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Erosion of beryllium by oxygen
Journal of Nuclear
Materials
845
176 & 177 (1990) 845-847
North-Holland
Erosion of beryllium by oxygen C.H. Wu I, E. Hechtl 2, H.R. Yang 3 and W. Eckstein 4 ’ The NET Team, c/o Max-Plonck-Institut
fulr Plasmaphysik, Boltzmonnstrasse 2, D-8046 Garching bei Miinchen, Fed. Rep. Germany ’ Technische Universitlit Miinchen, Physk Deportment, James-Franck-Strasse, O-8046 Garching bei M%chen, Fed. Rep. Germany 3 South- Western Institute of Physics, Leshon, P.R. China ’ Max-Planck-Institut fir Piasmaphysik, Boltzmonnstrosse 2, D-8046 Gorching bei Miinchen% Fed. Rep. Germany
Beryllium has been seriously considered as a plasma-facing material because of its low Z, favourable thermomechanical properties and expected low tritium retention. However, erosion by plasma and impurities is another important issue in assessing its suitability for this application. An experimental study of sputte~ng of beryllium by O+ was conducted for energies between 100 and 10500 eV and target temperatures from 300 to 1000 K. The erosion yield at, for example, an energy of 100 eV is around 9X fOe2, while for 3000 eV it is as high as 5 X IO-’ at room temperature. The experimental results on the erosion yields due to oxygen as a function of the energy and target temperature are reported and discussed. These are compared with analyses made with the TRIM-SP computer code.
1. IntroductioR In view of the radiation losses in thermonuclear fusion devices, low-2 materials have been seriously considered for use as plasma-facing components. In particular, graphite has been widely tested in present tokamak machines because it is a low-Z material with excellent mechanical properties. It has a low neutron absorption cross-section and it is thermal shock resistant at high temperature. However, graphite has the following li~tations [l]: high chemical erosion by D/T and oxygen, radiation-enhanced sublimation and high tritium retention. Recently, it was pointed out and demonstrated experimentally in JET and TPTR that graphite may exhibit runaway sputtering conditions at high temperature and at grazing angles of incidence, where the self-sputtering yield exceeds unity. This in combination with radiation-enhanced sublimation leads to the JET carbon catastrophe [Z]. Since the JET carbon catastrophe and TPTR “carbon bloom” have been observed, another low-Z material, beryllium, has become increasingly attractive. Operation with Be in JET shows that it is possible to maintain Z,,, I 1.5 at heating powers of the order of 30 MW. The density limit is substantially increased be~22-3115/~/$03.50
@ 1990 - Elsevier Science publishers
cause of the low radiative cooling rate and oxygen gettering of Be 131. It has been reported that oxygen is one of the major impurities in present tokamak devices such as JET, TPTR, ASDEX and TEXTOR [4]. Consequently, the nature of the interaction of oxygen with Be has to be understand to assess material erosion during discharges. The principal objective of the present work is to investigate Be erosion by oxygen as a function of the particle energy, target temperature and angle of incidence.
2. Experimental The ion irradiation of the beryllium targets was performed with the isotope separator of the Physics Department at Technische Universittit Mtinchen [5,6]. The targets were subjected to ohmic heating and the temperature of the targets was measured with an infrared thermometer, which in turn was calibrated with a micropyromerer. The oxygen ions were produced with CO2 as feed gas for the ion source. The particle flux on the target was about 6 x lo’* 0’/cm2 s. The base pressure in the irradiation chamber was 10e6 Pa. Dur-
B.V. (North-Holland)
C. H. Wu er ul. / Erosion
846
of hetylliumby oxygen
ing ion source operation the pressure rises to about lop5 Pa. The beryllium samples were sintered from powder and used as received from JET for the room temperature measurements, whereas the thin targets which were heated ohmically were machined by INTERATOM to a thickness of 0.25 mm. The sputtering yield was determined by the weight loss method [7]. The integrated beam current was measured with an accuracy better than 10% and the mass change of the target with better than 1 ug by an ultramicrobalance (Sartorius). The irradiation time was chosen to achieve a mass loss of about 100 pg. For the very low energies a mass loss of 20 pg was regarded as a compromise in order to keep the irradiation times acceptable. The uncertainty in the yield data according to the mass and current measurements is estimated to be 15% at most.
3. Results and discussions Fig. 1 shows the erosion yields of beryllium bombarded by oxygen as a function of the energy at room temperature and at a target temperature of 600 o C. The erosion yields at 600°C seem to be slightly higher than those at room temperature, but more detailed experimental investigations are needed at a target tem-
‘t
II
7
--_
ANGLE
-/*__-i--’ I
I’
8.
9
i IO'
ION ENERGY
[eV]
Fig. 1 The measured erosion yields of beryllium exposed to oxygen, as a function of energy: dashed line 0 -+ Be, solid line 0 ---)Be0 TRIM-SP code.
OF
INCIDENCE
Fig. 2 The calculated erosion yields 0 + Be0 as a function the energy and angle of incidence.
of
perature of > 600 o C to establish a temperature dependence of the erosion yields. It is seen that the erosion yields of beryllium irradiated by O+ first increase as the energy of the impinging particles increases to a maximum, and then they decline. The maximum erosion yield of - 5 x 10-l is reached at an energy of 3000 eV. The TRIM-SP code [8] was used to calculate the erosion of beryllium by oxygen, these results also being given in fig. 1. The dashed line represents the oxygen ions impinging on the beryllium, where a Be sublimation energy of 3.38 eV was used as binding energy, whilst the solid line represents the erosion characterisation of oxygen ions impinging on BeO, where the heat of formation of BeO, 6.33 eV, was used as binding energy. In the energy range from lo2 to lo4 eV, the calculated erosion yields for 0 -+ Be are higher than the measured values. At the lowest energy, lo2 eV, the discrepancy is a factor of 6, continuously decreasing to a factor of 1.3 at lo4 eV. Excellent agreement was found between the calculated 0 -+ Be0 values and the experimental results. This implies that the beryllium is converted to an oxide by being impinged on by oxygen ions. The fact that the measured erosion yields are very close to the calculated values of 0 -+ Be0 suggests that the 0 + Be0 mechanism is dominant during the experi-
C.H. Wu et al. / Erosion of beryllium by oxygen
ment. It is therefore assumed that once beryllium contacts the oxygen impurity, it will immediately form a beryllium oxide layer by gettering of oxygen. Further calculations were performed to investigate the erosion yields as a function of the energy and incident angle, namely for energies of 200, 300, 500 and 1000 eV. The results are given in fig. 2. It is seen that the erosion yields reach a maximum at incident angles between 60 and 70“ for the energy range 20%1000 eV. The maximum erosion yield already exceeds unity at an energy of 300 eV and an incident angle of 60 O. The maximum erosion yield is as high as 2.5 for an energy of E = 1000 eVand 0~70~. For NET/ITER devices, in the case of ignition mode, the edge plasma temperature T, is around 75 eV and the average charge state of oxygen is about 5. This indicates that the incident energy of the oxygen will be E > 300 eV. In particular, if Be is used as divertor plate material and the predominant angles of incidence are assumed to be between 45 o and 75 O, the erosion yields will normally be expected to exceed unity [9].
841
(3) Although
the erosion yields of beryllium exposed to oxygen at 6OOOC are slightly higher than those at room temperature, it can be stated that the temperature effect on erosion yields is not significant up to 600” C. Roth et al. [lO,ll] found an increase in yield at a target temperature of 650 o C for sputtering of beryllium with deuterium. The increase there is due to the disappearance of the oxide layer at elevated temperature, whereas here the oxide is continuously created during bombardment. Therefore no significant yield increase with temperature occurs. angles of incidence at the diver(4) If the predominant tor plates are assumed to be between 45 and 75’ and the edge plasma temperature is T, = 75 eV, the erosion yields will normally be expected to exceed unity. This implies that a very low oxygen impurity level must be kept in D-T tokamak devices. (5) There is a need for more detailed theoretical analyses and experimental investigations on the angular dependence of sputtering yields, as well as on ion current and on the associated influence of the surface topography of beryllium.
4. Conclusion This paper describes an experimental investigation of the erosion of beryllium by oxygen ions. The results indicate that beryllium oxide is formed by exposure of beryllium to oxygen ions. Consequently, the erosion process O++ Be0 was analysed by using the TRIM-SP code. The following conclusions can be drawn: (1) The coincidence of experiment and calculation appears to provide evidence that Be0 formation is occurring at the beryllium surface bombarded by oxygen. However, no experimental surface analysis work was performed to confirm this conclusion. (2) At a normal angle of incidence the erosion yields of beryllium exposed to oxygen first increase as the energy to a maximum, then decline as the energy continues to increase. The maximum erosion yield is y = 5 x 10-l at an energy of E = 3000 eV.
References [l] [2] [3] [4] [S] [6] [7]
C.H. Wu. NET/IN/85079. M. Hugon, P.P. Lallia and P.H. Rebut, JET-R (89) 14. M. Keilhacker and the JET Team, JET-P (89) 83. C.H. Wu, J. Nucl. Mater. 145-147 (1987) 448. E. Hechtl, Nucl. Instr. and Meth. 186 (1981) 453. E. Hechtl, Nucl. Instr. and Meth. B26 (1987) 37. E. Hechtl and J. Bohdansky, J. Nucl. Mater. 122 & 123 (1984) 1431. [8] J.P. Biersack and W. Eckstein, Appl. Phys. 34 (1984) 73. [9] J.N. Brooks, D.K. Brice, A.B. DeWald and R.T. McGrath, J. Nucl. Mater. 162-164 (1989) 363. [lo] J. Roth, J. Nucl. Mater. 145-147 (1987) 87. [ll] J. Roth, W. Eckstein and J. Bohdansky. J. Nucl. Mater. 165 (1989) 199.
Materials
845
176 & 177 (1990) 845-847
North-Holland
Erosion of beryllium by oxygen C.H. Wu I, E. Hechtl 2, H.R. Yang 3 and W. Eckstein 4 ’ The NET Team, c/o Max-Plonck-Institut
fulr Plasmaphysik, Boltzmonnstrasse 2, D-8046 Garching bei Miinchen, Fed. Rep. Germany ’ Technische Universitlit Miinchen, Physk Deportment, James-Franck-Strasse, O-8046 Garching bei M%chen, Fed. Rep. Germany 3 South- Western Institute of Physics, Leshon, P.R. China ’ Max-Planck-Institut fir Piasmaphysik, Boltzmonnstrosse 2, D-8046 Gorching bei Miinchen% Fed. Rep. Germany
Beryllium has been seriously considered as a plasma-facing material because of its low Z, favourable thermomechanical properties and expected low tritium retention. However, erosion by plasma and impurities is another important issue in assessing its suitability for this application. An experimental study of sputte~ng of beryllium by O+ was conducted for energies between 100 and 10500 eV and target temperatures from 300 to 1000 K. The erosion yield at, for example, an energy of 100 eV is around 9X fOe2, while for 3000 eV it is as high as 5 X IO-’ at room temperature. The experimental results on the erosion yields due to oxygen as a function of the energy and target temperature are reported and discussed. These are compared with analyses made with the TRIM-SP computer code.
1. IntroductioR In view of the radiation losses in thermonuclear fusion devices, low-2 materials have been seriously considered for use as plasma-facing components. In particular, graphite has been widely tested in present tokamak machines because it is a low-Z material with excellent mechanical properties. It has a low neutron absorption cross-section and it is thermal shock resistant at high temperature. However, graphite has the following li~tations [l]: high chemical erosion by D/T and oxygen, radiation-enhanced sublimation and high tritium retention. Recently, it was pointed out and demonstrated experimentally in JET and TPTR that graphite may exhibit runaway sputtering conditions at high temperature and at grazing angles of incidence, where the self-sputtering yield exceeds unity. This in combination with radiation-enhanced sublimation leads to the JET carbon catastrophe [Z]. Since the JET carbon catastrophe and TPTR “carbon bloom” have been observed, another low-Z material, beryllium, has become increasingly attractive. Operation with Be in JET shows that it is possible to maintain Z,,, I 1.5 at heating powers of the order of 30 MW. The density limit is substantially increased be~22-3115/~/$03.50
@ 1990 - Elsevier Science publishers
cause of the low radiative cooling rate and oxygen gettering of Be 131. It has been reported that oxygen is one of the major impurities in present tokamak devices such as JET, TPTR, ASDEX and TEXTOR [4]. Consequently, the nature of the interaction of oxygen with Be has to be understand to assess material erosion during discharges. The principal objective of the present work is to investigate Be erosion by oxygen as a function of the particle energy, target temperature and angle of incidence.
2. Experimental The ion irradiation of the beryllium targets was performed with the isotope separator of the Physics Department at Technische Universittit Mtinchen [5,6]. The targets were subjected to ohmic heating and the temperature of the targets was measured with an infrared thermometer, which in turn was calibrated with a micropyromerer. The oxygen ions were produced with CO2 as feed gas for the ion source. The particle flux on the target was about 6 x lo’* 0’/cm2 s. The base pressure in the irradiation chamber was 10e6 Pa. Dur-
B.V. (North-Holland)
C. H. Wu er ul. / Erosion
846
of hetylliumby oxygen
ing ion source operation the pressure rises to about lop5 Pa. The beryllium samples were sintered from powder and used as received from JET for the room temperature measurements, whereas the thin targets which were heated ohmically were machined by INTERATOM to a thickness of 0.25 mm. The sputtering yield was determined by the weight loss method [7]. The integrated beam current was measured with an accuracy better than 10% and the mass change of the target with better than 1 ug by an ultramicrobalance (Sartorius). The irradiation time was chosen to achieve a mass loss of about 100 pg. For the very low energies a mass loss of 20 pg was regarded as a compromise in order to keep the irradiation times acceptable. The uncertainty in the yield data according to the mass and current measurements is estimated to be 15% at most.
3. Results and discussions Fig. 1 shows the erosion yields of beryllium bombarded by oxygen as a function of the energy at room temperature and at a target temperature of 600 o C. The erosion yields at 600°C seem to be slightly higher than those at room temperature, but more detailed experimental investigations are needed at a target tem-
‘t
II
7
--_
ANGLE
-/*__-i--’ I
I’
8.
9
i IO'
ION ENERGY
[eV]
Fig. 1 The measured erosion yields of beryllium exposed to oxygen, as a function of energy: dashed line 0 -+ Be, solid line 0 ---)Be0 TRIM-SP code.
OF
INCIDENCE
Fig. 2 The calculated erosion yields 0 + Be0 as a function the energy and angle of incidence.
of
perature of > 600 o C to establish a temperature dependence of the erosion yields. It is seen that the erosion yields of beryllium irradiated by O+ first increase as the energy of the impinging particles increases to a maximum, and then they decline. The maximum erosion yield of - 5 x 10-l is reached at an energy of 3000 eV. The TRIM-SP code [8] was used to calculate the erosion of beryllium by oxygen, these results also being given in fig. 1. The dashed line represents the oxygen ions impinging on the beryllium, where a Be sublimation energy of 3.38 eV was used as binding energy, whilst the solid line represents the erosion characterisation of oxygen ions impinging on BeO, where the heat of formation of BeO, 6.33 eV, was used as binding energy. In the energy range from lo2 to lo4 eV, the calculated erosion yields for 0 -+ Be are higher than the measured values. At the lowest energy, lo2 eV, the discrepancy is a factor of 6, continuously decreasing to a factor of 1.3 at lo4 eV. Excellent agreement was found between the calculated 0 -+ Be0 values and the experimental results. This implies that the beryllium is converted to an oxide by being impinged on by oxygen ions. The fact that the measured erosion yields are very close to the calculated values of 0 -+ Be0 suggests that the 0 + Be0 mechanism is dominant during the experi-
C.H. Wu et al. / Erosion of beryllium by oxygen
ment. It is therefore assumed that once beryllium contacts the oxygen impurity, it will immediately form a beryllium oxide layer by gettering of oxygen. Further calculations were performed to investigate the erosion yields as a function of the energy and incident angle, namely for energies of 200, 300, 500 and 1000 eV. The results are given in fig. 2. It is seen that the erosion yields reach a maximum at incident angles between 60 and 70“ for the energy range 20%1000 eV. The maximum erosion yield already exceeds unity at an energy of 300 eV and an incident angle of 60 O. The maximum erosion yield is as high as 2.5 for an energy of E = 1000 eVand 0~70~. For NET/ITER devices, in the case of ignition mode, the edge plasma temperature T, is around 75 eV and the average charge state of oxygen is about 5. This indicates that the incident energy of the oxygen will be E > 300 eV. In particular, if Be is used as divertor plate material and the predominant angles of incidence are assumed to be between 45 o and 75 O, the erosion yields will normally be expected to exceed unity [9].
841
(3) Although
the erosion yields of beryllium exposed to oxygen at 6OOOC are slightly higher than those at room temperature, it can be stated that the temperature effect on erosion yields is not significant up to 600” C. Roth et al. [lO,ll] found an increase in yield at a target temperature of 650 o C for sputtering of beryllium with deuterium. The increase there is due to the disappearance of the oxide layer at elevated temperature, whereas here the oxide is continuously created during bombardment. Therefore no significant yield increase with temperature occurs. angles of incidence at the diver(4) If the predominant tor plates are assumed to be between 45 and 75’ and the edge plasma temperature is T, = 75 eV, the erosion yields will normally be expected to exceed unity. This implies that a very low oxygen impurity level must be kept in D-T tokamak devices. (5) There is a need for more detailed theoretical analyses and experimental investigations on the angular dependence of sputtering yields, as well as on ion current and on the associated influence of the surface topography of beryllium.
4. Conclusion This paper describes an experimental investigation of the erosion of beryllium by oxygen ions. The results indicate that beryllium oxide is formed by exposure of beryllium to oxygen ions. Consequently, the erosion process O++ Be0 was analysed by using the TRIM-SP code. The following conclusions can be drawn: (1) The coincidence of experiment and calculation appears to provide evidence that Be0 formation is occurring at the beryllium surface bombarded by oxygen. However, no experimental surface analysis work was performed to confirm this conclusion. (2) At a normal angle of incidence the erosion yields of beryllium exposed to oxygen first increase as the energy to a maximum, then decline as the energy continues to increase. The maximum erosion yield is y = 5 x 10-l at an energy of E = 3000 eV.
References [l] [2] [3] [4] [S] [6] [7]
C.H. Wu. NET/IN/85079. M. Hugon, P.P. Lallia and P.H. Rebut, JET-R (89) 14. M. Keilhacker and the JET Team, JET-P (89) 83. C.H. Wu, J. Nucl. Mater. 145-147 (1987) 448. E. Hechtl, Nucl. Instr. and Meth. 186 (1981) 453. E. Hechtl, Nucl. Instr. and Meth. B26 (1987) 37. E. Hechtl and J. Bohdansky, J. Nucl. Mater. 122 & 123 (1984) 1431. [8] J.P. Biersack and W. Eckstein, Appl. Phys. 34 (1984) 73. [9] J.N. Brooks, D.K. Brice, A.B. DeWald and R.T. McGrath, J. Nucl. Mater. 162-164 (1989) 363. [lo] J. Roth, J. Nucl. Mater. 145-147 (1987) 87. [ll] J. Roth, W. Eckstein and J. Bohdansky. J. Nucl. Mater. 165 (1989) 199.