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In-situ imaging of tungsten surface modification under ITER-like transient heat loads Vasiliev 12-553-2017

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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 [1]. 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 [6]. Lasers such as Nd: YAG can create radiation
heating similar to ELMs type 1 in reactor-size tokamaks [7]. 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 [7]).
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 [8].
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/)
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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 [11]. 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 [12].
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
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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 [13]. 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 [14]. 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 [15].
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
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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
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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 [16]. 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.)
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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.
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