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Penetration depths of injected/sputtered tungsten in the plasma edge layer of TEXTOR M. Laengner a,⇑,1, S. Brezinsek a, J.W. Coenen a, A. Pospieszczyk a, D. Kondratyev a, D. Borodin a, H. Stoschus b, O. Schmitz a, V. Philipps a, U. Samm a, the TEXTOR Team a b

Institute of Energy and Climate Research – Plasma Physics, Forschungszentrum Jülich GmbH, Association EURATOM-FZJ, Partner In the Trilateral Euregio Cluster, Jülich, Germany Oak Ridge Institute for Science Education, Oak Ridge, TN 37830, USA

a r t i c l e

i n f o

Article history: Available online 16 January 2013

a b s t r a c t To quantify tungsten (W) sputtering measurements by spectroscopy, two experiments have been carried out in the tokamak TEXTOR. The erosion of a W limiter in the plasma edge was studied spectrocopically under different plasma conditions. Conversion of the photon ﬂuxes of neutral W lines was performed [1] with the aid of inverse photon efﬁciencies, measured in situ in a second experiment with WF6 injection to realise a controllable W source. Penetration depths, particle velocities and line ratios of W I were determined and compared between the two sources of W. The velocities of injected and sputtered W differ by a factor of 3 as the analysis of the penetration depth shows, while the line ratios for different W I lines are comparable. A comparison of the e-folding length with the model code GKU was performed which reveals a deviation of at least a factor of 6. Inverse photon efﬁciencies at Te = 40 eV were determined to be about 44 for W I (400.88 nm) and about 63 for W I (429.46 nm). Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Tungsten is foreseen as the plasma-facing material in the ITER divertor due to its beneﬁcial properties like high melting temperature, low physical sputtering yield and small fuel retention. While the sputtering yield for the fuel gas is small, the sputtering is determined by impurities like oxygen, carbon and beryllium. Connected to the erosion of the target is the transport of W into the core plasma. As it is a high-Z material, it can there cause strong radiation losses and hamper the fusion burn. Only a small amount of W (105 W/D) [2] can be tolerated in the core plasma. Thus, it is of high importance to understand W as an impurity source and to determine the W source distribution. In this context two aspects are essential: in situ determination of the W source strength by spectroscopy means and the characterisation of the interaction of neutral W with the plasma by the penetration depths. To address these aspects two experiments have been set up at the tokamak TEXTOR. The ﬁrst experiment was performed to spectroscopically study the erosion of W under different plasma conditions [1]. However, to be able to spectroscopically quantify the amount of eroded W particles, inverse photon efﬁciencies – the so-called S/XB values (S: ioniSation rate, X: eXcitation rate, B: ⇑ Corresponding author. Address: Wilhelm-Johnen-Straße, D-52425 Jülich, Germany. E-mail address: [email protected] (M. Laengner). 1 Presenting author. 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.01.187

Branching ratio) [3] – are necessary which allow for the conversion of measured photon ﬂuxes into particles ﬂuxes. Therefore, a calibrated, controllable W source is needed which was realised in a second experiment by WF6 injection. Quantiﬁcation of S/XB values for W I has been performed previously uncontrolled by W(CO)6 sublimation [4] or W weight loss [5] in the low plasma temperature range, but only the WF6 injection experiment, pioneered in TEXTOR [1], allows the controlled injection as gaseous W source and the spectroscopic observation for a wide range of Te. This includes high Te up to 100 eV which is present during ELMs and causes the remaining W sputtering under cold divertor conditions [6]. To prove that the S/XB values determined using the WF6 injection method are as well applicable to sputtered W it has to be studied how far the injected W is comparable to sputtered W. In this paper a comparison of W originating from both release processes will be performed regarding the penetration depth of the W I (400.88 nm) line and – by means of the penetration depth – the particle velocities. Moreover will the temperature dependence of certain line ratios be determined for both processes. Thus we will show that the WF6 injection can be applied to simulate a W source for the purpose of S/XB value measurements. Finally the S/XB values determined in the described WF6 experiment will be presented. 2. Experimental setup and plasma conditions Both experiments were carried out in the TEXTOR tokamak (large radius: R = 1.75 m, small radius: a = 0.46 m) [7]. For all

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deuterium plasma discharges a constant plasma current Ip = 0.35 MA and a constant toroidal magnetic ﬁeld Bt = 2.25 T was used. The auxiliary heating was delivered by neutral-beam injection with a power of Paux = 1.2 MW. In the sputtering experiment a spherical so-called twin limiter consisting of one half made of W and the other half made of C was exposed to the boundary plasma in the scrape-off layer (SOL) with its apex at r = 46.5 cm which is 0.5 cm behind the last closed ﬂux surface (LCFS). During a number of reproducible discharges a stepwise change of the central density and temperature was performed within each single discharge. The edge density and temperature data were measured using an active He-beam diagnostic [8] positioned in section 12/13, equatorial plane. Although the plasma data were measured at a position differing toroidally and poloidally from the position of the limiter (section 10/11, bottom) it can be assumed that they hold for the limiter position as well as. This was shown with local measurements at the limiter/ gas inlet position in [9]. The edge plasma parameters measured at the emission peak position are shown in Fig. 1. Details referring to this experiment can be found in [1]. WF6 gas was introduced through a gas inlet protected by graphite (section 10/11, bottom). This gas known from semiconductor production has a low dissociation temperature varying in literature depending on the experimental conditions. Given numbers are e.g. Tdiss = 600–1100 K [10] (which is equivalent to 0.1 eV). The experiment was performed by positioning the gas inlet in the SOL between 2 and 4 cm behind the LCFS. The local plasma parameters were varied by altering the radial limiter position between r = 48.0–50.0 cm relative to the plasma centre which leads to a change in ne(r) and Te(r) due to their radial dependency. The edge density and temperature data were obtained by the super-sonic He-beam diagnostic [11] (section 1/2, top). In Fig. 2 the measured edge plasma parameters at the emission peak position are indicated. The spectroscopic in situ observation from a side view for both experiments was performed using a 2D CCD camera with image intensiﬁer and interference ﬁlters chosen depending on the wavelength of interest (W I (400.88 nm)). From that spatial information on the W emission patterns was obtained. In addition an overview spectrometer with an identical line-of-sight has been applied to observe the spectral range between 363 and 720 nm [12]. Furthermore, a set of Czerny–Turner compact spectrometers was used in the identical line-of-sight covering the wavelength range between 200 and 800 nm.

3. Results and discussion 3.1. Comparison of particles velocities By changing the radial position r of the injection gas inlet during the WF6 injection experiment it was possible to record W I line proﬁles for different ne(r) and Te(r). These proﬁles are shown in Fig. 2 with normalised intensities for the W I line at 400.88 nm. One can see how ne and Te (measured at the emission peak) stepwise decrease with changing the gas inlet position from r = 48.0 cm to r = 50.0 cm respective to the plasma centre. A stepwise increase of the distance between gas inlet surface and the position of the W I light proﬁle peak and second a broadening of the proﬁles can be observed with increase of the distance to the LCFS. By deﬁning the e-folding length keﬂ as the distance from the peak position to the position where the peak intensity Ipeak has decreased to Ipeak/e one gets a measure for the penetration depth of the line into the plasma. In a similar way the line proﬁles of the W sputtering on the twin limiter surface can be obtained (Fig. 1). However, here one has to take into account that the ne/Te variations were not induced by changing the limiter position but by changing the central density. While in the injection experiment both, ne,edge and Te,edge were decreasing when moving the limiter from the plasma centre, in the sputtering experiment Te,edge is decreasing when ne,centre and with that ne,edge are increasing. To make the experimental conditions in both experiments comparable, the change of both, ne,edge and Te,edge can be incorporated by calculating the inverse ionisation rates

1 1 ¼ S ne hme ri i

ð1Þ

which are linked to the ionisation length by

ki ¼

mW S

ð2Þ

where S is the ionisation rate, ne is the electron density, ve is the electron velocity, ri is the ionisation cross section for W I, hverii is the ionisation rate coefﬁcients and v W is the W particle velocity. As so far no experimental data for the W I ionisation rate coefﬁcients are available, the recent data used here (Table 1) were calculated by the code ATOM [13]. In Fig. 3 the measured keﬂ that were obtained from the emission proﬁles are plotted against 1/S for both, the injection and the

Fig. 1. Radial proﬁles of the W I (400.88 nm) line for sputtered W. From top to bottom in the picture the central plasma density and temperature were changed by deuterium fuelling in three steps within one discharge (#113112). This induced an inverse temperature/density change at the plasma edge. As a result from phase I to phase III the proﬁles become smaller due to an increase of the ionisation rate S.

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Fig. 2. Radial proﬁles of the W I (400.88 nm) line for injected W used to determine the e-folding lengths keﬂ as a measure for the penetration of neutral W into the plasma. From top to bottom in the picture the gas inlet was moved away from the plasma centre discharge by discharge. This induced a temperature/density decrease at the plasma edge. As a result the proﬁles become broader and the peak moves away from the place of injection. The gas inlet position is depicted as grey bars.

Table 1 Ionisation rate coefﬁcients hmerii (107 cm3 s1) (ATOM [13]) used to calculate the inverse ionisation rates 1/S (s) to determine the velocity ratio of the injected and sputtered W. They are shown depending on the respective plasma edge temperature (measured at the emission peak) for the sputtering/injection case and the GKU modelling. Sputtering

Te (eV) hmerii (107 cm3 s1)

45 5.5

60 6.5

85 7.3

Injection

Te (eV) hmerii (107 cm3 s1)

38 5.1

41 5.3

47 5.8

GKU

Te (eV) hmerii (107 cm3 s1)

29 4.3

37 5.0

48 5.8

sputtering experiment. What can be seen is that with decreasing ionisation rates S the e-folding length keﬂ increases. Due to the experimental conditions (overheating of the injection gas inlet at the twin limiter position, on the other hand only small erosion yield on the twin limiter at the gas inlet position) unfortunately no overlap of the curve showing keﬂ for the sputtered and the curve for the injected atoms could be achieved. Thus an extrapolation of the linearly behaving curve for the injected atoms was done to cover the inverse ionisation rates that were found in the sputtering experiment. Unlike the e-folding lengths of the emission proﬁles, the ionisation lengths are not experimentally accessible. Both, the radial proﬁle of the released neutrals being ionised and the measurable emission proﬁle decay exponentially with exp(r/ki) and exp(r/keﬂ), respectively. As the emission proﬁle must depend on the exponential decrease of neutrals, it is assumed that in ﬁrst order keﬂ = aki with a linear proportionality factor a. Using this assumption, Eq. (2) shows that calculating the slope ratio of both linear ﬁts means calculating the ratio of the W particle velocities

kefl;sput Ssput ki;sput Ssput msput ¼ ¼ kefl;inj Sinj ki;inj Sinj minj

ð3Þ

which is independent of the proportionality factor a. This ratio was found to be about vsput/vinj = 3.

55 6.2

Particles that leave a surface after a sputtering process are released with a Sigmund–Thompson velocity distribution [14]

fTh ¼

Ez ðEz þ Eb Þ3

ð4Þ

where Ez is the energy of ejected particle, Eb is the surface binding energy. From this one ﬁnds [14] that for a surface binding energy of Eb = 8.66 eV for W the most probable velocity vsput is equivalent to half the binding energy Eb/2 = 4.33 eV, i.e. vsput = 2122 m/s. Assuming this most probable velocity for the sputtered particles, together with the calculated slope ratio the velocity of the injected particles can now be found to be about vinj = vsput/3 = 707 m/s. As the mean velocity hvsputi of a Thompson-distributed W particle ensemble is higher than vsput = 2122 m/s this will lead to an even higher velocity of the injected particles than 707 m/s. However, this velocity is too large to be understood only by an energy gain during the dissociation process and therefore has to be regarded so far only as a parameter with dimensions of a velocity. It shows that a dissociation chain of the WF6 may have to be taken into account to explain the large penetration depths. Thus the dissociation cannot be assumed to happen instantaneously when the WF6 penetrates into the plasma. Detailed theoretical analysis of the break-up chain of WF6 is ongoing.

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Fig. 3. Measured e-folding lengths keﬂ for the injection (green squares, straight line)/sputtering (red rotated triangles, straight line) experiment versus the inverse ionisation rate 1/S. The thin dashed green and red lines represent a line ﬁt to the measured points. By comparing the slopes of the ﬁtted lines the calculation of the velocity of the injected particles is possible. The dashed and dotted lines show the modelling results applying the GKU code. A velocity of vsput = 2100 m/s for the sputtered W (orange triangles, dashed line) and vinj = 700 m/s for the injected W (olive points, dashed line) was assumed. In both cases only a modiﬁcation of the GKU calculation by a factor of at least 6 led to an approximation of the measured values (dotted lines). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

3.2. Comparison with modelling results In order to benchmark collisional-radiative model calculations done with the GKU code [15], a comparison of the measured efolding lengths obtained in the WF6 injections experiment and those extracted from the emission proﬁles calculated by GKU was performed (Fig. 3). It was found that by assuming particle velocities of vsput = 2100 m/s and vinj = 700 m/s, respectively, neither the penetration depths of the sputtered nor of the injected W particles could be reproduced. No velocity distribution of the W particles was taken into account. In both cases the modelling results are at least a factor of 6 below the measured penetration depths. Even assuming a velocity distribution, which would lead to a mean velocity of hvsputi = 3800 m/s by integrating a Thompson-velocity distribution, cannot explain this factor, as the e-folding length increases linearly with the velocity and can thus contribute only to a small part to broaden the proﬁles. In the model only calculated data [13] are used for the W I ionisation rate coefﬁcients hverii as no experimental data exist. It thus has to be taken into consideration that the used ionisation rate coefﬁcients may be overestimated which leads also to overestimated ionisation rates. 3.3. Comparison of line ratios Fig. 4 shows a comparison of line ratios (not absolutely calibrated), both for the injection and the sputtering case. The comparison was done for W I (400.88 nm), W I (429.46 nm), W I (498.26 nm) and W I (522.47 nm) with respect to W I (505.33 nm) and in addition for W I (400.88 nm) with respect to W I (429.46 nm). As can be seen, except for W I (522.47 nm), no difference in the behaviour of the studied line ratios for both release processes can be found within the uncertainty of the measurement, which indicates that the level population of the presented lines seems to be independent of the release process and only determined by the plasma parameters. The W I (522.47 nm) line is an exception which may be explained by a

disproportionally strong population of its high ground state energy level with respect to the other lines measured. For determining the line ratios, lines originating from very different energy levels were chosen, for lines emitted from energy levels close to each other would also produce similar ratios with respect to another line. This underlines that for the W I (400.88 nm) and W I (429.46 nm) line injected and sputtered atoms are comparable from that point of view. Thus the WF6 injection can be used to measure effective S/XB values for these lines. 3.4. Inverse photon efﬁciencies Besides the aim of realising a controllable W source by the aid of WF6, the method was used to determine the effective S/XB values. Fig. 5 shows the results for the lines W I (400.88 nm) and W I (429.46 nm) in the temperature range Te = 38–47 eV with a typical value of about (S/XB)eff,401nm = 44 and (S/XB)eff,429nm = 63 together with the GKU modelling calculated with a maximum of the W ground state energy distribution at TW = 0.3 eV. However, the measured values are systematically lower than the modelled ones which is consistent with the assumption that the W I ionisation rate coefﬁcients could have been overestimated. A more detailed analysis of the measured inverse photon efﬁciencies in a wider Te range and in comparison to the GKU modelling will be published in a separate paper. 4. Summary When comparing the W sputtering experiment to the W injection experiment two aspects were studied: the behaviour of the particle velocities and the line ratios in both processes. Using the experimentally determined e-folding lengths as a measure for the penetration of the neutral W atoms into the plasma the ratio of the particle velocities for injected/sputtered W could be found to be 3. Together with a most probable velocity

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Fig. 4. Line ratios (not absolutely calibrated) for W I (400.88 nm) (black squares), W I (429.46 nm) (red dots), W I (498.26 nm) (blue triangles) and W I (522.47 nm) (pink hexagons) with respect to the W I (505.33 nm) line for the injection (ﬁlled symbols) and the sputtering case (empty symbols), respectively, versus the plasma edge temperature. The ratio of the W I (400.88 nm) line with respect to the W I (429.46 nm) line is shown as rotated green squares. For both experiments the line ratios behave comparably, except for the W I (522.47 nm) line. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 5. Effective S/XB values for the W I (400.88 nm) (green squares, left scaling) and W I (429.46 nm) line (blue triangles, right scaling) depending on the plasma edge temperature. The experimental values were measured at TEXTOR applying the WF6 injection method. The GKU modelling that comes closest to the measurements is shown by the plotted curves assuming a maximum of the W ground state energy distribution at TW = 0.3 eV. The measured values in both cases lay systematically lower than the modelled ones. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

of the sputtered particles of vsput = 2122 m/s calculated from the Sigmund–Thompson distribution this leads to a velocity of the injected particles of vinj = 707 m/s. This experimental ﬁnding shows that the kinetic energies for injected/sputtered W are essentially different. However this velocity can up to now only be regarded as a parameter with dimensions of a velocity. So far it cannot be understood from the energy gain during the dissociation process. This may give a hint that a dissociation path length of the WF6 molecule has to be taken into consideration. The comparison of the e-folding lengths with the GKU modelling shows that the experimental results only can be reproduced by modifying the GKU calculation by a factor of at least 6 when particle velocities of vsput = 2100 m/s and vinj = 700 m/s are assumed. However, then both experimental ﬁndings can be approximated. It thus has to be taken into consideration that the

calculated data for the W I ionisation rate coefﬁcients may be overestimated. With regard to the ratios of lines originating from very distant energy levels it can be seen that in case of the W I (400.88 nm) and the W I (429.46 nm) line the same ratios exist for sputtered or injected W. Furthermore, the level population of these lines seems to be independent of the release process and mainly determined by the plasma parameters. This indicates that the effective S/XB values measured for the W I (400.88 nm) and the W I (429.46 nm) line with the aid of WF6 injection can be applied to sputtering experiments to determine absolute W ﬂuxes. Moreover is it now possible to measure the W erosion in a wider range of plasma conditions than was accessible up to now. Typical values at Te = 40 eV were determined to be about 44 for W I (400.88 nm) and about 63 for W I (429.46 nm).

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[10] E. Lassner, W.-D. Schubert, Tungsten – Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Springer, 1999. pp. 111-168, ISBN 0-306-45053-4. [11] U. Kruezi, Entwicklung einer Heliumstrahldiagnostik zur Messung der Elektronendichte und -temperatur mit hoher räumlicher und zeitlicher Auﬂösung, ISBN 978-3-89336-476-3. [12] S. Brezinsek et al., Plasma Fusion Res. 3 (2008) S1041. [13] L. Vainshtein et al., J. Phys. B: Atom. Mol. Opt. Phys. 44 (2011) 125201. [14] P.C. Stangeby, The Plasma Boundary of Magnetic Fusion Devices, Inst. of Physics Pub., 2000 (1 January). [15] L. Vainshtein et al., Plasma Phys. Control. Fusion 49 (2007) 1833.

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