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Experimental study on melting and evaporation of metal exposed to intense hydrogen ion beam
Fusion Engineering and Design 19 (1992) 193-202 North-Holland
193
Experimental study on melting and evaporation of metal exposed to intense hydrogen ion beam M a s u r o Ogawa a) M a s a n o r i Araki c, Masahiro Seki b, T o m o a k i K u n u g i a, Kiyoshi F u k a y a a and H i d e o Ise d a Department of High Temperature Engineering, Tokai Research Establishment, Japan Atomic Energy Research Institute (JAER1), Tokai-mura, Naka-gun, lbaraki-ken 319-11, Japan t, Office of Fusion Program, JAERI, Uchisaiwai-cho, Chiyoda-ku, Tokyo-to 100, Japan c Department of Fusion EnghzeeringResearch, Naka Fusion Research Establishment, JAERI, Naka-machi, Naka-gun, Ibaraki-ken 311-01, Japan a Kawasaki Heavy lndnstries, Ltd., Minamisuna 2-4-25, Koto-ku, Tokyo-to 136, Japan Submitted 19 February 1992, accepted 1 May 1992 Handling Editor: M. Ohta
This report describes experimental studies on melting and evaporation of metals, mainly stainless steels, subjected to high heat flux simulating a plasma disruption in a thermonuclear fusion reactor. The test pieces were heated by an intense hydrogen ion beam. The heated area was about 70 mm in diameter. The peak heat flux on the surface ranged from 68 to 261 MW/m 2, and the heating duration from 40 to 250 ms. The melting and evaporating process was observed by using a high-speed video camera. The melt layer convected from the center of the piece to the periphery and the thickness of the piece was decreased not only by the evaporation but also by the convection in the melt layer.
I. Introduction Plasma facing components are exposed to high heat load, high energy particles and intense electromagnetic forces when a plasma disruption takes place in a thermonuclear fusion reactor such as the International Thermonuclear Experimental Reactor (ITER) [1] and the Fusion Experimental Reactor (FER) [2]. The heat load during thermal quench in the plasma disruption is evaluated to range from 10 to 20 M J / m 2 for the duration from 0.1 to 3 ms in the ITER design [1]. In any experiment, it is difficult to simulate such very severe condition of the thermal quench. Accordingly, the developments of analytical methods and phenomenological models are required to understand thermal characteristics and to predict the durabi!ity of the plasma facing components against the severe heat load during the thermal quench. Correspondence to: Dr. Masuro Ogawa, Department of High Temperature Engineering, Tokai Research Establishment, Japan Atomic Energy Research Institute (JAERI), Tokaimura, Naka-gun, Ibaraki-ken 319-11, Japan.
Numerical analyses [3-8] have been conducted and continued even now. Recently Van der Laan et al. [8] pointed out that there were some discrepancies among the results calculated by using the existing thermal erosion analysis codes. These computer codes must be benchmarked by experimental results. Some experimental studies [9-16] have been performed on the thermal shock, the thermal deformation and the erosion behavior under the conditions of high heat loads. However, the data so far obtained were insufficient to develop or verify the analytical methods and models. Especially, in the metal wall of the plasma facing components, the effect of convection in the melt layer on the erosion is not well understood yet. Therefore, more experimental data on melting and evaporation of the metal wall are required for developing and benchmarking the computer codes. In ITER and FER, the first wall and divertor plate will be covered with armor materials like graphitic material because of its high thermal conductivity, high sublimation temperature and low-Z. Efforts on testing the graphitic materials have been actively undertaken at the Japan Atomic Energy Research Institute
0 9 2 0 - 3 7 9 6 / 9 2 / $ 0 5 . 0 0 © 1992 - Elsevier Science Publishers B.V. All rights reserved
194
M. Ogawa et al. / Mehing and evaporation of metal ~1800
( J A E R I ) [9]. On the other side, since the graphitic material may absorb much tritium and may break due to thermal shock, a metallic first wall might be selected if its good durability is able to be secured against erosion. The divertor plate in the technology phase of I T E R is planned to be covered with a tungsten plate. Thus, it is of basic importance to study melting and evaporation of the metal wall for predicting the life time of the plasma facing components. In the present report, experiments are described on melting and evaporation of metallic materials, mainly stainless steels, exposed to an intense hydrogen ion beam in order to acquire the data for developing and benchmarking the thermal erosion analysis codes.
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.... I -Hydrogen ion source
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High speed infrared thermometer ~Vocuum chamber (inner diameter ;1506)
o~ High speed video camera
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2. Experiments Figure 1 shows the Particle Beam Engineering Facility (PBEF) of J A E R I . The details of P B E F were described in ref. [9]. A metal test piece is heated by the hydrogen ion beam in the vacuum chamber of PBEF. The maximum acceleration voltage and current of the ion source are 100 kV and 50 A, respectively. The heat flux profiles on the surface of the test piece were measured by using a two-dimensional calorimeter before and after testing. The calorimeter consists of cylindrical probes made of copper. The diameter and height of the probe were both 10 mm. These probes were adiabatically mounted on the base plate at a latticelike interval of 5 mm. The heat flux at each probe was
I1 ~
(In mm)
Fig. 1. Schematic view of particle beam engineering facility (PBEF).
calculated from the energy balance of the probe. The spatial profile of the heat flux is nearly a Gaussian distribution. In the radial heat flux distribution, the e-holding half-width, at which the heat flux is decreased down to 1 / e of the peak heat flux, is held to be about 70 mm by adjusting the beam optics. About
Table 1 Contents of stainless steel (wt%) Element
Cr
Ni
Mo
Mn
Melting point (°C)
1890
1455
2625
1240
P
Si
Ti
C
1430
1820
3700
S
type-316 type-304 PCA-J type-316F
16.82 18.38 14.13 16.77
10.29 8.50 15.75 13.95
2.12 2.27 2.31
1.00 1.16 1.43 0.23
< 0.032 < 0.022 0.03 < 0.003
0.056 0.54 0.54 0.040
0.22 -
0.06 0.05 0.057 0.036
0.26 0.25 0.005 0.002
316F-T1 316F-T2 316F-T3
16.94 17.03 16.95
13.83 13.85 13.86
2.28 2.23 2.27
0.28 0.28 0.28
0.007 0.006 0.007
0.050 0.046 0.048
0.050 0.14 0.24
0.036 0.035 0.036
0.001 0.001 0.001
316F-P1 316F-P2 316F-P3
16.83 16.83 16.77
13.76 13.66 13.83
2.32 2.18 2.21
0.28 0.29 0.28
0.005 0.016 0.030
0.048 0.065 0.064
< 0.010 < 0.010 < 0.010
0.034 0.034 0.034
0.001 0.001 0.001
M. Ogawa et aL / Melting and evaporation of metal 40 ms was required for the heat flux to reach steady state in all cases. The materials used were mainly stainless steels (type-316, type-304, PCA-J, type-316F and modified 316F stainless steels) and pure metals (aluminum, copper, molybdenum, tungsten). The PCA-J is the Japanese Primary Candidate Alloy and the modified type-316 which has a high void swelling resistance. The type-316F is a modified form of type-316 which is being developed by JAERI for use in fusion reactors. The type-316F has reduced amounts of manganese, phosphorus, silicon and carbon compared with type-316. Table 1 shows the principal contents of stainless steels used. The geometry of test piece was a flat disc or flat rectangular plate. The diameter or width of the test piece ranged from 20 to 120 ram, and the thickness from 5 to 20 mm. The process of melting and evaporating was observed with a high-speed video camera which could take pictures at a speed of 12000 frames per second. The resolidified surfaces and cross-sections of the test piece were observed with the aid of a scanning electron microscope and an optical microscope. The evaporation loss and resolidified layer thickness were measured by an electric balance with an accuracy of + 10 mg and a reading microscope with an accuracy of + 10 /~m, respectively. Composition analyses were made with an electron probe micro analyzer (EPMA). Surface temperatures of the test piece were measured by using a high speed infrared thermometer. It can measure in the range of 800 to 3000°C with an accuracy of +50°C at a period of 1 /~s. The measured spot is a circular area of about 20 mm in diameter. About one hundred test pieces were heated by the ion beam under the following conditions: peak heat flux 68-261 M W / m 2 and heating duration 40-250 ms. Several test pieces were repeatedly irradiated under the same heating conditions. A few stainless steel test pieces inclined at sixty degrees to the ion beam were heated. The pressure in the vacuum chamber was measured by using an ionization gauge at the upper location of the vacuum chamber. The inside of the vacuum chamber was always evacuated with an exhausting speed of 3 ma/s. Figure 2 shows the pressure change with time during one test. The pressure before the test is about 2.5 × 10 -4 Pa. In fig. 2, hydrogen gas starts to be supplied from t = 0 rain, and the test piece is heated for 50 ms at t =3.8 min. The maximum pressure during the heating could riot be measured because the response time of the ionization gauge was 100 ms. The peak of pressure during the heating is seemed to result from the out-gas, including the vapor
0.10
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Fig. 2. Pressure change with time during the irradiation test. from the test piece and its support materials. In all tests, the pressure before the heating was kept at about 0.05 Pa.
3. Results and d i s c u s s i o n
3.1. Evaporation Evaporation loss from the test pieces of stainless steels is shown in fig. 3, where the abscissa denotes the product of peak heat flux qpeak and heating duration At. No dispersion of melt from the test piece was observed by the high-speed video camera. The present results do not include the cases in which the melting layer fell down from the test piece to the support plate. The three solid lines in fig. 3 are calculated by using a thermal erosion analysis code [6]. These two-dimen-
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304 I
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PCA 316F
20
0 A Q 17
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qpeok hi (MJ/m 2} Fig. 3. Evaporation loss of stainless steels.
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30
196
M. Ogawa et al. / Melthzg and evaporation of metal
sional transient calculations were conducted in consideration of evaporation. Whereas the calculated evaporation loss depends upon the heat flux, such a clear dependence was not found in the experiments. The measurement with higher accuracy is required in the low (qoeak'At) to make the dependence clear. The experimental results at a higher value of (qp~,~k"At) are lower than those calculated by the code. It should be noted that much vapor influences the electric current of t h e ion beam and restricts the very high heat flux tests. No difference in evaporation loss are found among the different stainless steels examined. 3.2. Resolidified surface The topography of the resolidified surfaces of stainless steels after heating has been mentioned in ref. [9]. It exhibited two features: a rough surface (typeZ316, PCA-J and type-304) and a smooth surface (type-316F). The content of phosphorus in the type-316F is considerably smaller than that in other stainless steels, and the PCA-J contains much titanium as shown in table 1. The test pieces of type-316F-T1-T3 and type-316FP1-P3 were made by varying the contents of titanium and phosphorus, respectively, to examine the effect of these contents on the formation of the rough surface. All resolidified surfaces of the test pieces became smooth after irradiation. Madarame et al. [14] reported that sulphur and oxygen contributed to the formation of the rough resolidified surface. It is known that the surface tension of molten iron is considerably reduced with the addition of alloying elements (oxygen, sulfur, selenium and tellurium belonging to the group VI B) [17]. The other thermal properties such as viscosity, which were thought to influence the formation of the rough surface, scarcely change with the composition. Therefore, it is seemed that the different features of the resolidifled surface of the stainless steels probably results from the difference in the surface tension. The surface tension of type-316F is inferred to be the largest among those of the stainless steels used here, because the amounts of minor elements such as sulfur in the type-316F are less than those in other stainless steels. The high-speed camera observations revealed that the small mountains were formed not during the resolidifying process but in the initial stage of the melting process. Fujimura et al. [18] provided the criteria for the formation of the rough surface by an instability analysis of the melt layer. In his theoretical study, the determinant factors are the surface tension and the temperature coefficient of surface tension. The tem-
perature coefficient is sensitive to the chromium content in molten iron. Madarame and Okamoto [19] analyzed this phenomena of formation of the rough surface, paying attention to the sign of the temperature coefficient of surface tension. The rough surface of the small disc of 15 mm diameter was, however, slightly different of those of the large discs with a diameter greater than 80 mm under the same heating conditions. This difference might be caused by the effect that the surface of the small disc was stretched by the surface tension from the peripheral edge. Thus, the size of the test piece affects the roughness of resolidified surface. The resolidified surface of aluminum exhibited a smooth surface, the same as the other pure metals under low heat flux conditions. However, it became rough, when the aluminum test piece was irradiated under the conditions of qpeak = 80 M W / m 2 and At = 100 ms. These heating conditions are severe to aluminum because the melting temperature of aluminum is 660°C, lower than those of other metals. No rough surfaces were formed with the electron beam of the JAERI Electron Beam Irradiation Stand (JEBIS) [9]. The heating conditions were the following: heated area of 13 mm × 13 mm, uniform heat flux of 72 M W / m 2, heating duration of 100 ms and type-316 test piece with 120 mm diameter and 20 mm thickness. The projected range (length penetrated by ion or electron) in the material for the hydrogen ion beam is considerably shorter than that for the electron beam. For example, the projected range in iron is about 0.5 /zm for the hydrogen ion beam and 27 /zm for the electron beam with an energy of 100 keV. Thus, the hydrogen ion beam gives rise to a steeper temperature gradient in the direction of the thickness than that by the electron beam. For example, in the melt layer of 27 /xm thickness, a rough thermal conduction calculation indicates that the temperature difference between the surface (x -- 0/zm) and the location of 27 p.m thickness (x = 27/zm) is about 110 K for the surface heating and 55 K for the internal heating. In this calculation, the heating by the electron beam and the heating by the hydrogen ion beam were assumed to be surface heating and the internal heating with uniform heat generation density from x = 0 to 27/xm, respectively. A heat flux of 72 M W / m 2 and a heat generation density corresponding to this heat flux were given for the surface heating and for the internal heating, respectively. A steady state was also assumed because the thermal diffusion time was very short compared with the heating duration. Moreover, the present heat flux of the hydrogen ion beam has a Gaussian distribution in the
M. Ogawa et al. / Melth)g and evaporation of metal
197
radius direction. These differences in heat generation profiles are seemed to cause the different resolidified surfaces. Photos l(a) and (b) show the resolidified surfaces of the test pieces of PCA-J and type-316F irradiated seven times under the same conditions. Many cracks are found on the rough resolidified surface of the PCA-J test piece, especially the peripheral surface of the test piece. On the other hand, cracks are scarcely observed on the smooth resolidified surface of the
Photo 2. Topographic change of resolidified layer inclined at 60° (peak heat flux = 210 MW/m 2, heating duration = 60 ms); (a) PCA-J, (b) type-316F.
Photo 1. Topographic change of resolidified layer exposed repeatedly seven times (peak heat flux = 92 MW/m 2, heating duration = 100 ms); (a) PCA-J, (b) type-316F.
316F test piece. The test piece of the type-316F has preferable characteristics against crack generation. Photos 2(a) and (b) show the topographies of the test pieces of PCA-J and type-316F inclined. The melting part near the center of the test piece hangs down to the bottom. Large erosion loss due to gravity loss of the melt layer is serious to the lifetime of the metallic materials. Figure 4 shows the temperature of the surface measured by using the high-speed infrared thermometer. The details of measurements were reported in ref. [20]. The temperatures were measured at a period of 30 ]~s in the center of the test piece of type-304 under the conditions: peak heat flux = 168 M W / m 2, heating duration = 70 ms. Since the emissivity of polished stainless steels is about 0.18-0.2, the measured data are calibrated by using the emissivities of 0.18 and 0.2. The broken lines in fig. 4 show the result calculated by the
M. Ogawa et al. / Me/ting and ecaporation of metal
198
code [6]. The measured temperatures exhibit a good time response, however those are lower than thosc calculated. The discrepancy between the experimental and calculated temperatures results from potential effects of the vapor shielding and the convection in the melt layer.
3000
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3.3. Melting and resolidified /ayer
ca. E
Figures 5(a) and (b) show the results of composition analyses with the aid of EPMA in the test pieces of PCA-J and type-316F. The analyses disclose depletion of only manganese in the resolidified layer of the stainless steel test piece, except for the type-316F. In the resolidified layer of PCA-J, titanium is found to fluctuate sharply. No change of other elements is observed in the resolidified layer. A copper pin with the diameter of 1 mm was inserted at 10 mm from the center of the test piece to clarify the occurrence of convection in the melt layer. Photo 3 shows the result of EPMA. The long strip of the center in photo 3 is the copper pin, and the left hand side is the center direction. The color bar at the right-hand side in photo 3 displays the copper concentration, and the upper color of the bar shows a higher concentration. The melting copper seen at the right-
1000
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--.....
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20
Experiment
Calculation I I I
40
60
80
Time (ms) Fig. 4. Temperature change with time on the surface of test piece (type-304, peak heat flux; 168 MW/m 2, heating duration; 70 ms).
hand surface of photo 3 is found to flow to the peripheral direction. In a short heating duration, the rough resolidified layer lies scattered on the base metal and the base metal is made bare at the part without the resolidified layer as shown in fig. 6. If such a test piece is repeatedly heated under the same condition, the bare base
t......a Center
lmm
Copper
pin
Periphery
Photo 3. Component analysis in cross-section of the resolidified layer to examine the convection, (type-316, peak heat flux = 89 MW/m 2, heating duration = 100 ms).
199
M. Ogawa et aL / Melthzg and evaporation of metal Melt zone
Bose melol
Fe (x2)
Melt zone
Cr (xl)
A
metol
Fe (X2}
Ni (Xl)
Cr (Xl)
Mn XI/IC
Ni
<~
(Xl)
Ti Xl/IC
O.)
Bose
Mn (Xl/lO)
Mo XI/IC
Si , (XI/lO)
Si
(Xt/t0) C (XI/i0)
C (Xl/lO)
100,m Distonce
Distonce
Fig. 5. Composition analysis with the aid of EPMA in test pieces (a, left) PCA-J, (b, right) type-316F.
metal would be subjected to strong melting. Figure 7 shows the comparison of the melt layer thickness after resolidification between the experiment and the thermal erosion calculation mentioned before. This difference in the thickness as shown in fig. 7 results from the convection in the melt layer. The volume of the resolidified layer in the experiment was greater 1.4 times than that in the calculation. This suggests that the convection in the melt layer transfers the heat from the center to the periphery, consequently the evaporation loss will be suppressed.
Seki et al. [12] have already reported the effect of the convection on the evaporation loss. In their experiment, the gradient of the evaporation loss to the absorbed energy became small and the melt layer became thick in the high absorbed energy region. They explained this tendency as a result of the convection effect because the result of the evaporation loss in their experiment agreed well with that calculated by the thermal erosion analysis code simulating the convection effect. Thus, the convection effect on the evap-
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Fig. 6. Resolidificd layer of the test piece with rough surface (type-304, peak heat flux = 92 IVlW/m 2, heating duration =
60 ms).
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Fig. 7. Comparison of melt layer thickness between results in experiment and calculation.
M. Ogawa et al. / Melting and evaporation of metal
200
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Fig. 8. Cross sectional deformation of test piece exposed repeatedly seven times (peak heat flux = 92 M W / m 2, heating duration = 100 ms); (a, left) PCA-J (see photo l(a)), (h, right) type-316F (see photo l(b)).
Photo 4. Metallographic photograph in cross-section of the resolidified layer exposed seven times (see photo l(b) and fig. 8(b)).
M. Ogawa et aL / Meltb~gand evaporation of metal oration loss was found to act as well as the vapor shielding effect as described at 3.1. Figures 8(a) and (b) show the cross-sectional resolidified layer of the test pieces of PCA-J and type-316F (see photo 1) heated repeatedly seven times. The solid lines in fig. 8 are drawn by smoothly connecting the data measured at an interval of 1 mm. The numbers denote each heating cycle. In the type-316F test piece, the thickness of the resolidified layer (0.24 mm) is the nearly same as that (0.23 mm) exposed one time. The thickness of the test piece at the center is decreasing at each shot, and the final thickness is 13.89 mm. The total eroded thickness is 0.92 mm. This erosion results from the evaporation and convection. U n d e r the present condition, the total eroded thickness of 0.92 mm is almost due to the convection, since the eroded thickness due to the evaporation is about 0.01 mm in one shot. The melt layer of each shot near the center of the test piece accumulates at the edge as shown in fig. 8. In the calculation code, the convection in the melt layer must be taken into consideration to predict the erosion loss. Photo 4 shows the resolidified layer stratifying at the edge shown in fig. 8(b). Seven resolidified layers were distinctly observed in photo 4. The structure of columnar dendrite grows continuously through each resolidified layer in the test piece irradiated seven times as shown in photo 4.
4. Conclusion Experimental studies were carried out on melting, evaporation and resolidification of metals, mainly stainless steels, subjected to intense hydrogen ion beam. The experimental results were obtained as follows: (1) Topography of the resolidified surfaces of metallic materials after heating exhibited two features: a rough surface and a smooth surface. The effects of the size of the test piece, the electron beam and the contents of stainless steel on the resolidified surface were examined. (2) The observations with the high-speed video camera clarified that the rough surface was formed in the initial stage of melting process. (3) The convection of the melt layer was observed and the thickness of the base metal was decreased by not only the evaporation also the convection. It is planned to develop or verify new analytical methods and models based on the present experimental results.
201
Acknowledgments The authors want to express their gratitude to Dr. K. Kiuchi and Dr. A. Hishinuma for valuable comments on materials. The authors are grateful to Mr. K. Yokoyama, Mr. M. Dairaku and Mr. K. Emori for their assistance in carrying out the experiments.
References [1] IAEA, ITER Conceptual Design Report, I A E A / I T E R / DS/No. 18 (1991). [2] S. Matsuda et al., Conceptual design study of the Fusion Experimental Reactor (FER), Proc. of 13th Int. Conf. on Plasma Physics and Controlled Nucl. Fusion Research, Washington DC, USA, IAEA-CN-53/G-2-2 (1990). [3] A.M. Hassanein, G.L Kulcinski and W.G. Wolfer, Vaporization and melting of materials in fusion devices, J. Nucl. Mater. 103/104 (1981) 321. [4] H.Th. Klippel, The thermal response of the first wall of a fusion reactor blanket to plasma disruptions, Research Report of the Netherlands Energy Research Foundation, ECN-137 (1983). [5] H. Nakamura, et al., First wall erosion during a plasma disruption in tokamak, Research Report of Japan Atomic Energy Research Institute, JAERI-M 83-058 (1983). [6] S. Yamazaki, M. Seki and T. Kobayashi, Two-dimensional disruption thermal analysis code DREAM, ibid., 88-163 (1988), (in Japanese). [7] J. Gilligan, D. Hahn and R. Mohanti, Vapor shielding of surfaces subjected to high heat fluxes during a plasma disruption, J. Nucl. Mater. 162/164 (1989) 957. [8] J.G. van der Laan et al., Prediction for disruption erosion of ITER PFC; a comparison of experimental and numerical results, Proc. ISFNT-2, Fusion Engrg. Des. 18 (1991) 135-144. [9] M. Seki et al., Thermal shock tests on various materials of plasma facing components for F E R / I T E R , Fusion Eng. Des. 15 (1991) 59. [10] J.R. Easoz, et al., Thermomechanical testing of first wall test pieces in ESURF, Nucl. Technol. Fusion 4 (1983) 780. [11] C.D. Croessmann, G.L Kulcinski and J.B. Whitley, Correlation of experimental and theoretical results for vaporization by simulated disruption et al., J. Nuel. Mater. 128/129 (1984) 816. [12] M. Seki et al., A simulated plasma disruption experiment using an electron beam as a heat source, J. Fusion Energy 5[3] (1986) 181. [13] C. Rigon, P. Moretto and F. Brossa, Experimental simulation of plasma disruption with an electron beam, Fusion Eng. Des. 5 (1987) 299. [14] H. Madarame, et al., Effect of impurity content in stainless steel on resolidified surface condition after disruption load, Fusion Eng. Des. 9 (1989) 213.
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M. Ogawa et al. / Melthzg and evaporation of metal
[15] S. Yamazaki, M. Seki and M. Uno, Crack initiation behaviors of Metallic walls subjected to high heat flux expected at plasma disruption, J. Atomic Energy Soc. Japan, 31111] (1989) 1251, (in Japanese). [16] M. Ogawa, et al., Experimental observations of metal surfaces exposed to intense hydrogen beam; simulation of plasma disruption, J. Nucl. Science Technol. 26[7] (1989) 721. [17] K. Ogino, K. Nogi and O. Yamase, The effect of selenium and tellurium on the surface tension of molten iron and wettability of solid oxide. Ion Steel, 66[2] (1980) 179, (in Japanese).
[18] K. Fujimura, in M. Seki, N. Miki, Y. Shibutani et al., Japanese contributions to IAEA INTOR workshop, Phase 2a, Part 2, Research Report of Japan Atomic Energy Research Institute, JAERI M85-075, 93-107 (1985). [19] H. Madarame and K. Okamoto, Numerical simulation of surface tension induced melt layer movement after disruption load, Fusion Engrg. Des. 19, No. 4 (1992). [20] M. Araki et al., High-speed surface temperature measurements of plasma facing materials under thermal shock loads for fusion applications, to be published in Review of Scientific Instruments.
193
Experimental study on melting and evaporation of metal exposed to intense hydrogen ion beam M a s u r o Ogawa a) M a s a n o r i Araki c, Masahiro Seki b, T o m o a k i K u n u g i a, Kiyoshi F u k a y a a and H i d e o Ise d a Department of High Temperature Engineering, Tokai Research Establishment, Japan Atomic Energy Research Institute (JAER1), Tokai-mura, Naka-gun, lbaraki-ken 319-11, Japan t, Office of Fusion Program, JAERI, Uchisaiwai-cho, Chiyoda-ku, Tokyo-to 100, Japan c Department of Fusion EnghzeeringResearch, Naka Fusion Research Establishment, JAERI, Naka-machi, Naka-gun, Ibaraki-ken 311-01, Japan a Kawasaki Heavy lndnstries, Ltd., Minamisuna 2-4-25, Koto-ku, Tokyo-to 136, Japan Submitted 19 February 1992, accepted 1 May 1992 Handling Editor: M. Ohta
This report describes experimental studies on melting and evaporation of metals, mainly stainless steels, subjected to high heat flux simulating a plasma disruption in a thermonuclear fusion reactor. The test pieces were heated by an intense hydrogen ion beam. The heated area was about 70 mm in diameter. The peak heat flux on the surface ranged from 68 to 261 MW/m 2, and the heating duration from 40 to 250 ms. The melting and evaporating process was observed by using a high-speed video camera. The melt layer convected from the center of the piece to the periphery and the thickness of the piece was decreased not only by the evaporation but also by the convection in the melt layer.
I. Introduction Plasma facing components are exposed to high heat load, high energy particles and intense electromagnetic forces when a plasma disruption takes place in a thermonuclear fusion reactor such as the International Thermonuclear Experimental Reactor (ITER) [1] and the Fusion Experimental Reactor (FER) [2]. The heat load during thermal quench in the plasma disruption is evaluated to range from 10 to 20 M J / m 2 for the duration from 0.1 to 3 ms in the ITER design [1]. In any experiment, it is difficult to simulate such very severe condition of the thermal quench. Accordingly, the developments of analytical methods and phenomenological models are required to understand thermal characteristics and to predict the durabi!ity of the plasma facing components against the severe heat load during the thermal quench. Correspondence to: Dr. Masuro Ogawa, Department of High Temperature Engineering, Tokai Research Establishment, Japan Atomic Energy Research Institute (JAERI), Tokaimura, Naka-gun, Ibaraki-ken 319-11, Japan.
Numerical analyses [3-8] have been conducted and continued even now. Recently Van der Laan et al. [8] pointed out that there were some discrepancies among the results calculated by using the existing thermal erosion analysis codes. These computer codes must be benchmarked by experimental results. Some experimental studies [9-16] have been performed on the thermal shock, the thermal deformation and the erosion behavior under the conditions of high heat loads. However, the data so far obtained were insufficient to develop or verify the analytical methods and models. Especially, in the metal wall of the plasma facing components, the effect of convection in the melt layer on the erosion is not well understood yet. Therefore, more experimental data on melting and evaporation of the metal wall are required for developing and benchmarking the computer codes. In ITER and FER, the first wall and divertor plate will be covered with armor materials like graphitic material because of its high thermal conductivity, high sublimation temperature and low-Z. Efforts on testing the graphitic materials have been actively undertaken at the Japan Atomic Energy Research Institute
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M. Ogawa et al. / Mehing and evaporation of metal ~1800
( J A E R I ) [9]. On the other side, since the graphitic material may absorb much tritium and may break due to thermal shock, a metallic first wall might be selected if its good durability is able to be secured against erosion. The divertor plate in the technology phase of I T E R is planned to be covered with a tungsten plate. Thus, it is of basic importance to study melting and evaporation of the metal wall for predicting the life time of the plasma facing components. In the present report, experiments are described on melting and evaporation of metallic materials, mainly stainless steels, exposed to an intense hydrogen ion beam in order to acquire the data for developing and benchmarking the thermal erosion analysis codes.
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o~ High speed video camera
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2. Experiments Figure 1 shows the Particle Beam Engineering Facility (PBEF) of J A E R I . The details of P B E F were described in ref. [9]. A metal test piece is heated by the hydrogen ion beam in the vacuum chamber of PBEF. The maximum acceleration voltage and current of the ion source are 100 kV and 50 A, respectively. The heat flux profiles on the surface of the test piece were measured by using a two-dimensional calorimeter before and after testing. The calorimeter consists of cylindrical probes made of copper. The diameter and height of the probe were both 10 mm. These probes were adiabatically mounted on the base plate at a latticelike interval of 5 mm. The heat flux at each probe was
I1 ~
(In mm)
Fig. 1. Schematic view of particle beam engineering facility (PBEF).
calculated from the energy balance of the probe. The spatial profile of the heat flux is nearly a Gaussian distribution. In the radial heat flux distribution, the e-holding half-width, at which the heat flux is decreased down to 1 / e of the peak heat flux, is held to be about 70 mm by adjusting the beam optics. About
Table 1 Contents of stainless steel (wt%) Element
Cr
Ni
Mo
Mn
Melting point (°C)
1890
1455
2625
1240
P
Si
Ti
C
1430
1820
3700
S
type-316 type-304 PCA-J type-316F
16.82 18.38 14.13 16.77
10.29 8.50 15.75 13.95
2.12 2.27 2.31
1.00 1.16 1.43 0.23
< 0.032 < 0.022 0.03 < 0.003
0.056 0.54 0.54 0.040
0.22 -
0.06 0.05 0.057 0.036
0.26 0.25 0.005 0.002
316F-T1 316F-T2 316F-T3
16.94 17.03 16.95
13.83 13.85 13.86
2.28 2.23 2.27
0.28 0.28 0.28
0.007 0.006 0.007
0.050 0.046 0.048
0.050 0.14 0.24
0.036 0.035 0.036
0.001 0.001 0.001
316F-P1 316F-P2 316F-P3
16.83 16.83 16.77
13.76 13.66 13.83
2.32 2.18 2.21
0.28 0.29 0.28
0.005 0.016 0.030
0.048 0.065 0.064
< 0.010 < 0.010 < 0.010
0.034 0.034 0.034
0.001 0.001 0.001
M. Ogawa et aL / Melting and evaporation of metal 40 ms was required for the heat flux to reach steady state in all cases. The materials used were mainly stainless steels (type-316, type-304, PCA-J, type-316F and modified 316F stainless steels) and pure metals (aluminum, copper, molybdenum, tungsten). The PCA-J is the Japanese Primary Candidate Alloy and the modified type-316 which has a high void swelling resistance. The type-316F is a modified form of type-316 which is being developed by JAERI for use in fusion reactors. The type-316F has reduced amounts of manganese, phosphorus, silicon and carbon compared with type-316. Table 1 shows the principal contents of stainless steels used. The geometry of test piece was a flat disc or flat rectangular plate. The diameter or width of the test piece ranged from 20 to 120 ram, and the thickness from 5 to 20 mm. The process of melting and evaporating was observed with a high-speed video camera which could take pictures at a speed of 12000 frames per second. The resolidified surfaces and cross-sections of the test piece were observed with the aid of a scanning electron microscope and an optical microscope. The evaporation loss and resolidified layer thickness were measured by an electric balance with an accuracy of + 10 mg and a reading microscope with an accuracy of + 10 /~m, respectively. Composition analyses were made with an electron probe micro analyzer (EPMA). Surface temperatures of the test piece were measured by using a high speed infrared thermometer. It can measure in the range of 800 to 3000°C with an accuracy of +50°C at a period of 1 /~s. The measured spot is a circular area of about 20 mm in diameter. About one hundred test pieces were heated by the ion beam under the following conditions: peak heat flux 68-261 M W / m 2 and heating duration 40-250 ms. Several test pieces were repeatedly irradiated under the same heating conditions. A few stainless steel test pieces inclined at sixty degrees to the ion beam were heated. The pressure in the vacuum chamber was measured by using an ionization gauge at the upper location of the vacuum chamber. The inside of the vacuum chamber was always evacuated with an exhausting speed of 3 ma/s. Figure 2 shows the pressure change with time during one test. The pressure before the test is about 2.5 × 10 -4 Pa. In fig. 2, hydrogen gas starts to be supplied from t = 0 rain, and the test piece is heated for 50 ms at t =3.8 min. The maximum pressure during the heating could riot be measured because the response time of the ionization gauge was 100 ms. The peak of pressure during the heating is seemed to result from the out-gas, including the vapor
0.10
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3. Results and d i s c u s s i o n
3.1. Evaporation Evaporation loss from the test pieces of stainless steels is shown in fig. 3, where the abscissa denotes the product of peak heat flux qpeak and heating duration At. No dispersion of melt from the test piece was observed by the high-speed video camera. The present results do not include the cases in which the melting layer fell down from the test piece to the support plate. The three solid lines in fig. 3 are calculated by using a thermal erosion analysis code [6]. These two-dimen-
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20
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196
M. Ogawa et al. / Melthzg and evaporation of metal
sional transient calculations were conducted in consideration of evaporation. Whereas the calculated evaporation loss depends upon the heat flux, such a clear dependence was not found in the experiments. The measurement with higher accuracy is required in the low (qoeak'At) to make the dependence clear. The experimental results at a higher value of (qp~,~k"At) are lower than those calculated by the code. It should be noted that much vapor influences the electric current of t h e ion beam and restricts the very high heat flux tests. No difference in evaporation loss are found among the different stainless steels examined. 3.2. Resolidified surface The topography of the resolidified surfaces of stainless steels after heating has been mentioned in ref. [9]. It exhibited two features: a rough surface (typeZ316, PCA-J and type-304) and a smooth surface (type-316F). The content of phosphorus in the type-316F is considerably smaller than that in other stainless steels, and the PCA-J contains much titanium as shown in table 1. The test pieces of type-316F-T1-T3 and type-316FP1-P3 were made by varying the contents of titanium and phosphorus, respectively, to examine the effect of these contents on the formation of the rough surface. All resolidified surfaces of the test pieces became smooth after irradiation. Madarame et al. [14] reported that sulphur and oxygen contributed to the formation of the rough resolidified surface. It is known that the surface tension of molten iron is considerably reduced with the addition of alloying elements (oxygen, sulfur, selenium and tellurium belonging to the group VI B) [17]. The other thermal properties such as viscosity, which were thought to influence the formation of the rough surface, scarcely change with the composition. Therefore, it is seemed that the different features of the resolidifled surface of the stainless steels probably results from the difference in the surface tension. The surface tension of type-316F is inferred to be the largest among those of the stainless steels used here, because the amounts of minor elements such as sulfur in the type-316F are less than those in other stainless steels. The high-speed camera observations revealed that the small mountains were formed not during the resolidifying process but in the initial stage of the melting process. Fujimura et al. [18] provided the criteria for the formation of the rough surface by an instability analysis of the melt layer. In his theoretical study, the determinant factors are the surface tension and the temperature coefficient of surface tension. The tem-
perature coefficient is sensitive to the chromium content in molten iron. Madarame and Okamoto [19] analyzed this phenomena of formation of the rough surface, paying attention to the sign of the temperature coefficient of surface tension. The rough surface of the small disc of 15 mm diameter was, however, slightly different of those of the large discs with a diameter greater than 80 mm under the same heating conditions. This difference might be caused by the effect that the surface of the small disc was stretched by the surface tension from the peripheral edge. Thus, the size of the test piece affects the roughness of resolidified surface. The resolidified surface of aluminum exhibited a smooth surface, the same as the other pure metals under low heat flux conditions. However, it became rough, when the aluminum test piece was irradiated under the conditions of qpeak = 80 M W / m 2 and At = 100 ms. These heating conditions are severe to aluminum because the melting temperature of aluminum is 660°C, lower than those of other metals. No rough surfaces were formed with the electron beam of the JAERI Electron Beam Irradiation Stand (JEBIS) [9]. The heating conditions were the following: heated area of 13 mm × 13 mm, uniform heat flux of 72 M W / m 2, heating duration of 100 ms and type-316 test piece with 120 mm diameter and 20 mm thickness. The projected range (length penetrated by ion or electron) in the material for the hydrogen ion beam is considerably shorter than that for the electron beam. For example, the projected range in iron is about 0.5 /zm for the hydrogen ion beam and 27 /zm for the electron beam with an energy of 100 keV. Thus, the hydrogen ion beam gives rise to a steeper temperature gradient in the direction of the thickness than that by the electron beam. For example, in the melt layer of 27 /xm thickness, a rough thermal conduction calculation indicates that the temperature difference between the surface (x -- 0/zm) and the location of 27 p.m thickness (x = 27/zm) is about 110 K for the surface heating and 55 K for the internal heating. In this calculation, the heating by the electron beam and the heating by the hydrogen ion beam were assumed to be surface heating and the internal heating with uniform heat generation density from x = 0 to 27/xm, respectively. A heat flux of 72 M W / m 2 and a heat generation density corresponding to this heat flux were given for the surface heating and for the internal heating, respectively. A steady state was also assumed because the thermal diffusion time was very short compared with the heating duration. Moreover, the present heat flux of the hydrogen ion beam has a Gaussian distribution in the
M. Ogawa et al. / Melth)g and evaporation of metal
197
radius direction. These differences in heat generation profiles are seemed to cause the different resolidified surfaces. Photos l(a) and (b) show the resolidified surfaces of the test pieces of PCA-J and type-316F irradiated seven times under the same conditions. Many cracks are found on the rough resolidified surface of the PCA-J test piece, especially the peripheral surface of the test piece. On the other hand, cracks are scarcely observed on the smooth resolidified surface of the
Photo 2. Topographic change of resolidified layer inclined at 60° (peak heat flux = 210 MW/m 2, heating duration = 60 ms); (a) PCA-J, (b) type-316F.
Photo 1. Topographic change of resolidified layer exposed repeatedly seven times (peak heat flux = 92 MW/m 2, heating duration = 100 ms); (a) PCA-J, (b) type-316F.
316F test piece. The test piece of the type-316F has preferable characteristics against crack generation. Photos 2(a) and (b) show the topographies of the test pieces of PCA-J and type-316F inclined. The melting part near the center of the test piece hangs down to the bottom. Large erosion loss due to gravity loss of the melt layer is serious to the lifetime of the metallic materials. Figure 4 shows the temperature of the surface measured by using the high-speed infrared thermometer. The details of measurements were reported in ref. [20]. The temperatures were measured at a period of 30 ]~s in the center of the test piece of type-304 under the conditions: peak heat flux = 168 M W / m 2, heating duration = 70 ms. Since the emissivity of polished stainless steels is about 0.18-0.2, the measured data are calibrated by using the emissivities of 0.18 and 0.2. The broken lines in fig. 4 show the result calculated by the
M. Ogawa et al. / Me/ting and ecaporation of metal
198
code [6]. The measured temperatures exhibit a good time response, however those are lower than thosc calculated. The discrepancy between the experimental and calculated temperatures results from potential effects of the vapor shielding and the convection in the melt layer.
3000
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Figures 5(a) and (b) show the results of composition analyses with the aid of EPMA in the test pieces of PCA-J and type-316F. The analyses disclose depletion of only manganese in the resolidified layer of the stainless steel test piece, except for the type-316F. In the resolidified layer of PCA-J, titanium is found to fluctuate sharply. No change of other elements is observed in the resolidified layer. A copper pin with the diameter of 1 mm was inserted at 10 mm from the center of the test piece to clarify the occurrence of convection in the melt layer. Photo 3 shows the result of EPMA. The long strip of the center in photo 3 is the copper pin, and the left hand side is the center direction. The color bar at the right-hand side in photo 3 displays the copper concentration, and the upper color of the bar shows a higher concentration. The melting copper seen at the right-
1000
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40
60
80
Time (ms) Fig. 4. Temperature change with time on the surface of test piece (type-304, peak heat flux; 168 MW/m 2, heating duration; 70 ms).
hand surface of photo 3 is found to flow to the peripheral direction. In a short heating duration, the rough resolidified layer lies scattered on the base metal and the base metal is made bare at the part without the resolidified layer as shown in fig. 6. If such a test piece is repeatedly heated under the same condition, the bare base
t......a Center
lmm
Copper
pin
Periphery
Photo 3. Component analysis in cross-section of the resolidified layer to examine the convection, (type-316, peak heat flux = 89 MW/m 2, heating duration = 100 ms).
199
M. Ogawa et aL / Melthzg and evaporation of metal Melt zone
Bose melol
Fe (x2)
Melt zone
Cr (xl)
A
metol
Fe (X2}
Ni (Xl)
Cr (Xl)
Mn XI/IC
Ni
<~
(Xl)
Ti Xl/IC
O.)
Bose
Mn (Xl/lO)
Mo XI/IC
Si , (XI/lO)
Si
(Xt/t0) C (XI/i0)
C (Xl/lO)
100,m Distonce
Distonce
Fig. 5. Composition analysis with the aid of EPMA in test pieces (a, left) PCA-J, (b, right) type-316F.
metal would be subjected to strong melting. Figure 7 shows the comparison of the melt layer thickness after resolidification between the experiment and the thermal erosion calculation mentioned before. This difference in the thickness as shown in fig. 7 results from the convection in the melt layer. The volume of the resolidified layer in the experiment was greater 1.4 times than that in the calculation. This suggests that the convection in the melt layer transfers the heat from the center to the periphery, consequently the evaporation loss will be suppressed.
Seki et al. [12] have already reported the effect of the convection on the evaporation loss. In their experiment, the gradient of the evaporation loss to the absorbed energy became small and the melt layer became thick in the high absorbed energy region. They explained this tendency as a result of the convection effect because the result of the evaporation loss in their experiment agreed well with that calculated by the thermal erosion analysis code simulating the convection effect. Thus, the convection effect on the evap-
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Fig. 7. Comparison of melt layer thickness between results in experiment and calculation.
M. Ogawa et al. / Melting and evaporation of metal
200
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Fig. 8. Cross sectional deformation of test piece exposed repeatedly seven times (peak heat flux = 92 M W / m 2, heating duration = 100 ms); (a, left) PCA-J (see photo l(a)), (h, right) type-316F (see photo l(b)).
Photo 4. Metallographic photograph in cross-section of the resolidified layer exposed seven times (see photo l(b) and fig. 8(b)).
M. Ogawa et aL / Meltb~gand evaporation of metal oration loss was found to act as well as the vapor shielding effect as described at 3.1. Figures 8(a) and (b) show the cross-sectional resolidified layer of the test pieces of PCA-J and type-316F (see photo 1) heated repeatedly seven times. The solid lines in fig. 8 are drawn by smoothly connecting the data measured at an interval of 1 mm. The numbers denote each heating cycle. In the type-316F test piece, the thickness of the resolidified layer (0.24 mm) is the nearly same as that (0.23 mm) exposed one time. The thickness of the test piece at the center is decreasing at each shot, and the final thickness is 13.89 mm. The total eroded thickness is 0.92 mm. This erosion results from the evaporation and convection. U n d e r the present condition, the total eroded thickness of 0.92 mm is almost due to the convection, since the eroded thickness due to the evaporation is about 0.01 mm in one shot. The melt layer of each shot near the center of the test piece accumulates at the edge as shown in fig. 8. In the calculation code, the convection in the melt layer must be taken into consideration to predict the erosion loss. Photo 4 shows the resolidified layer stratifying at the edge shown in fig. 8(b). Seven resolidified layers were distinctly observed in photo 4. The structure of columnar dendrite grows continuously through each resolidified layer in the test piece irradiated seven times as shown in photo 4.
4. Conclusion Experimental studies were carried out on melting, evaporation and resolidification of metals, mainly stainless steels, subjected to intense hydrogen ion beam. The experimental results were obtained as follows: (1) Topography of the resolidified surfaces of metallic materials after heating exhibited two features: a rough surface and a smooth surface. The effects of the size of the test piece, the electron beam and the contents of stainless steel on the resolidified surface were examined. (2) The observations with the high-speed video camera clarified that the rough surface was formed in the initial stage of melting process. (3) The convection of the melt layer was observed and the thickness of the base metal was decreased by not only the evaporation also the convection. It is planned to develop or verify new analytical methods and models based on the present experimental results.
201
Acknowledgments The authors want to express their gratitude to Dr. K. Kiuchi and Dr. A. Hishinuma for valuable comments on materials. The authors are grateful to Mr. K. Yokoyama, Mr. M. Dairaku and Mr. K. Emori for their assistance in carrying out the experiments.
References [1] IAEA, ITER Conceptual Design Report, I A E A / I T E R / DS/No. 18 (1991). [2] S. Matsuda et al., Conceptual design study of the Fusion Experimental Reactor (FER), Proc. of 13th Int. Conf. on Plasma Physics and Controlled Nucl. Fusion Research, Washington DC, USA, IAEA-CN-53/G-2-2 (1990). [3] A.M. Hassanein, G.L Kulcinski and W.G. Wolfer, Vaporization and melting of materials in fusion devices, J. Nucl. Mater. 103/104 (1981) 321. [4] H.Th. Klippel, The thermal response of the first wall of a fusion reactor blanket to plasma disruptions, Research Report of the Netherlands Energy Research Foundation, ECN-137 (1983). [5] H. Nakamura, et al., First wall erosion during a plasma disruption in tokamak, Research Report of Japan Atomic Energy Research Institute, JAERI-M 83-058 (1983). [6] S. Yamazaki, M. Seki and T. Kobayashi, Two-dimensional disruption thermal analysis code DREAM, ibid., 88-163 (1988), (in Japanese). [7] J. Gilligan, D. Hahn and R. Mohanti, Vapor shielding of surfaces subjected to high heat fluxes during a plasma disruption, J. Nucl. Mater. 162/164 (1989) 957. [8] J.G. van der Laan et al., Prediction for disruption erosion of ITER PFC; a comparison of experimental and numerical results, Proc. ISFNT-2, Fusion Engrg. Des. 18 (1991) 135-144. [9] M. Seki et al., Thermal shock tests on various materials of plasma facing components for F E R / I T E R , Fusion Eng. Des. 15 (1991) 59. [10] J.R. Easoz, et al., Thermomechanical testing of first wall test pieces in ESURF, Nucl. Technol. Fusion 4 (1983) 780. [11] C.D. Croessmann, G.L Kulcinski and J.B. Whitley, Correlation of experimental and theoretical results for vaporization by simulated disruption et al., J. Nuel. Mater. 128/129 (1984) 816. [12] M. Seki et al., A simulated plasma disruption experiment using an electron beam as a heat source, J. Fusion Energy 5[3] (1986) 181. [13] C. Rigon, P. Moretto and F. Brossa, Experimental simulation of plasma disruption with an electron beam, Fusion Eng. Des. 5 (1987) 299. [14] H. Madarame, et al., Effect of impurity content in stainless steel on resolidified surface condition after disruption load, Fusion Eng. Des. 9 (1989) 213.
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M. Ogawa et al. / Melthzg and evaporation of metal
[15] S. Yamazaki, M. Seki and M. Uno, Crack initiation behaviors of Metallic walls subjected to high heat flux expected at plasma disruption, J. Atomic Energy Soc. Japan, 31111] (1989) 1251, (in Japanese). [16] M. Ogawa, et al., Experimental observations of metal surfaces exposed to intense hydrogen beam; simulation of plasma disruption, J. Nucl. Science Technol. 26[7] (1989) 721. [17] K. Ogino, K. Nogi and O. Yamase, The effect of selenium and tellurium on the surface tension of molten iron and wettability of solid oxide. Ion Steel, 66[2] (1980) 179, (in Japanese).
[18] K. Fujimura, in M. Seki, N. Miki, Y. Shibutani et al., Japanese contributions to IAEA INTOR workshop, Phase 2a, Part 2, Research Report of Japan Atomic Energy Research Institute, JAERI M85-075, 93-107 (1985). [19] H. Madarame and K. Okamoto, Numerical simulation of surface tension induced melt layer movement after disruption load, Fusion Engrg. Des. 19, No. 4 (1992). [20] M. Araki et al., High-speed surface temperature measurements of plasma facing materials under thermal shock loads for fusion applications, to be published in Review of Scientific Instruments.