- Email: [email protected]
On secondary electron emission from solid H2 and D2
Journal of Nuclear Materials 76 St 77 (1978) 0 North-Holland Publishing Company
634-635
ON SECONDARY ELECTRON EMISSION FROM SOLID Hz AND D2
H. SQRENSEN and J. SCHOU Association Euratom, Rise National Laboratory,
DK-4000 Roskilde, Denmark
The emission of secondary electrons from solid hydrogens (H,, Da, T,) is often considered to be of importance for the interaction between a fusion plasma and pellets of solid hydrogens. A set-up was therefore built for studies of interactions between energetic particles and solid hydrogens [ 11. Studies of secondary electron emission (SEE) from solid Ha and Ds were made for incidence of electrons up to 3 keV [2,3] and for incidence of ions of hydrogen, deuterium, and helium up to 10 keV [3]. The measurements were made for normal incidence, and in some cases also for oblique incidence. The SEE coefficient for solid Ha is always 0.65-0.70 times that for solid Da. This difference is attributed to different losses to vibrational states in Ha and Dz for the low energy electrons [3]. Measurements were also made on solid para-Ha with both electrons and hydrogen ions. There was no difference from the results for normal Hz, which consists of 25% para-Hz and 75% o&o-Ha. These two states are related to the rotational states of the molecules, and we thus see that the rotational states are of minor importance for secondary electron emission. Some of the measurements were made in the orig inal set-up, where only the current collected by the target could be measured, and some in a newer set-up where the beam could be deflected into a Faraday cup placed right below the target [4]. Hence the beam current could be measured independently of the state of the target surface. For incidence of electrons, the emitted electrons are normally split up into two groups, the true secondary electrons having energies of a few eV and the reflected electrons having energies up to the energy of incidence E; the distinction between these two groups groups is normally made at around 50 eV. For materials with very low atomic numbers, the electron reflection (ER) coefficient is small and unimportant for normal incidence, and it will increase strongly with increasing 634
angle of incidence. In the older set-up we could thus only measure at normal incidence. Measurements were made for incidence of 0.5-3 keV electrons in the old set-up and for l-3 keV electrons in the new set-up. For normal incidence, the SEE coefficient for solid Da is then 6 = (0.201 + 0.0736E - 0.01 18E2)/E.
(1)
Fig. 1 shows the data for oblique incidence of electrons. The data have been multiplied by E and by (cos r9)3’2. The points fall together on two curves, one for H2 and one for Da; the factor (cos 0)3n thus gives a good description of the angular variation of the SEE coefficient up to 60°C. Fig. 2 shows the data for the ER coefficient for H2 and D2 and also for Be. There is good qualitative agreement with the data for Be. MoreI
I 0.3
I
1
D2
z
I
iA
I
3
z 0.2 u x I
HZ
% 8 :
0.1
2
1 ENERGY
3
IkeV)
Fig. 1. Secondary electron emission coefficients for oblique incidence of electrons on solid Hz and D2. The SEE coefficients have been multiplied by the electron energy E and by (cos @J/2.
H. Sqkensen, J. Schou /Secondary electron emission from solid Hz and D2
635
ber of SE per incident atom were plotted as a function of the energy per atomic mass unit. For incidence on solid D2, we thus find 0.L
S = -2.786 + 2.587(1 + 0.1073E)‘”
(2)
in the energy interval 0.6-10 keV. For incidence of He’ ions on solid D2, we obtained S=0.414t0.0311E F
0.2
0.1
0.0 8
IDEGREES)
Fig. 2. Electron reflection coefficients for oblique incidence of l-3 keV electrons on solid H2 and D2. Also shown are the experimental results for Be at 3 keV of Bronshtein et al. [5,6], and the theoretical results for Be at 20 keV obtained by Reimer by means of Monte Carlo calculations [ 71.
over, the ER coefficient is negligible at normal incidence and sizeable at angles above 45”. With ions, measurements have only been made in the old set-up. H+, Hi, H:, and D; were used [3], later also He+. For the first four ions it was seen that the results were in mutual agreement when the num-
(3)
in the energy interval 4-10 keV. Measurements at oblique incidence were also attempted with hydrogen ions. Secondary ions were also emitted in these measurements and a correction for them was only possible in one case, namely for H+ + D2. The beam current could here be measured by using the target as a calorimeter and a correction was then possible. The angular variation corresponded to a (cos e)-’ dependence up to 75”.
References [l] [ 21 [ 31 [4]
H. Sbrensen, Appl. Phys. 9 (1976) 321. R. Sbrensen, Nucl. Instrum. Methods 132 (1976) 377. H. Sdrensen, J. Appl. Phys. 48 (1977) 2244. H. Sdrensen and J. Schou, to be published. [ 51 I.M. Bronshtein and S.S. Denisov, Sov. Phys. - Solid State 7 (1965) 1484 (Fiz. Tverd. Tela 7 (1965) 1846). [6] I.M. Bronshtein and V.A. Dolinin, Sov. Phys. - Solid State 9 (1968) 2133 (Fiz. Tverd. Tela 9 (1968) 2718). [ 71 L.M. Reimer, Optik 27 (1968) 86.
634-635
ON SECONDARY ELECTRON EMISSION FROM SOLID Hz AND D2
H. SQRENSEN and J. SCHOU Association Euratom, Rise National Laboratory,
DK-4000 Roskilde, Denmark
The emission of secondary electrons from solid hydrogens (H,, Da, T,) is often considered to be of importance for the interaction between a fusion plasma and pellets of solid hydrogens. A set-up was therefore built for studies of interactions between energetic particles and solid hydrogens [ 11. Studies of secondary electron emission (SEE) from solid Ha and Ds were made for incidence of electrons up to 3 keV [2,3] and for incidence of ions of hydrogen, deuterium, and helium up to 10 keV [3]. The measurements were made for normal incidence, and in some cases also for oblique incidence. The SEE coefficient for solid Ha is always 0.65-0.70 times that for solid Da. This difference is attributed to different losses to vibrational states in Ha and Dz for the low energy electrons [3]. Measurements were also made on solid para-Ha with both electrons and hydrogen ions. There was no difference from the results for normal Hz, which consists of 25% para-Hz and 75% o&o-Ha. These two states are related to the rotational states of the molecules, and we thus see that the rotational states are of minor importance for secondary electron emission. Some of the measurements were made in the orig inal set-up, where only the current collected by the target could be measured, and some in a newer set-up where the beam could be deflected into a Faraday cup placed right below the target [4]. Hence the beam current could be measured independently of the state of the target surface. For incidence of electrons, the emitted electrons are normally split up into two groups, the true secondary electrons having energies of a few eV and the reflected electrons having energies up to the energy of incidence E; the distinction between these two groups groups is normally made at around 50 eV. For materials with very low atomic numbers, the electron reflection (ER) coefficient is small and unimportant for normal incidence, and it will increase strongly with increasing 634
angle of incidence. In the older set-up we could thus only measure at normal incidence. Measurements were made for incidence of 0.5-3 keV electrons in the old set-up and for l-3 keV electrons in the new set-up. For normal incidence, the SEE coefficient for solid Da is then 6 = (0.201 + 0.0736E - 0.01 18E2)/E.
(1)
Fig. 1 shows the data for oblique incidence of electrons. The data have been multiplied by E and by (cos r9)3’2. The points fall together on two curves, one for H2 and one for Da; the factor (cos 0)3n thus gives a good description of the angular variation of the SEE coefficient up to 60°C. Fig. 2 shows the data for the ER coefficient for H2 and D2 and also for Be. There is good qualitative agreement with the data for Be. MoreI
I 0.3
I
1
D2
z
I
iA
I
3
z 0.2 u x I
HZ
% 8 :
0.1
2
1 ENERGY
3
IkeV)
Fig. 1. Secondary electron emission coefficients for oblique incidence of electrons on solid Hz and D2. The SEE coefficients have been multiplied by the electron energy E and by (cos @J/2.
H. Sqkensen, J. Schou /Secondary electron emission from solid Hz and D2
635
ber of SE per incident atom were plotted as a function of the energy per atomic mass unit. For incidence on solid D2, we thus find 0.L
S = -2.786 + 2.587(1 + 0.1073E)‘”
(2)
in the energy interval 0.6-10 keV. For incidence of He’ ions on solid D2, we obtained S=0.414t0.0311E F
0.2
0.1
0.0 8
IDEGREES)
Fig. 2. Electron reflection coefficients for oblique incidence of l-3 keV electrons on solid H2 and D2. Also shown are the experimental results for Be at 3 keV of Bronshtein et al. [5,6], and the theoretical results for Be at 20 keV obtained by Reimer by means of Monte Carlo calculations [ 71.
over, the ER coefficient is negligible at normal incidence and sizeable at angles above 45”. With ions, measurements have only been made in the old set-up. H+, Hi, H:, and D; were used [3], later also He+. For the first four ions it was seen that the results were in mutual agreement when the num-
(3)
in the energy interval 4-10 keV. Measurements at oblique incidence were also attempted with hydrogen ions. Secondary ions were also emitted in these measurements and a correction for them was only possible in one case, namely for H+ + D2. The beam current could here be measured by using the target as a calorimeter and a correction was then possible. The angular variation corresponded to a (cos e)-’ dependence up to 75”.
References [l] [ 21 [ 31 [4]
H. Sbrensen, Appl. Phys. 9 (1976) 321. R. Sbrensen, Nucl. Instrum. Methods 132 (1976) 377. H. Sdrensen, J. Appl. Phys. 48 (1977) 2244. H. Sdrensen and J. Schou, to be published. [ 51 I.M. Bronshtein and S.S. Denisov, Sov. Phys. - Solid State 7 (1965) 1484 (Fiz. Tverd. Tela 7 (1965) 1846). [6] I.M. Bronshtein and V.A. Dolinin, Sov. Phys. - Solid State 9 (1968) 2133 (Fiz. Tverd. Tela 9 (1968) 2718). [ 71 L.M. Reimer, Optik 27 (1968) 86.