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Design and development of a LIBS system on linear plasma device PSI-2 for in situ real-time diagnostics of plasma-facing materials X. Jiang∗, G. Sergienko, B. Schweer, N. Gierse, M. Hubeny, A. Kreter, S. Brezinsek, Ch. Linsmeier Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung - Plasmaphysik, 52425 Jülich, Germany

a r t i c l e

i n f o

Article history: Received 29 July 2016 Revised 3 November 2016 Accepted 17 November 2016 Available online xxx Keywords: Plasma-wall interaction Laser induced breakdown spectroscopy Plasma facing component Material diagnostic

a b s t r a c t Laser induced breakdown spectroscopy (LIBS) is a strong candidate for detecting and monitoring the H/D/T content on the surface of plasma facing components (PFCs) due to its capability of fast direct in situ measurement in extreme environment (e.g., vacuum, magnetic field, long distance, complex geometry). To study the feasibilities and encounter the challenges of LIBS on plasma devices, a LIBS system has been set up on the linear plasma device PSI-2. A number of key parameters including laser energy, the influence of magnetic field and the persistence of laser induced plasma are studied. Real-time measurements of deuterium outgassing on tungsten samples exposed to deuterium plasma of 1025 D/m2 are performed in the first 40–130 min after plasma exposure. The experimental results are compared to the calculations in the literature. © 2016 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction In situ, real-time analysis of H/D/T content in plasma-facing materials is of great interest to ITER wall materials (e.g., Be, W) diagnostics. It is crucial to the safety of plasma operation and thus one of the most important research topics in this area. Different diagnostic tools have been developed for fuel monitoring and retention including laser induced desorption spectroscopy (LIDS), laser induced ablation spectroscopy (LIAS) and laser induced breakdown spectroscopy (LIBS) etc. [1]. As a powerful and versatile analytical technique, LIBS measures the element composition of bulk material and deposits with relatively low detection limit and high sensitivity. It can be adapted for plasma devices with various scales and provide analysis without plasma operation i.e., between plasma discharges. These features make LIBS favorable compared to other diagnostic tools that require plasma edge conditions or have lower sensitivity. Though an increasing number of studies have been carried out in the recent years [2–5], most of these studies are laboratory-based, post-mortem analysis [2,3], or merged with other tools [5] due to the complexity of in situ, real-time measurement. To achieve in situ real-time diagnostics of the target surface after

∗

plasma exposure on a plasma device, a LIBS system is installed on the linear plasma device PSI-2. PSI-2 is a linear plasma device used for the study of plasmawall interaction (PWI) [6]. A stationary plasma is produced in a low-pressure (10−7 mbar base pressure, 10−4 with plasma operation), high-current arc discharge between a heated cathode (1600 °C) consisting of lanthanum hexaboride (LaB6 ) and a molybdenum anode. Samples are exposed to linear plasma produced by one or a mix of the selected gases from H2 , D2 , Ar, Ne, N2 and He. PSI-2 has two major chambers for exposure and analysis purposes. The main chamber where samples are exposed to the plasma is about 1.5 m from the cathode and the analysis chamber which is for exchanging targets, diagnostics and analysis is another 1.5 m away from the main chamber. 6 magnetic coils surround the main chamber, producing a total magnetic field of 0.1 T. The plasma is confined by the magnetic field to form a ∅6 mm plasma cylinder on the target surface. A 3.5 m long sample manipulator holds the sample stage and move the samples between the two chambers. The current LIBS setup is on the plasma exposure chamber, another setup will be built on the target exchange chamber in the near future.

Corresponding author. E-mail address: [email protected] (X. Jiang).

http://dx.doi.org/10.1016/j.nme.2016.11.021 2352-1791/© 2016 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: X. Jiang et al., Design and development of a LIBS system on linear plasma device PSI-2 for in situ real-time diagnostics of plasma-facing materials, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.11.021

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2. Background

make sure that a sufficient amount of D atoms are implanted on the tungsten surface.

2.1. Laser ablation 3. Experimental The ablation of target material by a pulsed laser is a complex process involving laser-solid interaction, plasma formation and expansion, laser-plasma interaction. Inverse Bremsstrahlung and photoionization are considered to be the main absorption mechanisms of the laser light within the ablation plume produced on a metallic target [7]. In the case of W sample ablated by a nanosecond laser pulse, the penetration depth of a laser pulse interacting with the material is determined by the optical and thermal penetration depths, depending on which is larger: l = max{lopt , lthm } [8]. Tungsten is not an opaque material the ablation depth is defined by the thermal penetration depth. Thermal penetration depth lthm = 2(at p )1/2 depends on the laser pulse duration tp . a = K/ρC is the thermal diffusivity which equals to the thermal conductivity K divided by the material density ρ and specific heat C [8]. The W samples used in the current experiments have a purity greater than 99.97% with a few trace elements (Mo, Fe, C, Cr, etc.) less than 30 μ g/g. For a ns laser pulse, the power density has to be above a certain threshold to cause  the ablation. The ablation threshold is expressed as Fth = ρ Lν at p . Lν is the latent heat of vaporization. The mass ablated per pulse can be estimated as m = E/[C (Tb − T0 ) + Lν ]. E is the energy of the laser pulse that is absorbed by the target, C is the specific heat capacity in J K−1 g−1 , Tb and T0 are the boiling and initial temperature of the material, Lv is the latent heat of vaporization (J g−1 ). When a laser pulse of 600 mJ, focused to a ∅4 mm spot on the W surface, the ablation parameters are calculated as follows: Heat penetration depth: 0.9 μ m; Power threshold to vaporize the target: 8.6 × 108 W/cm2 ; Estimated mass removal by a single pulse: 12 μ g. In fact, the LIBS experiments on W sample, the averaged ablation depth is estimated to be 25 nm/pulse on a ∅4 mm spot and ablation mass m = ρW π (d/2 )2 h is roughly 8 μ g/pulse, which is less due to the fact that part of the laser pulse is reflected by the tungsten surface and part of the it is absorbed by the laser induced plasma. 2.2. LIBS Detection limit In LIBS literature, limit of detection (LOD) is defined by the International Union of Pure and Applied Chemistry IUPAC [9] as 3σ B /S which is 3 times the ratio of standard deviation of the background and the slope of the calibration curve. The lowest LOD of LIBS is reported to be about 0.1 ppm. In practice, this value is usually above 1–10 ppm [10]. In the case of in situ measurement of H, D on W, detection limit is described as the lowest measurable D atoms per m2 . Using the calculations in Section 2.1, in a single measurement where one laser pulse ablates 12 μ g of W material out of a ∅4 mm crater, to reach 10 ppm LIBS detection limit, the D atoms should reach a value of 3× 1018 D/m2 . According to a study on deuterium retention in tungsten samples [11], the retained D atoms on the tungsten surface depends on the incident fluence at a certain temperature. With 200 eV/D+ on polycrystalline tungsten (PCW) at 300 K, when the incident fluence is between 1024 and 1025 D+ /m2 , the retained D is between 1020 and 1021 D/m2 . Since the current linear plasma operates with a fluence usually above 1024 D+ /m2 , the retained D (1020 D/m2 ) is above the estimated detection limit of 3 × 1018 D/m2 . This result reassures that the retained D on W surface is above the LIBS detection limit. The retention rate depends on the crystal structure, damage and other factors. The current work will not discuss the material aspects of tungsten. The purpose of this work is to study the feasibilities of LIBS and all the samples are exposed in the plasma for 3–4 h to

The LIBS system consists of the laser , the lenses and mirrors for laser beam focusing and guiding, plasma light collection optics and the spectrometer based detecting system. The laser is an INNOLAS Nd:YAG laser (model Spitlight20 0 0/10 Hz). It provides four possible wavelengths (1064, 532, 355, 266 nm) and operates at second harmonic 532 nm for the current experiments. The corresponding maximum output energy for 532 nm is 600 mJ. The pulse width is 6 ns with jitter less than 1 ns. The power density on a ∅4 mm spot is (0.6 J / 6 ns)/ (π · 22 mm2 ) ≈ 0.8 GW/cm2 . The repetition rate is 10 Hz with dividing options to reduce the frequency to 5 Hz, 2 Hz, 1 Hz and single shot mode. Three sets of mirrors along a 30 m beam path are used to guide the laser light from the laser room to the PSI-2 stage. Each set of mirrors consists 2 mirrors. Each mirror has efficient reflection at 355 nm, 532 nm and 1064 nm at 45°, with a transmission of 0.226% at 532 nm. Before the laser beam enter the plasma chamber, a spherical metallic mirror (Cu coated by Al) of ∅100 mm, f = 1.9 m is used to focus the laser beam at the center of the chamber, where the target is located. A 30 m optic fiber transmits LIBS signal from the PSI-2 chamber to the spectrometer. One SOLIGOR TV lens with f = 50 mm, F # = 1.3 is placed near the chamber window to collect the plasma light and focus it into a ∅1.5 mm quartz glass fiber. One ISCO CCTV lens with f = 25 mm, F # = 1.4 is used on the other end of the fibre to focus the light into the spectrometer. The spectrometer is a Czerny Turner spectrometer (Acton SpectroPro-500) equipped with three gratings (60 0/120 0/240 0 grooves/mm). 2400 grooves/mm setting is used for all the experiments in this work unless specified. The camera is an image intensified CCD camera (Pike F032b with micro channel plate (MCP) image intensifier). The experimental structure for LIBS on main chamber setup are shown in Fig. 1. In PSI-2 exposure chamber, tungsten samples are exposed to deuterium plasma with a maximum flux density of 1023 m−2 s−1 . The deuterium gas flow is 100 sccm. The plasma flux is measured by Langmuir probe, the plasma ion flux profile is shown in Fig. 2. The parallel flux is the flux measured in the direction of the plasma flow. In the current case, the W sample surface is perpendicular to the plasma direction, which means that the ion flux on the sample surface equals to the parallel flux measured by langmuir probe. In the figure, the W sample surface is parallel to the x axis and thus directly facing the plasma. The size of each W sample is 10 × 10 mm and is fully covered by the plasma flux. The maximum ion flux is 3.5 × 1021 m−2 s−1 and 4 h exposure time gives a total fluence of 5× 1025 m−1 . LIBS measurements are carried out in plasma exposure chamber so that the environment (vacuum condition, chamber temperature etc.) for W samples before and after plasma exposure are roughly the same and the measurements provide the outgassing behavior in vacuum. 4. Results and discussion 4.1. Influence of laser pulse energy The laser beam path is fixed on PSI-2 and so focusing condition of the laser beam is a fixed parameter for LIBS. As a result, features of the laser induced plasma depend solely on the laser pulse energy on a given sample. The power density on the sample surface has to be above the ablation threshold to vaporize and ionize the material. At lower laser energies, the electron density ne and temperature Te are lower and the dominant emission is from neutral atoms or singly charged ions. Within a certain range above the threshold, increasing the laser energy results in increased emission

Please cite this article as: X. Jiang et al., Design and development of a LIBS system on linear plasma device PSI-2 for in situ real-time diagnostics of plasma-facing materials, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.11.021

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3

Target exchange (analysis) chamber Plasma exposure (main) chamber

Fig. 1. Experimental set up of LIBS on PSI-2.

Sample posion

Fig. 2. Ion flux of deuterium plasma on PSI-2 measured by langmuir probe.

intensity. When the laser energy keep increasing, emission intensity of one particular spectral line will be saturated and eventually reduced when the dominant emission is shifted to the highly charged ions. In the meanwhile, the continuum background of the plasma emission is also increased and sometimes overwhelms the spectral line and make it difficult for spectral analysis. To examine the dependence of LIBS signal on the laser pulse energy, C samples from TEXTOR tokamak first wall tiles are used. One reason is that these wall tiles have been exposed to the plasma during TEXTOR operation and there are codeposited and implanted H and D in the sample, which make a good material for testing the newly setup LIBS system. Another reason is that C has a relatively

simple atomic structure and is a well calibrated element for spectroscopy. The intensity and signal-to-background ratio (SBR) of C II 426.7 nm line are recorded at various laser energies as shown in Figs. 3 and 4. The time delay between the laser pulse and the camera is 50 ns, the exposure time is 300 ns. In the range of 250– 600 mJ, the intensity increases almost linearly with the laser energy. The SBR curve shows a hint of saturation at energies above 550 mJ. The green line denotes SBR = 1, indicating that the spectral signal is only greater than the background when the laser energy is above 350 mJ. Considering both intensity and SBR, 450–500 mJ is a suitable energy range for LIBS measurement. To avoid poten-

Please cite this article as: X. Jiang et al., Design and development of a LIBS system on linear plasma device PSI-2 for in situ real-time diagnostics of plasma-facing materials, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.11.021

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Fig. 3. Influence of laser pulse energy on spectral line intensity.

Fig. 4. Influence of laser pulse energy on signal-to-background ratio.

tial damage to the focusing metallic mirror, 450 mJ is use for the experiments after this section unless specified.

The second image shows the plume with two magnetic coils on the exposure chamber. These two magnetic coils are closer to the target and provide stronger magnetic field than the other 4 coils combined. The third image shows the plume with full magnetic field of 0.1 T. One can see that the core area of the LIP plume is larger and brighter in magnetic field but the plume expansion is largely restrained. This results in a dense plasma core but with limited characteristic emission. Frame size of the fast camera is reduced to 384 × 24 to increase the recording speed to 530,0 0 0 fps. The shutter is triggered every 1.9 μ s and opens for 1.58 μ s for each frame. The shutter time covers the lifetime of a typical LIBS plasma (0.5–1 μ s). Fig. 6 shows the image intensity distribution along the plasma expansion direction (x axis). The result is an direct interpretation of the imaging results in Fig. 5. The intensity is a sum of the total emission and it does not represent any particular emission line. The x axis has the unit of pixel and covers the area from 250 to 350 which corresponds to the plume area in Fig. 5. The intensity peak between pixel 260 and 290 corresponds to the plasma core area which has a high thermal radiation. From pixel 290 to 320, the plasma plume in the magnetic field is restrained, the plume in vacuum without magnetic field expands further and characteristic emission lines are emitted from this area. A time scan of the intensity of C II 426.7 nm line (which denotes the emission intensity) is shown in Fig. 7. The camera delay time is varied from 50 ns to 480 ns in the steps of 25 ns with exposure time of 25 ns. From 0 to 100 ns, the plasma is full of continuum emission and the intensity is directly related to the plasma temperature. From 100 to 400 ns, where the plasma expands and emits characteristic lines, the intensity with magnetic field is much lower than the intensity without magnetic field. The magnetic coils on PSI-2 are used to restrain the plasma and so it is in the direction of the linear plasma column. In LIBS experiments, the target surface is rotated and tilted to face the laser beam. As a result, the plume of the laser induced plasma on the target surface is restrained by the surrounded magnetic field. As can be seen from the imaging result in Fig. 5, the plasma core area (yellow part) is bigger in magnetic field but the plume area (light blue part) is smaller compared to that without magnetic field. It is shown that the laser induce plasma expansion is restrained by the magnetic field which leads to a reduction in the emission from C II (singly charged) line. After 500 ns, the plasma decays and all intensities drop to about the same level.

4.2. Influence of magnetic field 4.3. D outgassing on W samples Diagnostics of the plasma facing wall in the complex ITER environment encounters a number of challenges. These challenges include high vacuum, awkward optical path, small observing angles, high excitation energy (10 eV) for hydrogen isotopes [3], as well as high magnetic field (1 - 5 T). On PSI-2, there are 6 magnetic coils around the vacuum chamber. These coils are placed about 40– 50 cm away from each other in a distance of 2–3 m, spread from the heating source to the end of the exposure chamber. Altogether, the 6 magnetic coils provide 0.1 T magnetic field. During a normal plasma operation, all 6 magnetic coils are turned on at the same time to confine the plasma with a settled shape and position. LIBS experiment is carried out when the linear plasma is off, it offers a chance to study the influence of the magnetic field on the laser induced plasma emission. A fast camera Phantom v711 is used to capture images and compare the patterns of LIBS plasma in vacuum with and without magnetic field. The camera faces the target in the same direction as the optic fibre as shown in Fig. 1, that is about 30° away from z direction. The top image in Fig. 5 is a picture showing the camera view of the target in the chamber. The 3 images below the camera picture are the images taken during the laser operation. The first image shows the plasma plume without magnetic field.

The capability of fast in situ measurement allows LIBS to monitor the process of D outgassing on W samples immediately after plasma exposure. 3 measurements of D outgassing on W samples are carried out on PSI-2. The preliminary results are reported in this section. In each experiment, LIBS measurement started about 40 min or longer after the deuterium plasma is turned off. There are a few things to consider after turning off the deuterium gas input and before the measurement: •

•

it takes 60 ms–0.5 s (measured with a 1300 l/s Pfeiffer Hipack 1200 turbo-molecular pump for m/z = 4 at 630 Hz and 210 Hz) to pump the chamber from 10−4 mbar to 10−7 mbar, which is the required base pressure for PSI-2 vacuum chamber. with the current setting, the target holder needs to be turned and rotated to face the incident laser beam, consequently, the light collecting fibre optics needs to be adjusted every time before the measurement. These movements and adjustments are all done manually and it takes about 20–30 min at the moment.

The pumping-down time is negligible compare to the other factors. The optic adjustment time can be much improved. Computer

Please cite this article as: X. Jiang et al., Design and development of a LIBS system on linear plasma device PSI-2 for in situ real-time diagnostics of plasma-facing materials, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.11.021

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Chamber

5

Target holder 2 cm Target

B = 0 (no magnec field)

B = 0.06 T (magnec field on main chamber)

B = 0.1 T (full magnec field)

Fig. 5. Fast camera images of LIP plume in magnetic field. X axis and Y axis are both pixels. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Intensity distribution of laser induced plasma plume in the direction of plasma expansion with and without magnetic field.

programs are under development to allow fast and precise control of those movements. A much reduced time (< 5 min) is expected for future measurements. Further more, a new setup is expected on PSI-2 with a completely new optic path will be available through two side ports of the main chamber. In that case, the sample stage or the collecting fibre optics no longer need adjustment before each measurement and the measurement time range can be reduced down to 0.1 s to a few seconds. With precise programming of target positioning, the next measurement can be performed less than 1 second. LIBS measurement with this setup will provide valuable information of the D outgassing on the target sample in the first few seconds after plasma exposure. In the current measurement, the laser spot size is a little less than ∅4 mm, the surface of one W sample is 10 × 10 mm, which allows roughly 2 measurement spots on each sample. 3 samples in the lower part of the target holder are available for one measurement, due to the fixed focusing position of the laser beam and limited movement of the target holder. Therefore, 6 measuring points are collected for each experiment. Studies on hydrogen outgassing following plasma exposure [12– 14] have revealed a universal power law decay of the released flux with time and the flux is described as:

F (t ) ∼ A · t −α

(1)

The power coefficient α is between 0.7 and 0.8. LIBS measures the remained D content on the W surface, which can be described as:

 N = N0 −

0

t

F (t )dt

(2)

N0 is the initial D content on the W surface at the time of plasma off. The D content at the time of LIBS measurement N equals to the initial amount of D subtract the outgassed D over the time period of t. Thus the flux F(t) is comparable to dN/dt. To compare LIBS results with the above reported result, each measurement is fitted by a single power function and the results are shown in Fig. 8. The three power fittings are listed in Table 1. The measured D content by LIBS can be expressed as:

N (t ) = N (t0 ) · (t/t0 )−β + N (∞ )

Fig. 7. Influence of magnetic field on CII 426.7 nm line intensity.

(3)

The equation describes the measured D content at the time t (t0 ≤ t ≤ ∞). t0 is the time of first LIBS measurement. For one exposure experiment, N(t0 is determined by the plasma exposure parameters and is fix value for all measurements in this exposure condition. In

Please cite this article as: X. Jiang et al., Design and development of a LIBS system on linear plasma device PSI-2 for in situ real-time diagnostics of plasma-facing materials, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.11.021

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Fig. 8. D outgassing on W in the first 2 h after plasma exposure.

Table 1 Power fit applied to 3 measurements of D outgassing on W samples. Measurement

Time after exposure

Power Fit

1 2 3

55–85 min 46–78 min 82–130 min

I = 1.969 × 106 · t−1.905 I = 1.063 × 105 · t−1.175 I = 1.117 × 106 · t−1.651

the first measurement, t = t0 , N (t ) = N (t0 ) + N (∞ ). When the time is long enough, the D residual in the W sample reaches a stable state, t = ∞, N (t ) = N (∞ ). This value changes very little over time and can be considered as a constant. The flux derived from the measured result (Eq. 3) is dN (t )/dt ∼ t −(β +1 ) , which is compared to the reported power law (Eq. 1), and so α is compared to β + 1. The value of the measured power coefficient β is between 1 and 2 and is already higher than the above reported α . β + 1 is between 2 and 3 and is about 2–3 times more than α . There are a few possible reasons: (a) LIBS measures D content in the first 1–2 h after plasma exposure, the reported calculation is the result of a much longer time period. (b) the laser pulse does not only ablate the sample but also heats up a considerable area around the crater. The outgassing process is accelerated due to the thermal effect, thus the D content drops faster than expected and gives a lower α value. (c) the 3 measurements were taken on 9 different W samples in 3 different days, the preliminary result includes only 6 data points for each plot, more data points will be included when better sample geometry is available. 5. Conclusions and future work As a versatile and flexible analytical tool, LIBS can be adapted for real-time, in situ measurements of D/H/T content in plasma facing wall materials. The current work reports several experiments on PSI-2 and shows the viability of LIBS in material diagnostics. A number of parameters are examined. The laser power density range (0.6–0.8 GW/cm2 ) for LIBS on graphite measurement is determined by considering both line intensity and signal-tobackground ratio. The expansion of plasma plume in vacuum with and without magnetic field are studied by fast camera imaging and spectra analysis. The most recent work on the time scan of D outgassing on W samples demonstrates one of the most notable features of LIBS: real-time in situ measurement of PFC materials. In

this work, a scan of D content on W samples is performed in the first 40 min to a couple of hours immediately after plasma exposure without vacuum interruption. With improved settings and pre-conditioning, the detection limit of the current LIBS system is expected to be improved, the time delay from plasma off to first measurement to be reduced to a few seconds and the measuring speed to be increased. Acknowledgment This project has received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement number 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] B. Schweer, G. Beyene, S. Brezinsek, N. Gierse, A. Huber, F. Irrek, V. Kotov, V. Philipps, U. Samm, M. Zlobinski, Laser techniques implementation for wall surface characterization and conditioning, Phys. Scr. 2009 (T138) (2009) 014008. [2] A. Huber, B. Schweer, V. Philipps, N. Gierse, M. Zlobinski, S. Brezinsek, W. Biel, V. Kotov, R. Leyte-Gonzales, P. Mertens, et al., Development of laser-based diagnostics for surface characterisation of wall components in fusion devices, Fusion Eng. Des. 86 (6) (2011) 1336–1340. [3] A. Semerok, C. Grisolia, Libs for tokamak plasma facing components characterisation: perspectives on in situ tritium cartography, Nucl. Instrum. Methods Phys. Res., Sect. A 720 (2013) 31–35. [4] Q. Xiao, A. Huber, G. Sergienko, B. Schweer, P. Mertens, A. Kubina, V. Philipps, H. Ding, Application of laser-induced breakdown spectroscopy for characterization of material deposits and tritium retention in fusion devices, Fusion Eng. Des. 88 (9) (2013) 1813–1817. [5] N. Gierse, B. Schweer, A. Huber, O. Karger, V. Philipps, U. Samm, G. Sergienko, In situ characterisation of hydrocarbon layers in textor by laser induced ablation and laser induced breakdown spectroscopy, J. Nucl. Mater. 415 (1) (2011) S1195–S1198. [6] A. Kreter, C. Brandt, A. Huber, S. Kraus, S. Möller, M. Reinhart, B. Schweer, G. Sergienko, B. Unterberg, Linear plasma device psi-2 for plasma-material interaction studies, Fusion Sci. Technol. 68 (1) (2015) 8–14. [7] M. Stafe, C. Negutu, N.N. Puscas, I. Popescu, Pulsed laser ablation of solids, Rom. Rep. Phys 62 (4) (2010). [8] D.W. Bäuerle, Laser Processing and Chemistry, Springer Science & Business Media, 2013. [9] G.L. Long, J.D. Winefordner, Limit of detection. a closer look at the iupac definition, Anal. Chem. 55 (7) (1983) 712A–724A. [10] D.A. Cremers, A.K. Knight, Laser-Induced Breakdown Spectroscopy, Wiley Online Library, 20 0 0. [11] Z. Tian, J. Davis, A. Haasz, Deuterium retention in tungsten at fluences of up to 10 26 d+/m 2 using d+ ion beams, J. Nucl. Mater. 399 (1) (2010) 101–107.

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