Nuclear Materials and Energy 12 (2017) 553–558 Contents lists available at ScienceDirect Nuclear Materials and Energy journal homepage: www.elsevier.com/locate/nme In-situ imaging of tungsten surface modiﬁcation under ITER-like transient heat loads A.A. Vasilyev a,b,∗, A.S. Arakcheev a,b,c, I.A. Bataev c, V.A. Bataev c, A.V. Burdakov a,c, I.V. Kandaurov a, A.A. Kasatov a,b, V.V. Kurkuchekov a,b, K.I. Mekler a, V.A. Popov a,b, A.A. Shoshin a,b, D.I. Skovorodin a,b, Yu.A. Trunev a,b, L.N. Vyacheslavov a,b a Budker Institute of Nuclear Physic SB RAS, Novosibirsk, 630090, Russia Novosibirsk State University, Novosibirsk, 630090, Russia c Novosibirsk State Technical University, Novosibirsk, 630092, Russia b a r t i c l e i n f o Article history: Received 14 July 2016 Revised 19 October 2016 Accepted 18 November 2016 Available online 5 December 2016 a b s t r a c t Experimental research on behavior of rolled tungsten plates under intense transient heat loads generated by a powerful (a total power of up to 7 MW) long-pulse (0.1–0.3 ms) electron beam with full irradiation area of 2 cm2 was carried out. Imaging of the sample by the fast CCD cameras in the NIR range and with illumination by the 532 nm continuous-wave laser was applied for in-situ surface diagnostics during exposure. In these experiments tungsten plates were exposed to heat loads 0.5–1 MJ/m2 with a heat ﬂux factor (Fhf ) close to and above the melting threshold of tungsten at initial room temperature. Crack formation and crack propagation under the surface layer were observed during multiple exposures. Overheated areas with excessive temperature over surrounding surface of about 500 K were found on severely damaged samples more than 5 ms after beam ending. The application of laser illumination enables to detect areas of intense tungsten melting near crack edges and crack intersections. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) 1. Introduction The problem of plasma surface interaction is one of the challenges in the construction of future fusion reactors with magnetic conﬁnement. High-energy neutron irradiation, potential retention of the radioactive fuel and severe transient heat loads due to different plasma transients (ELMs, VDEs, and disruptions) impose strong restriction on plasma facing components. Tungsten has been selected as the material of the divertor plates for its resistivity to neutron activation, high thermomechanical properties and relatively low tritium retention. Currently, the energy loads due to ELMs type 1 are expected to be 0.5–10 MJ/m2 with duration 0.1– 0.5 ms . Such heat loads result in strong erosion of the plasma facing components including cracking. Recent theoretical models of the tungsten behavior predict crack formation at low limit of ITER-like transients events [2,3]. Also, experimental simulation of such heat loads exposed cracking and recrystallization of surface layer [4,6,7]. Crack formation on tungsten surface under transient heating has been studied on different types of devices. Quasi-stationary ∗ Corresponding author. E-mail address: [email protected] (A.A. Vasilyev). plasma accelerators (QSPAs) are used for simulation of plasma loads on surface with a dense accelerated plasma ﬂux. QSPAs can produce ∼2 MJ/m2 with duration of up to 0.5 ms [4,5]. However, such devices create excessive plasma pressure on the target surface, which affects the melted tungsten behavior. In addition, accelerated dense plasma emits a lot of radiation which complicates optical diagnostics of the surface. The scanning electron beam facility JUDITH-1 produces heat loads of ∼1 MJ/m2 with duration of a millisecond and a total irradiation area of ∼0.16 cm2 for such operating modes . Lasers such as Nd: YAG can create radiation heating similar to ELMs type 1 in reactor-size tokamaks . Although test installations based on electron beams and lasers have no constraints due to shielding effects, these machines cannot provide desired combination of power density, pulse duration and exposure area (bigger than 10 cell sizes for tungsten cracking ). Linear plasma generator PILOT-PSI is developed to gain transient plasma loads of up to 1 MJ/m2 with a pulse duration of 1.5 ms . It also has capabilities for steady-state heat loads simulation. On the GOL-3 facility, electron beam loads are applied simultaneously with plasma exposure. The installation produces energy densities of up to 4 MJ/m2 with duration about 10 μs [9,10]. A specialized test installation for research on material behavior under the impact of the powerful thermal shock was developed http://dx.doi.org/10.1016/j.nme.2016.11.017 2352-1791/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) 554 A.A. Vasilyev et al. / Nuclear Materials and Energy 12 (2017) 553–558 Fig. 1. Schematic of the facility for generation of transient heat loads. at the Budker Institute. It grants new capabilities for experimental simulation of transient heat loads corresponding to ITER-relevant ELMs type I. Application of intense electron beam for target heating almost eliminate shielding effects of target plasma that gives possibilities to achieve high heat ﬂuxes on the surface in this device. The electron beam provide a suﬃciently large exposure area with spatial uniformity of the absorbed power density in a wide range of operating modes. Absence of the collateral light during heating pulse enables using of the in-situ surface optical diagnostics. 2. Test facility description The schematic of the facility is shown in Fig. 1. The device consists of a high-power submillisecond electron beam injector with a plasma emitter, the vacuum and magnetic ﬁeld systems, the target exposure unit and ports for optical diagnostics. The emitted electron beam passes through a converging magnetic ﬁeld to the target and creates an intense heat load on its surface. 2.1. Electron beam accelerator parameters The electron beam source is based on a wide-area multiaperture electron beam diode with a plasma cathode . It is able to produce an electron beam with a current up to 100 A and has an acceleration voltage of up to 100 kV. The typical pulse duration on target is 10 0–30 0 μs. The beam source is immersed in a guiding Fig. 3. Comparison of the system spectral sensitivity and WI and WII spectra. Lone peaks: spectra of tungsten ions, smooth curve: sensitivity of CCD camera and IR ﬁlter. external magnetic ﬁeld, which is used to transport and to compress the electron beam, producing desired current densities on a target. The magnetic ﬁeld is about 7 mT at the diode and about 0.2 T in the target location. Measurements with Faraday cup located further the target region showed that ∼85% of the emitted current passes through the system. The spatial distribution of the current density at the diode was obtained with X-ray diagnostic and was found rather ﬂat and uniform. 2.2. Optical diagnostics speciﬁcation This installation is ﬁtted with a set of optical diagnostics of the target erosion as shown in Fig. 2. A front view of the target surface is imaged through tilted mirror by lens (f = 200 mm) into a CCD camera supplied with an infrared ﬁlter. The ﬁnal spectral sensitivity enables to receive a picture of thermal radiation of the target in the NIR range without any interference from the light of tungsten ablation plume near the target surface. The total system sensitivity is depicted in Fig. 3. The CCD matrix has 6.45 × 6.45 μm pixels, so with the optical magniﬁcation 1/2.7 the spatial resolution is about 20 μm. The 2D temperature distribution is reconstructed through absolute calibration using a tungsten ribbon lamp and variable value of emissivity taken from the literature . Fig. 2. Schematics of the optical diagnostics. Surface erosion is observed using CCD camera in NIR range and CCD camera with continuous wave laser light illumination (λ = 532 nm) and narrowband interferential ﬁlter. A.A. Vasilyev et al. / Nuclear Materials and Energy 12 (2017) 553–558 555 Fig. 4. Images of the target surface in the NIR range after heat load. a) ﬁrst shot, b) second shot, c) seventh shot. A continuous wave laser (λ = 532 nm) is used for illumination of the target surface. A narrowband optical ﬁlter is applied for rejection of plasma light and thermal radiation of the surface. This diagnostics set is used for detection of melted layer formation on crack edges and crack intersections. 3. Experimental results and discussion 3.1. Exposure conditions Target samples with size of 25 × 25 mm and 3–4 mm thick of rolled tungsten were used in these experiments. Sample were manufactured by electric spark cutting from a single plate without following grinding or polishing. All specimens were exposed at room temperature without additional cooling or heating. Exposures were performed with low repetition rate (one pulse per several minutes) because of the features of the heating device. Target cooled down to the initial temperature between exposures due to thermal conductivity to target holder. A typical shape of the heat load had a rectangular proﬁle with sharp edges. Experimental calorimetric measurements showed that the target absorbed 50% of the emitted electron beam energy. This value is well correlated with the facts that 85% of the compressed beam current passes through the magnetic system to the target region and 60% of the electron energy is reﬂected from the tungsten sample surface as shows numerical calculations . The power density on the target was calculated through these data and the measurement of current density at a distance of ∼20 mm from target region taking into accout compressing in magnetic ﬁeld. The machine is able to provide up to 13 GW/m2 . The energy load cross-sectional distribution has dome-like form . About half of the absorbed heat ﬂux is located in the central area with 9 mm in diameter, which is bigger almost in order than characteristic size of the crack network. The tungsten samples were exposed to heat loads in the range of 0.5–1 MJ/m2 , close to and above the melting threshold, which was assumed ∼ 50 MJ/m2 s0.5 at room temperature . 3.2. Crack formation on tungsten surface A tungsten target surface was imaged in the NIR range during heating with an average energy load of 0.6 MJ/m2 and Fhf ∼50 MJ/m2 s0.5 , close to the melting threshold. Average pulse duration was ∼130 μs. Initially, the target surface was clear and undamaged. Images obtained during ﬁrst, second and seventh pulses are shown in Fig. 4. A slight crack network was formed after the ﬁrst pulse. With subsequent pulses this net became denser due to appearance of new cracks on the surface and the thermal radiation inhomogeneities became more distinguishable as a consequence of crack widening and their propagation into the bulk. The magnitude Fig. 5. Magnitude of the thermal radiation inhomogeneities in the central area. The graph shows the growth of the root mean square deviation with ﬁrst four shots; then the increase becomes much slower. of the root mean square deviation (RMSD) of the inhomogeneities over intensity of background radiation of tungsten surface in the central area is shown in Fig. 5. It was discovered that value of RMSD increased signiﬁcantly after ﬁrst three heating pulses and did not grow noticeably after several subsequent heat loads. 3.3. Crack propagation along the sample surface The tungsten target was irradiated 24 times with an average energy load 1 MJ/m2 and average Fhf up to 70 MJ/m2 s0.5 . Average exposure duration was about 120 μs. A network of inhomogeneities with typical distance of ∼1 mm between them was detected in the thermal radiation, as shown in Fig. 6(b). This structure matches exactly with the crack net on the surface as displayed on the photo of the sample and its SEM images in Fig. 6(a,c). The bright areas on the thermal image correspond to edges and intersections of the cracks. The SEM survey of the surface revealed intense melting of the tungsten layer near the bright spots. A 2D absolute temperature plot is presented in Fig. 6(d). It was derived from the thermal radiation image made 20 μs after heating ending and exposed that the tungsten temperature exceeded melting point on the crack edges in the center of the target. A transverse microsection is depicted in Fig. 7(b,d,e). It revealed cracks propagated along the surface at a depth of 0.1–0.2 mm. Correlation of the surface temperature increase and propagation of longitudinal cracks is showed on Fig. 7. Such cracks start from perpendicular ruptures and propagate into the bulk. This target damage is responsible for decrease of the heat transport into the bulk of the material, which lead to appearance of the overheated areas on the surface and intensiﬁcation of the target erosion. Also these crack can develop into detachment 556 A.A. Vasilyev et al. / Nuclear Materials and Energy 12 (2017) 553–558 of some parts of the sample, what is discussed at chapter 3.5 of this paper. 3.4. Imaging of the target surface with the laser light illumination In the heating experiments, the target surface illuminated with the green laser light was imaged using the CCD camera. A tungsten plate was irradiated 72 times with an average heat load of about 0.55 MJ/m2 and Fhf ∼60 MJ/m2 s0.5 . Average pulse length was about 90 μs. The images of the green laser spot on the target were obtained during and well after the heating pulse, when the surface was at room temperature. They are shown in Fig. 8(b,c) superimposed with the thermal image of the target during the heating process. Without heating, the image of the laser spot on the target had a detailed structure. The reﬂection pattern of the laser spot during the heating pulse was smoother, which can be associated with appearance of tungsten melt in these areas. Comparison with the thermal radiation image and SEM images (Fig. 9) showed that this effect appeared on hot spots of the target surface: edges and intersections of the cracks. 3.5. Hot areas on severely damaged tungsten sample on cooling stage Fig. 6. Images of the target surface. a) photo after extraction from vacuum chamber, b) IR image during heat load, c) SEM images of the cracks, d) 2D temperature distribution, dimensions are in mm, temperature is in K degrees. The black box on a) represent area of temperature picture calculation on d). The blue and red rectangulars on b) mark SEM image on c). The black line on d) denotes line of the following cross section. The crack network obtained with photo and SEM survey matches with the thermal radiation inhomogeneities. The temperature picture shows melting of the tungsten in the center of the target near the crack edges. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) The tungsten target was imaged while its surface was cooling after the electron beam termination. The target was initially exposed to more than 100 pulses with heat loads of about 1 MJ/m2 and average duration of 130 μs. Heat ﬂux factor was ∼90 MJ/m2 s0.5 , i.e. was above the melting threshold. Local hot areas with temperature of about 20 0 0 K were found on the target surface more than 5 ms after the heating end (Fig. 10). Excessive temperature of this areas over surrounding surface was about 500 K. The subsequent SEM survey uncovered a heavily damaged surface with detached parts on it (Fig. 11). The discovered hot areas caused by the tungsten layers detachment are a consequence of development of cracks parallel to the surface. The absorbed heat cannot penetrate into the bulk of the material and leads to formation of melted spots at relatively low heat loads. The melted tungsten can promote ejection of many Fig. 7. Matching of the local surface temperature increase and crack propagation into the material bulk. a) temperature distribution along the cross section line, b) SEM images of the transverse microsections, c) photo of the half of the target surface after cutting, d,e) SEM images of the cracks propagated under the surface. Dashed black line marks the same place on the target surface. Missing material in the center on b) and c) is associated with total detaching of the tungsten ﬂake during preparatory cutting. Formation of cracks parallel to the surface was revealed at a depth 0.1–0.2 mm. Surface temperature increases near cracks which can be associated with propagation of cracks along the surface. A.A. Vasilyev et al. / Nuclear Materials and Energy 12 (2017) 553–558 557 Fig. 8. Images of the target obtained with laser light illumination. a) photo of the target irradiated to 72 heat pulses after extraction from vacuum chamber; b) image of structure of the laser light reﬂection from the target surface obtained at room temperature after heating event, the image is overlaid on the IR pictures of the target recorded during the electron beam pulse; c) same image during the heating process. The blue and red boxes show the area of SEM images depicted on Fig. 9. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) droplet particles that was observed experimentally . The detached parts can break away from the divertor plates, accumulate and ﬁnally increase the tritium retention and risks of tungsten penetration into the plasma core. 4. Summary Fig. 9. SEM images of the target surface after irradiation. The green circles mark spots of the intense laser light reﬂection in Fig. 8(c). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) A novel test facility with a high-power long-pulse electron beam injector is being developed at the Budker Institute. It enables simulation of ITER-relevant transient heat loads like ELMs type I. Unique features (irradiation area bigger than 10 characteristic of the crack net, low plasma radiation and pressure) of the heating device gives opportunities for conduction of research on target surface erosion during and soon after electron beam exposure. Application of the fast CCD cameras and IR ﬁlter allows observation of the thermal radiation during the heating process and reconstruction of 2D distribution of the surface temperature. Imaging of the target surface with green laser illumination grants possibility of investigation on melting and re-solidiﬁcation dynamics. It was found that crack net was formed after the ﬁrst pulse and continued to develop with subsequent shots. After a few exposures, the thermal radiation in homogeneities caused by the crack propagation did not grow signiﬁcantly. It was shown that the occurrence of cracks propagated along the surface suppressed heat transfer into the bulk and caused a substantial local temperature increase, which began melting of the material near crack edges. Local areas with melted tungsten trigger intensiﬁcation of the laser light reﬂection, so the surface imaging with green laser light illumination can be applied to analysis of melting dynamics. Hot spots with excessive temperature of about 500 K over surrounding Fig. 10. Images of the damaged target surface after more than 100 shots. a) IR image during heat load, b) photo after extraction from vacuum chamber, c) SEM image after irradiation. The red rectangular on a) and b) marks SEM image on c). SEM survey showed existence of the detached parts. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) 558 A.A. Vasilyev et al. / Nuclear Materials and Energy 12 (2017) 553–558 Fig. 11. 2D temperature distribution of the damaged target surface at different times after heat load ending. a) 350μs after electron beam termination, b) 1850 μs, c) 7850 μs; d) enlarged location of temperature image from (a) that corresponds to SEM image on Fig. 10(c). Pictures (a), (b) and (c) have the same scale and color key. The dimensional coordinates are in mm, temperature is in K degrees. The black rectangular marks the SEM image in Fig. 10(c). surface were found on tungsten target exposed to more than 100 pulses with an energy load of 0.8 MJ/m2 . These hot areas correlate with detached parts of the surface. Application of the novel electron beam device for sample heating together with original diagnostic set provide unique in-situ experimental data that can reveal mechanisms of surface damage under powerful heat loads. Acknowledgements The work at the electron beam facility was supported by Russian Science Foundation (project N 14-50-0 0 080). Study of target surface modiﬁcation was partially supported by RFBR, research project No. 15-32-20669. References  A. Loarte, et al., Transient heat loads in current fusion experiments, extrapolation to ITER and consequences for its operation, Phys. Scr. T128 (2007) 222– 228, doi:10.1088/0 031-8949/20 07/T128/043.  A.S. Arakcheev, et al., Theoretical investigation of crack formation in tungsten after heat loads, J. Nucl. Mater. V463 (2015) 179–246, doi:10.1016/j.jnucmat. 2014.10.090.  A.S. Arakcheev, et al., Calculation of cracking under pulsed heat loads in tungsten manufactured according to ITER speciﬁcations, J. Nucl. Mater. V467 (2015) 165–171, doi:10.1016/j.jnucmat.2015.09.034.  V.P. Budaev, et al., Tungsten recrystallization and cracking under ITER-relevant heat loads, J. Nucl. Mater. V463 (2015) 237–240, doi:10.1016/j.jnucmat.2014.11. 129.  V.A. Makhlaj, et al., Dust generation mechanisms under powerful plasma impacts to the tungsten surfaces in ITER ELM simulation experiments, J. Nucl. Mater. V438 (2013) 233–236, doi:10.1016/j.jnucmat.2013.01.034.  M. Wirtz, et al., Thermal shock response of deformed and recrystallised tungsten, Fus. Eng. Design V88 (2013) 1768–1772, doi:10.1016/j.fusengdes.2013.05. 077.  A. Huber, et al., Investigation of the impact of transient heat loads applied by laser irradiation on ITER-grade tungsten, Phys. Scripta T159 (2014) 014005, doi:10.1088/0031-8949/2014/T159/014005.  H.J.N. van Eck, et al., ELM simulation experiments on Pilot-PSI using simultaneous high ﬂux plasma and transient heat/particle source, Nucl. Fus. V51 (2011) 073008, doi:10.1088/0029-5515/51/7/073008.  A.V. Burdakov, et al., Surface modiﬁcation and droplet formation of tungsten under hot plasma irradiation at the GOL-3, J. Nucl. Mater. V438 (2013) 677– 680, doi:10.1016/j.jnucmat.2013.01.143.  A.A. Shoshin, et al., Plasma-surface interaction during ITER Type I ELMs: comparison of simulation with QSPA Kh-50 and the GOL-3 facilities, Fusion Sci. Technol. V59 (No.1T) (2011) 57–60.  V.V. Kurkuchekov, Novel injector of intense long pulse electron beam for linear plasma devices, Fusion Sci. Technol. V63 (1T) (2013) 292–294.  C. Cargan, Spectra emissivities and emissivity X-Points of pure Molybdenum and tungsten, Intern. J. Thermophys. V26 (4) (2005) 1001–1015, doi:10.1007/ s10765- 005- 6680- 1.  Y.G. Li, et al., Theoretical simulation of thermal behavior in transient heat loads of plasms-facing materials, Fus. Eng. Design V86 (2011) 2812–2820.  Yu.A. Trunev, et al., Heating of tungsten target by intense pulse electron beam, AIP Conf. Proc. 1771 (2016) 060016, doi:10.1063/1.4964224.  V.I. Tereshin, Repetitive plasma loads typical for ITER Type-I ELMS: simulation in QSPA Kh-50, AIP Conf. Proc. 812 (2006) 128, doi:10.1063/1.2168807.  A.A Kasatov, Observation of dust particles ejected from tungsten surface under impact of intense transient heat load, AIP Conf. Proc. 1771 (2016) 060 0 07, doi:10.1063/1.4964215.