PSFC/JA-12-82 Depth profiles of helium and hydrogen in tungsten nano-tendril surface morphology using Elastic Recoil Detection K.B. Woller, D.G. Whyte, G.M. Wright, R.P. Doerner*, G. de Temmerman** * Center for Energy Research, UCSD, La Jolla CA ** FOM Institute DIFFER, Nieuwegein, The Netherlands June, 2014 Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge MA 02139 USA This work was supported by the U.S. Department of Energy, Grant Nos. DE-SC00-02060. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.
Depth profiles of helium and hydrogen in tungsten nano-tendril surface morphology using Elastic Recoil Detection K.B. Woller a *, D.G. Whyte a, G.M. Wright a, R.P. Doerner b, G. de Temmerman c a Plasma Surface Interactions Science Center MIT, 175 Albany St., Cambridge, MA, USA, 02139 b Center for Energy Research, University of California in San Diego, 9500 Gilman Dr, La Jolla, CA, 92093-0417, USA c FOM Institute DIFFER, Dutch Institute For Fundamental Energy Research, Association EURATOM-FOM, Trilateral Euregion Cluster, Postbus 1207, 3430BE, Nieuwegein, The Netherlands Abstract: Helium (He) and Hydrogen (H) depth profiles in tungsten nano-tendrils (W fuzz) have been measured for the first time using Elastic Recoil Detection (ERD). Fuzzy and non-fuzzy W surfaces were analyzed in order to illuminate the role of He in the transition in surface morphologies. Samples grown in the PISCES-A and PILOT-PSI experiments allowed a survey of surface temperature ranging from T s =470-2595 K and of He fluence on the order of Φ He ~10 24-10 27 ions/m 2. He concentrations measured in W fuzz layers are roughly uniform throughout the measurable layer depth at 1-4 at.% while samples with just He bubbles in the near surface measured at 1-2%. This confirms that voids in the nano-tendrils are filled with high pressure He. Both fuzzy and non-fuzzy surfaces show similar He concentration values. This indicates that the ~1000 K temperature fuzz-growth threshold is determined by the response of the W, not the near-surface He concentration. PACS: 52.40.Hf, 82.80.Yc, 81.07-b, 81.05.Je PSI-20 keywords: W fuzz, Helium, Surface analysis, Bubbles & Blisters, Divertor material * Corresponding author address: MIT Plasma Science and Fusion Center, 175 Albany St., NW17-210, Cambridge, MA, USA, 02139 * Corresponding author e-mail: kbwoller@mit.edu Presenting author: Kevin B. Woller 1
1. Introduction Tungsten (W) is a leading material for plasma facing components (PFCs) due to its high melting point, low sputtering yield, and resilience to nuclear damage. However, the surface morphology of the tungsten with surface temperatures T s > 1000 K changes when subjected to plasmas containing He of sufficient energy (E He > 20 ev). At these conditions, nano-tendrils extrude from the surface of the bulk W substrate and intertwine to form a layer commonly called W fuzz. The conditions that are necessary for this structure to develop are likely to occur at a W strikepoint of the ITER divertor and hot walls in future fusion reactors. W fuzz is potentially detrimental to the lifetime of W components in future devices. The nano-tendrils are brittle and easily removed from the surface, and unipolar arcing is intensified, contributing to dust production and erosion [1]. This could also lead to greater radiated power loss. Thus, analyzing how W fuzz develops can guide future considerations of W as a PFC material. It is speculated that the interaction of implanted He, namely the coalescing of implanted He into bubbles in the near surface region, drives fuzz growth [2]. Also, the W fuzz layer thickness increases with He fluence [3]. The purpose of our research is to make the first nondestructive measurements of He concentration in a variety of W fuzzy and non-fuzzy surfaces in order to illuminate the role of He in the growth of W fuzz and help guide modeling of the processes taking place. Elastic Recoil Detection (ERD) is a common ion beam analysis technique used to measure the concentration of low-z materials in high-z matrices. Appropriate selection of experimental parameters can provide simultaneous measurement of the concentrations of the plasma species of interest (He, H, and D) with respect to the W density as a function of depth perpendicular to the surface. 2. Experimental 2.1. Elastic Recoil Detection Measurement Setup 2
Measurements were carried out with the 1.7 MV tandem accelerator of the Cambridge Laboratory for Accelerator Study of Surfaces (CLASS). Due to issues with raising the terminal potential of the accelerator to desired operational values, samples were analyzed with various beam energies and corresponding stopping foil thicknesses depending on the highest terminal potential that was available. An oxygen-16 (O) ion beam of an appropriate energy (4 to 7 MeV) was directed at an angle of 75 degrees to the normal to the surface of the samples. A 2 mm wide curved slit aperture accurately followed the nominal scattering angle of 30 degrees from the incident beam path. An aluminum (Al) stopping foil of an appropriate thickness to stop the highest energy O scattered from the sample was placed after the aperture. A Passivated Implanted Planar Silicon (PIPS) detector was positioned behind the stopping foil. The aperture and active area of the detector defined the detection solid angle of 2.418 msr. The resolution of the detection electronics was measured using a polonium-210 alpha emitter and has a value of 22 kev FWHM for 5.3 MeV alpha particles. The energy scale was calibrated using the alpha emitter and various stopping foil thicknesses in front of the detector with an error of 200 kev. A summary of the ERD setup is shown in fig. 1. Secondary electrons generated during the measurement were suppressed by a grid surrounding the sample holder biased to -500 V. Thus, the beam dose was measured directly as the charge collected on the sample. 2.2. Concentration Depth Profile Calculation 2.2.1. Depth Scale Calculations In each spectrum, the energy axis is related to the integrated energy loss through the various materials that the detected particle passes through. Since the electronic stopping power of a material is proportional to the square of the atomic number of the elements composing that material, ions passing through the W fuzz slow down mainly on the W atoms. The stopping powers of W and Al for O, H, He, and D were calculated using SRIM 2010 3
experimental data. Since each recoiled species has a different energy loss on its way to the detector, the total spectrum has to be broken up into the spectra of the various elements collected. Then each sub spectrum of a species is converted into a volumetric number density versus areal density of W. For each energy channel, the areal density of W representing that channel of linear thickness, τ, is calculated in four steps. First, the energy of the recoiled species as it exits the sample is derived from the detected energy using the known thickness and stopping power of the Al stopping foil. Second, the energy of the incident O ion required to produce a recoiled particle with such an energy upon exiting the sample is calculated by matching the areal density of W required to slow the O ion down from its incident energy to that required to slow the recoiled species down from its recoil energy to its exit energy. Since the collision is elastic, the impacting energy of the O ion is equal to the recoil energy of the detected species multiplied by the forward recoil kinematic factor. Third, the areal density of W required is calculated knowing the energy loss of either the incident O ions or the recoiled species and the stopping power of W. Finally, the areal density of W represented by each energy channel can be realized as the difference between the channels. The linear depth of the layer containing the light elements can only be calculated if the mass density of that layer is known or assumed. For fully developed W fuzz layers, the porosity has been measured to be between 20-80% [4,5] and as much as 90% [6]. Given this large variation in porosity, the linear depth scale can only be computed for a given measurement if the porosity is known for that measurement. 2.2.2. Concentration Calculations ERD was performed on W unexposed to plasma to measure the background counts of the measurement. Once normalized, a fit to the background was made and subtracted from the normalized spectrum of interest. 4
If there is no overlap in the signals of different species on the ERD spectrum, then the yield from each channel is related to the areal density of the recoil species of that channel. The incident O energy for all of the samples was E i 7.0 MeV, so the differential cross section for the interaction is assumed to be Rutherford [7]. In this case, the areal density of each channel is calculated directly, using the measured detection geometry and beam dose. Once the areal density of both the W and the recoiled species are known, then the atomic density of that recoiled species in that channel can be easily calculated. 2.3. Examples 2.3.1. Polyamide Foil To test the measurement and calculation procedures, the known H content of polyamide foil was sought to be experimentally reproduced. The measurement was setup to use a 6.5 MeV O 4+ ion beam and a 6 μm thick Al stopping foil. The H concentration calculated from the experimental data was within 5 % of the stoichiometric value to a depth of approximately 250 nm. The measurement was simulated using SimNRA [8] and both the measured and simulated data can be found in fig. 2. Measurements for He content is assumed to be of similar accuracy as the H measurement. 2.3.2. W Fuzz Pilot-PSI is a high-density (n e =0.1-10x10 20 m -3 ), high-flux (>10 23 m -2 s -1 ) linear plasma generator [9] that operates in an axial magnetic field (0.4-1.6T). A W sample was exposed to He plasma with a central flux density of 1.5 ± 0.5 He + /m 2 s for 500 s. This resulted in a central maximum surface temperature of 1470 ± 100 K. ERD was performed on this sample after being shipped to the CLASS facilities from the Netherlands. Upon exposure to atmosphere, water vapor adsorps to the surface of the sample, thus contributing H that could be measured simultaneously with the He resultant from plasma exposure. The He and H were measured using a 7.0 MeV O 4+ ion beam and a 6 μm Al stopping foil. Concentration depth profiles were 5
calculated as outlined in subsections 2.2.1 and 2.2.2. A target profile was created in SimNRA using the calculated values. The details of the ERD measurement were also used as input to SimNRA and a simulated spectrum of the W fuzz sample was generated. This simulated spectrum is plotted along with the measured ERD spectrum with the background removed in fig. 3. For this sample, the porosity was approximately 80%, so the linear depth of detectable He was calculated to be approximately 350 nm. 2.4. Surface Temperature and He Fluence Variation The He flux density and the surface temperature vary across the samples exposed in Pilot- PSI, which offers the opportunity to measure the concentration of the plasma species retained at various stages of morphology development by performing ERD at different radial locations on one sample. 8 samples from Pilot-PSI were analyzed for a total of 40 different measurements. PISCES-A produces a plasma column with uniform ion flux density typically around 5x10 22 m -2 s -1 and also keeps the sample at a uniform temperature. 7 samples from PISCES-A were analyzed with 4 of the 7 samples having only He bubbles/blisters in the near surface (<100 nm) and no nano-tendril development. The surface temperature variation across all of these samples was 470 K < T s < 2595 K while the He fluence varied in the range 3x10 24 He/m 2 < Φ He < 1.3x10 27 He/m 2. A summary of the He concentration measured in this parameter space can be found in fig. 4. 3. Results and Discussion Although there is a large variation in surface temperature and He fluence from sample to sample, the He concentration remains near 1-4 at. % in both the nano-tendril morphology and the nano-bubbles that form when the surface temperature is outside of the range for W fuzz growth. This value is very high compared to the natural solubility of He in W. This amount of He in the fuzzy and non-fuzzy samples confirms the presence of high pressure helium within the voids observed in the surface of He irradiated W and the voids in the nano- 6
tendrils themselves observed by Kajita [10] and others. However, the He concentration measured is too low for pressure equilibrium and for current speculation of void creation mechanisms. The presence of high pressure He certainly provides fuel to sustain W nanotendril development. One sample from Pilot-PSI had three different regions of morphology where a simple radial scan provided a scan transitioning from He surface bubbles to W nano-tendrils. The He concentration depth profile was measured for each region and the result can be found in fig. 5. When the surface temperature is T s < 1000 K as in plot a), the W contains He bubbles just in the near surface. If a porosity of 20% is assumed for this layer, then the He depth scales to approximately 80 nm. In the third profile c), the surface temperature is T s > 1000 K and the W nano-tendril morphology is fully developed. The He concentration is not only above 1 at. %, but is also uniform through the measurable depth. A porosity of 80% for this region produces a measurable depth of 350 nm. When the temperature is very near the threshold for nano-tendrils to develop, as in the center plot b), the He profile starts to broaden as the layer deepens. Viewing these regions in succession confirms the importance of the surface temperature to the advent of nano-tendril development. 4. Summary Elastic Recoil Detection (ERD) has been used for the first time to measure the concentration depth profiles of light elements (Z 4) retained in W that has developed the characteristic nano-tendril morphology called W fuzz. In particular, the He concentrations of various samples from Pilot-PSI and PISCES-A were measured to be in the range of 1-4 at. % despite a surface temperature variation of T s = 2000 K among the samples and various He fluences across three orders of magnitude. This is a high value when compared to natural solubility of He in W, but does not adequately represent the necessary concentration to support theories of void creation and fuzz development. The individual He concentration 7
depth profiles of fully developed W fuzz show the He concentration to be uniform through the measurable depth, which varies with the porosity of the W fuzz layer. By monitoring He depth profiles, ERD is an indirect method to follow W fuzz development. 5. Acknowledgements This work is supported by US DOE award DE-SC00-02060. 6. References [1] M. Tokitani, et al., Nucl. Fusion 51 (2011) 102001. [2] S. Kajita, et al., Nucl. Fusion 49 (2009) 095005. [3] M.J. Baldwin, R.P. Doerner, Nucl. Fusion 48 (2008) 035001. [4] S. Kajita et al. J. Nucl. Mater. 421 (2012) 22. [5] M. Miyamoto, et al., J. Nucl. Mater. 415 (2011) S657. [6] M.J. Baldwin, R.P. Doerner, J. Nucl. Mater. 404 (2010) 165. [7] E. Markina, et al., Nucl. Instrum. Methods Phys. Res. B 269 (2011) 3094 3097. [8] M. Mayer, SIMNRA User s Guide, Report IPP 9/113, Max-Planck-Institut für Plasmaphysik, Garching, Germany, 1997. [9] G. J. van Rooij, et al., Appl. Phys. Letters 90 (2007) 121501. [10] S. Kajita et al. J. Nucl. Mater. 421 (2012) 22. 8
Figure captions: Fig. 1. The forward recoil geometry of ERD using a stopping foil. For this work, θ inc = θ exit = 75 and θ recoil = 30. The stopping foil thickness is chosen to stop the highest energy O scattered into the solid angle of the aperture. Fig. 2. The H concentration of polyamide foil as measured by ERD. The measured profile is shown as the dotted line and the SimNRA simulation of the measurement is shown as the solid line. Fig. 3. H and He concentration profiles of a W fuzz sample grown in Pilot-PSI as the dashed lines measured with ERD. The W was exposed to pure He plasma, so the H content measured is completely from exposure to atmosphere. The calculated concentration profiles were simulated in SimNRA and plotted over the measured profiles as the solid line. The linear depth to which He could be independently detected was approximately 350 nm assuming the W fuzz void fraction was 80%. Fig. 4. Saturated and peak He concentrations of W samples from both Pilot-PSI and PISCES- A of various surface temperatures exposed to various He fluences. Fig. 5. He concentration profile of W irradiated with He at different stages in W fuzz development. a) He bubbles in the near surface ( 80 nm). b) W nano-tendrils beginning to form. c) W fuzz layer grown deeper than ERD can measure ( 350 nm in this sample). 9
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