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Backscattering of H and He from W and WO3
Journal of Nuclear Materials 79 (1979) 420-422 0 North-Holland Publishing Company
LETTER TO THE EDITORS - LETTRE AUK REDACTEURS BACKSCATTERING
OF H AND He FROM W AND WOs
A knowledge of the reflection coefficients (RN), energy distributions and charge fractions of low energy (5-20 keV) light ions backscattered from solid surfaces is of considerable interest in connection with the recycling of plasma particles from the walls of plasma machines in nuclear fusion research. Most of the relevant materials have already been studied experimentally [ 1,2] and by computer simulation [3]. In a plasma machine there is a continuous ion implantation into the walls which leads to the modification of the wall material. This modification gives rise to changes in the energy distributions and reflection coefficients. For instance, it has been shown [2] that when titanium was implanted with large doses of hydrogen or deuterium, the reflection coefficients were reduced by about 20% and a change occurred in the energy distributions of reflected particles. This reduction is thought to be due to an increase in the electronic stopping power after hydrogen implantation. In this paper, we compare the reflection from W and WOa with the purpose of studying the effect of light atom admixture in a heavy metal in continuation of our earlier work [2]. The experimental method is described earlier [2]. A mass analyzed ion beam of energies between 5 to 10 keV impinges normally on the target. The backscattered ions and neutrals are analyzed at a scattering angle of 0 = 135’ by an electrostatic cyclindrical spectrometer. The neutrals are ionized by stripping in a nitrogen gas cell, the efficiency of which has been determined separately. W targets are taken from 99.9% metal sheeting which are mechanically polished. One of them was oxidised anodically to a thickness of z 1000 A. All measurements have been carried out at room temperature. The pressure in the target chamber was 1 X lo-’ Pa during the measurement of charged particles and rose to 4 X lo-’ Pa during the neutral particle analysis. Figs. la-c show the typical energy spectra of all
particles (neutrals + ions) backscattered from W and WOs for 5 keV H+, 10 keV H+ and 10 keV He+ ions respectively. It is clear from the figures that much fewer particles are reflected from WOa as compared to W and that the shapes of the spectra are quite different. Between the high energy edge and the maxima, the distributions are convex for W and concave for WOa in the case of both 5 and 10 keV H+ ions. The curvature in the energy spectrum from W has flattened out in the case of 10 keV l-l+ ions compared to the other two cases. The particle reflection coefficients (RN), i.e. the ratio of all reflected to incident particles can be calculated by integrating the energy spectra of all particles over the whole half space assuming a cosine distribution. These values are listed in table 1. It is observed that RN is significantly lower for WOs compared to that for W. The difference between the RN values for W and WOs is larger for 10 keV beams compared to that for 5 keV beams. The most probable reason for this is: An oxide layer was also present on the W target which results in a lower reflection from the surface as compared to that expected from a clean target. As the incident energy decreases, the mean penetration depth decreases and the contribution of the surface region on the reflection coefficient increases. Therefore, the differences in RN for W and WOa are larger for 10 keV than for 5 keV beams. The shapes of the WOs spectra are in qualitative agreement with those for targets with low Z measured [2] and calculated [3] previously, It appears that the RN values for WOa can be obtained merely by scaling according to a mean Z. In table 1 we have included RN values for stainless steel, the Z of which is close to the mean Z of WOs. Although they are in good agreement, we cannot suggest this to be a reliable guideline for deducingRN for compound targets until some more experiments are performed. As discussed in ref. [2], the systematic uncertainties in the 420
R.S. Bhattacharya et al. / Backscattering of H and He J?vm Wand WO3 5 keV
H
---+
W
10 keV
wo3
421 H +---*
3
z
4
ENERGY
ENERGY
(keV)
10 keV
He--+
to
IkeVJ
W wo,
A
EN E RGY
Fig. 1. Energy d~tributions of particles backscattered 10 keV H+ and (c) 10 keV He+.
tkc’ff
from W and WO3 for (a) 5 KeV H+ (measmed with 10 keV H$ ions), (b)
absolute RN values are -20% for H and -30% for He. But this does not effect the differences in RN for W and WOs. It is interesting to note that the spectra do not show any edge corresponding to the energy loss in the surface oxygen of WOs. This can, in principle, be due to the contribution of multiple collision in the high energy edge of the spectra as well as due to the large
difference in the cross sections between W and 0. While bombar~g SiaN4 we have, however, observed an edge corresponding to N which was rather broad because of multiple collisions. The absence of 0 edge in the WOs spectra seems, therefore, to be the result mainly of large difference of cross sections between WandO.
R.S. 3h~ttucka~u et ul. / B~~k~~tte~~
422
Acknowledgement
Table 1
Energy Type of (kW
Target
RN
RN
(present) (other results)
projec-
tie 5 5 10 10
5 5 10 10
of H and He fromW nnd W03
H H H H He He He He
W wo3
W wo3
W wo3
W wo3
0.17
0.13 0.13 0.014 0.17 0.13 0.14 0.077
Received 10 November 1978
0.17 ref. 121 0.13 (SS) ref. il j 0.12 ref. [2] 0.07 (SS) ref. [I] 0.18 ref. [2] 0.16 -
This experiment resulted from a discussion with J. B&tiger (Aarhus) who also supplied the WOS target.
References [l] W. Eckstein, F.E.P. Matschke and H. Verbeek, J. Nucl. Mat. 63 (1976) 199. [2] W. Eckstein and H. Verbeek, J. Nucl. Mat. 76 and 77 (1978) 365. [ 31 O.S. Oen and M.T. Robinson, Nucl. instr. and Methods 132 (1976) 647;and J. Nucl. Mat. 76 and 77 (1978) 370.
R.S. Bhattacharya, W. Eckstein and H. Verbeek Max-PIanck-Inst~tut fir Pla~aFhy~k, Association EuratomIPP*O-8046 Garciting/Miinchen, Fed. Rep. Germany
LETTER TO THE EDITORS - LETTRE AUK REDACTEURS BACKSCATTERING
OF H AND He FROM W AND WOs
A knowledge of the reflection coefficients (RN), energy distributions and charge fractions of low energy (5-20 keV) light ions backscattered from solid surfaces is of considerable interest in connection with the recycling of plasma particles from the walls of plasma machines in nuclear fusion research. Most of the relevant materials have already been studied experimentally [ 1,2] and by computer simulation [3]. In a plasma machine there is a continuous ion implantation into the walls which leads to the modification of the wall material. This modification gives rise to changes in the energy distributions and reflection coefficients. For instance, it has been shown [2] that when titanium was implanted with large doses of hydrogen or deuterium, the reflection coefficients were reduced by about 20% and a change occurred in the energy distributions of reflected particles. This reduction is thought to be due to an increase in the electronic stopping power after hydrogen implantation. In this paper, we compare the reflection from W and WOa with the purpose of studying the effect of light atom admixture in a heavy metal in continuation of our earlier work [2]. The experimental method is described earlier [2]. A mass analyzed ion beam of energies between 5 to 10 keV impinges normally on the target. The backscattered ions and neutrals are analyzed at a scattering angle of 0 = 135’ by an electrostatic cyclindrical spectrometer. The neutrals are ionized by stripping in a nitrogen gas cell, the efficiency of which has been determined separately. W targets are taken from 99.9% metal sheeting which are mechanically polished. One of them was oxidised anodically to a thickness of z 1000 A. All measurements have been carried out at room temperature. The pressure in the target chamber was 1 X lo-’ Pa during the measurement of charged particles and rose to 4 X lo-’ Pa during the neutral particle analysis. Figs. la-c show the typical energy spectra of all
particles (neutrals + ions) backscattered from W and WOs for 5 keV H+, 10 keV H+ and 10 keV He+ ions respectively. It is clear from the figures that much fewer particles are reflected from WOa as compared to W and that the shapes of the spectra are quite different. Between the high energy edge and the maxima, the distributions are convex for W and concave for WOa in the case of both 5 and 10 keV H+ ions. The curvature in the energy spectrum from W has flattened out in the case of 10 keV l-l+ ions compared to the other two cases. The particle reflection coefficients (RN), i.e. the ratio of all reflected to incident particles can be calculated by integrating the energy spectra of all particles over the whole half space assuming a cosine distribution. These values are listed in table 1. It is observed that RN is significantly lower for WOs compared to that for W. The difference between the RN values for W and WOs is larger for 10 keV beams compared to that for 5 keV beams. The most probable reason for this is: An oxide layer was also present on the W target which results in a lower reflection from the surface as compared to that expected from a clean target. As the incident energy decreases, the mean penetration depth decreases and the contribution of the surface region on the reflection coefficient increases. Therefore, the differences in RN for W and WOa are larger for 10 keV than for 5 keV beams. The shapes of the WOs spectra are in qualitative agreement with those for targets with low Z measured [2] and calculated [3] previously, It appears that the RN values for WOa can be obtained merely by scaling according to a mean Z. In table 1 we have included RN values for stainless steel, the Z of which is close to the mean Z of WOs. Although they are in good agreement, we cannot suggest this to be a reliable guideline for deducingRN for compound targets until some more experiments are performed. As discussed in ref. [2], the systematic uncertainties in the 420
R.S. Bhattacharya et al. / Backscattering of H and He J?vm Wand WO3 5 keV
H
---+
W
10 keV
wo3
421 H +---*
3
z
4
ENERGY
ENERGY
(keV)
10 keV
He--+
to
IkeVJ
W wo,
A
EN E RGY
Fig. 1. Energy d~tributions of particles backscattered 10 keV H+ and (c) 10 keV He+.
tkc’ff
from W and WO3 for (a) 5 KeV H+ (measmed with 10 keV H$ ions), (b)
absolute RN values are -20% for H and -30% for He. But this does not effect the differences in RN for W and WOs. It is interesting to note that the spectra do not show any edge corresponding to the energy loss in the surface oxygen of WOs. This can, in principle, be due to the contribution of multiple collision in the high energy edge of the spectra as well as due to the large
difference in the cross sections between W and 0. While bombar~g SiaN4 we have, however, observed an edge corresponding to N which was rather broad because of multiple collisions. The absence of 0 edge in the WOs spectra seems, therefore, to be the result mainly of large difference of cross sections between WandO.
R.S. 3h~ttucka~u et ul. / B~~k~~tte~~
422
Acknowledgement
Table 1
Energy Type of (kW
Target
RN
RN
(present) (other results)
projec-
tie 5 5 10 10
5 5 10 10
of H and He fromW nnd W03
H H H H He He He He
W wo3
W wo3
W wo3
W wo3
0.17
0.13 0.13 0.014 0.17 0.13 0.14 0.077
Received 10 November 1978
0.17 ref. 121 0.13 (SS) ref. il j 0.12 ref. [2] 0.07 (SS) ref. [I] 0.18 ref. [2] 0.16 -
This experiment resulted from a discussion with J. B&tiger (Aarhus) who also supplied the WOS target.
References [l] W. Eckstein, F.E.P. Matschke and H. Verbeek, J. Nucl. Mat. 63 (1976) 199. [2] W. Eckstein and H. Verbeek, J. Nucl. Mat. 76 and 77 (1978) 365. [ 31 O.S. Oen and M.T. Robinson, Nucl. instr. and Methods 132 (1976) 647;and J. Nucl. Mat. 76 and 77 (1978) 370.
R.S. Bhattacharya, W. Eckstein and H. Verbeek Max-PIanck-Inst~tut fir Pla~aFhy~k, Association EuratomIPP*O-8046 Garciting/Miinchen, Fed. Rep. Germany