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Ablation properties of plasma facing materials using thermal plasmas
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FUSION-9027; No. of Pages 4
Fusion Engineering and Design xxx (2017) xxx–xxx
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Ablation properties of plasma facing materials using thermal plasmas H. Kim a , P.Y. Oh a , B.R. Kang b , H.M. Lim b , S.Y. Moon c , B.G. Hong c,∗ a
High Enthalpy Plasma Research Center, Chonbuk National University, Republic of Korea Department of Applied Plasma Engineering, Chonbuk National University, Republic of Korea c Department of Quantum System Engineering, Chonbuk National University, Republic of Korea b
h i g h l i g h t s • Ablation characteristics of plasma facing materials are investigated using thermal plasma facilities with high heat and particle flux. • Heat/particle load produced the cracks in W layer of the W/FMS PFM while they produced only pores in W layer of the W/Graphite PFM. • No delamination or cracks were seen at the bonding layer, indicating soundness of the coating.
a r t i c l e
i n f o
Article history: Received 1 October 2016 Received in revised form 14 January 2017 Accepted 16 January 2017 Available online xxx Keywords: Plasma facing material Thermal plasma Ablation property
a b s t r a c t We investigated the ablation characteristics of carbon/carbon composites and tungsten coated plasma facing materials (PFMs) using a 400 kW plasma wind tunnel (PWT) and a 55 kW vacuum plasma spraying system. These thermal plasma facilities allow a particle flux greater than 1024 /(m2 s) and a heat flux greater than 10 MW/m2 , which are the levels relevant for testing PFMs under fusion reactor conditions. We identified the ablation properties through measurement of the ablation rate and investigation of the microstructures of the PFMs before and after the ablation test. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The plasma facing materials (PFMs) of a fusion reactor are subject to various mechanical, thermal, chemical, and radiation loads. They must function in extreme environments such as in disruption and ELMs [1,2], and new PFMs need to be developed. Physical and chemical sputtering occurs due to interactions between the plasma and the PFM. Sputtering erodes the PFM and impurities may lead to degradation of plasma confinement. The PFM experiences both ion fluxes greater than 1024 /(m2 s) and heat fluxes greater than 10 MW/m2 [3,4]. Carbon-based materials, such as graphite and carbon/carbon (C/C) composites, have been used as PFMs due to their low atomic number (Z) and high sublimation temperature [5,6]. C/C composites possess an excellent strength at high temperatures, a high thermal conductivity, and a good thermal shock resistance. However, carbon-based PFMs have problems like the high erosion rate
∗ Corresponding author. E-mail address: [email protected] (B.G. Hong).
at elevated temperatures, degradation of the thermal conductivity caused by neutron irradiation, and a high tritium retention [7]. Tungsten has been studied as a candidate of PFMs because of its good thermal conductivity, low tritium retention, high sputtering threshold, and low erosion rate during ion bombardment [8]. A high ductile to brittle transition temperature and difficulties in machining are the main drawbacks in using tungsten as the PFM [9]. Direct tungsten coating on the heat sink or on the structural material can mitigate the disadvantages associated with bulk tungsten. A vacuum plasma spraying (VPS) method is more favored for tungsten coating as PFMs due to the high porosity and large columnar crystals present in layers coated by an atmospheric plasma spraying (APS) system [10,11]. 400 kW PWT and 55 kW VPS plasma facilities were constructed at Chonbuk National University (CBNU) in Korea. The PWT can produce high enthalpy plasma flows sufficient for transferring a steady-state heat flux greater than 10 MW/m2 and particle flux greater than 1024 /(m2 s), which are relevant for testing the PFMs of the fusion reactor. In the VPS facility, thermal plasma is generated by the arc between the central cathode and the annular anode. The plasma gun produces a flame with a temperature above a few
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Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
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Fig. 1. A picture of the 400 kW plasma wind tunnel.
Fig. 2. A picture of the VPS system.
thousand K and a velocity in a range of hundreds of m/s to several thousand m/s. Heat fluxes greater than 10 MW/m2 and particle fluxes greater than 1024 /(m2 s) are possible. The combined effects of the heat and the particles are not well understood [12], and this study will shed light on the combined effects. We investigated the ablation characteristics of C/C composites and tungsten coated PFMs using thermal plasma facilities.
2. Thermal plasma facilities
Table 2 Physical and thermal properties of the materials. Material
Bulk density (g/cm3 )
Compressive strength (MPa)
Thermal conductivitya (W/m K)
CC-NP CC-3D
1.70 1.94
100–120 160–170
13–18 80–90
a
At room temperature.
2.1. High enthalpy plasma wind tunnel As shown in Fig. 1, the 400 kW PWT consists of a segmented arc plasma torch, a test chamber with a substrate manipulation system, a diffuser, a removable section, a heat exchanger, and vacuum equipment. The test chamber is made of stainless steel with a water-cooled double wall. A substrate manipulation system in the chamber allows for operation with three degrees of freedom under a vacuum. Its four arms are cooled with water, and each arm is designed to hold a substrate, a heat flux probe and an enthalpy probe. The specifications of the 400 kW PWT are shown in Table 1.
2.2. Vacuum plasma spraying facility
Power Gas flow rate Nozzle Chamber pressure Heat flux Burn-through time Distance
420 kW (@390A) Air 16 g/s Mach 3 40 mbar 9.6 MW/m2 10 s 85 mm
3. Ablation test of the plasma facing material
Fig. 2 shows a picture of the VPS system. It consists of a plasma gun (with a maximum power of 55 kW, Metco F4VB), a powder feeder (with a maximum feed rate of 4 kg/h for particle sizes in the range of 4–200 m), a vacuum chamber (with a diameter of 1800 mm, a length of 2350 mm, and a pressure in the range of 10–900 mbar), and a plasma gun manipulator (with a six axes robot). Substrates with a diameter of up to 1200 mm can be coated with this setup.
Table 1 Specifications of the 400 kW plasma wind tunnel. Parameter
Specification
Electrode material Max. Arc torch power Max current/electrode Velocity at exhaust Enthalpy Electrode contamination Gas supply pressure Segment gas flow
Oxygen free copper 400 kW 250 A (total 500 A) Mach 2–3 10–20 MJ/kg <0.05% mass ratio of the plasma gas 4 bar Air (5–15) g/s, Argon (0.25–1) g/s <13 mbar
Chamber pressure
Table 3 Conditions of the ablation test using the PWT.
3.1. C/C PFMs C/C composites that are currently under development as PFMs were used for the ablation test. The C/C composites were made from a three-dimensionally reinforced fiber and a pitch-based carbon matrix. They consist of two preforms: a needle-punched preform for CC-NP and a 3D rod preform for CC-3D. The C/C composite preforms are composed of different sorts of PAN-based carbon fiber. M46JB fiber, a pitch carbon matrix and pyrocarbon were used for the CC-NP, while T700SC fiber and a pitch carbon matrix without pyrocarbon were used for the CC-3D. Physical and thermal properties of the materials are shown in Table 2. The materials were machined into cylindrical specimens 11 mm in diameter and 20 mm in length to fit in the arm of the substrate manipulation system. The conditions of the ablation test using the PWT are shown in Table 3. For the ablation test, the specimens were inserted into the plasma with a burn-through time of 10 s. At an air flow rate of 16 g/s, the air particle flux is 4.13 × 1026 /(m2 s). To compare the effect of the particle flux on the ablation characteristics, we performed the ablation test using the VPS with the same heat flux of 9.6 MW/m2 and the same
Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
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Table 4 Conditions of the ablation test using the VPS. Experimental Condition
Value
Plasma gas (Ar) flow rate (lpm) Chamber pressure (mbar) Current (A) Net power (kW) Burn-through time (s)
50 50 580 9.7 10
Table 5 Results of the ablation test. Before (g)
After (g)
Erosion rate (g/s)
PWT
CC-NP CC-3D
12.0999 14.125
11.3779 13.6291
0.07220 0.04959
VPS
CC-NP CC-3D
10.3525 11.3514
10.3251 11.3268
0.00274 0.00246
burn-through time of 10 s. The test conditions are shown in Table 4. At an Ar flow rate of 50 lpm, the particle flux of Ar is 5.6 × 1024 /(m2 s), which is much lower than the air flux in the PWT test. Table 5 shows the erosion rate of the C/C composites for the ablation tests in the PWT and the VPS. Mass loss is 0.072 g/s for the CC-NP, 0.050 g/s for the CC-3D in the PWT test, and 0.0027 g/s for the CC-NP and 0.0025 g/s for the CC-3D in the VPS test. The erosion rate is larger in the PWT test than in the VPS test since both thermochemical and thermomechanical erosion in a high pressure, rapid heat stream occurred in the PWT test, while only thermal ablation occurred in the VPS test. Fig. 3 shows microstructures of the specimens before and after the test. Small (<100 m) pores were seen in the matrix before the test. In the PWT test, the ablation caused the pores to grow (∼200 m) and cracks occurred due to the difference in coefficient of thermal expansion required for propagation (∼20 m). The CC3D sample lost less of the matrix than CC-NP due to its higher density. In addition, the edges of the fibers changed from a round shape to an uneven shape. In CC-NP, erosion of matrices was faster than that of fibers, resulting in sharp and tapered tips. In the VPS test, there was no significant difference between the microstructures of the C/Cs. Some matrices were lost and some fibers were broken due to the Ar particle impact. 3.2. Tungsten coated PFMs Tungsten coated on the FM steel substrate (W/FMS) with dimensions of 50 mm × 50 mm × 40 mm and tungsten coated on the graphite substrate (W/Graphite) with dimensions of 50 mm × 50 mm × 30 mm were prepared. The tungsten powder had an average particle size of 10 m. The optimal conditions for achieving a good coating quality were developed by varying the operational conditions, including feeding powder size, wettability control and a pre/post heating process [13]. The hardness of the coated layer was 380 HV for the W/FMS and 350 HV for the W/Graphite, and the porosity of the coated layer was less than 1% for both cases. The microstructures of the tungsten coating layers for both cases indicated good coating quality. The ablation tests of the tungsten coated PFMs were performed by using the VPS system with a heat flux of 10 MW/m2 for 10–60 s. An Ar flow rate of 50 lpm produced an Ar particle flux of 5.7 × 1024 /(m2 s).
Fig. 3. Microstructures of (a) CC-NP and (b) CC-3D before and after the test.
The erosion of the tungsten coating layer for both PFMs was found to be negligible. Surface roughness increased to 4.0 m, which is twice that of bulk tungsten. Fig. 4 shows microstructures of the specimens before and after the test. For the W/FMS, the cracks produced after 30 s of propagation (∼10 m). For the W/Graphite, pores produced after 30 s grew (∼5 m), but no cracks were produced due to the higher thermal conductivity of the graphite than the FM steel. WC formation at the bonding layer was observed for the W/Graphite. However, no delamination or cracks were seen at the bonding layer up to 60 s, indicating that the coating was robust.
4. Conclusion We investigated the ablation characteristics of PFMs using thermal plasma facilities. For C/C composites, both thermochemical and thermomechanical erosion by a high pressure, rapid heat stream occurred in the PWT test, while only thermal ablation occurred in the VPS test. For tungsten-coated PFMs, a heat/particle load produced cracks in the W layer of the W/FMS, while they produced only pores in W layer of the W/graphite due to the difference in the thermal conductivity of the substrate. However, no delamination or cracks were observed at the bonding layer, indicating that the coating was sound.
Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
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Fig. 4. Microstructures of (a) the W/FMS and (b) the W/Graphite before and after the test.
Acknowledgments This research was supported by the “Research Base Construction Fund Support Program” funded by Chonbuk National University in 2016. This study was also supported by the National Research Foundation of Korea under contract 2014M1A3A3A03066611 and by research facilities in the Plasma Application Institute at Chonbuk National University. References [1] J.W. Coenen, et al., ELM induced tungsten melting and its impact on tokamak operation, J. Nucl. Mater. 463 (2015) 78–84. [2] G. Federici, et al., Effects of ELMs and disruptions on ITER divertor armour materials, J. Nucl. Mater. 337–339 (2005) 684–690. [3] G. Federici, et al., Plasma–material interactions in current tokamaks and their implications for next step fusion reactors, Nucl. Fusion 41 (2001) 1967–2137. [4] V. Philipps, J. Roth, A. Loarte, Key issues in plasma–wall interactions for ITER: a European approach, Plasma Phys. Control. Fusion 45 (2003) A17–A30.
[5] S. Pestchanyi, V. Safronov, I. Landman, Estimation of carbon fibre composites as ITER divertor armour, J. Nucl. Mater. 329–333 (2004) 697–701. [6] H. Würz, B. Bazylev, I. Landman, S. Pestchanyi, V. Safronov, Macroscopic erosion of divertor and first wall armour in future tokamaks, J. Nucl. Mater. 307–311 (2002) 60–68. [7] K. Tokunaga, et al., High heat load properties of tungsten coated carbon materials, J. Nucl. Mater. 258–263 (1998) 998–1004. [8] Y. Niu, et al., Microstructure and thermal property of tungsten coatings prepared by vacuum plasma spraying technology, Fusion Eng. Des. 85 (2010) 1521–1526. [9] C. Ruset, et al., The impact of thermal fatigue and carbidization on the W coatings deposited on CFC tiles for the ITER-like wall project at JET, Fusion Eng. Des. 88 (2013) 1690–1693. [10] Z. Salhi, et al., Development of coating by thermal plasma spraying under very low-pressure condition <1 mbar, Vacuum 77 (2005) 145–150. [11] G. Kumar, K.N. Prabhu, Review of non-reactive and reactive wetting of liquids on surfaces, Adv. Colloid Interface Sci. 133 (2007) 61–89. [12] Y. Ueda, et al., Research status and issues of tungsten plasma facing materials for ITER and beyond, Fusion Eng. Des. 89 (2014) 901–906. [13] S.Y. Moon, et al., Thick tungsten coating on ferritic–martensitic steel applied with a vacuum plasma spray coating method, Surf. Coat. Technol. 280 (2015) 225–231.
Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
ARTICLE IN PRESS
FUSION-9027; No. of Pages 4
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Ablation properties of plasma facing materials using thermal plasmas H. Kim a , P.Y. Oh a , B.R. Kang b , H.M. Lim b , S.Y. Moon c , B.G. Hong c,∗ a
High Enthalpy Plasma Research Center, Chonbuk National University, Republic of Korea Department of Applied Plasma Engineering, Chonbuk National University, Republic of Korea c Department of Quantum System Engineering, Chonbuk National University, Republic of Korea b
h i g h l i g h t s • Ablation characteristics of plasma facing materials are investigated using thermal plasma facilities with high heat and particle flux. • Heat/particle load produced the cracks in W layer of the W/FMS PFM while they produced only pores in W layer of the W/Graphite PFM. • No delamination or cracks were seen at the bonding layer, indicating soundness of the coating.
a r t i c l e
i n f o
Article history: Received 1 October 2016 Received in revised form 14 January 2017 Accepted 16 January 2017 Available online xxx Keywords: Plasma facing material Thermal plasma Ablation property
a b s t r a c t We investigated the ablation characteristics of carbon/carbon composites and tungsten coated plasma facing materials (PFMs) using a 400 kW plasma wind tunnel (PWT) and a 55 kW vacuum plasma spraying system. These thermal plasma facilities allow a particle flux greater than 1024 /(m2 s) and a heat flux greater than 10 MW/m2 , which are the levels relevant for testing PFMs under fusion reactor conditions. We identified the ablation properties through measurement of the ablation rate and investigation of the microstructures of the PFMs before and after the ablation test. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The plasma facing materials (PFMs) of a fusion reactor are subject to various mechanical, thermal, chemical, and radiation loads. They must function in extreme environments such as in disruption and ELMs [1,2], and new PFMs need to be developed. Physical and chemical sputtering occurs due to interactions between the plasma and the PFM. Sputtering erodes the PFM and impurities may lead to degradation of plasma confinement. The PFM experiences both ion fluxes greater than 1024 /(m2 s) and heat fluxes greater than 10 MW/m2 [3,4]. Carbon-based materials, such as graphite and carbon/carbon (C/C) composites, have been used as PFMs due to their low atomic number (Z) and high sublimation temperature [5,6]. C/C composites possess an excellent strength at high temperatures, a high thermal conductivity, and a good thermal shock resistance. However, carbon-based PFMs have problems like the high erosion rate
∗ Corresponding author. E-mail address: [email protected] (B.G. Hong).
at elevated temperatures, degradation of the thermal conductivity caused by neutron irradiation, and a high tritium retention [7]. Tungsten has been studied as a candidate of PFMs because of its good thermal conductivity, low tritium retention, high sputtering threshold, and low erosion rate during ion bombardment [8]. A high ductile to brittle transition temperature and difficulties in machining are the main drawbacks in using tungsten as the PFM [9]. Direct tungsten coating on the heat sink or on the structural material can mitigate the disadvantages associated with bulk tungsten. A vacuum plasma spraying (VPS) method is more favored for tungsten coating as PFMs due to the high porosity and large columnar crystals present in layers coated by an atmospheric plasma spraying (APS) system [10,11]. 400 kW PWT and 55 kW VPS plasma facilities were constructed at Chonbuk National University (CBNU) in Korea. The PWT can produce high enthalpy plasma flows sufficient for transferring a steady-state heat flux greater than 10 MW/m2 and particle flux greater than 1024 /(m2 s), which are relevant for testing the PFMs of the fusion reactor. In the VPS facility, thermal plasma is generated by the arc between the central cathode and the annular anode. The plasma gun produces a flame with a temperature above a few
http://dx.doi.org/10.1016/j.fusengdes.2017.01.019 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
G Model FUSION-9027; No. of Pages 4
ARTICLE IN PRESS
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H. Kim et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
Fig. 1. A picture of the 400 kW plasma wind tunnel.
Fig. 2. A picture of the VPS system.
thousand K and a velocity in a range of hundreds of m/s to several thousand m/s. Heat fluxes greater than 10 MW/m2 and particle fluxes greater than 1024 /(m2 s) are possible. The combined effects of the heat and the particles are not well understood [12], and this study will shed light on the combined effects. We investigated the ablation characteristics of C/C composites and tungsten coated PFMs using thermal plasma facilities.
2. Thermal plasma facilities
Table 2 Physical and thermal properties of the materials. Material
Bulk density (g/cm3 )
Compressive strength (MPa)
Thermal conductivitya (W/m K)
CC-NP CC-3D
1.70 1.94
100–120 160–170
13–18 80–90
a
At room temperature.
2.1. High enthalpy plasma wind tunnel As shown in Fig. 1, the 400 kW PWT consists of a segmented arc plasma torch, a test chamber with a substrate manipulation system, a diffuser, a removable section, a heat exchanger, and vacuum equipment. The test chamber is made of stainless steel with a water-cooled double wall. A substrate manipulation system in the chamber allows for operation with three degrees of freedom under a vacuum. Its four arms are cooled with water, and each arm is designed to hold a substrate, a heat flux probe and an enthalpy probe. The specifications of the 400 kW PWT are shown in Table 1.
2.2. Vacuum plasma spraying facility
Power Gas flow rate Nozzle Chamber pressure Heat flux Burn-through time Distance
420 kW (@390A) Air 16 g/s Mach 3 40 mbar 9.6 MW/m2 10 s 85 mm
3. Ablation test of the plasma facing material
Fig. 2 shows a picture of the VPS system. It consists of a plasma gun (with a maximum power of 55 kW, Metco F4VB), a powder feeder (with a maximum feed rate of 4 kg/h for particle sizes in the range of 4–200 m), a vacuum chamber (with a diameter of 1800 mm, a length of 2350 mm, and a pressure in the range of 10–900 mbar), and a plasma gun manipulator (with a six axes robot). Substrates with a diameter of up to 1200 mm can be coated with this setup.
Table 1 Specifications of the 400 kW plasma wind tunnel. Parameter
Specification
Electrode material Max. Arc torch power Max current/electrode Velocity at exhaust Enthalpy Electrode contamination Gas supply pressure Segment gas flow
Oxygen free copper 400 kW 250 A (total 500 A) Mach 2–3 10–20 MJ/kg <0.05% mass ratio of the plasma gas 4 bar Air (5–15) g/s, Argon (0.25–1) g/s <13 mbar
Chamber pressure
Table 3 Conditions of the ablation test using the PWT.
3.1. C/C PFMs C/C composites that are currently under development as PFMs were used for the ablation test. The C/C composites were made from a three-dimensionally reinforced fiber and a pitch-based carbon matrix. They consist of two preforms: a needle-punched preform for CC-NP and a 3D rod preform for CC-3D. The C/C composite preforms are composed of different sorts of PAN-based carbon fiber. M46JB fiber, a pitch carbon matrix and pyrocarbon were used for the CC-NP, while T700SC fiber and a pitch carbon matrix without pyrocarbon were used for the CC-3D. Physical and thermal properties of the materials are shown in Table 2. The materials were machined into cylindrical specimens 11 mm in diameter and 20 mm in length to fit in the arm of the substrate manipulation system. The conditions of the ablation test using the PWT are shown in Table 3. For the ablation test, the specimens were inserted into the plasma with a burn-through time of 10 s. At an air flow rate of 16 g/s, the air particle flux is 4.13 × 1026 /(m2 s). To compare the effect of the particle flux on the ablation characteristics, we performed the ablation test using the VPS with the same heat flux of 9.6 MW/m2 and the same
Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
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Table 4 Conditions of the ablation test using the VPS. Experimental Condition
Value
Plasma gas (Ar) flow rate (lpm) Chamber pressure (mbar) Current (A) Net power (kW) Burn-through time (s)
50 50 580 9.7 10
Table 5 Results of the ablation test. Before (g)
After (g)
Erosion rate (g/s)
PWT
CC-NP CC-3D
12.0999 14.125
11.3779 13.6291
0.07220 0.04959
VPS
CC-NP CC-3D
10.3525 11.3514
10.3251 11.3268
0.00274 0.00246
burn-through time of 10 s. The test conditions are shown in Table 4. At an Ar flow rate of 50 lpm, the particle flux of Ar is 5.6 × 1024 /(m2 s), which is much lower than the air flux in the PWT test. Table 5 shows the erosion rate of the C/C composites for the ablation tests in the PWT and the VPS. Mass loss is 0.072 g/s for the CC-NP, 0.050 g/s for the CC-3D in the PWT test, and 0.0027 g/s for the CC-NP and 0.0025 g/s for the CC-3D in the VPS test. The erosion rate is larger in the PWT test than in the VPS test since both thermochemical and thermomechanical erosion in a high pressure, rapid heat stream occurred in the PWT test, while only thermal ablation occurred in the VPS test. Fig. 3 shows microstructures of the specimens before and after the test. Small (<100 m) pores were seen in the matrix before the test. In the PWT test, the ablation caused the pores to grow (∼200 m) and cracks occurred due to the difference in coefficient of thermal expansion required for propagation (∼20 m). The CC3D sample lost less of the matrix than CC-NP due to its higher density. In addition, the edges of the fibers changed from a round shape to an uneven shape. In CC-NP, erosion of matrices was faster than that of fibers, resulting in sharp and tapered tips. In the VPS test, there was no significant difference between the microstructures of the C/Cs. Some matrices were lost and some fibers were broken due to the Ar particle impact. 3.2. Tungsten coated PFMs Tungsten coated on the FM steel substrate (W/FMS) with dimensions of 50 mm × 50 mm × 40 mm and tungsten coated on the graphite substrate (W/Graphite) with dimensions of 50 mm × 50 mm × 30 mm were prepared. The tungsten powder had an average particle size of 10 m. The optimal conditions for achieving a good coating quality were developed by varying the operational conditions, including feeding powder size, wettability control and a pre/post heating process [13]. The hardness of the coated layer was 380 HV for the W/FMS and 350 HV for the W/Graphite, and the porosity of the coated layer was less than 1% for both cases. The microstructures of the tungsten coating layers for both cases indicated good coating quality. The ablation tests of the tungsten coated PFMs were performed by using the VPS system with a heat flux of 10 MW/m2 for 10–60 s. An Ar flow rate of 50 lpm produced an Ar particle flux of 5.7 × 1024 /(m2 s).
Fig. 3. Microstructures of (a) CC-NP and (b) CC-3D before and after the test.
The erosion of the tungsten coating layer for both PFMs was found to be negligible. Surface roughness increased to 4.0 m, which is twice that of bulk tungsten. Fig. 4 shows microstructures of the specimens before and after the test. For the W/FMS, the cracks produced after 30 s of propagation (∼10 m). For the W/Graphite, pores produced after 30 s grew (∼5 m), but no cracks were produced due to the higher thermal conductivity of the graphite than the FM steel. WC formation at the bonding layer was observed for the W/Graphite. However, no delamination or cracks were seen at the bonding layer up to 60 s, indicating that the coating was robust.
4. Conclusion We investigated the ablation characteristics of PFMs using thermal plasma facilities. For C/C composites, both thermochemical and thermomechanical erosion by a high pressure, rapid heat stream occurred in the PWT test, while only thermal ablation occurred in the VPS test. For tungsten-coated PFMs, a heat/particle load produced cracks in the W layer of the W/FMS, while they produced only pores in W layer of the W/graphite due to the difference in the thermal conductivity of the substrate. However, no delamination or cracks were observed at the bonding layer, indicating that the coating was sound.
Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019
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Fig. 4. Microstructures of (a) the W/FMS and (b) the W/Graphite before and after the test.
Acknowledgments This research was supported by the “Research Base Construction Fund Support Program” funded by Chonbuk National University in 2016. This study was also supported by the National Research Foundation of Korea under contract 2014M1A3A3A03066611 and by research facilities in the Plasma Application Institute at Chonbuk National University. References [1] J.W. Coenen, et al., ELM induced tungsten melting and its impact on tokamak operation, J. Nucl. Mater. 463 (2015) 78–84. [2] G. Federici, et al., Effects of ELMs and disruptions on ITER divertor armour materials, J. Nucl. Mater. 337–339 (2005) 684–690. [3] G. Federici, et al., Plasma–material interactions in current tokamaks and their implications for next step fusion reactors, Nucl. Fusion 41 (2001) 1967–2137. [4] V. Philipps, J. Roth, A. Loarte, Key issues in plasma–wall interactions for ITER: a European approach, Plasma Phys. Control. Fusion 45 (2003) A17–A30.
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Please cite this article in press as: H. Kim, et al., Ablation properties of plasma facing materials using thermal plasmas, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.01.019