Journal of Nuclear Materials 452 (2014) 51–56 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat Damage studies on tungsten due to helium ion irradiation N.J. Dutta, N. Buzarbaruah, S.R. Mohanty ⇑ Centre of Plasma Physics-Institute for Plasma Physics, Sonapur, Kamrup 782402, India h i g h l i g h t s Used plasma focus helium ion source to study radiation induced damage on tungsten. Surface analyses conﬁrm formation of micro-crack, bubbles, blisters, pinholes, etc. XRD patterns conﬁrm development of compressive stress due to thermal load. Reduction in hardness value is observed in the case of exposed sample. a r t i c l e i n f o Article history: Received 27 January 2014 Accepted 23 April 2014 Available online 5 May 2014 a b s t r a c t Energetic and high ﬂuence helium ions emitted in a plasma focus device have been used successfully to study the radiation induced damage on tungsten. The reference and irradiated samples were characterized by optical microscopy, ﬁeld emission scanning electron microscopy, X-ray diffraction and by hardness testers. The micrographs of the irradiated samples at lower magniﬁcation show uniform mesh of cracks of micrometer width. However at higher magniﬁcation, various types of crystalline defects such as voids, pinholes, bubbles, blisters and microcracks are distinctly noticed. The prominent peaks in X-ray diffraction spectrum of irradiated samples are seen shifted toward higher Bragg angles, thus indicating accumulation of compressive stress due to the heat load delivered by helium ions. A marginal reduction in hardness of the irradiated sample is also noticed. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Materials that are proposed to be used as plasma facing component (PFC) in future fusion devices must work in adverse environment that includes combinations of high temperatures, intense damaging radiations, heat and mechanical loadings, reactive chemicals, etc. Therefore, the task of constructing and operating fusion reactors with materials that survive the aforesaid adverse environment and meet the objectives for safety, environment and performance is still very challenging. The development of the new materials for such reactors would require fundamental understanding of processes like radiation damage, plasma-surface interactions, sputtering, corrosion and erosion . Tungsten has been considered as one of the most important material as a PFC in the divertor and bafﬂe regions of a tokamak because of its intrinsic advantages such as high atomic number, high melting point, high threshold energy towards sputtering and good thermomechanical properties . The PFC of fusion reactors are in generally prone to damage due to its direct contact with the edge plasma and high particle ﬂux of neutrals, ions and impurity particles possessing ⇑ Corresponding author. Tel.: +91 9435464759. E-mail address: [email protected] (S.R. Mohanty). http://dx.doi.org/10.1016/j.jnucmat.2014.04.032 0022-3115/Ó 2014 Elsevier B.V. All rights reserved. broad energy spectrum. Hence, in recent years, there has been an increasing interest in testing radiation induced damage on fusion reactor materials through deployment of small scale laboratory experiments [3–8]. During the last decade, some notable experiments were carried out to understand the radiation effect on fusion materials using divertor simulator , plasma interaction with surface and components experimental simulator , high ﬂux ion beam test device , inertial electrostatic conﬁnement fusion device , plasma focus [7,8], etc. Formation of surface defects such as bubbles, microcracks, blisters, voids and nanostructures on tungsten due to irradiation with helium ions having energy of few eV to keV have been reported in literature [9–12]. Kajita et al.  observed the formation of nanoﬁbers on tungsten surface when irradiated with 75 eV helium ions. While studying the response of polycrystalline tungsten to 30 keV helium ions, Zenobia et al.  observed the formation of a highly oriented grass like structure for a certain ﬂuence of helium ions and for a speciﬁc substrate temperature. In case of exposure under moderate energy (a few keV) helium ions on tungsten surface, the changes mainly occur in terms of hole and bubble formation which critically depends on the exposure temperature . This reduces the strength of surface and causes the material to release impurities in response to transient heat load such as edge localized modes 52 N.J. Dutta et al. / Journal of Nuclear Materials 452 (2014) 51–56 . A systematic study of helium retentions and surface blistering in terms of helium dose, temperature, pulse implantation and tungsten microstructure was conducted by Gilliam et al. . They reported that maintaining the helium ion ﬂux per fusion event below a threshold level may reduce the radiation damage effect and further stressed the necessity to investigate the effect of variable helium ion energy and simultaneous lattice damage on helium retention and blistering. On the background of the investigations reported above, we have studied here the effect of energetic helium ions on tungsten using an ingenious plasma focus (PF) device. The low energy PF device is a simple and cost effective pulsed coaxial plasma accelerator that makes use of the self-generated magnetic ﬁeld for compressing the plasma to a very high density (1025–1026 m3) and high temperature (>1 keV). Being a rich source of charged particles and electromagnetic (EM) radiations, it has been successfully used in numerous applications namely as X-ray source of microlithography  and microscopy , electron source for microlithography , X-ray interstitial radio surgery , EUV source for EUV lithography  and as ion source for processing of materials  and thin ﬁlm deposition . The uniqueness of a PF device is that it produces pulsed ion beams with highly charged states having wide range of energies and it does not require any external conﬁnement system or extraction grid. Generally, this device emits ions of energies in the ranges from a few keV to a few MeV depending upon its operating energy . Using such a PF device, Bostick et al.  studied blister formation on various metallic plates (Al, Cu and Si) by using deuteron beams liberated from a PF device and obtained an information about the ﬁne structure of the deuterium beam by making statistical analyses of blister parameters. On the other hand, Pimenov et al.  investigated the damage induced on tungsten plates by using axially emitted high energy ion beam and dense plasma stream at different capacitor banks energy. Due to the anisotropic nature of ion emission from a PF device, it would be better to study the ion irradiation on materials not only at axial position but also at different angular positions . In this paper, therefore, we have investigated the main features of damages that occurred on tungsten material when irradiated by wide spectrum helium ion pulses that emanated from our PF device. Ex situ characterization of irradiated as well as reference materials has been carried out using optical microscopy, ﬁeld emission scanning electron microscopy, X-ray diffraction and hardness tester. 2. Experimental procedures The helium ion source employed in the present investigation is basically a 2.2 kJ Mather geometry PF device which is powered by a low inductance 7.1 lF high energy storage capacitor charged up to 25 kV. A schematic drawing of the device is given in Fig. 1. The storage capacitor is connected by a low inductance spark gap to the co-axial and concentric electrode system which is enclosed in a stainless steel chamber having helium gas ﬁlling at pressures of several mTorr. The inductance of the capacitor along with the external connections and the switch are about 130 nH. The details of the device can be seen elsewhere [26,27]. The electrical energy of the capacitor bank is ﬁrst transferred to the electrodes and, as a result, the gas breakdown occurs across the electrodes forming the current sheaths that propagate along the electrodes by J B Lorentz force. When the current sheaths reach at the open end of the electrodes, the sheaths undergo radial pinching forming a highly dense plasma column. This plasma column is short lived and unstable. Moreover, due to instabilities it breaks up and emits X-rays, energetic ions and electrons, and neutrons . The ions emitted from the plasma column are basically instability Fig. 1. Schematic of experimental set up. accelerated ions having energy range of tens of keV to MeV and moves mostly towards the top of the chamber. The ion density distribution, ﬂuences were measured at a height of 6 cm from top of the anode using ion collector . It is well-known that operating gas type does not affect many of the key characteristics such as plasma density, plasma temperature and ion spectra of an optimized PF device  and therefore, for a single PF shot, we have considered the expected ion ﬂuence and ﬂux at the location of sample as 1018 m2 and 1025 m2 s1, respectively. The thermal evolution of samples during and after ion irradiation is studied mainly theoretically and reported in literatures [30,31]. Since in this experiment, the ion beams interacted with the sample for a few hundred of nsec, therefore, it is observed that the sample temperature variation is transient in nature with extremely high temperature rise rate (40 K/nsec) followed by fast cooling . Exact measurement of thermal evolution of sample during ion bombardment requires a detector or sensor having time resolution superior to 200 nsec which is impossible from technological point of view. Therefore, we have made use of a thermocouple and found that the bulk temperature of samples varies from 70 to 100 °C depending upon ion pulses. High purity (99.95 wt.%) polycrystalline tungsten plates were procured from Goodfellow, United Kingdom. The concentrations of impurities in tungsten material as determined by the manufacturer are given in Table 1. The plates were cut into square sizes (typically 5 5 and 10 10 mm2) with the help of a precession cutter and mechanically polished in a Buehler make polisher  using different grits (180, 240, 360, 420, 600 and 1200) of abrasive paper. Samples were then polished with a nylon cloth for getting a mirror ﬁnish surface. The polished tungsten samples were then introduced into the PF chamber and mounted axially and also in other angular positions with the help of a movable sample holder placed at a distance 6 cm from the top of the anode. The samples were irradiated at room temperature to single and multi PF shots (i.e., pulses of helium ions) keeping helium ﬁlling gas pressure at 0.5 Torr inside the chamber. Inclination of samples with respect to ion trajectories was kept perpendicular in our case. This fact has been ascertained by observing the mostly circular ion tracks 53 N.J. Dutta et al. / Journal of Nuclear Materials 452 (2014) 51–56 Table 1 Concentrations of impurities in tungsten material. Impurity Content (ppm) Ca <20 Cu <20 Fe 20 Mg <10 Mo 150 Ni <20 Pb <50 Si <50 Sn <30 Ti <20 C 30 O 30 H 6 N 10 recorded on nuclear track detectors while investigating the spatial distribution of ions of source . The irradiated and reference tungsten samples were characterized by Olympus BX51M optical microscopy (OM), Sigma Zeiss ﬁeld emission scanning electron microscopy (FESEM) for ascertaining the changes in their surface morphology. Similarly, structural analyses and hardness measurements of the reference and the irradiated samples were carried out by employing Bruker D8 X-ray diffraction (XRD) system and Buehler Micromet 2101 microhardness tester, respectively. 3. Results and discussion The energetic ions from the device were allowed to impinge on the tungsten samples which were kept at different angular positions as mentioned earlier. After getting irradiated by helium ions, a clear difference in surface morphology is observed visually. The smooth, shiny and bright surface of reference sample got transformed into uneven surface with dull appearance and reﬂecting yellowish gray color. When viewed under the optical microscope to investigate the exact surface morphological changes caused due to helium ion irradiation, a signiﬁcant difference has been marked on the surface morphology. As evident from the micrograph shown in Fig. 2a, the reference sample is quite smooth without any major surface imperfections. On the contrary, the irradiated samples reveal a uniform network or mesh of cracks that appeared in the samples that were kept at axis or nearby to it. The cracks are of micron size width (1–4 lm) having primary as well as secondary branches as illustrated in Fig. 2b and c. Moreover, interconnected cracks or loop like patterns are mainly observed either in the samples kept at axis and close to it or the samples irradiated by larger number of PF shots. In some irradiated samples, bubble formation is seen near the cracks which are mainly due to diffusion of helium ions in tungsten and subsequent trapping in bubbles. In order to study the detailed damage pattern at higher magniﬁcation, both the irradiated and reference samples were viewed under FESEM. While observing the surface texture of reference sample one can notice distinct tiny microstructures arranged in clusters that are similar to dendrite type of structure as shown in Fig. 3a. These clusters get smeared out due to ion bombardment as evident from the micrograph depicted in Fig. 3b. Moreover, at certain places small cracks appeared in reference sample which are produced during the mechanical polishing of the sample. But these crack patterns are quite different from the patterns that are observed after ion irradiation as shown in Fig. 3c. In our study, microcracks having width of a few hundred nanometer to micrometer are noticed in irradiated samples. Such type of cracks on tungsten surface is also reported earlier [7,8]. Shirokova et al.  observed similar type of microcracks patterns on tungsten and in doped tungsten samples by bombarding these samples with deuterium plasma. Microcracks patterns that are quite similar to the pattern obtained in the present study are also noticed on tungsten surface by Bhuyan et al.  when such samples were irradiated with proton beams. A look at the exposed samples (Figs. 4 and 5) at higher magniﬁcation clearly indicates various crystalline defects such as voids, pinholes, bubbles, blisters and microcracks which arise due to multiple heat load on tungsten surface by helium ion pulses. It is reported elsewhere that 500 eV helium ions can Fig. 2. Optical micrographs of samples: (a) reference, (b) irradiated to 5 pulses of helium ions and (c) irradiated to 10 pulses of helium ions. displace atoms from tungsten crystal . Since in the present study tungsten is bombarded with helium ions of much higher energy, the vacancies and interstitials are created in the crystal structure. Successive exposure of the samples to helium ion pulses results in the migration of interstitials that leave vacancies behind. Subsequently, these vacancies coalesce and form small voids which 54 N.J. Dutta et al. / Journal of Nuclear Materials 452 (2014) 51–56 Fig. 3. FESEM images of (a) reference sample, (b) irradiated sample showing smearing of clusters and (c) irradiated sample indicating cracks. Fig. 4. FESEM image of irradiated sample with magniﬁed view of blisters. afterward allow some helium ions to go into the lattice resulting in the formation of helium bubbles. It is observed that the number of radiation induced defects is comparable to the number of helium ions implanted into the surface . As the estimated ion density per ion pulse impinging on the sample is of the order of 1019–1020/ m3 [23,28], a high density of defects namely pinholes and voids have been noticed in samples which were irradiated to single ion pulse. Other defects such as voids, pinholes, cracks bubbles and blisters. are observed in samples irradiated by multi ion pulses (5 and 10 pulses). Therefore, the experiment reasonably establishes that ion ﬂuences on tungsten are primary controlling factor for the defect formation. Moreover, the exposed sample temperature is a key factor in controlling parameters like the helium diffusion rate, thermal vacancy density and thermal migration for the bubble formation and growth . It may be noted that in the process of PF ion bombardment no separate heating arrangement is needed since the heat deposited by the single ion pulse is enough to raise the temperature of near surface layer of sample up to a few hundred degrees centigrade . Nevertheless, in the case of the samples irradiated by 5 and 10 pulses of helium ions, it is expected that due to the multi shot exposure the near surface layer of sample must have attained much higher temperature of the order of thousand degrees centigrade. Owing to successive ion pulses bombardment, the migration of vacancies and helium bubbles occurs along thermal gradient which results in the growth of bubbles and inter bubble fracture. It is understood that formation of some micropath takes place along the inter bubble cracks which collectively coalescence resulting in the formation of surface blisters as shown in window of Fig. 4 . The formation of subsurface cracks can be interpreted from the inter bubble fracture model proposed by Evans  according to which the bubbles grow due to increase in the internal pressure, which decreases their spacing. Eventually several adjacent bubbles will create a local stress that is sufﬁcient to form subsurface cracks and these may lead to the formation of surface cracks as shown in Fig. 4. The formation of holes and bubbles decreases drastically for the samples placed at off axis which is mainly due to lesser heat load deposited by ion beams . As illustrated in Fig. 5, growth of nanoﬂakes (having 28 nm diameter and 191 nm length) and nanoglobules structures have been also observed inside some pinholes and microcracks, respectively. The formation of these nanoﬂakes and nanoglobules structures may be due to the reduction in surface heat deposition around the wall of pinholes and microcracks by ion pulses. It is widely reported that the bombardment of low energy helium ions on tungsten surface favors the formation nanoﬁbers . In order to ascertain the detailed structural changes that have occurred due to the helium ion irradiation, both reference and irradiated tungsten samples were analyzed using XRD. The diffraction patterns were recorded from 5° to 90° at a scan rate of 0.0199° and the result is shown in Fig. 6a. Sharp peaks at 2h equals to 40.21°, 58.22°, 73.17° and 86.94° appear in the spectrum of the reference sample that are recognized as (1 1 0), (2 0 0), (2 1 1) and (2 2 0) planes of the tungsten. These peaks correspond to the body centered cubic crystal structure of the tungsten. Another feeble peak centered at 25.54° is also noticed in the spectrum of the reference sample. The peak is seen to become prominent when the sample was irradiated to helium ion pulses (Fig. 6b). Moreover, the four sharp peaks that appeared in reference sample also appear in the spectrum of the irradiated sample but these peaks get shifted towards higher Bragg angles. On the contrary, the feeble peak observed in the irradiated sample gets shifted towards lower Bragg angles in comparison to the reference sample. The feeble peak at (1 1 2) plane  may be attributed to the formation of tungsten oxide (WO3) in both the samples and the intensity is seen to have increased in the irradiated sample due to the inclusion of more impurity ions (namely oxygen coming out from the chamber) into tungsten sample. The possible explanation of shifting of the peaks towards the higher Bragg angles is due to build up of ion irradiation compressive internal stress in the exposed samples . We N.J. Dutta et al. / Journal of Nuclear Materials 452 (2014) 51–56 55 Fig. 5. FESEM image of irradiated sample with magniﬁed view of nanoﬂakes and nanoglobules structures. have also calculated the average grain size of both the samples by using Scherrer’s formula as given below, D¼ 0:94:k b2h cos h ð1Þ where D is the grain size, k is wavelength of X-ray, b2h is FWHM of the plane, and h is Bragg’s angle. It is found that the average grain size of the sample decrease from 31.26 nm to 27.50 nm due to the bombardment of helium ions. Thermal load due to helium ion exposure of tungsten sample is seen to change the hardness of sample surface. In order to determine the surface hardness, we employed Vickers hardness tester with 500 g load to estimate hardness values at ﬁve different positions on reference as well as on the exposed samples. The microhardness values so obtained are given in Table 2. A marginal reduction in hardness values is observed at certain positions in case of the irradiated sample than the reference one. This is because of the formation of microcracks, pinholes and other defects in the irradiated samples. 4. Conclusions Fig. 6. XRD patterns of reference and irradiated sample having 2h value (a) 5–90° and (b) 5–30°. Table 2 Microhardness of samples. Sample Hardness, HV0.5 Reference Exposed 402 354 356 261 373 298 Hardness measured at ﬁve different places along the samples. 362 367 359 256 From the above investigations it may be concluded that the helium ions emanated from our PF device can be successfully utilized to study the ion induced damage to the PFC material namely tungsten. The experiment conclusively establishes the difference in surface morphology of the irradiated and reference samples when exposed with helium ions. Optical micrographs of the irradiated sample exhibit a uniform mesh of cracks having width around 1– 4 lm that spreads throughout the exposed surface. FESEM pictures of irradiated sample clearly indicate the different types of crystalline defects such as voids, pinholes, microcracks, bubbles and blisters which seems to have appeared due to ion bombardment. Moreover, formation of nanoﬂakes and nanoglobules structures has also been observed at certain places. The development of crystalline defects decreases signiﬁcantly in the case of the samples irradiated at off axis region because of lesser heat load deposition by helium ion pulses. In the XRD spectrum of irradiated samples, the major peaks exhibit a shift towards the higher Bragg angles indicating build up of compressive stress due to heat load. A marginal reduction in hardness value of irradiated sample is noticed which may be due to formation of crystalline defects. 56 N.J. 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