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Surface and structural analyses of helium ion irradiated beryllium
Vacuum 170 (2019) 108962
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Surface and structural analyses of helium ion irradiated beryllium N.J. Dutta a, 1, S.R. Mohanty a, b, *, K.P. Sooraj c, M. Ranjan b, c a
Center of Plasma Physics-Institute for Plasma Research, Sonapur, Kamrup, Assam, 782402, India Homi Bhabha National Institute, Anushaktinagar, Mumbai, Maharashtra, 400094, India c FCIPT, Institute for Plasma Research, Gandhinagar, Gujarat, 382016, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma focus Pulsed helium ion Beryllium Ostwald ripening Helium bubbles
We present the findings of pulsed helium ion irradiation on beryllium that is emitted from a tabletop pulsed ion source. The source delivers a broad range of helium ion energy having most probable energy of 60 keV with a flux of ~1025 m 2s 1. The FESEM micrographs of irradiated samples depict the formation of helium bubbles having tri-modal size distribution. The appearance of larger size bubbles is supposed to be resulted due to Ostwald ripening phenomenon. The calculated rate of irradiation swelling for the beryllium sample due to the development of helium bubbles is found to ~0.24%. The three dimensional AFM micrographs suggest the for mation of the hill like structures. An increase in the roughness value of irradiated samples is observed when the helium ion pulse number increases (up to 5 shots) and finally the roughness value decreases for 10 shots helium ion irradiation sample. The XRD patterns reveal the polycrystalline nature of the beryllium samples. In the case of irradiated samples, two feeble planes of beryllium oxide are speculated and most intense peak (002) confirms the formation of compressive stress due to helium ion irradiation.
1. Introduction The lightest material beryllium, also popularly named as the wonder material for future [1] has numerous applicability in the field of industry and research. Because of its unique mechanical and thermal properties like light in weight, high flexural rigidity, high thermal conductivity and stability [2], it is listed as a suitable material for the application in space and aircraft industries, satellite gyroscope, satellite scan mirrors, elec tronic industries, etc [2–4]. Moreover, due to favorable nuclear prop erties such as the low atomic number and high neutron scattering cross section [5–7], beryllium and its alloy are proposed to be used as plasma blanket and plasma facing material for the future generation nuclear reactors namely International Thermonuclear Experimental Reactor (ITER) [8]. In fusion reactors, beryllium produces two alpha particles by undergoing the neutron multiplication (n, 2n) reaction. Therefore, there is a wide possibility that the metallic beryllium surface is exposed with energetic charged particles (e.g. alpha particle) along with the neutrons and high heat fluxes of plasma. Hence, the charged particle-material interactions will play a key role in the successful run of these next-generation fusion reactors. In the past, different research groups have reported their important works on this domain of ion-beryllium
material interactions [9–11]. P. P. Liu et al. [9] have performed irradi ation of 30 keV helium ion on beryllium samples with a dose of 1017 cm 2. In their study, they have observed the helium bubbles of the average size of 2.7 nm. Also, at higher ion doses, they have speculated different types of dislocation loops in case of irradiated samples. While on irradiation of hot pressed beryllium samples by 650 keV hydrogen ions with a dose of 6.7 � 1016 cm 2, dislocation loops and oriented gas-filled bubbles were observed when viewed under TEM [10]. On the other hand, 8 keV helium ions irradiation on beryllium samples have been carried out by K. Morishita et al. [11] as a function of different temperatures (that lies between room temperature and 873 K). In their study, they have marked different types of defects namely tiny bubbles, blisters, exfoliation, flakes and pinholes, etc on the surface of the irra diated samples and those structures were found to be strongly dependent on temperature. Further, they also found that the formation of tiny bubbles on the irradiated samples is based on the gas driven process and radiation-induced vacancy migration phenomenon which is actually an athermal process. Although this field of charged particle interaction on beryllium material has been started in early eighties [12,13], but there is a scarcity in the reported data on it compared to that of other fusion relevant materials. The limitations in studies of beryllium are mainly
* Corresponding author. Center of Plasma Physics-Institute for Plasma Research, Sonapur, Kamrup, Assam, 782402, India. E-mail address: [email protected] (S.R. Mohanty). 1 Present address: Department of Physics, B. N. College, Dhubri, 783324, Assam, India. https://doi.org/10.1016/j.vacuum.2019.108962 Received 10 July 2019; Received in revised form 13 August 2019; Accepted 19 September 2019 Available online 20 September 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
N.J. Dutta et al.
Ag
<8000 <10 <400
Si N
<400 <200
Ni Mo
<20 <120
Mn Mg
<500 <3
Li Pb
<20 <700
Fe Cu
<100 <10
Co Cr
<100 <700
C Ca Cd
<500 Content (ppm)
B Al Impurity
Table 1 Concentration of impurities in the reference beryllium sample. 2
<100
due to (i) the toxic nature of beryllium that restricts the researcher to carry out the permutation and combination with a variety of charged particle sources and beryllium materials, and (ii) ease of availability of the beryllium materials to perform research and development work. Since, the prime objective of these ions/charged particles interactions study on fusion materials is to achieve a strong database of fusion ma terials under different experimental conditions that can able to simulate an environment which mimics the actual fusion reactor operating con ditions such as in terms of particle flux, ion species, etc. In turn, using this database, the researcher can able to acquire better understanding on the damage patterns of these fusion relevant materials to overcome any type of safety-related issues as well as gathering some information for designing an advanced fusion reactor materials or alloys that are less prone to the radiation damages. Hence, keeping these important key points in our mind, we have performed the helium ion irradiation on commercial grade pure beryllium samples by using a plasma-based pulsed ion source named as plasma focus (PF) device. The self-generated magnetic field allows the device to produce a high density and high temperature plasma column. The plasma column disrupts within a small time span of about hundreds of nanoseconds and resulting in the production of different types of radiations (starting from ions, electrons, electromagnetic radiations, etc) that are driven by the plasma instabilities [14]. A brief description of the working of the device is discussed in the next section of this manuscript. In last few decades, being an important tabletop radiation source, the PF device found its application in material research [15], nanofabrication [16], x-ray radi ography [17], UV lithography [18], etc. Also, the ions produced in the device were employed in various research experiments such as in thin film deposition [19], surface modification [20,21], the nuclear material study [22,23], etc. Recently, in order to simulate ITER related particle-material interactions, many linear accelerators are set up or proposed to be set up around the globe having incident ion flux within the range of 1020–1025 m 2s 1 [24]. In reference to that, our device is a suitable option for studying fusion material because of its advantages such as availability of high ion flux ~1025m 2s 1 at a relatively small time interval and size with the added benefit of reasonably lower cost and power consumption [25]. Moreover, it is also reported that the dense magnetized plasma devices [26] have emerged as one of the basic tools for the fusion material testing in connection with the designing of
<2
Fig. 1. Schematic of experimental set up.
<3
BeO
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Fig. 2. (a) A typical Faraday cup signal along with current (di/dt) and voltage signals, (b) helium ion distribution as a function of energy.
larger fusion irradiation facilities. Hence, plasma focus device, belonging to the family of dense magnetized plasma devices enable us to carry out the present ion-material interaction study related to fusion research.
helium gas. In our case, we have used helium gas of purity 99.99% to carry out the present helium ion irradiation study on beryllium samples. The emission of the ions is depending on the working pressure of the device. The details of the ion characterization of the device by using biased ion collector and nuclear track detector are found elsewhere [27]. The high purity (99%) beryllium samples were procured from Goodfellow, United Kingdom. The concentration of impurities in the bulk reference sample as supplied by the manufacturer is listed in Table 1. The geometry of the irradiated samples is square in shape having a dimension of 1 cm x 1 cm and the thickness of the sample is 0.3 cm. Before mounting the samples inside the chamber, these were polished with the help fine grain (grit of 1200) abrasive paper. The polished samples were then mounted inside the chamber with the help of a movable sample holder as shown in Fig. 1. The trajectory of the ion was kept perpendicular to the surface of the samples throughout the whole experimentation. The samples were irradiated with different experi mental conditions mainly by changing the numbers of helium ion pulses or numbers of plasma focus shots imposed on it. During multiple PF shots helium ion irradiation experimentation, each PF shot was taken with a fresh helium gas filling to minimise the contribution of impurities on the surface of the irradiated samples. The helium ion emitted from the plasma focus device at a working pressure of 0.66 mbar is studied by using a fast response Faraday cup (FC). The Faraday cup is operated in biased ion collector mode (with a biased voltage of 200V). The velocity, energy and density of helium ions are estimated by using time of flight method and the details of the same is available in Refs. [28,29]. A typical Faraday cup signal along with current (di/dt) signal and voltage signal as well as ion distribution as a function of energy is shown in Fig. 2. The expected ion fluence and flux emitted from the source as calculated from FC signal for a single PF shot or a single helium ion pulse
2. Experimentation The schematic of the experimental set up used as a helium ion source (a Mather type plasma focus device) is shown in Fig. 1. The main components of this ion source are an energy bank capacitor having a rated voltage of 25 kV and capacitance of 7.1 μF and a stainless steel (SS) chamber. Inside the SS chamber, an electrode system is housed and that consists of a central anode and twelve squirrel-cage arranged cathodes. In simple, the basic principle of the device is that at first the electrical energy is stored inside the capacitor and then this stored electrical en ergy is converted to magnetic energy by means of a low inductance circuit and finally part of this energy is converted to plasma energy. The dynamics of the device can be explained briefly as follows. As mentioned earlier, initially the capacitor is charged to its maximum value and then it is discharged by means of the spark gap switch between annul of the two electrodes system. When the capacitor is discharged, the current sheath initially developed across the insulator sleeve due to the break down of gas medium and the sheath is axially accelerated towards the open end of the anode by the axial component of self-generated J x B force. At the open end of electrodes, the sheath undergoes radial pinching and produces a highly dense plasma column. This plasma so produced lasts for only a few hundreds of nanoseconds because of m ¼ 0 instabilities. These instabilities disrupt the plasma column resulting in the formation of electromagnetic radiations (ions, electrons, x-rays, UV rays, etc). The ions that are produced inside the device move towards the top of the anode due to the geometry of the electrode system. Thus, one can able to generate helium ions by simply filling the chamber with 3
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Fig. 3. Plot of incident helium ion energy versus (a) electronic and (b) nuclear energy loss.
3. Theoretical aspect of helium ions irradiation on beryllium When an energetic charged particle interacts with a material, the charged particle loses its energy in the material matrix via two processes namely elastic collisions or nuclear energy loss and inelastic collisions or electronic energy loss. The contribution of energy loss mechanism is dependent on the incident energy of the projectile charged particle. We have generated the ion stopping ranges data (both electric and nuclear) of helium ion (having a range of 25–250 keV) that are irradiated on a beryllium sample by using Monte Carlo Program (SRIM 2008). Fig. 3 illustrates the plot of incident ion energy versus energy loss in the ma terial beryllium through electronic and nuclear stopping process. Since, in our case the most probable irradiated helium ion energy is ~60 keV, therefore, at this energy value, from Fig. 3, it is observed the contribution of nuclear energy loss is ~3.23 keV/μm and that of the electronic energy loss is ~1.86 � 102 keV/μm and the calculated maximum ion ranges for the above mentioned energy value is found to be ~443 nm. In other words, within the projected range (443 nm) of 60 keV helium ions on beryllium sample losses, it’s one part of energy (1.45 keV) by undergoing elastic collisions and another part of energy (8.37 keV) by the process of inelastic collisions. Also by using the same software we have calculated the helium ion content on the beryllium sample due to irradiation of 60 keV helium ions. From Fig. 4 it is found that the peak helium content is ~8.25 � 1018 per cm3. In view of studying damage of nuclear materials the process, nuclear energy loss is very important as in this process elastic collisions is taken place between the incident ion and target atoms. An important complexity that suffers from the fusion reactors are the erosions of plasma facing materials. These erosions cause the loss of the materials and that in turn act as plasma impurities creating obstacle in the suc cessful run of the plasma inside the reactors. The physical sputtering is an important erosion process, which is based on the momentum trans port in a collision cascade between the incident ion and target material. Sputtering yield is the parameter that measures the quantity of physical sputtering which can be defined as the number of target atoms that are ejected as a result of the interaction of single incident ion. Here, we have tried to estimate analytically the sputtering yield of beryllium material due to helium ion irradiation. The most widely used theory to determine the sputtering yield was based on Sigmund analyzing sputtering theory published by J. Bohdansky [31]. In our paper, we have used an advanced and modified empirical formula as reported in Ref. [32]. The final expression for the calculation of sputtering yield is obtained as,
Fig. 4. Helium content in beryllium sample.
is found to be ~1018 m 2 and 1025 m 2s 1 respectively, at an optimum working pressure of 0.66 mbar. The ion energy spectrum exhibits a broad band spectrum having peak at ~60 keV. It also follows the spec tral law dNi =dE � E K (where Ni are the numbers of ions having energy E) with the exponent value K in the range of 2–5. It is reported that the ion energy of a few tens of keV to a few MeV is achieved in case of a plasma focus device that operated in kJ to MJ energy [30]. After irradiation, the reference and irradiated samples are examined under different types of characterization tools to know about the changes that have taken place due to the ion irradiation. For the surface morphological study the samples are viewed under Sigma Zeiss field effect scanning electron microscope (FESEM) and NTEGRA PRIMA (NTMDT) atomic force microscope (AFM) (operating in non-contact mode). Whereas to ascertain the structural characterization of the samples Bruker D8 x-ray diffraction (XRD) machine with Cu K α (wavelength 1.54 Å) source is used. The XRD data are collected at 3� grazing inci dence angle with a step size of 0.019917. The elemental analyses of the beryllium sample surfaces before and after ion irradiation are deter mined by INCA X-Max 250, energy dispersive x-ray (EDX) spectroscopy.
4
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Table 2 Estimation of sputtering yield and numbers of beryllium atoms eject out per PF shot with different incident ion energies. Incident ion energy (in keV)
Sputtering yield (YE)
Numbers of beryllium atoms eject out per PF shot (X1016)
25 50 100 125 150 175 200 225 250
0.0571 0.0247 0.0136 0.0113 0.0094 0.0081 0.0071 0.0061 0.0056
5.71 2.47 1.36 1.13 0.94 0.81 0.71 0.61 0.56
ε¼
0:03255 M2 0 112 M1 þ M2 EðeVÞ 2 B 2 C Z1 Z2 @Z 31 þ Z 32 A
(3)
After evaluating the reduced energy, we have then calculated the reduced nuclear stopping power by the equation given by (4), pffiffi 3:441 εlnðε þ 2:718Þ pffiffi pffiffi STF (4) n ðεÞ ¼ 1 þ 6:355 ε þ εð6:882 ε 1:708Þ Further, the value of Lindhard electronic stopping co-efficient is estimated by using the following expression, 2
3
ke ¼ 0:079
ðM1 þ M2 Þ2 3 2
1 2
M1 M2
0
1
Z 31 Z 22
134
(5)
B C @Z 1 þ Z 2 A 2 3
2 3
Then, we have calculated the nuclear stopping cross section by using the relation given below [32],
Fig. 5. FESEM micrograph of reference beryllium samples at two different magnifications.
" QðZ2 Þα* ðM2 =M1 Þ Sn ðEÞ 1 YE ¼ 0:042 US 1 þ Γke ε0:3
rffiffiffiffiffiffi #s Eth E
(1)
Where, α* and Q(Z2) are the best fit values. The parameters Sn ðEÞ, ke , ε, US, M2, M1, Eth and E are nuclear stopping cross section, Lindhard elec tronic stopping coefficient, reduced energy term, surface binding energy term, the mass of the target, the mass of the projectile, threshold energy and projectile energy, respectively. Moreover, the factor Г is represented by, Γ¼
WðZ2 Þ � �3 1 þ M71
(2)
To calculate the sputtering yield we have to estimate the reduced energy term ε by using the following equation [32], Fig. 7. Size distribution of helium bubbles.
Fig. 6. FESEM micrographs of helium ion irradiated beryllium samples (a) 2 shots, (b) 5 shots and (c)–(d) 10 shots. 5
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Fig. 8. AFM micrographs of helium ion irradiated beryllium samples (a) reference, (b) 2 shots, (c) 5 shots and (d) 10 shots.
Table 4 Atomic percentage of different elements present in beryllium samples before and after ion irradiation. Elements
Atom percentage in samples
CK OK Fe K Cu K WM Cr K Ni K
Reference
2 shots
5 shots
10 shots
53.72 46.28 Nil Nil Nil Nil Nil
39.93 58.40 0.98 0.43 0.26 Nil Nil
36.62 61.81 0.43 Nil 0.18 0.37 0.59
32.65 65.90 0.46 0.46 Nil Nil 0.53
Z1 Z2 M1 TF Sn ðEÞ ¼ 84:78 8 912 M þ M Sn ðεÞ 1 2 > > < 2 = 2 Z 31 þ Z 32 > > : ;
After that, we have estimated the best fit value α* by using the following equation, � � 0:15 � � M M2 α* ¼ 0:088 2 þ 0:165 (7) M1 M1
Fig. 9. XRD pattern of reference and helium ion irradiated beryllium samples.
Table 3 Average grain size of reference and ion irradiated beryllium samples. Sample Reference 2 shots 5 shots 10 shots
Grain size in nm
Further, we have calculated the energy transfer function and threshold energy by using the two relations as given by equations (8) and (9) respectively,
Average grain size in nm
Plane(002)
Plane(101)
Plane(102)
23.06 22.48 22.47 22.29
17.85 17.20 16.21 15.47
15.02 14.91 14.72 14.72
(6)
18.6 18.2 17.8 17.5
γ¼
4M1 M2 ðM1 þ M2 Þ2
Eth ¼
� � 1 1 þ 5:7 M M2 γ
(8)
(9)
In the calculation, we have assumed the surface binding energy term US and best-fit values of Q(Z2), W(Z2) and s(Z2) as 3.32eV, 1.66, 2.32 and 2.5, respectively [32]. The estimated sputtering yield (as given by equation (1)) and numbers of beryllium atoms that ejected out for a 6
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Fig. 10. EDX spectrum of reference and helium ion irradiated beryllium samples.
single PF shot within a range of incident helium ion energy (25–250 keV) are tabulated in Table 2. It is observed that the value of sputtering yield decreases by almost ten times when the incident ion energy increased by one order. Moreover, to get the information about sputtering yield in case of higher ion incident energy, we have calculated the sputtering yield of beryllium target due to irradiation of 1 MeV helium ion. The calculated value of YE for 1 MeV (not shown in the table) is found to be ~0.0014, which is also about ten times lower than the value that is obtained for incident helium ion energy of 100 keV (YE ¼ 0.0136). As mentioned earlier, in our experiment, the most probable helium ion energy is 60 keV and the sputtering yield for that ion energy and it is found to be 0.026. The numbers of beryllium atoms that are ejected out for incident helium ion energy of 60 keV is ~2.6 � 1016 m-2.
places. The average size of the holes is about 1 μm and that of crack width is about 0.5 μm. Moreover, we have also observed some tiny point like bubbles at some certain places (not shown in the figure). The density of these bubbles is found to be more near to the places where cracks and holes are located. Whereas the sample irradiated to 5 shots of helium ion illustrates a molten like surface having splash out along with exploit dust grains at definite place on the sample surface. Also, the sample displays a surface with secondary cracks that are arranged in some loops. On the other hand, in case of sample irradiated to 10 shots of pulsed helium ion shows a surface containing holes, cracks and bubbles. The size of the bubbles shows tri-modal distributions (as shown in Fig. 6 (c)) consisting of very small (~20–50 nm), medium (~50–100 nm) and very large (>200 nm). We propose that the formation of large bubbles can be explained with the help of Ostwald ripening phenomenon [33,34]. It is a thermodynamically driven spontaneous process where the small bubbles coalesce together to form a bigger size bubble to minimise the overall surface energy. From Fig. 6 (c), it is observed that the process of Ostwald ripening has happened in the centre of grain but they are not able to migrate across the cracks or pits. That is why the larger size bubbles are always found near the centre region of the grain and not near the boundaries. In Fig. 6 (d) a magnified view of these bubbles are shown. The mean diameter of the bubbles is calculated from the above micro graph (shown in Fig. 6 (d)) by using Image J software. The sizes of the bubbles follow a distribution as shown in Fig. 7 with the maximum diameter of the bubbles ~350 nm. The mean diameter of the bubbles that calculated from the distribution is ~168 � 42.06 nm. The damage formation on the surface of the helium ion irradiated samples is believed to occur by the mechanism of relief of ion irradiation induced thermal stress via the process of plastic deformation or crack formation. E.A. Tkachenko et al. [35] in their work describe the opti mum condition to produce cracks on AISI A2 steel surface due to lon gitudinal and transverse stresses arising from irradiation of charged particle beam. They nicely proposed a model for simulation of longitu dinal and transverse stresses on the surface of the material to develop the cracks. Besides these cracks, in case of the surface of the irradiated sample shown in Fig. 6 (b) we observed some hill-like structures (marked in red circle). B. Spilker et al. [36] reported the formation of the hill-like structures on beryllium at a pulse having 260 MW m 2 power and 10 ms duration. They described that the higher temperature and
4. Result and discussion 4.1. FESEM characterization As mentioned earlier, the beryllium samples are irradiated with different numbers of helium ion pulses (2, 5 and 10 shots). As obvious, we can expect a clear difference between the surface morphologies of the different irradiated samples as there is a difference in the accumu lation of sputtered beryllium atoms with respect to the irradiated ion pulse numbers or PF shots. To have a better insight into the samples surface morphology at a higher resolution as well as magnification, we have characterized both the reference and irradiated samples by using FESEM. Fig. 5 depicts the pictures of shiny grey metal color reference beryllium sample. At relatively lower magnification the reference sam ple (Fig. 5 (a)) shows an uneven surface structure. On the other hand, at higher magnification some individual dust or dust clusters (as shown in Fig. 5 (b)) are observed at specific places on the surface for the reference sample. This dust or dust clusters may have arisen during the mechanical polishing of the samples. But the samples that are irradiated to various numbers of helium ion pulses represent a distinctly different surface morphology as compared to that of the reference sample. The samples irradiated to 2 shots (1018 helium ions per m2 in each shot) of helium ion show a surface having cracks and holes at some 7
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pulse duration have facilitated the surface tension force to act suffi ciently for a long while to overcome the damage that originates from plastic deformation. As a result, the liquid beryllium in the presence of surface tension agglomerates to produce such type of hill structure. In light of their understanding, we can also realize the formation of the hill structures as shown in Fig. 6 (b). When the thermal energy delivered on the surface of the sample is sufficiently high (>1287 � C) then the surface layers get melted and becomes beryllium liquid. However, it is also possible to achieve the local melting of the sample at a relatively lower temperature than the threshold value that arises due to the accumula tion of plastic deformation as well as disengagement of single grains at the surface [36]. Further, the surface tension force leads this liquid or melted beryllium material to develop some structures as observed on the surface of the sample as shown in Fig. 6 (b) followed by the rapid quenching process. On the other hand, Fig. 6 (d) shows a magnified view of helium ion irradiated sample to 10 shots which reveals the formation of spheroidal helium bubbles and that are uniformly distributed over the surface of the sample. The growth mechanism of individual bubbles is supposed to be a pressure driven process. The process can be explained with the help of J.H Evan [37] model. In his model of bubble growth mechanism, he explained that when there is an excess of internal pres sure in a bubble, then this excess pressure deforms the surrounding of atom planes and thus results in the creation of interstitial loop by punching out the planes. The consequence of the formation of gas bubbles on the surface of helium ion irradiated samples is the swelling of the material that turns up due to the increase in the volume of the material. The swelling rate in the material due to irradiation can be estimated by the following simple equation that is based on the change in volume of the material, number density and average size of the bubbles. S¼
ΔV � 100% V
4.2. AFM characterization The three dimensional AFM micrographs of reference and irradiated samples are obtained for a scan area of 2 � 2 μm and are shown in Fig. 8. The surface of the reference sample illustrates relatively a smooth sur face. The r.m.s and average roughness of the sample is found to be 2.2 and 1.6 nm, respectively. Fig. 8 (b) shows an AFM micrograph of 2 shots helium ion irradiated beryllium sample. The surface of the sample clearly indicates the changes in structure while comparing with the surafe structure of reference sample. Here, in Fig. 8 (b), surface is found to be more or less smooth one but having some spike like structures at certain places (mostly near to the boundary). The recorded r.m.s and average roughness of this sample are 20.7 and 16.3 nm, respectively. This roughness value is nearly ten times greater than that of the refer ence sample. The micrograph of 5 shots helium ion irradiated sample is depicted in Fig. 8 (c).The sample surface informed us about the forma tion of the heterogeneously distributed hill-like structures. For the sample, we found r.m.s and average roughness value as 64.3 and 50.2 nm, respectively. This increase in the roughness value of the irra diated sample is almost 30 times than that of the reference sample. On the other hand, sample irradiated to 10 numbers of helium ion shots suggests a surface having almost uniformly distributed hill-like structures as shown in Fig. 8 (d). It is observed that the recorded roughness value of the sample is slightly less than that of 5 shots ion irradiated sample. This lowering in the roughness value in case of higher ion irradiated sample is marked because of formation of uniform structures on the surface of the sample during irradiation due to higher heat load delivered and in turns which facilitated the higher mobility to the surface layer of the sample to form such type structures. The r.m.s and average roughness value for 10 shots irradiated sample are 59.2 and 47.1 nm, respectively and which is about 28 times as compared to the reference sample.
(10)
Where, ΔV is the increased in volume of the irradiated material and is nothing but the total volume occupied by the bubbles corresponding to the volume V. For the simplicity of calculation it is assumed that all the bubbles are spherical in shape then equation (10) can be rewritten as follows, P4 3 πr S¼ 3 � 100% (11) V
4.3. XRD characterization XRD characterization is used to carry out the structural study of the samples. The XRD patterns of both reference and irradiated samples (as shown in Fig. 9) reveal the polycrystalline nature of the beryllium samples. The reference sample exhibits the plane (100), (002), (101), (102) and (110) of hexagonal closed pack alpha phase of beryllium at two-theta value 45.620, 50.830, 52.690, 70.760 and 84.420, respectively. After irradiation, two feeble peaks at two-theta value 41.150 and 43.590 are observed in case of all the ion irradiated samples and that is assigned to the peak of (002) and (101) of BeO (beryllium oxide). Moreover, in the case of irradiated samples, the crystallinity is found to be decreased for the planes (100) and (110). The most intense peak (002) of the sample recorded a slight shift towards the higher Braggs angle region suggesting the development of compressive stress in the material matrix [40]. Whereas in case of 10 shots helium ion irradiated sample, (102) plane recorded a shift towards the lower Bragg’s angle region suggesting an expansional stress in that specifically oriented lattice plane. Further, the average grain size of reference, as well as the irradiated samples, is
Where r is the radius of the spherical bubble. The estimated swelling rate for the bubbles with a mean radius of 84 nm and having a number density of 1018per m3 within an area of thickness 360 nm is found to be about 0.24%. The calculated swelling rate in the present study for beryllium sample is well inside the range that is observed in the earlier work of neutron irradiation (that is from almost zero to 10%). P.P. Liu et al. [9] have also recorded a swelling of 0.1% when they irradiated 30 keV helium ions on the beryllium sample. It is observed that our swelling rate is almost twice than that they have reported. This increase in the value of swelling rate is due to the depo sition of relatively high heat load on the beryllium samples during the irradiation. Since ion beam of this PF device interacts with the sample for a few hundred of nsec, therefore, the sample temperature variation is a transient phenomenon with extremely high temperature rise rate (~40 K/nsec) that followed by fast cooling [38]. It is also believed that at lower heat load on the surface of the sample, the smaller bubbles are unable to coalesce through the process of Ostwald ripening to form a bigger bubble. To relate the swelling rate with temperature we can cite the example of work done by Klimenkov et al. [39]. In their paper, they reported that the swelling of beryllium falls within the range of 0.6%– 6.5% when temperature increases from 686K to 986 K as a result of neutron irradiation.
calculated by using Scherrer’s formula as given by equation (12) and the value obtained using the formula for different planes are listed in Table 3. D¼
0:9λ β2θ cos θ
(12)
Where, D is the grain size, λ is the wavelength of X-ray, β2θ is FWHM of the plane, and θ is Bragg’s angle. The computed values of the same propose a slight decrease (6.16%) in the average grain size of the irra diated sample (10 shots). The possible explanation for the decrease in the grain size is because of the hindering in the movement of grain boundary that arises due to the higher value of the retarding force 8
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produced by irradiated ions than the driving force of the grain growth [41].
Research, Gandhinagar, India and Centre Director, Centre of Plasma Physics- Institute for Plasma Research, Assam, India, for the financial support to carry out the present work. We are also thankful Dr. N. Adhikary, Dr. D. Chowdhury and Dr. S. Kundu of IAAST, Guwahati, India for their help in XRD, EDX and AFM characterization, respectively. One of the authors Mr. N. J. Dutta extends his heartiest thanks to P.P. Liu of School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China, for a fruitful discussion related to the analysis of results. We thanked Mr. M.K.D Sarma for his technical help.
4.4. EDX characterization Since, in our case, the ion irradiation effect is only limited to the surface and near-surface region (with maximum ion range of ~875 nm) of the sample, therefore we can assume that the impurity concentrations of the bulk beryllium sample (as tabulated in Table 1) before and after the helium ion irradiation remain more or less unchanged. However, there is a finite probability of the presence of impurities on the surface of samples that may be contributed from the chamber wall and electrode system. The elemental impurity compositions of the reference and irradiated samples are investigated by using EDX. The atomic percent ages of different elements present in the sample before and after irra diation to different numbers of PF shots are listed in Table 4. The EDX results suggest an increase (~42%) in the atomic percent age of oxygen content in the irradiated samples while we increased the helium ion pulse number (or PF shots) to 10 shots. On the other hand, in case of all the irradiated samples, the presence of elements namely C and Fe are also found. Besides above mentioned impurities, the irradiated samples contain some traces of Cu, W, Cr and Ni. These impurity ele ments are originated from the SS vacuum chamber wall and electrode system. The distribution of impurities in the surface of the irradiated samples that are imposed to different PF shots does not follow any specific trend rather it follows some random distribution nature. In Fig. 10, EDX spectrum of reference as well as irradiated samples to different PF shots is shown.
References [1] E.A. Smith, The big breakthrough in Beryllium, Financ. Anal. J. 16 (6) (1960) 51–54. [2] G.E. Darwin, J.H. Buddery, Beryllium, Academic Press, New York, 1960. [3] C.E. Windecker, D.W. White, J.E. Burke, The Metal Beryllium, ASM, Cleveland, 1955. [4] R.J.M. Konings, T.R. Allen, R. Stoller, S. Yamanaka, Comprehensive Nuclear Materials, Elsevier, Amsterdam, 2012. [5] J. Hackmann, J. Uhlenbusch, Test of a beryllium limiter in the tokamak unitor, J. Nucl. Mater. 128–129 (1984) 418–421. [6] A. Goldberg, Atomic, Crystal, Elastic, Thermal, Nuclear, and Other Properties of Beryllium, Lawrence Livermore National Laboratory, Livermore, 2006. [7] E.B. Deksnis, A.T. Peacock, H. Altmann, C. Ibbot, H.D. Falter, Beryllium plasmafacing components: JET experience, Fusion Eng. Des. 37 (1997) 515–530. [8] F.S. Argentina, G.R. Longhurst, V. Shestakov, H. Kawamura, The status of beryllium technology for fusion, J. Nucl. Mater. 283–287 (2000) 43–51. [9] P.P. Liu, L.W. Xue, L.P. Yu, J.L. Liu, W. Hu, Q. Zhan, F.R. Wan, Microstructure change and swelling of helium irradiated beryllium, Fusion Eng. Des. 140 (2019) 62–66. [10] S.P. Vagin, P.V. Chakrov, B.D. Utkelbayev, L.A. Jacobson, R.D. Field, H. Kung, Microstructural study of hydrogen-implanted beryllium, J. Nucl. Mater. 258–263 (1998) 719–723. [11] K. Morishita, T. Inoue, N. Yoshida, Microstructural evolution in beryllium by fusion-relevant low energy helium ion irradiation, J. Nucl. Mater. 266–269 (1999) 997–1002. [12] R. Vianden, E.N. Kaufmann, J.W. Rodgers, Impurity lattice location in ionimplanted beryllium: measurements and systematic, Phys. Rev. B. 22 (1) (1980) 63–79. [13] M.B. Liu, I. Sheft, D.M. Gruen, Deuterium trapping in ion-bombarded beryllium, J. Nucl. Mater. 79 (1979) 267–269. [14] H. Bhuyan, S.R. Mohanty, N.K. Neog, S. Bujarbarua, R.K. Rout, Magnetic probe measurements of current sheet dynamics in a coaxial plasma accelerator, Meas. Sci. Technol. 14 (2003) 1769–1776. [15] S. Javadi, M. Ghoranneviss, R.S. Rawat, A.S. Elahi, Topographical, structural and hardness changes in surface layer of stainless steel-AISI 304 irradiated by fusionrelevant high energy deuterium ions and neutrons in a low energy plasma focus device, Surf. Coat. Technol. 313 (2017) 73–81. [16] R.S. Rawat, High-energy-density pinch plasma: a unique nonconventional tool for plasma nanotechnology, IEEE Trans. Plasma Sci. 41 (2013) 701–713. [17] P. Knoblauch, V. Raspa, F.D. Lorenzo, A. Clausse, C. Moreno, Hard X-ray dosimetry of a plasma focus suitable for industrial radiography, Radiat. Phys. Chem. 145 (2018) 39–42. [18] S.R. Mohanty, T. Sakamoto, Y. Kobayashi, I. Song, M. Watanabe, T. Kawamura, A. Okino, K. Horioka, E. Hotta, Miniature hybrid plasma focus extreme ultraviolet source driven by 10kA fast current pulse, Rev. Sci. Instrum. 77 (2006), 043506 (7pp). [19] M.T. Hosseinnejad, M. Ghoranneviss, G.R. Etaati, F. Shahgoli, Fabrication of magnesium silicide thin films by pulsed ion beam ablation in a 1.6 kJ plasma focus device, Vacuum 94 (2013) 57–63. [20] M. Hassan, R. Ahmad, A. Qayyum, G. Murtaza, A. Waheed, M. Zakaullah, Surface modification of AlFe1.8 Zn0.8 alloy by using dense plasma focus, Vacuum 81 (2006) 291–298. [21] S.A. Hawat, M. Soukieh, M.A. Kharoub, W.A. Sadat, Using Mather-type plasma focus device for surface modification of AISI304 steel, Vacuum 84 (7) (2010) 907–912. [22] S. Javadi, B. Ouyang, Z. Zhang, M. Ghoranneviss, A.S. Elahi, R.S. Rawat, Effects of fusion relevant transient energetic radiation, plasma and thermal load on PLANSEE double forged tungsten samples in a low-energy plasma focus device, Appl. Surf. Sci. 443 (2018) 311–320. [23] S.H. Saw, V. Damideh, J. Ali, R.S. Rawat, P. Lee, S. Lee, Damage Study of Irradiated Tungsten using fast focus mode of a 2.2 kJ plasma focus, Vacuum 144 (2017) 14–20. [24] C. Linsmeier, B. Unterberg, J.W. Coenen, R.P. Doerner, H. Greuner, A. Kreter, J. Linke, H. Maier, Material testing facilities and programs for plasma-facing component testing, Nucl. Fusion 57 (2017), 092012 (34pp). [25] R.S. Rawat, Dense plasma focus – from alternative fusion source to versatile high energy density plasma source for plasma nanotechnology, J. Phys. Conf. Ser. 591 (25pp) (2015), 012021.
5. Conclusion The impact of helium ions emanated from a pulsed plasma device on beryllium samples have been investigated as a function of ion pulses or PF shots. A clear difference in surface morphologies (as obtained from FESEM micrographs) of the irradiated samples with different pulses has been observed and that displays the formation of different types of de fects such as holes, cracks, bubbles, melting splash, etc. The threshold helium ion pulses for the formation of tiny bubbles and secondary crack loops, as well as hill-like structures, are found to be two and five numbers (1018 helium ions per m2 in each pulse), respectively. At higher pulses (10 shots) the sample depicts a surface with helium bubbles of trimodal distributed size. The bigger size bubbles are supposed to form by the process of Ostwald ripening phenomenon. In this paper, we have estimated the irradiation-induced swelling on beryllium material and which is about 0.24%. The AFM investigation corroborates the surface morphology pattern obtained by FESEM. Also from AFM characteriza tion initially it is found that the roughness value of irradiated samples increase up to 30 times when pulse number increased to 5 shots but finally in case of 10 shots ion irradiation the samples suggest a slight decrease in the roughness and is around 28 times. The XRD results reveal the polycrystalline nature of the samples. All the diffraction peaks pre sent in case of reference sample are also appeared in case of irradiated samples along with two low intensity peaks at two-theta values 41.150 and 43.590. These peaks are assigned to be the peak of beryllium oxide. In the case of 10 shots ion irradiated sample, the calculated average grain size shows a decrease of about 6% than the average grain size of the reference sample. EDX spectrums suggest an increase of oxygen content (~42%) in irradiated samples when the PF shot is increased to 10 shots. In conclusion, it is noteworthy to mention that the ion-material interaction study presented in this paper will definitely provide some insights to the defects developed on beryllium surface because of ion irradiation. Acknowledgments Authors would like to acknowledge the Director, Institute for Plasma 9
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[26] reportIntegrated Approach to Dense Magnetized Plasma Applications in Nuclear Fusion Technology, International Atomic Energy Agency: Vienna, Report of Coordinated Research Project 2007-20112013. [27] M. Bhuyan, N.K. Neog, S.R. Mohanty, C.V.S. Rao, P.M. Raole, Temporal and spatial study of neon ion emission from a plasma focus device, Phys. Plasmas 18 (2011), 033101 (8pp). [28] G. Gerdin, W. Stygar, F. Venneri, Faraday cup analysis of ion beams produced by a dense plasma focus, J. Appl. Phys. 52 (1981) 3269. [29] S.R. Mohanty, H. Bhuyan, N.K. Neog, R.K. Rout, E. Hotta, Development of multi Faraday cup assembly for ion beam measurements from a low energy plasma focus device, Jpn. J. Appl. Phys. 44 (7A) (2005) 5199–5205. [30] H. Bhuyan, S.R. Mohanty, T.K. Borthakur, R.S. Rawat, Analysis of nitrogen ion beams produced in a dense plasma focus device using Faraday cup, Indian J. Pure Appl. Phys. 39 (2001) 698–703. [31] J. Bohdansky, A universal relation for the sputtering yield of monatomic solids at normal ion incidence, Nucl. Instrum. Methods Phys. Res. B. 2 (1984) 587–591. [32] Y. Yamamura, H. Tawara, Energy dependence of ion induced sputtering yields from monoatomic solids at normal incidence, NIFS DATA 23 (1995) 1–110 (ISSN 0915-6304). [33] H.S. Paulo, F.P. Fichtner, On the coarsening mechanisms of helium bubbles — Ostwald ripening versus migration and coalescence, J. Nucl. Mater. 179–181 (1991) 1007–1010.
[34] D.V. Yuryev, M.J. Demkowicz, Modeling growth, coalescence, and stability of helium precipitates on patterned interfaces, Model. Simul. Mater. Sci. Eng. 25 (15pp) (2017), 015003. [35] E.A. Tkachenko, D.V. Postnikov, I.A. Logachev, A.I. Logacheva, A.I. Blesman, D. A. Polonyankin, Simulation of internal stresses at the metals and alloys radiation with charged particle beams, Procedia Eng. 152 (2016) 589–593. [36] B. Spilker, J. Linke, G. Pintsuk, M. Wirtz, Investigation of damages induced by ITER-relevant heat loads during massive gas injections on Beryllium, Nucl. Mater. Energy 9 (2016) 145–152. [37] J.H. Evans, The role of implanted gas and lateral stress in blister formation mechanisms, J. Nucl. Mater. 76 –77 (1978) 228–234. [38] G. Sanchez, J. Feugeas, The thermal evolution of targets under plasma focus pulsed ion implantation, J. Phys. D Appl. Phys. 30 (1997) 927–936. [39] M. Klimenkov, V. Chakin, A. Moeslang, R. Rolli, TEM study of beryllium pebbles after neutron irradiation up to 3000appm helium production, J. Nucl. Mater. 443 (2013) 409–416. [40] B.D. Cullity, Elements of X-Ray Diffraction, second ed., Addison-Wesley, Palo Alto, 1978. [41] R.W. Kelsall, I.W. Hamley, M. Geoghegan, Nanoscale Science and Technology, JohnWiley & Sons, Chichester, 2006.
10
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Surface and structural analyses of helium ion irradiated beryllium N.J. Dutta a, 1, S.R. Mohanty a, b, *, K.P. Sooraj c, M. Ranjan b, c a
Center of Plasma Physics-Institute for Plasma Research, Sonapur, Kamrup, Assam, 782402, India Homi Bhabha National Institute, Anushaktinagar, Mumbai, Maharashtra, 400094, India c FCIPT, Institute for Plasma Research, Gandhinagar, Gujarat, 382016, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma focus Pulsed helium ion Beryllium Ostwald ripening Helium bubbles
We present the findings of pulsed helium ion irradiation on beryllium that is emitted from a tabletop pulsed ion source. The source delivers a broad range of helium ion energy having most probable energy of 60 keV with a flux of ~1025 m 2s 1. The FESEM micrographs of irradiated samples depict the formation of helium bubbles having tri-modal size distribution. The appearance of larger size bubbles is supposed to be resulted due to Ostwald ripening phenomenon. The calculated rate of irradiation swelling for the beryllium sample due to the development of helium bubbles is found to ~0.24%. The three dimensional AFM micrographs suggest the for mation of the hill like structures. An increase in the roughness value of irradiated samples is observed when the helium ion pulse number increases (up to 5 shots) and finally the roughness value decreases for 10 shots helium ion irradiation sample. The XRD patterns reveal the polycrystalline nature of the beryllium samples. In the case of irradiated samples, two feeble planes of beryllium oxide are speculated and most intense peak (002) confirms the formation of compressive stress due to helium ion irradiation.
1. Introduction The lightest material beryllium, also popularly named as the wonder material for future [1] has numerous applicability in the field of industry and research. Because of its unique mechanical and thermal properties like light in weight, high flexural rigidity, high thermal conductivity and stability [2], it is listed as a suitable material for the application in space and aircraft industries, satellite gyroscope, satellite scan mirrors, elec tronic industries, etc [2–4]. Moreover, due to favorable nuclear prop erties such as the low atomic number and high neutron scattering cross section [5–7], beryllium and its alloy are proposed to be used as plasma blanket and plasma facing material for the future generation nuclear reactors namely International Thermonuclear Experimental Reactor (ITER) [8]. In fusion reactors, beryllium produces two alpha particles by undergoing the neutron multiplication (n, 2n) reaction. Therefore, there is a wide possibility that the metallic beryllium surface is exposed with energetic charged particles (e.g. alpha particle) along with the neutrons and high heat fluxes of plasma. Hence, the charged particle-material interactions will play a key role in the successful run of these next-generation fusion reactors. In the past, different research groups have reported their important works on this domain of ion-beryllium
material interactions [9–11]. P. P. Liu et al. [9] have performed irradi ation of 30 keV helium ion on beryllium samples with a dose of 1017 cm 2. In their study, they have observed the helium bubbles of the average size of 2.7 nm. Also, at higher ion doses, they have speculated different types of dislocation loops in case of irradiated samples. While on irradiation of hot pressed beryllium samples by 650 keV hydrogen ions with a dose of 6.7 � 1016 cm 2, dislocation loops and oriented gas-filled bubbles were observed when viewed under TEM [10]. On the other hand, 8 keV helium ions irradiation on beryllium samples have been carried out by K. Morishita et al. [11] as a function of different temperatures (that lies between room temperature and 873 K). In their study, they have marked different types of defects namely tiny bubbles, blisters, exfoliation, flakes and pinholes, etc on the surface of the irra diated samples and those structures were found to be strongly dependent on temperature. Further, they also found that the formation of tiny bubbles on the irradiated samples is based on the gas driven process and radiation-induced vacancy migration phenomenon which is actually an athermal process. Although this field of charged particle interaction on beryllium material has been started in early eighties [12,13], but there is a scarcity in the reported data on it compared to that of other fusion relevant materials. The limitations in studies of beryllium are mainly
* Corresponding author. Center of Plasma Physics-Institute for Plasma Research, Sonapur, Kamrup, Assam, 782402, India. E-mail address: [email protected] (S.R. Mohanty). 1 Present address: Department of Physics, B. N. College, Dhubri, 783324, Assam, India. https://doi.org/10.1016/j.vacuum.2019.108962 Received 10 July 2019; Received in revised form 13 August 2019; Accepted 19 September 2019 Available online 20 September 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
N.J. Dutta et al.
Ag
<8000 <10 <400
Si N
<400 <200
Ni Mo
<20 <120
Mn Mg
<500 <3
Li Pb
<20 <700
Fe Cu
<100 <10
Co Cr
<100 <700
C Ca Cd
<500 Content (ppm)
B Al Impurity
Table 1 Concentration of impurities in the reference beryllium sample. 2
<100
due to (i) the toxic nature of beryllium that restricts the researcher to carry out the permutation and combination with a variety of charged particle sources and beryllium materials, and (ii) ease of availability of the beryllium materials to perform research and development work. Since, the prime objective of these ions/charged particles interactions study on fusion materials is to achieve a strong database of fusion ma terials under different experimental conditions that can able to simulate an environment which mimics the actual fusion reactor operating con ditions such as in terms of particle flux, ion species, etc. In turn, using this database, the researcher can able to acquire better understanding on the damage patterns of these fusion relevant materials to overcome any type of safety-related issues as well as gathering some information for designing an advanced fusion reactor materials or alloys that are less prone to the radiation damages. Hence, keeping these important key points in our mind, we have performed the helium ion irradiation on commercial grade pure beryllium samples by using a plasma-based pulsed ion source named as plasma focus (PF) device. The self-generated magnetic field allows the device to produce a high density and high temperature plasma column. The plasma column disrupts within a small time span of about hundreds of nanoseconds and resulting in the production of different types of radiations (starting from ions, electrons, electromagnetic radiations, etc) that are driven by the plasma instabilities [14]. A brief description of the working of the device is discussed in the next section of this manuscript. In last few decades, being an important tabletop radiation source, the PF device found its application in material research [15], nanofabrication [16], x-ray radi ography [17], UV lithography [18], etc. Also, the ions produced in the device were employed in various research experiments such as in thin film deposition [19], surface modification [20,21], the nuclear material study [22,23], etc. Recently, in order to simulate ITER related particle-material interactions, many linear accelerators are set up or proposed to be set up around the globe having incident ion flux within the range of 1020–1025 m 2s 1 [24]. In reference to that, our device is a suitable option for studying fusion material because of its advantages such as availability of high ion flux ~1025m 2s 1 at a relatively small time interval and size with the added benefit of reasonably lower cost and power consumption [25]. Moreover, it is also reported that the dense magnetized plasma devices [26] have emerged as one of the basic tools for the fusion material testing in connection with the designing of
<2
Fig. 1. Schematic of experimental set up.
<3
BeO
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Fig. 2. (a) A typical Faraday cup signal along with current (di/dt) and voltage signals, (b) helium ion distribution as a function of energy.
larger fusion irradiation facilities. Hence, plasma focus device, belonging to the family of dense magnetized plasma devices enable us to carry out the present ion-material interaction study related to fusion research.
helium gas. In our case, we have used helium gas of purity 99.99% to carry out the present helium ion irradiation study on beryllium samples. The emission of the ions is depending on the working pressure of the device. The details of the ion characterization of the device by using biased ion collector and nuclear track detector are found elsewhere [27]. The high purity (99%) beryllium samples were procured from Goodfellow, United Kingdom. The concentration of impurities in the bulk reference sample as supplied by the manufacturer is listed in Table 1. The geometry of the irradiated samples is square in shape having a dimension of 1 cm x 1 cm and the thickness of the sample is 0.3 cm. Before mounting the samples inside the chamber, these were polished with the help fine grain (grit of 1200) abrasive paper. The polished samples were then mounted inside the chamber with the help of a movable sample holder as shown in Fig. 1. The trajectory of the ion was kept perpendicular to the surface of the samples throughout the whole experimentation. The samples were irradiated with different experi mental conditions mainly by changing the numbers of helium ion pulses or numbers of plasma focus shots imposed on it. During multiple PF shots helium ion irradiation experimentation, each PF shot was taken with a fresh helium gas filling to minimise the contribution of impurities on the surface of the irradiated samples. The helium ion emitted from the plasma focus device at a working pressure of 0.66 mbar is studied by using a fast response Faraday cup (FC). The Faraday cup is operated in biased ion collector mode (with a biased voltage of 200V). The velocity, energy and density of helium ions are estimated by using time of flight method and the details of the same is available in Refs. [28,29]. A typical Faraday cup signal along with current (di/dt) signal and voltage signal as well as ion distribution as a function of energy is shown in Fig. 2. The expected ion fluence and flux emitted from the source as calculated from FC signal for a single PF shot or a single helium ion pulse
2. Experimentation The schematic of the experimental set up used as a helium ion source (a Mather type plasma focus device) is shown in Fig. 1. The main components of this ion source are an energy bank capacitor having a rated voltage of 25 kV and capacitance of 7.1 μF and a stainless steel (SS) chamber. Inside the SS chamber, an electrode system is housed and that consists of a central anode and twelve squirrel-cage arranged cathodes. In simple, the basic principle of the device is that at first the electrical energy is stored inside the capacitor and then this stored electrical en ergy is converted to magnetic energy by means of a low inductance circuit and finally part of this energy is converted to plasma energy. The dynamics of the device can be explained briefly as follows. As mentioned earlier, initially the capacitor is charged to its maximum value and then it is discharged by means of the spark gap switch between annul of the two electrodes system. When the capacitor is discharged, the current sheath initially developed across the insulator sleeve due to the break down of gas medium and the sheath is axially accelerated towards the open end of the anode by the axial component of self-generated J x B force. At the open end of electrodes, the sheath undergoes radial pinching and produces a highly dense plasma column. This plasma so produced lasts for only a few hundreds of nanoseconds because of m ¼ 0 instabilities. These instabilities disrupt the plasma column resulting in the formation of electromagnetic radiations (ions, electrons, x-rays, UV rays, etc). The ions that are produced inside the device move towards the top of the anode due to the geometry of the electrode system. Thus, one can able to generate helium ions by simply filling the chamber with 3
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Fig. 3. Plot of incident helium ion energy versus (a) electronic and (b) nuclear energy loss.
3. Theoretical aspect of helium ions irradiation on beryllium When an energetic charged particle interacts with a material, the charged particle loses its energy in the material matrix via two processes namely elastic collisions or nuclear energy loss and inelastic collisions or electronic energy loss. The contribution of energy loss mechanism is dependent on the incident energy of the projectile charged particle. We have generated the ion stopping ranges data (both electric and nuclear) of helium ion (having a range of 25–250 keV) that are irradiated on a beryllium sample by using Monte Carlo Program (SRIM 2008). Fig. 3 illustrates the plot of incident ion energy versus energy loss in the ma terial beryllium through electronic and nuclear stopping process. Since, in our case the most probable irradiated helium ion energy is ~60 keV, therefore, at this energy value, from Fig. 3, it is observed the contribution of nuclear energy loss is ~3.23 keV/μm and that of the electronic energy loss is ~1.86 � 102 keV/μm and the calculated maximum ion ranges for the above mentioned energy value is found to be ~443 nm. In other words, within the projected range (443 nm) of 60 keV helium ions on beryllium sample losses, it’s one part of energy (1.45 keV) by undergoing elastic collisions and another part of energy (8.37 keV) by the process of inelastic collisions. Also by using the same software we have calculated the helium ion content on the beryllium sample due to irradiation of 60 keV helium ions. From Fig. 4 it is found that the peak helium content is ~8.25 � 1018 per cm3. In view of studying damage of nuclear materials the process, nuclear energy loss is very important as in this process elastic collisions is taken place between the incident ion and target atoms. An important complexity that suffers from the fusion reactors are the erosions of plasma facing materials. These erosions cause the loss of the materials and that in turn act as plasma impurities creating obstacle in the suc cessful run of the plasma inside the reactors. The physical sputtering is an important erosion process, which is based on the momentum trans port in a collision cascade between the incident ion and target material. Sputtering yield is the parameter that measures the quantity of physical sputtering which can be defined as the number of target atoms that are ejected as a result of the interaction of single incident ion. Here, we have tried to estimate analytically the sputtering yield of beryllium material due to helium ion irradiation. The most widely used theory to determine the sputtering yield was based on Sigmund analyzing sputtering theory published by J. Bohdansky [31]. In our paper, we have used an advanced and modified empirical formula as reported in Ref. [32]. The final expression for the calculation of sputtering yield is obtained as,
Fig. 4. Helium content in beryllium sample.
is found to be ~1018 m 2 and 1025 m 2s 1 respectively, at an optimum working pressure of 0.66 mbar. The ion energy spectrum exhibits a broad band spectrum having peak at ~60 keV. It also follows the spec tral law dNi =dE � E K (where Ni are the numbers of ions having energy E) with the exponent value K in the range of 2–5. It is reported that the ion energy of a few tens of keV to a few MeV is achieved in case of a plasma focus device that operated in kJ to MJ energy [30]. After irradiation, the reference and irradiated samples are examined under different types of characterization tools to know about the changes that have taken place due to the ion irradiation. For the surface morphological study the samples are viewed under Sigma Zeiss field effect scanning electron microscope (FESEM) and NTEGRA PRIMA (NTMDT) atomic force microscope (AFM) (operating in non-contact mode). Whereas to ascertain the structural characterization of the samples Bruker D8 x-ray diffraction (XRD) machine with Cu K α (wavelength 1.54 Å) source is used. The XRD data are collected at 3� grazing inci dence angle with a step size of 0.019917. The elemental analyses of the beryllium sample surfaces before and after ion irradiation are deter mined by INCA X-Max 250, energy dispersive x-ray (EDX) spectroscopy.
4
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Table 2 Estimation of sputtering yield and numbers of beryllium atoms eject out per PF shot with different incident ion energies. Incident ion energy (in keV)
Sputtering yield (YE)
Numbers of beryllium atoms eject out per PF shot (X1016)
25 50 100 125 150 175 200 225 250
0.0571 0.0247 0.0136 0.0113 0.0094 0.0081 0.0071 0.0061 0.0056
5.71 2.47 1.36 1.13 0.94 0.81 0.71 0.61 0.56
ε¼
0:03255 M2 0 112 M1 þ M2 EðeVÞ 2 B 2 C Z1 Z2 @Z 31 þ Z 32 A
(3)
After evaluating the reduced energy, we have then calculated the reduced nuclear stopping power by the equation given by (4), pffiffi 3:441 εlnðε þ 2:718Þ pffiffi pffiffi STF (4) n ðεÞ ¼ 1 þ 6:355 ε þ εð6:882 ε 1:708Þ Further, the value of Lindhard electronic stopping co-efficient is estimated by using the following expression, 2
3
ke ¼ 0:079
ðM1 þ M2 Þ2 3 2
1 2
M1 M2
0
1
Z 31 Z 22
134
(5)
B C @Z 1 þ Z 2 A 2 3
2 3
Then, we have calculated the nuclear stopping cross section by using the relation given below [32],
Fig. 5. FESEM micrograph of reference beryllium samples at two different magnifications.
" QðZ2 Þα* ðM2 =M1 Þ Sn ðEÞ 1 YE ¼ 0:042 US 1 þ Γke ε0:3
rffiffiffiffiffiffi #s Eth E
(1)
Where, α* and Q(Z2) are the best fit values. The parameters Sn ðEÞ, ke , ε, US, M2, M1, Eth and E are nuclear stopping cross section, Lindhard elec tronic stopping coefficient, reduced energy term, surface binding energy term, the mass of the target, the mass of the projectile, threshold energy and projectile energy, respectively. Moreover, the factor Г is represented by, Γ¼
WðZ2 Þ � �3 1 þ M71
(2)
To calculate the sputtering yield we have to estimate the reduced energy term ε by using the following equation [32], Fig. 7. Size distribution of helium bubbles.
Fig. 6. FESEM micrographs of helium ion irradiated beryllium samples (a) 2 shots, (b) 5 shots and (c)–(d) 10 shots. 5
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Fig. 8. AFM micrographs of helium ion irradiated beryllium samples (a) reference, (b) 2 shots, (c) 5 shots and (d) 10 shots.
Table 4 Atomic percentage of different elements present in beryllium samples before and after ion irradiation. Elements
Atom percentage in samples
CK OK Fe K Cu K WM Cr K Ni K
Reference
2 shots
5 shots
10 shots
53.72 46.28 Nil Nil Nil Nil Nil
39.93 58.40 0.98 0.43 0.26 Nil Nil
36.62 61.81 0.43 Nil 0.18 0.37 0.59
32.65 65.90 0.46 0.46 Nil Nil 0.53
Z1 Z2 M1 TF Sn ðEÞ ¼ 84:78 8 912 M þ M Sn ðεÞ 1 2 > > < 2 = 2 Z 31 þ Z 32 > > : ;
After that, we have estimated the best fit value α* by using the following equation, � � 0:15 � � M M2 α* ¼ 0:088 2 þ 0:165 (7) M1 M1
Fig. 9. XRD pattern of reference and helium ion irradiated beryllium samples.
Table 3 Average grain size of reference and ion irradiated beryllium samples. Sample Reference 2 shots 5 shots 10 shots
Grain size in nm
Further, we have calculated the energy transfer function and threshold energy by using the two relations as given by equations (8) and (9) respectively,
Average grain size in nm
Plane(002)
Plane(101)
Plane(102)
23.06 22.48 22.47 22.29
17.85 17.20 16.21 15.47
15.02 14.91 14.72 14.72
(6)
18.6 18.2 17.8 17.5
γ¼
4M1 M2 ðM1 þ M2 Þ2
Eth ¼
� � 1 1 þ 5:7 M M2 γ
(8)
(9)
In the calculation, we have assumed the surface binding energy term US and best-fit values of Q(Z2), W(Z2) and s(Z2) as 3.32eV, 1.66, 2.32 and 2.5, respectively [32]. The estimated sputtering yield (as given by equation (1)) and numbers of beryllium atoms that ejected out for a 6
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Fig. 10. EDX spectrum of reference and helium ion irradiated beryllium samples.
single PF shot within a range of incident helium ion energy (25–250 keV) are tabulated in Table 2. It is observed that the value of sputtering yield decreases by almost ten times when the incident ion energy increased by one order. Moreover, to get the information about sputtering yield in case of higher ion incident energy, we have calculated the sputtering yield of beryllium target due to irradiation of 1 MeV helium ion. The calculated value of YE for 1 MeV (not shown in the table) is found to be ~0.0014, which is also about ten times lower than the value that is obtained for incident helium ion energy of 100 keV (YE ¼ 0.0136). As mentioned earlier, in our experiment, the most probable helium ion energy is 60 keV and the sputtering yield for that ion energy and it is found to be 0.026. The numbers of beryllium atoms that are ejected out for incident helium ion energy of 60 keV is ~2.6 � 1016 m-2.
places. The average size of the holes is about 1 μm and that of crack width is about 0.5 μm. Moreover, we have also observed some tiny point like bubbles at some certain places (not shown in the figure). The density of these bubbles is found to be more near to the places where cracks and holes are located. Whereas the sample irradiated to 5 shots of helium ion illustrates a molten like surface having splash out along with exploit dust grains at definite place on the sample surface. Also, the sample displays a surface with secondary cracks that are arranged in some loops. On the other hand, in case of sample irradiated to 10 shots of pulsed helium ion shows a surface containing holes, cracks and bubbles. The size of the bubbles shows tri-modal distributions (as shown in Fig. 6 (c)) consisting of very small (~20–50 nm), medium (~50–100 nm) and very large (>200 nm). We propose that the formation of large bubbles can be explained with the help of Ostwald ripening phenomenon [33,34]. It is a thermodynamically driven spontaneous process where the small bubbles coalesce together to form a bigger size bubble to minimise the overall surface energy. From Fig. 6 (c), it is observed that the process of Ostwald ripening has happened in the centre of grain but they are not able to migrate across the cracks or pits. That is why the larger size bubbles are always found near the centre region of the grain and not near the boundaries. In Fig. 6 (d) a magnified view of these bubbles are shown. The mean diameter of the bubbles is calculated from the above micro graph (shown in Fig. 6 (d)) by using Image J software. The sizes of the bubbles follow a distribution as shown in Fig. 7 with the maximum diameter of the bubbles ~350 nm. The mean diameter of the bubbles that calculated from the distribution is ~168 � 42.06 nm. The damage formation on the surface of the helium ion irradiated samples is believed to occur by the mechanism of relief of ion irradiation induced thermal stress via the process of plastic deformation or crack formation. E.A. Tkachenko et al. [35] in their work describe the opti mum condition to produce cracks on AISI A2 steel surface due to lon gitudinal and transverse stresses arising from irradiation of charged particle beam. They nicely proposed a model for simulation of longitu dinal and transverse stresses on the surface of the material to develop the cracks. Besides these cracks, in case of the surface of the irradiated sample shown in Fig. 6 (b) we observed some hill-like structures (marked in red circle). B. Spilker et al. [36] reported the formation of the hill-like structures on beryllium at a pulse having 260 MW m 2 power and 10 ms duration. They described that the higher temperature and
4. Result and discussion 4.1. FESEM characterization As mentioned earlier, the beryllium samples are irradiated with different numbers of helium ion pulses (2, 5 and 10 shots). As obvious, we can expect a clear difference between the surface morphologies of the different irradiated samples as there is a difference in the accumu lation of sputtered beryllium atoms with respect to the irradiated ion pulse numbers or PF shots. To have a better insight into the samples surface morphology at a higher resolution as well as magnification, we have characterized both the reference and irradiated samples by using FESEM. Fig. 5 depicts the pictures of shiny grey metal color reference beryllium sample. At relatively lower magnification the reference sam ple (Fig. 5 (a)) shows an uneven surface structure. On the other hand, at higher magnification some individual dust or dust clusters (as shown in Fig. 5 (b)) are observed at specific places on the surface for the reference sample. This dust or dust clusters may have arisen during the mechanical polishing of the samples. But the samples that are irradiated to various numbers of helium ion pulses represent a distinctly different surface morphology as compared to that of the reference sample. The samples irradiated to 2 shots (1018 helium ions per m2 in each shot) of helium ion show a surface having cracks and holes at some 7
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pulse duration have facilitated the surface tension force to act suffi ciently for a long while to overcome the damage that originates from plastic deformation. As a result, the liquid beryllium in the presence of surface tension agglomerates to produce such type of hill structure. In light of their understanding, we can also realize the formation of the hill structures as shown in Fig. 6 (b). When the thermal energy delivered on the surface of the sample is sufficiently high (>1287 � C) then the surface layers get melted and becomes beryllium liquid. However, it is also possible to achieve the local melting of the sample at a relatively lower temperature than the threshold value that arises due to the accumula tion of plastic deformation as well as disengagement of single grains at the surface [36]. Further, the surface tension force leads this liquid or melted beryllium material to develop some structures as observed on the surface of the sample as shown in Fig. 6 (b) followed by the rapid quenching process. On the other hand, Fig. 6 (d) shows a magnified view of helium ion irradiated sample to 10 shots which reveals the formation of spheroidal helium bubbles and that are uniformly distributed over the surface of the sample. The growth mechanism of individual bubbles is supposed to be a pressure driven process. The process can be explained with the help of J.H Evan [37] model. In his model of bubble growth mechanism, he explained that when there is an excess of internal pres sure in a bubble, then this excess pressure deforms the surrounding of atom planes and thus results in the creation of interstitial loop by punching out the planes. The consequence of the formation of gas bubbles on the surface of helium ion irradiated samples is the swelling of the material that turns up due to the increase in the volume of the material. The swelling rate in the material due to irradiation can be estimated by the following simple equation that is based on the change in volume of the material, number density and average size of the bubbles. S¼
ΔV � 100% V
4.2. AFM characterization The three dimensional AFM micrographs of reference and irradiated samples are obtained for a scan area of 2 � 2 μm and are shown in Fig. 8. The surface of the reference sample illustrates relatively a smooth sur face. The r.m.s and average roughness of the sample is found to be 2.2 and 1.6 nm, respectively. Fig. 8 (b) shows an AFM micrograph of 2 shots helium ion irradiated beryllium sample. The surface of the sample clearly indicates the changes in structure while comparing with the surafe structure of reference sample. Here, in Fig. 8 (b), surface is found to be more or less smooth one but having some spike like structures at certain places (mostly near to the boundary). The recorded r.m.s and average roughness of this sample are 20.7 and 16.3 nm, respectively. This roughness value is nearly ten times greater than that of the refer ence sample. The micrograph of 5 shots helium ion irradiated sample is depicted in Fig. 8 (c).The sample surface informed us about the forma tion of the heterogeneously distributed hill-like structures. For the sample, we found r.m.s and average roughness value as 64.3 and 50.2 nm, respectively. This increase in the roughness value of the irra diated sample is almost 30 times than that of the reference sample. On the other hand, sample irradiated to 10 numbers of helium ion shots suggests a surface having almost uniformly distributed hill-like structures as shown in Fig. 8 (d). It is observed that the recorded roughness value of the sample is slightly less than that of 5 shots ion irradiated sample. This lowering in the roughness value in case of higher ion irradiated sample is marked because of formation of uniform structures on the surface of the sample during irradiation due to higher heat load delivered and in turns which facilitated the higher mobility to the surface layer of the sample to form such type structures. The r.m.s and average roughness value for 10 shots irradiated sample are 59.2 and 47.1 nm, respectively and which is about 28 times as compared to the reference sample.
(10)
Where, ΔV is the increased in volume of the irradiated material and is nothing but the total volume occupied by the bubbles corresponding to the volume V. For the simplicity of calculation it is assumed that all the bubbles are spherical in shape then equation (10) can be rewritten as follows, P4 3 πr S¼ 3 � 100% (11) V
4.3. XRD characterization XRD characterization is used to carry out the structural study of the samples. The XRD patterns of both reference and irradiated samples (as shown in Fig. 9) reveal the polycrystalline nature of the beryllium samples. The reference sample exhibits the plane (100), (002), (101), (102) and (110) of hexagonal closed pack alpha phase of beryllium at two-theta value 45.620, 50.830, 52.690, 70.760 and 84.420, respectively. After irradiation, two feeble peaks at two-theta value 41.150 and 43.590 are observed in case of all the ion irradiated samples and that is assigned to the peak of (002) and (101) of BeO (beryllium oxide). Moreover, in the case of irradiated samples, the crystallinity is found to be decreased for the planes (100) and (110). The most intense peak (002) of the sample recorded a slight shift towards the higher Braggs angle region suggesting the development of compressive stress in the material matrix [40]. Whereas in case of 10 shots helium ion irradiated sample, (102) plane recorded a shift towards the lower Bragg’s angle region suggesting an expansional stress in that specifically oriented lattice plane. Further, the average grain size of reference, as well as the irradiated samples, is
Where r is the radius of the spherical bubble. The estimated swelling rate for the bubbles with a mean radius of 84 nm and having a number density of 1018per m3 within an area of thickness 360 nm is found to be about 0.24%. The calculated swelling rate in the present study for beryllium sample is well inside the range that is observed in the earlier work of neutron irradiation (that is from almost zero to 10%). P.P. Liu et al. [9] have also recorded a swelling of 0.1% when they irradiated 30 keV helium ions on the beryllium sample. It is observed that our swelling rate is almost twice than that they have reported. This increase in the value of swelling rate is due to the depo sition of relatively high heat load on the beryllium samples during the irradiation. Since ion beam of this PF device interacts with the sample for a few hundred of nsec, therefore, the sample temperature variation is a transient phenomenon with extremely high temperature rise rate (~40 K/nsec) that followed by fast cooling [38]. It is also believed that at lower heat load on the surface of the sample, the smaller bubbles are unable to coalesce through the process of Ostwald ripening to form a bigger bubble. To relate the swelling rate with temperature we can cite the example of work done by Klimenkov et al. [39]. In their paper, they reported that the swelling of beryllium falls within the range of 0.6%– 6.5% when temperature increases from 686K to 986 K as a result of neutron irradiation.
calculated by using Scherrer’s formula as given by equation (12) and the value obtained using the formula for different planes are listed in Table 3. D¼
0:9λ β2θ cos θ
(12)
Where, D is the grain size, λ is the wavelength of X-ray, β2θ is FWHM of the plane, and θ is Bragg’s angle. The computed values of the same propose a slight decrease (6.16%) in the average grain size of the irra diated sample (10 shots). The possible explanation for the decrease in the grain size is because of the hindering in the movement of grain boundary that arises due to the higher value of the retarding force 8
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produced by irradiated ions than the driving force of the grain growth [41].
Research, Gandhinagar, India and Centre Director, Centre of Plasma Physics- Institute for Plasma Research, Assam, India, for the financial support to carry out the present work. We are also thankful Dr. N. Adhikary, Dr. D. Chowdhury and Dr. S. Kundu of IAAST, Guwahati, India for their help in XRD, EDX and AFM characterization, respectively. One of the authors Mr. N. J. Dutta extends his heartiest thanks to P.P. Liu of School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China, for a fruitful discussion related to the analysis of results. We thanked Mr. M.K.D Sarma for his technical help.
4.4. EDX characterization Since, in our case, the ion irradiation effect is only limited to the surface and near-surface region (with maximum ion range of ~875 nm) of the sample, therefore we can assume that the impurity concentrations of the bulk beryllium sample (as tabulated in Table 1) before and after the helium ion irradiation remain more or less unchanged. However, there is a finite probability of the presence of impurities on the surface of samples that may be contributed from the chamber wall and electrode system. The elemental impurity compositions of the reference and irradiated samples are investigated by using EDX. The atomic percent ages of different elements present in the sample before and after irra diation to different numbers of PF shots are listed in Table 4. The EDX results suggest an increase (~42%) in the atomic percent age of oxygen content in the irradiated samples while we increased the helium ion pulse number (or PF shots) to 10 shots. On the other hand, in case of all the irradiated samples, the presence of elements namely C and Fe are also found. Besides above mentioned impurities, the irradiated samples contain some traces of Cu, W, Cr and Ni. These impurity ele ments are originated from the SS vacuum chamber wall and electrode system. The distribution of impurities in the surface of the irradiated samples that are imposed to different PF shots does not follow any specific trend rather it follows some random distribution nature. In Fig. 10, EDX spectrum of reference as well as irradiated samples to different PF shots is shown.
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5. Conclusion The impact of helium ions emanated from a pulsed plasma device on beryllium samples have been investigated as a function of ion pulses or PF shots. A clear difference in surface morphologies (as obtained from FESEM micrographs) of the irradiated samples with different pulses has been observed and that displays the formation of different types of de fects such as holes, cracks, bubbles, melting splash, etc. The threshold helium ion pulses for the formation of tiny bubbles and secondary crack loops, as well as hill-like structures, are found to be two and five numbers (1018 helium ions per m2 in each pulse), respectively. At higher pulses (10 shots) the sample depicts a surface with helium bubbles of trimodal distributed size. The bigger size bubbles are supposed to form by the process of Ostwald ripening phenomenon. In this paper, we have estimated the irradiation-induced swelling on beryllium material and which is about 0.24%. The AFM investigation corroborates the surface morphology pattern obtained by FESEM. Also from AFM characteriza tion initially it is found that the roughness value of irradiated samples increase up to 30 times when pulse number increased to 5 shots but finally in case of 10 shots ion irradiation the samples suggest a slight decrease in the roughness and is around 28 times. The XRD results reveal the polycrystalline nature of the samples. All the diffraction peaks pre sent in case of reference sample are also appeared in case of irradiated samples along with two low intensity peaks at two-theta values 41.150 and 43.590. These peaks are assigned to be the peak of beryllium oxide. In the case of 10 shots ion irradiated sample, the calculated average grain size shows a decrease of about 6% than the average grain size of the reference sample. EDX spectrums suggest an increase of oxygen content (~42%) in irradiated samples when the PF shot is increased to 10 shots. In conclusion, it is noteworthy to mention that the ion-material interaction study presented in this paper will definitely provide some insights to the defects developed on beryllium surface because of ion irradiation. Acknowledgments Authors would like to acknowledge the Director, Institute for Plasma 9
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