Remote-LIBS characterization of ITER-like plasma facing materials

Journal of Nuclear Materials 421 (2012) 73–79

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Remote-LIBS characterization of ITER-like plasma facing materials S. Almaviva a,⇑, L. Caneve b, F. Colao b, R. Fantoni b, G. Maddaluno a a b

Associazione EURATOM-ENEA sulla Fusione, C.R. Frascati, P.O. Box 65, 00044 Frascati, Roma, Italy ENEA, UTAPRAD-DIM, C.R. Frascati, P.O. Box 65, 00044 Frascati, Roma, Italy

a r t i c l e

i n f o

Article history: Received 29 July 2011 Accepted 16 November 2011 Available online 23 November 2011

a b s t r a c t The occurrence of several plasma-wall interaction processes, eventually affecting the overall system performances, is expected in a working fusion device chamber. Monitoring the changes in the composition of the plasma facing component (PFC) surface layer, as a result of erosion and redeposition mechanisms, can provide useful information on the possible plasma pollution and fuel retention. To this aim, suitable diagnostic techniques able to perform depth profiling analysis of the superficial layers on the PFCs have been developed. Due to the constraints commonly found in fusion devices, the measuring apparatus must be non invasive, remote and sensitive to light elements. These requirements make LIBS (Laser Induced Breakdown Spectroscopy) an ideal candidate for on-line monitoring the walls of current and of next generation (as ITER) fusion devices. LIBS is a well established tool for qualitative, semi-quantitative and quantitative analysis of surfaces, with micro-destructive characteristics and some capabilities for stratigraphy. In this work, LIBS depth profiling capability has been verified for the determination of the composition of multilayer structures simulating plasma facing components covered with deposited impurity layers. A new experimental setup has been designed and realized in order to optimize the characteristics of a LIBS system working in vacuum conditions and remotely, two noticeable properties for an ITER-relevant diagnostics. A quantitative analysis has been carried out in determining the elemental composition of the ITER-like samples. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction In fusion devices the confinement of the hot central plasma is never complete and the surrounding walls are subjected to particle and energy fluxes [1]. In addition they are hit by electromagnetic radiation and neutrons created in the plasma [1–3]. Due to these harsh conditions the choice of the plasma facing materials is crucial for present and even more for future fusion devices like ITER and DEMO. The presence of different materials inside the same system (as foreseen in the initial phase of ITER operation, with C, Be and W PFCs [4]) makes chemical characterization of the in vessel material surfaces a crucial issue in monitoring the real composition of the PFCs. Most of the techniques commonly used for the chemical characterization of thin films requires working conditions that prevent them to be useful for an on line analysis, in vacuum and remotely (see, for example, secondary ion mass spectrometry (SIMS) [5], X-ray photoelectron spectros-

⇑ Corresponding author. Address: ENEA, C.R. Frascati, V.E. Fermi 45, Office box 65, I-00044 Frascati, Italy. Tel.: +39 06 9400 5320; fax: +39 06 9400 5314. E-mail address: [email protected] (S. Almaviva). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.11.050

copy (XPS) [6], Auger electron spectroscopy (AES) [7], energy dispersive X-ray spectroscopy (EDX) [8] and Rutherford back scattering (RBS) [9]). On the other hand, these requirements make LIBS an ideal candidate for monitoring elemental impurities deposited on and fuel retained in PFCs of present and of next generation tokamaks. LIBS is a well assessed characterization tool for chemical composition of bulk materials [10]. It resulted to be particularly suitable for the analysis of various kinds of samples like metal alloys [11], artworks [12] and soils [13,14]. In fusion environment the use of LIBS could be advantageous since it is a relatively cheap technique that allows analysis of samples without any pre-treatment [10–15]. Furthermore, almost all materials can be examined and a wide variety of elements can be detected once present either as main constituents or at a trace level [10]. This study is just devoted to check the depth profiling capabilities of the LIBS technique as a remote on-line diagnostics for the determination of the composition of multilayer structures simulating the surface of a PFC covered with deposited impurity layers. The atomic concentration of the main chemical elements composing the samples surface layer has been derived from the LIBS spectroscopic data and the results were compared with the nominal values.

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2. Experimental

2.2. Sample description

2.1. Experimental set-up

The samples have been provided by the National Institute for Laser, Plasma and Radiation Physics of Magurele–Bucharest (NILPRP), Romania. They are made of a metallic substrate of pure Ti of 4  2.5  0.5 cm3 on which metallic coatings of fusionistic interest have been deposited using Combined Magnetron Sputtering and Ion Implantation (CMSII) technique [16]. The samples were characterized after deposition with Glow Discharge Optical Spectroscopy (GD-OES) technique [17], which provided the reference values for the chemical species concentration. Two different kinds of coatings were deposited on the substrate: the first is a W coating, 12.8 lm thick with 2.4 lm thick Mo interlayer as the one used as marker of W erosion in JET divertor [18]; the second is a mixed C–W coating (C–W coating in the following), deposited with two different thickness (8.5 lm and 12.2 lm) and atomic concentrations. The main properties of the sample coatings are summarized in Table 1.

In the framework of the EFDA task WP08-TGS-01-04: ‘‘material deposition and composition of walls’’ this work is focused on the feasibility study for application of the LIBS technique on a tokamak. The optimization criteria considered rely the capability of a LIBS system in working at low pressure and remotely, at distances of some meters from the target, to spectrally detect thin coatings of materials of fusionistic interest like the tiles here used as target. To match these requirements a new experimental setup has been designed and realized in order to optimize the characteristics of a LIBS system working at low pressure and remotely. The system layout is shown in Fig. 1a and b. The apparatus is composed of a vacuum chamber, 35 cm diameter, in which the sample is positioned on a two-axis motorized translation stage, remotely controlled for changing the measurement point to find optimal focusing and to expose a new zone of the target after a few laser shots. Laser ablation of samples was achieved using a Q-switched Nd:YAG laser (model Handy, Quanta System) emitting the fundamental wavelength at 1064 nm. Laser pulses of 8 ns duration, having a Gaussian-like spatial profile and nominal energies of 275, 363, 500, 643, 788, 925 mJ, were focused horizontally onto the samples by a 200-cm focal length quartz lens (25 mm diameter) through a 150 cm length vacuum arm closed at the end by an optical window highly transparent to infrared radiation. Repetition rate was 10 Hz. All the measurements were carried out in vacuum with a residual atmosphere of 5  105 mbar. Plasma emission was collected by a 30-cm focal length fused quartz lens, 76 mm aperture, placed behind a 75 mm diameter fused quartz window mounted at an angle of about 45° with respect to the laser beam axis. The lens focuses the plasma light onto a telescopic coupler, 25.4 mm aperture, consisting of two quartz lenses whose total focal length is 50 mm. It drives the optical signal into an optical fiber bundle, composed of 12 fibers, 100 lm core, F# 4, connected to a 100 lm entrance slit of a TRIAX 550 ISA JOBIN-IVON spectrograph, f# 6.4. For a diffractive grating of0 2400 g/mm the wavelength resolution at 500 nm is around 0.1 Å A. Spectra were recorded using a gated ICCD (DH532-18F, Andor), with a minimum time resolution of 10 ns, whose gate aperture was synchronized with the laser burst by mean of an electronic trigger. In order to test the possibility of remote acquisition, the collecting optics were positioned at a distance of 150 cm from the sample surface.

3. Results and discussion 3.1. Plasma parameters Some preliminary tests were performed to infer the critical plasma parameters in this conditions. According to previous works on LIBS plasmas in vacuum [19], we observed a faster plasma decay than in ambient pressure with atomic and ionic emission lines intensities decreasing very rapidly. This is shown in Fig. 2 where the decay constants of some atomic and ionic Ti lines are fitted in the 343–353 nm range in a sequence of 100 ns time-resolved spectra, collected with a gate width of 100 ns. It can be argued that very slight differences in time decay between atomic and ionic species are observed. This is due to the low probability of three body recombination for ions in vacuum because the ablated material can easily leave the sample surface and expand into the surrounding low pressure region [20]. At the settled low pressure condition the emission peaks are better resolved than in air at atmospheric pressure and the background is greatly reduced, nevertheless the net intensities of each emission line is reduced and some of the weaker expected emissions [21] could not be detected. A window suitable for signal maximization was chosen in the temporal range where conditions close to Local Thermodynamic Equilibrium (LTE) could be assumed [22], as described in the following, and the best signal-to-noise ratio (S/N) was achieved. To this aim the electron density was evaluated by the FWHM Stark broadening

Fig. 1. (a) Sketch of the LIBS apparatus for remote inspection of plasma facing components, and (b) picture of the set-up, from right to left: the vacuum chamber, its long arm towards the laser (on the back), the spectrograph and the ICCD with signal collection optics.

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S. Almaviva et al. / Journal of Nuclear Materials 421 (2012) 73–79 Table 1 Samples description. Sample

Coating thickness (lm) C–W

1 2 3 a

Coating composition (atom%)

W

Mo (interlayer)

12.8 ± 1

2.4 ± 1

8.5 ± 1 12.2 ± 1

C

W

Mo (interlayer)

O

100

82 70

100 15 27

–a 3 3

Oxygen contamination at the surface occurring only for less than 1 lm thickness.

The results are shown in Fig. 3, where spectra from the Ti substrate, acquired as specified, are compared. As expected, the S/N is significantly decreased by reducing the lens aperture, nevertheless with the current set-up major spectral features are expected to be still detectable at 5.5 m distance from the collecting lens with an experimental set-up similar to the presented one. However, the increase in collection distance affects the measurement accuracy, particularly the detection of the weaker emission peaks, which can appear unresolved or smeared in the background. A quantitative estimation of the S/N loss was done comparing the background noise amplitude with the signal of the higher peak in each spectrum and a loss of 11 dB/m was estimated. This results suggest that a proper collecting telescope of great aperture would be necessary for remote analysis of targets at distances longer than 5 m. Fig. 2. Temporal evolution of the intensity of atomic and ionic titanium lines; their wavelengths are given in the legend together with respective decay constants (as calculated from a single exponential fit). Relevant uncertainty are about 6%.

18

3

of the Ti II line at 346.15 nm (w = 0.06 nm at ne = 10 cm [23]); on the basis of the temporal behavior of the ne value a gate delay of 750 ns and width of 200 ns have been chosen, corresponding to an almost constant ne value of 5 ± 0.3  1017 cm3. With the aim to study the effects of laser ablation on the coated tiles, the laser pulse energy was varied in six steps (275, 363, 500, 643, 788, 925 mJ), as detected with a Gentec (Markham, Canada) ED-500 energy meter. In order to determine the laser fluence on the sample, the areas of the laser-produced craters were measured using an optical microscope. Values between 0.018 and 0.026 cm2 were obtained, corresponding to fluencies values in the range 15– 35 J/cm2 as specified in Table 2. 3.2. Remote detection For a remote LIBS setup, whose optical assembly is designed to be used at distances comparable with the fusion vessel sizes, the signal from the plasma is expected to be greatly reduced, compared with a conventional apparatus. To get quantitative information on this point we placed the collecting lens at a distance of 500 mm from the plasma and we partially screened it with circular diaphragms of different diameters, in order to simulate the occurrence of plasma light detection at increasing distances.

Table 2 Laser energies and corresponding fluencies released on the samples surfaces per single laser shot. The spot area considered is the one where the power density is above the ablation threshold of the materials. Laser shot energy (J)

Laser spot area (cm2)

Fluence (J/cm2)

0.275 0.363 0.5 0.643 0.788 0.925

0.018 0.0197 0.0254 0.0257 0.0258 0.0261

15.2 18.4 19.7 25 30.5 35.4

3.3. Profilometry The elemental composition and the stratigraphic structure of the first coating have been studied recording the emission intensity of the W, Mo, Ti lines as a function of the laser shots number. To this aim, the background signal was averaged over 20 pixels in a spectral region free from emission lines of the elements and then subtracted to the peak intensities. The spectral region of interest was selected between 416 and 441 nm, because intense and well resolved W, Mo, Ti emission lines are present there [24], thus allowing to clearly detect the transition from the superficial layer to the substrate. A sequence of 50 shots was applied for each set of measurements, in order to detect the whole transition from the surface layer to the sample substrate. In the case of sample 1 (W–Mo coating) the detection of the thin Mo interlayer is particularly interesting because this layer is of great importance as marker in monitoring the erosion of the W superficial coating of the vacuum vessel tiles [18]. So the intensity of the Mo I emission line at 438.16 nm was monitored together with the W I line at 434.7 nm and the Ti II line at 439.16 nm which identified the surface layer and the substrate, respectively. The intensities of the monitored lines are shown as a function of laser shots and laser fluencies in Fig. 4. From the intensity behavior of the W I line the superficial coating appears to be quite uniform, no traces of Ti or Mo have been detected by LIBS inspection. As expected, the LIBS intensity increase with laser fluence; to this respect it should be noted that it was not possible to detect the Mo I line for fluencies lower than 19.7 mJ/cm2. On the contrary, in the first shot we observed an unexpected reduction of the W I LIBS signal and an increase of the Ti I LIBS signal with respect to the following shots. We attribute this effect to the presence of contamination from either the deposition process or former ablation at different sample locations. To confirm this assumption we performed an elemental analysis by Energy Dispersive X-ray analysis (EDX) [8,25] on two laser-produced holes on samples with C–W and W–Mo coating respectively. With EDX it has been studied the distribution of C, Ti, W and Mo along two linear profiles through the center of the considered laser

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Fig. 3. Sequence of spectra from the titanium substrate, obtained partially screening the collecting lens (note the different vertical scale).

spots. For both samples it was possible to verify that Ti is present in traces as impurity in the area near the laser-induced holes (Fig. 5 a and b) up to 1 cm away, whereas the average distance among the investigated points was about 5 mm. From the nominal thickness of the layers we estimated the ablation rate at the adopted laser fluencies for each sample, as reported in Table 3. Submicrometric values were found in each case, demonstrating the good depth profiling capabilities of the apparatus. In detecting the thin Mo interlayer in sample 1 the signal starts to appear with a number of laser shots decreasing with increasing fluence, as expected because of the increased ablation rate of the superficial coating. However, irrespective of the larger ablation, the same signal tends to be more persistent at higher fluencies as can be inferred from Fig. 6 in which is shown the intensity of the Mo I at 438.16 nm as a function of the number of laser shots at different fluencies. We attribute this trend to an increased lateral ablation which results in a intermixing of different layers. This hypothesis would be consistent with the results of the previous EDX analysis carried out in the middle of the two laser spot, where the pure Ti substrate is reached. In fact, due to a persistent lateral ablation, probably present until the last laser shot, traces of re-deposited elements from the superficial layers (C, W for the samples with C–W coating and W, Mo for the sample with W–Mo coating) have been also detected in the center of the laser-induced spots (Fig. 5a and b), whereas only Ti is expected to be detected. We attribute the occurrence of lateral ablation to several factors like, for example, the focal length of the focusing lens used that affects the energy distribution of the incident laser beam [26] and reduces the efficiency of beam shaping techniques, the angle of incidence of the laser beam on the sample surface [27], some physical and mechanical

properties of the target itself like reflectivity, density, specific heat and boiling point of the target material which have an important influence on the shape and size of the craters and could led to different ablation rates for different materials [28]. All these factors are critical in LIBS because they could modify the sampling volume, and consequently the depth resolved information. More in general, intermixing of materials coming from different layers could arise due to the phenomena occurring during laser-material interactions in a nanosecond laser pulsed that include heating, melting and vaporization of the sample [29]. Such phenomena could be suppressed or minimized using femtosecond laser pulse durations. Ablation with femtosecond laser pulses is considered as a direct solid–vapor (or solid plasma) transition [30]. During the pulse, thermal conduction into the sample can be neglected and some of the benefits of this fast interaction are more reproducible ablation processes [30]. In spite of the advantages, the use of a fs laser source in the on-line LIBS set-up add remarkable complexity, so the current experiments have been carried on with the available ns source, as currently used in all commercial LIBS set-up. However in optimizing the depth profiling capabilities of the current ns LIBS system two opposing needs must be considered: reducing laser fluence means reduce lateral ablation but LIBS signal also decreases. 3.4. Detection of trace elements We have also detected hydrogen and oxygen on the samples surface with C–W coatings. oxygen is present as low concentration element in the first two micron of the superficial layer, (as reported from the GDOS spectra of the C–W coatings, not shown here). The detection of hydrogen isotopes is of great interest for the inventory of tritium in the plasma facing materials, limiting machine operation because of safety problems, and it will be one of the greatest

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Fig. 4. LIBS intensity of W I at 434,7 nm, Mo I at 438.16 nm, Ti II at 439.5 nm lines at different laser fluencies.

Fig. 5. (a) EDX analysis of one the laser spot on sample 2 (C–W coating on the left), and (b) sample 1 (W–Mo coating on the right). The linear profiles investigated are superimposed to the spots images. In (a) counts in red are related to carbon Ka1 emission, counts in green are related to titanium Ka1 emission, counts in blue are related to tungsten La1 emission. In (b) counts in red are related to titanium Ka1 emission, counts in green are related to molibdenum Ka1 emission and counts in blue are related to tungsten La1 emission. Reference bar are 800 lm in (a) and 500 lm in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

technological challenges of next generation fusion facilities like ITER [2]. To better highlight the different contamination of hydrogen and oxygen the normalized intensities of Ha hydrogen line at 656.28 nm

and the O I lines at 777.19 nm, 777.42 nm are reported in Fig. 7 as a function of number of laser shots. While the hydrogen signal, due to the sample contamination from air is present only in the first laser shot, the oxygen signal

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Table 3 Ablation rates for different coatings at different laser fluencies. The error is due to the uncertainty on the thickness of the superficial layer. Laser fluence (J/ cm2)

Ablation rate (lm/shot) sample 1 (W coating)a

Ablation rate (lm/shot) sample 2 (C–W coating)

Ablation rate (lm/shot) sample 3 (C–W coating)

15.2 18.2 19.7 25 30.5 35.4

// // 0.45 ± 0.04 0.53 ± 0.04 0.56 ± 0.04 0.63 ± 0.05

0.29 ± 0.03 0.32 ± 0.04 0.34 ± 0.04 0.37 ± 0.04 0.37 ± 0.04 0.4 ± 0.05

0.3 ± 0.02 0.34 ± 0.03 0.38 ± 0.03 0.4 ± 0.03 0.43 ± 0.04 0.44 ± 0.04

a The symbol ‘‘//’’ stands for ‘‘not retrieved from data’’ being impossible to clearly detect the Mo interlayer.

persists for some successive laser shots, thus revealing a slightly deeper contamination.

Fig. 7. Normalized intensities vs laser shot for Ha line at 656.28 nm, O I lines at 777.19 nm, 777.42 nm.

3.5. Quantitative analysis

Iki k ¼ Ns

A LIBS quantitative analysis usually begins with determining the response of the detection system for a given concentration or mass of the analyte of interest and this is accomplished in making calibration curves, using several samples containing the analyte at different concentrations. In the case of the present application the described approach appears difficult because of the large quantity of different mixed layers possibly found in the fusion devices and therefore of the corresponding large number of needed calibration samples (that should be analyzed remotely). Due to these constraints we applied the Calibration Free (CF) procedure as alternative LIBS quantitative analysis approach [31,32]. The method, suitable for a remote and in situ analysis, does not require any calibration sample, provided that experimental spectroscopic data are available for each chemical species present in the plasma. As in all procedures for LIBS quantitative analysis, the CF method assumes the occurrence of the following assumptions within the LIBS signal acquisition time window [31]:  Local Thermodynamic Equilibrium holds, with a unique value for temperature and electron density.  All the ablated material is in neutral or singly ionized form, (e.g. higher ionization stages are negligible).  Plasma emission is optically thin. Once these assumptions are satisfied, the spectrally integrated line intensity corresponding to an atomic or ionic transition between energy levels Ek and Ei of the element s is given by:

Fig. 6. LIBS intensity of Mo I line at 438.16 nm at different number of laser shots.

g k Aki eEk =kB T U S ðTÞ

ð1Þ

where k is the transition wavelength, Ns is the number density (particle/cm3) of the emitting atom in plasma, gk is the level degeneracy, Aki is the transition probability, kB is Boltzmann constant and US(T) is the partition function of the emitting species at the plasma temperature. From (1) it can be observed that, once the plasma temperature is determined and the atomic data for the emissions are known, in principle the concentration of each species could be obtained from the measurements of a single line. However, because of the uncertainties existing in literature on the atomic data, a precise concentration measure must be obtained using several spectral lines at different energies. Peculiar to CF method, the hypothesis of detecting all different elements present is formulated in utilizing the closure property [31,32] which simply means that the sum of all species concentrations (in percentage) must equals unity. In the case of the present measures the plasma temperature was estimated by the Boltzmann plot method [10] applied to the emission lines of tungsten in the 416–441 nm spectral region where characteristic emissions of this element appear from both neutral and singly ionized species. Depending on fluencies temperatures between 19,000 ± 1200 K and 20,000 ± 1800 K were found for samples 2 and 3. In order to check the existence of an optically thin plasma [33] we compared the intensity ratio of WI emission lines at 424.1 nm, 426.94 nm and 430.21 nm with the expected values reported in NIST electronic database [24]. The absence of line quenching has also been tested checking the linear dependence of the WI line intensity at 429.46 nm against the laser fluence. It was found that at higher fluencies (more than 25 J/cm2) quenching phenomena starts to be consistent, resulting in a saturation of the line intensity and a non-linear behavior against laser fluence. On the other side, low laser fluencies should be avoided, due to a reduced LIBS signal and a possible non-stoichiometric ablation [34]. To satisfy these two opposing requirements we applied CF to data obtained with laser fluencies of 19.7 and 25.0 J/cm2 that, with a 8-ns laser pulse duration correspond to power densities of 2.4 GW/cm2 and 3.1 GW/cm2 per laser shot. These values satisfy the necessary condition for stoichiometric ablation for the elements composing the coatings because it is well beyond the process threshold of about 1.0 GW/cm2 as can also be inferred from [34]. With the CF procedure we evaluated the carbon/tungsten atomic concentration ratio, presents as main constituents in the C–W coatings and we compared the obtained results with the nominal concentration ratio provided by GD-OES. The results are summarized in Table 4.

S. Almaviva et al. / Journal of Nuclear Materials 421 (2012) 73–79 Table 4 Comparison between the nominal and the calculated elemental composition (atom% concentration) as obtained by CF for two different laser fluencies. Error bar is mainly due to uncertainties on temperature. Sample

Element

Nominal value

CF-LIBS (19.7 J/cm2)

CF-LIBS (25.0 J/cm2)

1 1 1 2 2 2

C W Others C W Others

70 27 3 82 15 3

68 ± 12.5 29 ± 12.5 // 80 ± 12.5 17 ± 12.5 //

70 ± 12.5 27 ± 12.5 // 76 ± 12.5 21 ± 12.5 //

A good agreement was obtained between the measured and the nominal values, although the uncertainty on the experimental measurement is not lower than 25%. This uncertainty is mainly due to the uncertainty in determining the plasma parameters, temperature and electron density [29]. 4. Conclusions A new experimental apparatus has been set up to test LIBS as remote and in situ diagnostic for nuclear fusion application. In vacuum LIBS measurements have been done on samples coated with layers of fusion relevant materials with the aim to monitor the effective elemental composition of the in vessel PFCs. The results show that thin layers of different materials have been clearly detected and the nominal concentration of elements in a mixed layer has been quantitatively reproduced. Although determining absolute thickness of a re-deposited layer with unknown composition is not possible since the ablation rates are function of the layer composition, the performance achieved are nevertheless significant to the problem of on line monitoring the elemental composition of the layer. We also detected trace elements present on the sample surface as environmental contaminants such as hydrogen and oxygen, whereas the second was also detected as trace impurity in the growth process of the films, highlighting the high sensitivity of the system and the possibility of a qualitative and quantitative on-line evaluation of the gaseous species codeposited on the in vessel PFCs during operation. The conclusion is that, a LIBS remote system operating in realtime without internal standards would be very useful for an on-line elemental analysis of the fusion device PFCs, provided that both optical configuration and electronic acquisition are optimized for high sensitivity detection. Once quantitative analysis by CF procedure are routinely proposed, an accurate determination of plasma parameters should be attempted in order to reduce the uncertainty on concentration measurements. To this respect missing spectroscopic data (line-widths, lifetimes, accurate knowledge of the partition functions of heavy elements in the investigated temperature range) should be provided in updated data bases for nuclear fusion materials. Acknowledgments This work was carried out in the frame of the European Fusion Development Agreement, Task WP08-TGS-01-04 ‘‘Material deposition

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