Ion beam analysis methods in the studies of plasma facing materials in controlled fusion devices

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1 Vacuum 70 (2003) Ion beam analysis methods in the studies of plasma facing materials in controlled fusion devices M. Rubel a, *, P. Wienhold b, D. Hildebrandt c a Alfv!en Laboratory, Royal Institute of Technology, Association EURATOM VR, Teknikringen 31, S Stockholm, Sweden b Institute of Plasma Physics, Forschungszentrum J.ulich, Association EURATOM, D J.ulich, Germany c Max Planck Institute of Plasma Physics, Diagnostic Division, Association EURATOM, D Berlin, Germany Abstract Application of ion beam analysis techniques in the studies of material transport and fuel inventory in the controlled fusion devices is exemplified. Enhanced proton scattering on the carbon isotopes 12 C(p,p) 12 C, 13 C(p,p) 13 C and secondary ion mass spectrometry allowed for determination of carbon erosion and re-deposition on the wall components following the experiments with a tracer ( 13 CH 4 ) injection into the plasma edge at the TEXTOR tokamak. For the assessment of the deuterium fuel accumulation in the plasma facing components depth profiling by means of nuclear reaction analysis, 3 He(d,p) 4 He, was performed. Advantages and limitations of those nuclear methods in solving experimental problems are addressed. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ion beam analysis; Erosion; Deposition; Hydrogen inventory; Tokamak 1. Introduction High particle fluxes and resulting heat loads cause erosion and significant modification of material surfaces surrounding the plasma in the controlled fusion devices. Plasma material interaction (PMI) leads to degradation of plasma performance (power radiation losses) and properties of wall materials [1,2]. The contact of particles escaping a fusion plasma with the wall is unavoidable because the discharge duration is much longer than the particle confinement time (t p ). In spite of these negative effects, PMI is also a *Corresponding author. Tel.: ; fax: address: rubel@fusion.kth.se (M. Rubel). necessary condition for a future reactor operation. The wall has to extract injected power and to thermalise helium ash produced in the D T fusion process [3]. The major mechanisms of erosion are physical sputtering, chemical erosion and evaporation [1,2]. Eroded species become ionised in the plasma edge, they are transported along the magnetic field lines and eventually redeposited (co-deposited, co-implanted) in another location together with fuel atoms, i.e. hydrogen isotopes. Re-deposition results in material mixing [4,5] and retention of hydrogen isotopes. Fuel inventory is especially pronounced in tokamaks with the plasma facing components (PFC) made of carbon [2,6 8]. Accumulation of radioactive tritium in a next-step device must not exceed safety limits (350 g). Therefore, the studies of PMI in X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi: /s x(02)

2 424 M. Rubel et al. / Vacuum 70 (2003) Table 1 Ion beam analysis methods used in studies of plasma facing materials Isotope/ tracer Method/reaction Beam/energy (MeV) Information depth (mm) Remarks: advantages and limitations D NRA/ 3 He(d,p) 4 He 3 He + / Depth profiling (+) Very high selectivity (+) Sensitivity cm 2 (+) Ion-induced detrapping ( ) D d(p,p)d H + / Cross sections not well defined ( ) Depth profiling in thin layers 11 B NRA p( 11 B,a) 8 Be H + / High selectivity & sensitivity (+) No depth profiling ( ) 12 C EPS 12 C(p.p) 12 C H + /1.5 8 H + /1.748 (resonance) Around 0.1 * In case of mixed layers * 13 C EPS 13 C(p.p) 13 C H + /1.25 H + /1.442 (resonance) 6 Around 0.1 * In the presence of C-12, * quantification of C-13 possible only in thin layers and only at the resonance energy Si RBS 4 He + / Below 1 Mo RBS 4 He + / Below 1 Sensitivity: cm 2 W, Ta RBS 4 He + / Below 1 Very high sensitivity: cm 2 present-day experiments, operated predominantly with deuterium, are focused on material transport in order to determine the distribution and composition of erosion and deposition zones. This, in turn, allows for conclusions regarding the selection of wall materials and development of methods for fuel removal. The thickness of modified layers ranges from some nanometers to hundreds of microns dependent on the location in a fusion device and on the plasma operation time [9]. Characterisation of such layers is a mosaic work requiring application of various spectroscopy and microscopy techniques. Among them, the accelerator-based ion beam analysis (IBA) methods prove to be the most effective ones because of their sensitivity, selectivity, good lateral and depth resolution and, last but not least, the speed of analysis (important in mapping of large surfaces) as several various detectors can be simultaneously used. This paper is focused on the investigation of fusion reactor materials with nuclear reaction analysis (NRA), Rutherford backscattering spectroscopy (RBS) and enhanced proton scattering (EPS). Special emphasis is given to the application of IBA in the determination of carbon transport by means of a tracer technique ( 13 C marker) and, to the assessment of deuterium retention and distribution in PFC. In Table 1 we summarise some practical aspects regarding analyses of carbon, deuterium and some other elements (B, Si, W) used for wall conditioning [10] or power handling [2] in tokamaks. 2. Experimental procedure Experiments were carried out in the TEXTOR tokamak (Forschungszentrum J.ulich, Germany), a controlled fusion device with a circular plasma (a ¼ 46 cm) and several arrays of graphite limiters. The structure of inner wall components can be found in [11]. Carbon ( 12 C) is the major plasma impurity species and, therefore, for the studies of its transport mechanism a predetermined amount of 13 C tracer ( molecules of 13 CH 4 ) was injected into the plasma edge through an inlet hole in an aluminium plate located in the scrape-off layer (SOL) [12]. The aim was to determine the carbon deposition efficiency, i.e. the ratio of species deposited locally after a short range

3 M. Rubel et al. / Vacuum 70 (2003) transport in the vicinity of the gas inlet to the amount transported in the torus (long-range transport): 13 C s / 13 C l. The plate inclination by 201 with respect to the magnetic field lines, as shown in Fig. 1a, allowed for the determination of radial distributions of carbon species. To assess the ratio of 13 C/ 12 C after the long-range transport, a flux catcher plate was mounted on the same holder perpendicularly to the field lines. There was no magnetic connection between the two plates. Analysis of 13 C [13], 12 C [14] and deuterium [15,16] in co-deposits was performed at a scattering angle of 1701 by means of EPS at resonance energies and by NRA, respectively (see Table 1 for details). Resonance energies for protons scatteredoff on the two carbon isotopes are well separated (B300 kev), but relatively narrow (B1 kev) making the quantitative analysis and depth profiling in non-uniform mixed layers rather difficult because of the energy drop (stopping factor [e] B7eV cm 2 ) and corresponding sharp change of the reaction cross-section. Fortunately, equally high reaction cross-sections (1 b sr 1 at resonance in both cases) allow for the determination of the isotope ratio ( 13 C/ 12 C) with a high degree of confidence. The results were cross-checked by means of secondary ion mass spectrometry (SIMS) with a 10 kev oxygen beam. SIMS was also used to measure the total layer thickness. These results were compared with the NRA depth profiling of co-deposited deuterium. A 3 He + beam of energy of 0.75 MeV was used. Determination of deuterium inventory in PFC was carried out on tiles of the toroidal belt limiter. For probing thick layers formed in the deposition zone (structure shown in [9]) an energy scan from 0.7 to 2.5 MeV with a 3 He + beam was required to recognise the D content and its distribution. High energy protons (11 13 MeV range) emerging from the 3 He(d,p) 4 He reaction assure high selectivity of deuterium tracing and allow for high-resolution depth profiling [17]. The accessible analysis depth in a carbon matrix extends to around mm with a 2.5 MeV beam. These features make the reaction attractive not only in fusion science but also in other fields whenever deuterium may be used as a non-radioactive marker in the studies of hydrogen behaviour in solids. Another reaction PLASMA last closed flux surface Al plate α = 20 o 13 CH4 inlet for deuterium analysis, D(p,p)D [18,19], is cheaper than the one involving 3 He, but depth profiling is limited. 3. Results and discussion flux catcher plate Scrape-off layer Fig. 1. Schematics of the experimental set-up used for the studies of carbon transport at TEXTOR. Images in Fig. 2a and b show the plates after their exposure during the plasma discharges with 13 CH 4 injection. An optical analysis of interference fringes (method explained in [20]) indicates the layer thickness ranging from at least 40 nm to over 170 nm. From the isotopic ratio 13 C/ 12 C determined with EPS (results plotted in Fig. 3a) one infers a distribution of locally re-deposited 13 C (injected) and 12 C eroded from the wall and deposited from the plasma. The ratio reaches its maximum value near the gas inlet and then decreases, both towards the plasma and further into the SOL, indicating a high background deposition of C-12 from the plasma. Already at the distance of a few centimetres from the inlet a very small amount of 13 C is detected. The integrated amount of 13 C found on the plate corresponds to only 1% of the total gas input ( 13 C s / 13 C l =0.01) [12,21]. The rest becomes ionised and transported to other locations in the torus, i.e. to the deposition zones on the blades and neutraliser plates of a toroidal belt limiter [22]. Indeed, surface analysis of the flux catcher probe revealed a significant fraction of C-13 involved in the long-range transport, 13 C/ 12 C=0.27 independent of the radial distance with respect to the plasma. The results prove that the prevailing fraction of eroded carbon is transported by the plasma. This is unlike in case of eroded high-z

4 426 M. Rubel et al. / Vacuum 70 (2003) Carbon deposit (a) Gas inlet (b) Fig. 2. Deposition pattern on the aluminium plates following the 13 CH 4 injection into the plasma edge: (a) Plate with the gas inlet hole. White dotted line corresponds to the line of analysis; (b) Flux catcher plate. metals, which become promptly and locally redeposited [4,23,24]. While the measurements of the isotope ratio with EPS and SIMS yielded the same results, the total thickness of mixed carbon deposits could not be quantified with EPS alone (at resonance energies) for the layers thicker than 100 nm for the reasons explained in Section 2. This discrepancy is exemplified in Fig. 3b revealing that the agreement in the determination of the total layer thickness, i.e. 13 C+ 12 C, is obtained for thin layers, only. Moreover, the plots prove that the maximum of carbon content is detected closer to the plasma edge and deposition is strongly dominated by 12 C impurities eroded from PFC. The distribution of co-deposited deuterium has been found similar to the thickness profile shown in Fig. 3b. Inspection of the TEXTOR in-vessel components indicates that the re-deposition of eroded material occurs mainly on some areas of the blades (layer growth rate B3 nm/s) and on the neutraliser plates (B11 nm/s) of the toroidal belt limiter. Therefore, during hours of plasma operation the layers reached thickness of up to 1 mm [9]. Though the composition of co-deposits is a complex mixture of elements [23,25], the analysis of such structures is mainly focused on the total fuel inventory and distribution (lateral and depth) of hydrogen isotopes. While the quantitative IBA measurements of hydrogen 15 N(p,a) 12 C and tritium 3 He(t,d) 4 He in thick layers meet some practical difficulties, the deuterium content can be conclusively determined. The plots in Fig. 4a and b exemplify deuterium depth distribution in the deposition and erosion zones on the limiter tiles. In the deposition zone the deuterium content reaches nearly D at cm 2 and the distribution of the isotope is fairly uniform indicating that the presence of deuterium extends outside the accessible analysis depth with the applied He-3 beam (primary energy of 1.5 MeV in this case). The analysis with the beam energy increased to 2.5 MeV performed on both sides (front and rear sides) of the layer detached from the tile allowed for the determination of the amount of deuterium fuel trapped in the co-deposit. Knowing the layer thickness and the total area of the deposition zone (B1m 2 ) on that limiter, the total content of deuterium could be estimated ( D atoms). The result was cross-checked with thermal desorption spectrometry measurements showing an agreement within 8%. In the erosion zone, the narrow distribution and small amount of deuterium detected predominantly in the near-surface layer is attributed to the implanted species. The tail extended to several microns is related both to

5 M. Rubel et al. / Vacuum 70 (2003) RATIO C 13 /C 12 (a) THICKNESS [nm] (b) DISTANCE ACROSS THE SURFACE [mm] 49.5 RADIAL DISTANCE FROM PLASMA CENTER [cm] inlet hole RADIAL DISTANCE FROM PLASMA CENTER [cm] inlet hole SIMS EPS DISTANCE ACROSS THE SURFACE [mm] Fig. 3. Distribution of carbon isotopes in the deposit on the aluminium plate exposed to the scrape-off layer plasma during the 13 CH 4 injection: (a) 13 C/ 12 C isotope ratio determined with EPS; (b) comparison of total amount of carbon ( 13 C+ 12 C) on the Al plate measured with EPS at resonance energies and with SIMS. the surface roughness and to the in-depth migration of implanted fuel into the carbon substrate [26,27]. By repetition of this analytical procedure for studying material re-deposited in other locations of TEXTOR (i.e. other limiters and neutraliser plates) the overall deuterium inventory in the device was determined. Deuterium atomic density (10 22 cm _ 3 ) (a) E(0) 3 He + = 1.5 MeV Deposition 3zone C(D) (microns) = 4.78 x cm -2 Erosion zone 3 C(D) (microns) = 2.02 x cm -2 (b) Depth (microns) Fig. 4. Deuterium depth distribution and concentration in: (a) deposition, and (b) erosion zones on the toroidal belt limiter at TEXTOR. The results of analysis allow for the conclusions regarding fuel inventory (amount and distribution), surface modification of PFC and material transport in fusion devices. The 13 C tracer experiment followed by measurements of carbon isotopes resulted in the quantitative distinction of the long and short range impurity transport. It has been clearly shown that in the erosion of carbon, long-range transport prevails in the formation of co-deposits. From the analytical point of view, the proton scattering technique applied to the mixed carbon films has yielded conclusive results regarding the isotope ratio, but the determination of the total layer thickness (over 100 nm) must be complemented by sputter-assisted measurements, e.g. SIMS as applied in this work. In the analysis of deuterium in co-deposits 3 He(d,p) 4 He reaction yields reliable results. The challenge to determine the total inventory in a controlled fusion device is not on the physics side, but the necessity of mapping of a large number of large components from various locations. The results help the assessment of tritium inventory in a future D T operated reactor. 4. Summary and conclusions Acknowledgements This work was partly supported by NFR Contract F /2000.

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