represent the increase of fluence. The thin arrows serve for the classification of surface deformations depending upon the experimental conditions after reaching the critical dose.
In the light of these results, the broad energy spectrum of helium in a tokamak reactor does not necessarily prevent flaking. It would be, however, interesting to make simultaneous bombardment with multiple energy helium and low energy deuterium. On the other hand, in a tokamak reactor, deposition of sputtered wall material may locally diminish the erosion speed of the first wall favouring blister formation. The blistering can be reduced by (i) preparing an appropriately rough surface, (ii) keeping the material at a certain temperature and (iii) using a material with small grain size.
G Mezey et al: Surface deformation due to high fluence helium irradiation
4. Summary W e have l e a r n e d a lot o f the n a t u r e of surface d e f o r m a t i o n s b u t a full u n d e r s t a n d i n g of the m e c h a n i s m o f f o r m a t i o n is still far away. M a n y f u r t h e r e x p e r i m e n t s are n e e d e d to w o r k o u t a c o n s i s t e n t theoretical model. But w h a t e v e r the m e c h a n i s m of the blister f o r m a t i o n is, blistering does n o t n o w a p p e a r to be as relevant in fusion as was initially t h o u g h t .
References i G M McCracken and P E Stott, Nucl Fusion, 19, 889 (1978). 2 M Kaminsky, CRC Critical Review in Solid State Sci, 4, 433 (1976). 3 K F Alexander, H Grote, D Hildebrandt, M Laux, P Pech, H-D Reiner, H Strusny, H Wolf, E K6tai, A Manuaba and F P~iszti, presented in Controlled Fusion and Plasma Physics, Aachen (1983). 4 D Hildebrandt, M Laux, J Lingertadt, P Pech, H-D Reiner, H Strusny, H Wolf, V M Chicherov, A Manuaba, G Mezey and F P~iszti, ZIE Preprint 82-6. 5 H Grodete, H Hildebrandt, H Strusny, M Fried, F Pfiszti and Gy Vizkelethy, ZIE Preprint 83 5. 6 G Staudenmaier, J Vac Sci, to be published (1985). 7 D M Manos and G M McCracken, to appear in Plasma Wall Interaction in Controlled Fusion Devices (Edited by Behrish and Post). s T Leffers, B B Singh, W V Green and M Victoria, EIR-Bericht Nr 518 (1984). 9 D E Post, Princeton Plasma Physics Laboratory, Report PPPL 1815 (1981). ,o B B Kadomtsev and O P Pogutse, Nucl Fusion, 11, 67 (1971). 11 L A Archimovich Nucl Fusion, 12, 215 (1972). 12 T W Petrie and G H Miley, Nucl Sci Eno, 64, 151 (1977). 13 H P Furth, Nucl Fusion, 15, 487 (1975). 1,~ L M Hively and G M Miley, Nucl Fusion, 76[77, 389 (1978). 15 R Behrisch and B M U Scherzer, Rad Eft, 78, 3939 (1983). 16 K Erlich and D Kaletta, Proc Rad Def and Tritium Techn for Fusion Reactors, Gattlinburg, p 289 (1979). 17 S E Donnelly, J C Rife, J M Gilles and A A Lucas, J Nucl Mat, 93[94, 767 (1983). la M Kaminsky, Adv Mass Spectrum, 3, 69 (1964). 19 j H Evans, J Nucl Mat, 68, 129 (1977). zo R S Barnes and D J Mazey, Proc Roy Soc, A275, 47 (1963). 21 j B Holt, W Bauer and G J Thomas, Rad Eft, 7, 269 (1971). 22 G J Thomas and W Bauer, Rad Eft, 17, 221 (1973).
23 R D Daniels, J appl Phys, 42, 417 (1971) 24 G M Greenwood, A J Foreman and D E Rimmer, J Nucl Mat, 4, 305 (1959). 25 L M Brown and D J Mazey, Phyl Ma9, 10, 140 (1963) 26 S K Erents and G M McCracken, Rad Eft, 18, 1919 (1973). 27 j H Evans, Nature, 256, 299 (1975). 28 j Roth, R Behrisch and B M U Scherzer, J Nucl Mat, 57, 365 (1974). 29 S K Das and M Kaminsky, J appl Phys, 44, 260 (1973). 30 V M Gusev, M I Guseva, Yu V Martinenko, A N Mansurova, V N Morosov and O I Chelnikov, Rad Eft, 40, 37 (1979). 31 M Kaminsky and S K Das, Rad Eft, 15, 245 (1973). 32 j Roth, R Behrisch and B M U Scherzer, J Nucl Mat, 57, 365 (1974). 33 R Behrisch, J Bottiger, W Eckstein, U Littmark, J Roth and B M U Scherzer, Appl Phys Lett, 27, 199 (1975). 34 j Roth, R Behrisch and B M U Scherzer, J Nucl Mat, 53, 257 (1974). 35 E P Eernisse and S T Picraux, J appl Phys, 48, 9 (1977). 36 E P Eernisse, Appl Phys Lett, 15, 581 (1971). 37 N E W Hartley, J Vac Sci Technol, 12, 485 (1975). 3s j Roth, Inst ConfSeries, 28, 280 (1976). 39 O Auciello, Rad Eft, 30, 11 (1976). 40 F Pfiszti, L Pogfiny, G Mezey, E K6tai, A Manuaba, L P6cs, J Gyulai and T Lohner, J Nucl Mat, 98, 11 (1981). 41 G Mezey, F P~.szti, L Pog~iny, A Manuaba, M Fried, E K6tai, T Lohner, L P6cs and J Gyulai, Ion Implantation Into Metals, p 293, Pergamon Press, Oxford (1982). 42 F P~iszti, G Mezey, L Pogfiny, M Fried, A Manuaba, E K6tai, T Lohner, and L P6cs, Nucl Instr Meth, 209]210, 1001 (1983). 43 A Manuaba, F P/tszti, L Pog~.ny, M Fried, E K6tai, G Mezey, T Lohner, I Lovas, L P6cs and J Gyulai, Nucl Instr Meth, 199, 409 (1982). 44 F Pfiszti, M Fried, L Pog~iny, A Manuaba, G Mezey, E K6tai, I Lovas, T Lohner and L P6cs, Phys Rev, B25 (10), 5688 (1983). 4s F P~tszti, M Fried, L Pog~iny, A Manuaba, G Mezey, E Kbtai, I Lovas, T Lohner and L P6cs, Nucl Insr Meth, 209[210, 273 (1983). 46 F Pfiszti, Cs Hajdu, A Manuaba, N T My, E K6tai, L Pogfiny, G Mezey, M Fried, G Vizkelethy and J Gyulai, Nucl Instr Meth, B7]8, 371 (1985). 47 Cs Hajdu, F Pfiszti, G Mezey and I Lovas, submitted to Phys Rev Lett. 48 F P~.szti, M Fried, A Manuaba, L Pog~.ny, G Mezey, E K6tai and T Lohner, J Nucl Mat, 114, 330 (1983). 49 F Pfiszti, E K6tai, H V Suu, T Lohner, L P6cs and G Mezey J Nucl Mat, 119, 26 (1983). 50 B Navinsek, Fus Technol, 6, 491 (1984). 51 K Kamada, Fus Technol, 6, 483 (1984). 52 O Auciello, Ion Bombardment Modification of Surfaces (Edited by O Auciello and R Kelly), chap 1-3, Elsevier, Amsterdam (1984).
45
0042-207X/8753.00+.00 Pergamon Journals Ltd
Printed in Great Britain
Surface deformation due to high fluence helium irradiation G Mezey, F Pfiszti, L Pog6ny, M Fried, A Manuaba, G Vizkelethy, Cs Hajdu and E Kbtai, Central Research Institute for Physics, H- 1525 Budapest 114, PO Box 49, Hungary
During the past ten years a new branch of materials science has been developed. This field is called plasmasurface interaction (PSI). PSI itself is a broad field of increasing importance and includes both in situ impurity observations in different machines (i.e. deposition and erosion probes) and model experiments to understand the basic processes such as physical and chemical sputtering and recycling. This paper makes an attempt to summarize the results concerning surface deformations (blistering, exfoliation, flaking) and special pattern formation accompanying them. Mainly of the effects due to high energy bombardment (0.8-3.52 MeV) are reviewed. A theoretical explanation based on a stress model for the pattern formation is also discussed.
1. Introduction
Extremely high fluences of particles (D, T, He, electrons, fast neutrons, medium and heavy mass ions) and photons will hit the first wall components of a fusion type reactor (CTR machine). Approximately 60 elementary processes take place on the walls. Of course, their relative importance in a given machine will depend upon the operation regime and the location of the first wall component inside the machine. For example, limiters, divertor plates or shields are mainly subjected to erosion type processes. The common feature of the elementary processes is that they tend to contaminate the plasma by impurity production and recycling; introducing cold working gas atoms into the plasma. These processes either tend to prevent ignition in the existing big tokamaks (TFTR, JET, JT-60) or diminish the future reactor efficiency. In the past ten years a new branch of materials science which is called plasma surface interaction (PSI) has been developed. PSI studies have concentrated on the above mentioned problems. As the building of the first demonstrator type reactor machine approaches, the importance of this research is increasing 1,2. Roughly one can divide the PSI studies into two groups: - - i n situ observations in present-day machines (erosion and deposition probes, sample transfer systems, surface analysis stations, etc.) 3 7. --model experiments for both the understanding of the mechanism of elementary processes and for seeking new classes of materials together with an optimum first wall structure (for example the honeycomb structure) to minimize the erosion and recycling and improve the tritium inventory. Most of the PSI studies should be carried out in close cooperation with plasma physicists and construction engineers. But for model experiments a relative independence is necessary for better understanding of the physics of elementary processes since
a firm basis will help a lot in 'tailoring' the first wall components in the future controlled thermonuclear reactors. This paper tries to outline experiments concerning a narrow field of PSI, namely surface deformations induced by high fluence helium implantation. Some experiments which are reviewed here will, at first sight, be peculiar as the targets were not chosen among the 'candidate' first wall materials of fusion reactors but experiments made on these materials could contribute to a better understanding of the evolution of surface deformations and their contribution to surface erosion and plasma contamination. The model experiments have an obvious danger because equally an over or underestimation of the real physical situation could take place since in tokamaks more than one process affects the first wall simultaneously. So the relevance of results to the practical situation (tokamak conditions) should always be investigated.
2. Helium generation rate and its effect on surface deformation
In the plasma, deuterium and tritium will react producing 14.1 MeV neutrons and 3.52 MeV alpha particles. The neutrons will leave the plasma and go directly to the wall of the vacuum vessel, while the alpha particle trajectories will be guided by the magnetic field and they will mostly be trapped in the plasma. A small fraction of the alpha particles, typically a few per cent, will be created at such location of the plasma and with such direction of motion (loss cones) that they will drift with nearly their full energy to the vessel wall. The other part of the alpha particles will transfer their energy to plasma ions and electrons. The thermalized alpha particles will arrive at the wall presumably with an energy corresponding to the temperature of the plasma boundary. If the mean energy of the plasma boundary stays below some 100 eV, the He particles hitting the vessel wall with this energy will 41
G Mezey et al. Surface deformation due to high fluence helium irradiation
mainly contribute to wall sputtering. However, the He particles arriving at the wall with their full energy can cause heavy surface modifications like blistering, exfoliation, flaking and spongy structures. In order to estimate this damage the fluence, the current density, the energy and angular distribution of He particles have to be known. The behaviour of the particles produced in a magnetically confined fusion plasma has been studied extensively for tokamak reactors 9 ,4 For an estimate of wall damage in a fusion reactor it was assumed I s (i) the He ions go to the vessel wall in the same way as the hydrogen plasma, (ii) the fractional burn up of tritium and deuterium is about 5%, (iii) the recycling can be neglected and (iv) the He flux is uniformly distributed over the first wall. This gives a composition for the wall bombardment of about 5% He 47.5% T and 47.5 D. The total He flux can be estimated from today's fusion reactor design studies. Here, generally, an average neutron load to the vessel walls of 1.3 to 2 M W m 2 is taken. This corresponds to an average 14.1 MeV neutron current of about 0.6-1 × 10 TM n m 2 s 1, which also gives the average number of alpha particles going to the first wall. Recycling may increase the He flux by more than a factor of 2. This estimation suggests that blistering will appear in the first 2-10 days of the reactor operation. The available simulation techniques have already demonstrated that the helium generation (or introduction) rate has a decisive effect on the cavity structure and hence on the whole microstructural evolution of the reactor wall. For example, the weakening of grain boundaries is very sensitive to the He generation rate via the effect of He generation rate on cavity structure, particularly on the width of the cavity-denuded zone from which He diffuses to the boundaries. For the simulation of radiation effects, where the He generation or introduction rate is an important parameter, it is obviously essential that He is introduced continuously (rather than by some pre-implantation procedure). In the characterization and evaluation of a simulation technique the ratio between helium generation and displacement damage rate (the appm to dpa ratio) is often taken to be the most important single parameter. On the other hand the particular importance of this parameter has been questioned on theoretical as well as experimental grounds. It has been argued that, for many damage accumulation processes the individual rates are more important than their ratio. It appears that the above mentioned grain boundary weakening (by preferential nucleation and growth of He bubbles) is mainly governed by the He generation rate and less by displacement damage s . 3. Surface deformation due to He bombardment The first consequence of the high fluence of helium implantation into the first wall is the appearance of bubbles near to the projected range 16 18. O n increasing the helium f l u e n c e ~ e p e n d ing on the experimental conditions (target composition, bombarding flux, energy, target temperature)--three basic types of surface deformation will develop. (i) Blistering. This formation dominates in the low energy region (10 keV-1 MeV depending upon the target material and temperature). (ii) Exfoliation is a large formation covering almost the whole implanted spot. (iii) Flaking takes place when almost the total area of the implanted spot leaves the surface with practically uniform thickness. 42
In the model experiments monoenergetic helium beams with perpendicular angle of incidence are the most frequently used for implantation. Two fundamental theoretical models have been developed to explain blister formation: the gas pressure model and the stress model. The first direct demonstration of the presence of gas in bubbles, hence in blisters was done by Kaminsky ~8, who used a mass spectrometer to detect bursts of He gas released from ruptured blisters, This was subsequently confirmed by several authors2O 23 Blistering was initially attributed to a sudden coalescence of bubbles within the range distribution of the incident ions. This coalescence would lead to a high pressure exerted on the surface 24'25. The idea was developed in greater detail. Erents and McCracken 2~, for example, assumed that all the implanted gas was collected in the coalescence process. They found that the pressure inside a blister was directly proportional to the total dose of implanted helium and to the average depth of the void in the solid. Their calculations suggested that a proportionality between the blister radius and the energy of the incident ions should be expected. Their own experiments indicated that the expected relationship was approximately correct. The experimental evidence on which the gas pressure model was based were: (i) the observation of gas bursts and their correlation with the number of pits observed on the surface e7 {ii) the spherical symmetry of blisters 2<2s, (iii) the exfoliations (the extremely large blisters) due to the lateral coalescence of small blisters with high density 2<3°'4° and (iv) the fact that, at implantation energies exceeding 100 keV, the thickness of the blister covers ('Deckeldicke') was the same as the mean range of the implanted inert gas 3~. In the middle of the seventies as an alternative, the stress model was developed 32'33. The main observations on which this model was based were the following, (v) the 'deckeldicke' was 2 3 times larger than the calculated mean range of implanted ions in amorphous materials. It remains unclear, however, why this effect was observed mainly for energies below 15 keV 33-~4. The observed thickness of the blister covers were interpreted as an indication that blisters did not break at the depths of the maximum implanted ion concentration but rather that the implanted ions produced large stresses in the implanted layer. This was interpreted as support to the idea that the major driving force for blister formation was stress release and not the gas pressure. Furthermore, (vi) large stresses were measured in the implanted surfaces 3s 3~ and the blister diameters were proportional to (deckeldicke) 32 ref 38. Thus both models are able to explain part of the experimental observations. Probably the final theoretical description will be a kind of in-between. Some attempts have already been made in this direction 3°. 4. Surface deformation due to MeV energy helium implantation Some investigations of this phenomenon on "non candidate" materials resulted in a better understanding of this complicated process. For example, gold proved to be useful to describe the inner morphology of exfoliations (large size of blisters). Gold was chosen because of its good thermal conductivity, hence problems of high enerygy bombardment (E~> 1 MeV) such as beam heating of the target could be minimized and room temperature irradiation conditions could be easily fulfilled even for the increased beam power. Secondly the gold has better secondary
G Mezey et al:
Surface deformation due to high fluence helium irradiation 5
39 1 2;
~
6
7
; I
1
.9
8 l
///11
Figure 1. Schematic drawing of SEM observations after mechanically opening up a gold exfoliation of size 0.8 mm: (l) the edge of exfoliation, border zone under formation, (2) quasi-circular dip on the bottom and the missing material on the skin, (3) secondary blister on the bottom, (4) lamellae border zone, (5) secondary blister on the skin, (6) surface scratch and its print on the bottom, (7) radiation hardened part of the cover and crack due to the radiation hardening, (8) slip on the cover, (9) soft, thick layer of the cover, (10) thin, hardened part of the cover, (11) inner volume of exfoliation, (12) the bottom of the exfoliation.
electron emission properties than the candidate materials, so more details of the surface morphology can easily be observed. Figure 1 shows a summary of observations on gold exfoliation induced by 3.52 MeV helium bombardment. The SEM micrographs confirm that the larger formation is due to lateral coalescence of blisters 4°. Also on gold, a transition energy between blistering and exfoliation was found at 2 MeV 41. To generalize the observations for other materials together with exclusion of properties specific to gold further experiments are necessary. The transition energy for inconel and stainless steel was below 1 MeV 42. But the investigation of different types ofinconels showed extremely different behaviour under helium irradiation. While INCO-625 exhibited exfoliation, EP-753, with nominal composition of 19 Cr, 1.5 Mn, 33 Fe, 41 Ni, 0.5 Nb, 5 Mo, flaked immediately the critical dose was reached (Figure 2(a), (b)). Peculiar phenomena may accompany the surface deformations.
For example, wave pattern formation was observed on the flaked surface of several types of metallic glasses and silicon 43-46. The waves consisted of elevations of asymmetric triangular cross section (Figure 3). One side of these elevations slopes up to a certain height with a small angle and its surface is rather smooth whereas the other descends steeply and not so smoothly, and it is
Figure 2(a). Exfoliation on INCO 625 after 1 MeV 4He + bombardment.
Figure 3. Pattern formation on Metglass 2826A after flaking induced by 2 MeV4He + formation.
Figure 2(b). Flaking on inconel type EP-753 after 1 MeV implantation.
43
G Mezey et al: Surface deformation due to high fluence helium irradiation
often extended in a zigzag line. Generally the waves regularly extend for a relatively long distance (for example, in one case some hundreds of wave lines 1 mm in length followed each other in a parallel manner). This surprising regularity is, however, broken in some places by imperfections. The surface at the central region of the flaked spot is rough in every case and, going outwards, the waves appear gradually. The region of wave appearance is still uncertain. Two directions compete with each other and, as a result, an interference-like structure is developed. Later on, one of these directions gradually disappears and a region of pure waves is formed. The pattern formation was also observed when more than one generation of flaking took place on the implanted spot. Three further observations seem to be important: (i) the border of the last zone covered by waves is often given by the frame of the flaked layer and on the broken surface of the remaining part of the flaked layer the waves continue, (ii) the wavelength varied between 0.9 and 1.8/~m and its mean value did not change significantly although the sample composition was rather different, (iii) in an experiment where two metallic glass strips were put together and the He implantation was performed along the edges of the ribbons, the waves crossed continuously the border line between the two stripes. No pattern formation was observed on the polycrystalline modifications of the metallic glasses, which were obtained by annealing. Suprisingly a similar pattern was observed after flaking on single crystal silicon wafers of (111) orientation by 2 MeV helium irradiation. The pattern followed the border of the flaked spot 46. To explain these observations a model based on mechanical stress was proposed. The idea was to consider the implanted zone as a thin sheet that tries to swell due to the large amount of implanted helium. The surrounding material will, of course, try to prevent the lateral expansion, so huge stresses are developed within the sheet ¢6. The detailed formalism is in publication4v. Exceeding a critical stress value the layer becomes bumpy and the new surface after flaking bears the mark of it. This process could take place only in the case when the lateral stress is not able to relax in another way, for example, by forming blisters. The model suggests that no pattern will be observed in the case of polycrystalline materials where the different orientation grains and the presence of grain boundaries smear out the periodicity. For amorphous materials and single crystals, however, the initial homogeneity allows a wave pattern to form before flaking. A further prediction of the model is that the wavelength of the pattern is proportional to the range straggling of the implanted specimen. Further experiments are necessary to clarify what classes of materials and experimental conditions produce such effects. The observation of such 'side effects' may, however, bring us closer to a better understanding of the evolution of surface deformations. In a reactor a broad spectrum of alpha particles will hit the first walls mostly at oblique incidence with a distribution between 0 and 20" to the surface. A way to approach this situation is to simulate the multiple energy bombardment, i.e. to make implantation where the helium beam energy varies between zero to several MeV-s quasi-simultaneously by transmitting a monoenergetic helium beam through a continuously tilted absorber foil4s. This method seems to have vital importance for checking theoretical considerations suggesting that blistering and flaking will not take place on the first wall due to the sputtering and channel formation (the coalesence of bubbles perpendicular to the surface) of the low energy part of the helium distribution. On a system, where single crystal silicon was covered by 44
aluminium foil, the quasi-simultaneous multiple energy irradiation was applied. The He distribution was monitored by proton backscattering during implantation. It was found that the He concentration gradually increases up to 80 at% in silicon while in aluminium the peak concentration was only 30 at%. The accelerated He re-emission in aluminium may be explained by the formation of an interconnected channel network which intercepts the surface. This is confirmed by the porous structure observed by SEM. On the other hand, flaking also took place according to SEM micrographs. To interpret these results one can either assume that flaking and channel formation simultaneously took place which means that the 30 at% is the critical concentration for both phenomena or at the moment of flaking, the channels have already formed but they have not reached the surface. The second version may be supported by the presence of surface oxide that could cause depletion of helium in the surface layer or the beam could accelerate the out-diffusion4'. Figure 4 summarizes the evolution of surface deformations under different experimental conditions49.
"7
T
fo,iation I I Porous structure]
¢) :%
ii i Potter2~orsmulla!ed~d~eform~Qtion IChonne[ formotion I 1 ~
I
II
Bubble coalescence ]
11
Bubble formation and stress build up
1"
[ He bombardment I
Experimental conditions (more than one dimensions)
Figure 4. The evolution of surface deformations. The thick arrows represent the increase of fluence. The thin arrows serve for the classification of surface deformations depending upon the experimental conditions after reaching the critical dose.
In the light of these results, the broad energy spectrum of helium in a tokamak reactor does not necessarily prevent flaking. It would be, however, interesting to make simultaneous bombardment with multiple energy helium and low energy deuterium. On the other hand, in a tokamak reactor, deposition of sputtered wall material may locally diminish the erosion speed of the first wall favouring blister formation. The blistering can be reduced by (i) preparing an appropriately rough surface, (ii) keeping the material at a certain temperature and (iii) using a material with small grain size.
G Mezey et al: Surface deformation due to high fluence helium irradiation
4. Summary W e have l e a r n e d a lot o f the n a t u r e of surface d e f o r m a t i o n s b u t a full u n d e r s t a n d i n g of the m e c h a n i s m o f f o r m a t i o n is still far away. M a n y f u r t h e r e x p e r i m e n t s are n e e d e d to w o r k o u t a c o n s i s t e n t theoretical model. But w h a t e v e r the m e c h a n i s m of the blister f o r m a t i o n is, blistering does n o t n o w a p p e a r to be as relevant in fusion as was initially t h o u g h t .
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