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Thermal desorption of graphite during deuterium ion bombardment
ELSEVIER
Journal of Nuclear Materials 217 (1994) 206-208
Letter to the Editors
Thermal desorption of graphite during deuterium ion bombardment J.W. Davis, A.A. Haasz Fusion Research Group, University of Toronto Institute for Aerospace Studies, 4925 Dufferik Street, North York, Ontario, Canada M3H 5T6
Received 16 May 1994; accepted 6 July 1994
Carbon and carbon-based materials have found wide acceptance for use as plasma-facing materials in fusion experiments. Many recent studies have greatly improved our understanding of the processes involved, however, a sufficient understanding to be able to predict hydrogen recycling in tokamaks is not yet available. Thermal desorption spectroscopy (TDS) experiments show virtually no release of trapped hydrogen from graphite until temperatures of u 800 K are reached; see, for example, Fig. 1 [l]. In a fusion reactor, such as TFTR, operating at near room temperature, this result implies that no thermally activated release of wall hydrogen inventory is likely to occur during discharges. In fact, however, it is observed that large amounts of H are released into the plasma, depending on the power loading, with the wall being the obvious source [2]. This has led to speculation that the desorption process may be significantly different during plasma bombardment than in the absence of bombardment. Experiments were performed with our dual-beam ion accelerator facility which is described elsewhere [3]. Only one of the accelerators was operated, with a beam of 3 keV D: ions (i.e., 1 keV/D; _ 3 x 10” D/m’s> incident on an “as deposited” pyrolytic graphite sample (Union Carbide, HPG99) at an angle of 35” with respect to the sample normal. After loading the sample to a fluence of N 3 X lo** D/m*, the sample was heated at - 20 K/s to temperatures > 1900 K by resistive heating. Temperature measurement was made by optical pyrometry, with pyrometers in the 350-470 K and 470-1870 K ranges. Limited viewing access prevented both pyrometers from being used together. For some of the runs, the ion beam remained on as the temperature was raised to simulate conditions in a reactor. The released deuterium was
monitored via quadrupole mass spectrometer residual gas analysis (QMS-RGA). The D, (M/e = 4) signal, along with the low temperature pyrometer readings are shown in Fig. 2. During Dl bombardment, as soon as the sample temperature is observed to rise, D, desorption is observed; see Fig. 2b. In the absence of D: irradiation, no such effect was observed; see Fig. 2a. The total D, desorption profiles, as a function of temperature, are presented in Fig. 3. In Fig. 3, the steady-state reemission signal has been subtracted from the Dl beam-on profile, to make the differences between the two cases (i.e., D: on and off) more clear. Also, in the beam-on case, the D, signal at high temperature drops below the level of the signal observed before heating due to the emission of D” atoms [l], which are pumped by the chamber walls and thus are not recorded by the mass spectrometer in the RGA mode. In the absence of the D: beam, the deuterium release begins at - 800 K, consistent with previous TDS results. Due to the close proximity of the quadrupole ionizer to the sample ( _ 5 cm), the sample ambient temperature was typically N 400 K before heating. The fact that the total amounts of D, released for the two cases for T < 1400 K (i.e., the areas under the desorption profiles in Fig. 3) are similar indicates that the released deuterium for T < 1400 K is predominantly in the form of molecules, and not atoms or directly ejected energetic species, which would be pumped by the walls and not be observed. Thus the release mechanism must involve detrapping, subsequent recombination of deuterium atoms and emission of molecules. This release mechanism is consistent with models of hydrogen transport in graphite [4-7] which consider ion-induced detrapping as a local process within the graphite.
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-3115(94)00349-l
J. W. Dauis, AA. Haasz /Journal
The immediate release of deuterium from graphite upon heating while under Dl bombardment, can in fact be predicted if one assumes that a dynamic equilibrium is established between ion-induced detrapping, retrapping and molecular release as the temperature is being raised. Data for hydrogen retained in graphite as a function of temperature show that even for T < 300 K graphite is not fully saturated, but may trap more hydrogen as the temperature is lowered, see Fig. 4 [8-121. The rate at which deuterium is released will depend on the temperature ramping rate, the ion flux, and the effective detrapping cross section; the faster
a 5 zJ
3 keV Da+ (1023Dlm~)--, HPG99 (300K)
. ‘I ‘:’ :i D2 ‘i .*.
10 6
1
Ds+on D3+off
\; a
400
600
600
I 1000 1200 1400 1600 1600 2000
TEMPERATURE/(K)
Fig. 3. Comparison of TDS profiles as a function of temperature for the beam-on and beam-off cases. The total amounts of released D, for T < 1400 K are similar for the two cases.
14 12
201
of Nuclear Materials 217 (1994) 206-208
$6 z 8
4
x
2 0 1
400
600
600
1000 1200 1400 1600 1600 2000
TEMPERATURE/(K)
Fig. 1. Thermal desorption of D”, D, and CD, from pyrolytic graphite as measured by line-of-sight quadrupole mass spectrometry [l].
300
500
700
900
1100
1300
I
15100
TEMPERATURE/(K)
Fig. 4. Retained amounts of deuterium and hydrogen in graphite as a function of temperature during loading, taken from various sources [8-121.
500
25
100
b) D3+on ailthetime 460 s
the temperature rise, the larger the flux required to establish dynamic equilibrium. It is not known whether the present beam flux of N 3 X 1019 D/m* s is sufficient to produce this equilibrium or whether a larger desorption could be achieved with a larger flux. In a tokamak, fluxes would be much higher and faster temperature increases might be kept in equilibrium.
25 a) D3+off
Acknowledgements 160 Y 460 ii z 640 2 ,,, B 420 F
0’
-10
0
10
20
30
40
2.
400
50
This work was supported by the Canadian Fusion Fuels Technology project and the Natural Sciences and Engineering Research Council of Canada. We acknowledge useful comments from Drs. G.M. McCracken, P. Franzen and C.S. Pitcher.
References
TIME/(s)
Fig. 2. D, thermal desorption signals (QMS-RGA) and low temperature pyrometer signals for (a) normal TDS with the D: beam off, and (b) TDS with D; beam on.
111J.W. Davis and A.A. Haasz, Reemission and thermal desorption of D”, D, and CD, from graphite, to be published.
J. W Daviq A.A. Haasz /Journal
208 [2] C.S.
[3] [4] [5] [6] [7]
of Nuclear Materials 217 (1994) 206-208
Pitcher, R.V. Budny, K.W. Hill, S.J. Kilpatrick, D.M. Manos, S.S. Medley and A.T. Ramsey, J. Nucl. Mater. 176&177 (1990) 285. A.A. Haasz and J.W. Davis, Nucl. Instr. and Meth. B83 (1993) 117. W. Mijller and B.M.U. Scherzer, J. Appl. Phys. 64 (1988) 4860. K. Morita and Y. Hasebe, J. Nucl. Mater. 176&177 (1990) 213. D.K. Brice, Nucl. Instr. and Meth. B44 (1990) 302. A.A. Haasz, P. Franzen, J.W. Davis, S. Chiu and C.S.
Pitcher, Two region model for hydrogen trapping in and release from graphite, submitted, 1994. 181J. Roth, unpublished data in Ref. [4]. [9] W. Miiller, P. Borgensen and B.M.U. Scherzer, Nucl. Instr. and Meth. B19/20 (1987) 826. IlO] M. Braun and B. Emmoth, J. Nucl. Mater. 128&129 (1984) 657. [II] B.L. Doyle, W.R. Wampler and D.K. Brice, J. Nucl. Mater. 1038~104(1981) 513. [12] J.W. Davis, University of Toronto, unpublished data, 1991.
Journal of Nuclear Materials 217 (1994) 206-208
Letter to the Editors
Thermal desorption of graphite during deuterium ion bombardment J.W. Davis, A.A. Haasz Fusion Research Group, University of Toronto Institute for Aerospace Studies, 4925 Dufferik Street, North York, Ontario, Canada M3H 5T6
Received 16 May 1994; accepted 6 July 1994
Carbon and carbon-based materials have found wide acceptance for use as plasma-facing materials in fusion experiments. Many recent studies have greatly improved our understanding of the processes involved, however, a sufficient understanding to be able to predict hydrogen recycling in tokamaks is not yet available. Thermal desorption spectroscopy (TDS) experiments show virtually no release of trapped hydrogen from graphite until temperatures of u 800 K are reached; see, for example, Fig. 1 [l]. In a fusion reactor, such as TFTR, operating at near room temperature, this result implies that no thermally activated release of wall hydrogen inventory is likely to occur during discharges. In fact, however, it is observed that large amounts of H are released into the plasma, depending on the power loading, with the wall being the obvious source [2]. This has led to speculation that the desorption process may be significantly different during plasma bombardment than in the absence of bombardment. Experiments were performed with our dual-beam ion accelerator facility which is described elsewhere [3]. Only one of the accelerators was operated, with a beam of 3 keV D: ions (i.e., 1 keV/D; _ 3 x 10” D/m’s> incident on an “as deposited” pyrolytic graphite sample (Union Carbide, HPG99) at an angle of 35” with respect to the sample normal. After loading the sample to a fluence of N 3 X lo** D/m*, the sample was heated at - 20 K/s to temperatures > 1900 K by resistive heating. Temperature measurement was made by optical pyrometry, with pyrometers in the 350-470 K and 470-1870 K ranges. Limited viewing access prevented both pyrometers from being used together. For some of the runs, the ion beam remained on as the temperature was raised to simulate conditions in a reactor. The released deuterium was
monitored via quadrupole mass spectrometer residual gas analysis (QMS-RGA). The D, (M/e = 4) signal, along with the low temperature pyrometer readings are shown in Fig. 2. During Dl bombardment, as soon as the sample temperature is observed to rise, D, desorption is observed; see Fig. 2b. In the absence of D: irradiation, no such effect was observed; see Fig. 2a. The total D, desorption profiles, as a function of temperature, are presented in Fig. 3. In Fig. 3, the steady-state reemission signal has been subtracted from the Dl beam-on profile, to make the differences between the two cases (i.e., D: on and off) more clear. Also, in the beam-on case, the D, signal at high temperature drops below the level of the signal observed before heating due to the emission of D” atoms [l], which are pumped by the chamber walls and thus are not recorded by the mass spectrometer in the RGA mode. In the absence of the D: beam, the deuterium release begins at - 800 K, consistent with previous TDS results. Due to the close proximity of the quadrupole ionizer to the sample ( _ 5 cm), the sample ambient temperature was typically N 400 K before heating. The fact that the total amounts of D, released for the two cases for T < 1400 K (i.e., the areas under the desorption profiles in Fig. 3) are similar indicates that the released deuterium for T < 1400 K is predominantly in the form of molecules, and not atoms or directly ejected energetic species, which would be pumped by the walls and not be observed. Thus the release mechanism must involve detrapping, subsequent recombination of deuterium atoms and emission of molecules. This release mechanism is consistent with models of hydrogen transport in graphite [4-7] which consider ion-induced detrapping as a local process within the graphite.
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-3115(94)00349-l
J. W. Dauis, AA. Haasz /Journal
The immediate release of deuterium from graphite upon heating while under Dl bombardment, can in fact be predicted if one assumes that a dynamic equilibrium is established between ion-induced detrapping, retrapping and molecular release as the temperature is being raised. Data for hydrogen retained in graphite as a function of temperature show that even for T < 300 K graphite is not fully saturated, but may trap more hydrogen as the temperature is lowered, see Fig. 4 [8-121. The rate at which deuterium is released will depend on the temperature ramping rate, the ion flux, and the effective detrapping cross section; the faster
a 5 zJ
3 keV Da+ (1023Dlm~)--, HPG99 (300K)
. ‘I ‘:’ :i D2 ‘i .*.
10 6
1
Ds+on D3+off
\; a
400
600
600
I 1000 1200 1400 1600 1600 2000
TEMPERATURE/(K)
Fig. 3. Comparison of TDS profiles as a function of temperature for the beam-on and beam-off cases. The total amounts of released D, for T < 1400 K are similar for the two cases.
14 12
201
of Nuclear Materials 217 (1994) 206-208
$6 z 8
4
x
2 0 1
400
600
600
1000 1200 1400 1600 1600 2000
TEMPERATURE/(K)
Fig. 1. Thermal desorption of D”, D, and CD, from pyrolytic graphite as measured by line-of-sight quadrupole mass spectrometry [l].
300
500
700
900
1100
1300
I
15100
TEMPERATURE/(K)
Fig. 4. Retained amounts of deuterium and hydrogen in graphite as a function of temperature during loading, taken from various sources [8-121.
500
25
100
b) D3+on ailthetime 460 s
the temperature rise, the larger the flux required to establish dynamic equilibrium. It is not known whether the present beam flux of N 3 X 1019 D/m* s is sufficient to produce this equilibrium or whether a larger desorption could be achieved with a larger flux. In a tokamak, fluxes would be much higher and faster temperature increases might be kept in equilibrium.
25 a) D3+off
Acknowledgements 160 Y 460 ii z 640 2 ,,, B 420 F
0’
-10
0
10
20
30
40
2.
400
50
This work was supported by the Canadian Fusion Fuels Technology project and the Natural Sciences and Engineering Research Council of Canada. We acknowledge useful comments from Drs. G.M. McCracken, P. Franzen and C.S. Pitcher.
References
TIME/(s)
Fig. 2. D, thermal desorption signals (QMS-RGA) and low temperature pyrometer signals for (a) normal TDS with the D: beam off, and (b) TDS with D; beam on.
111J.W. Davis and A.A. Haasz, Reemission and thermal desorption of D”, D, and CD, from graphite, to be published.
J. W Daviq A.A. Haasz /Journal
208 [2] C.S.
[3] [4] [5] [6] [7]
of Nuclear Materials 217 (1994) 206-208
Pitcher, R.V. Budny, K.W. Hill, S.J. Kilpatrick, D.M. Manos, S.S. Medley and A.T. Ramsey, J. Nucl. Mater. 176&177 (1990) 285. A.A. Haasz and J.W. Davis, Nucl. Instr. and Meth. B83 (1993) 117. W. Mijller and B.M.U. Scherzer, J. Appl. Phys. 64 (1988) 4860. K. Morita and Y. Hasebe, J. Nucl. Mater. 176&177 (1990) 213. D.K. Brice, Nucl. Instr. and Meth. B44 (1990) 302. A.A. Haasz, P. Franzen, J.W. Davis, S. Chiu and C.S.
Pitcher, Two region model for hydrogen trapping in and release from graphite, submitted, 1994. 181J. Roth, unpublished data in Ref. [4]. [9] W. Miiller, P. Borgensen and B.M.U. Scherzer, Nucl. Instr. and Meth. B19/20 (1987) 826. IlO] M. Braun and B. Emmoth, J. Nucl. Mater. 128&129 (1984) 657. [II] B.L. Doyle, W.R. Wampler and D.K. Brice, J. Nucl. Mater. 1038~104(1981) 513. [12] J.W. Davis, University of Toronto, unpublished data, 1991.