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Reflection of low energy deuterium ions from W
Vacuum/volume 44jnumber Printed in Great Britain
Reflection U U Bandurko
S/pages
937 to 938/l
0042-207X/93$6.00+.00
993
@ 1993
of low energy
and U A Kurnaev,
Moscow
Engineering
deuterium Physics
Institute,
Pergamon
Press Ltd
ions from W
Moscow
7 15 409, Russia
Reflection coefficients RN of deuterium ions from W are measured at low ion energies (EO A IO-400 eV per deuteron) and at both normal and glancing angles So of ion incidence. At glancing 9,, the reflection coefficient close to unity in the range of 70-700 eV, but a small RN reduction with a decrease in energy is observed at E, < 30 eV per deuteron. The latter is thought to be related to the chemical interaction of deuterons with the surface.
is
3r
1. Introduction
hydrogen isotopic ions from solids at energies from several to several hundred electron volts is an important problem for evaluation of recycling processes in a thermonuclear reactor. Retlcction coefficients R, of 30P1200 eV D; ions from W at different angles of bombardment are investigated in this work. The experiments arc performed on a mass separator with an ion retardation system. Angles of ion incidcncc 3,, vary from 0 to 75 Elcctropolished and vacuum-annealed W (99.9%) targets are used. Refcction
of
2. Experimental
T,
Two cxpcrimental procedures are used in this work for R, measurement. The first one is based on the measurement of a dcuterium pressure jump in a small volume round the target during switch-on of the ion beam. The registration of the pressure changes is performed in the ‘dynamic’ regime where the rate of gas input due to ion reflection is proportional to the gas pressure increase above the base equilibrium pressure. The absolute R, values are obtained using a calibration proccdure at high ion fluences, when z 100% of incoming ions rc-emit into the vacuum. so the net partial pressure increase corresponds to the number of primary particles. To prevent dcuterium trapping on Ni walls, surrounding the target, the wall is kept at z 500 K. This procedure was shown to give reasonable results at energies about 1 kcV. At ion energies as low as lOPlOO eV per deutcron the ion tlux is too small to ensure the quantitativc registration of the partial pressure jump due to reflection. Therefore an altcrnutivc procedure of R, measurement is used. The trapping coefficient ~7 is measured and the reflection cocficient is calculated as R, = I -PI.
500
450
400
procedures
K
Figure I. Deuteriuln thermodesorption
spectra from W after D; ion implication at 300 K, fluence C/I,,= 3.4 x IOIJ cm ‘. (I) 30 eV D:, 3,, = 60 : (2) 90 eV DI . 3,, = 60 ; and (3) 90 eV D: . S,,= 75 3. Results Typical trapping glancing reflection
thermodesorption spectra are shown in Figure I. The probability at an energy of 3U eV per deuteron and a angle of 75 is seen to be very small. Therefore, the coefficient at these parameters may be taken as R, 1 1 though the error (- 15%) in these measurements is rather high. The energy dependences in the reflection coefficient for different angles of incidence are shown in Figure 2. At glancing, 9,, = 60 and 75 incidence one may see that R, is close to unity over all
(1)
This procedure is valid only if all non-reflected ions are trapped in the target and no rc-emission from the target is observed. Thcrcforc. these experiments are performed at low ion fluences where expression ( I) still applies. The number of trapped particles representing the quantitative characteristics of ‘1 is measured using thcrmodesorption after ion bombardment together with calibration at high fluences. In all experiments the partial pressure of HD and Dz molecules is monitored to find the number of dcuterium atoms in the volume around the target.
0.6
0.4
t
1 0
I
I
I
I
20
40
60
80
I 100
E, (eV)
Figure 2. Reflection coefficient from W as a function of ion energy per deuteron (E,,) at different angles of incidence 3,,: 0’ (O), 60’ (O), 75 (A). The solid line represents calculations upon formula (2) for 9,) = 0 , the broken lines are those for 9, = 60 937
V V Bandurko
938
and VA
Kurnaev
Low
energy
deuterlum
Reflection U U Bandurko
S/pages
937 to 938/l
0042-207X/93$6.00+.00
993
@ 1993
of low energy
and U A Kurnaev,
Moscow
Engineering
deuterium Physics
Institute,
Pergamon
Press Ltd
ions from W
Moscow
7 15 409, Russia
Reflection coefficients RN of deuterium ions from W are measured at low ion energies (EO A IO-400 eV per deuteron) and at both normal and glancing angles So of ion incidence. At glancing 9,, the reflection coefficient close to unity in the range of 70-700 eV, but a small RN reduction with a decrease in energy is observed at E, < 30 eV per deuteron. The latter is thought to be related to the chemical interaction of deuterons with the surface.
is
3r
1. Introduction
hydrogen isotopic ions from solids at energies from several to several hundred electron volts is an important problem for evaluation of recycling processes in a thermonuclear reactor. Retlcction coefficients R, of 30P1200 eV D; ions from W at different angles of bombardment are investigated in this work. The experiments arc performed on a mass separator with an ion retardation system. Angles of ion incidcncc 3,, vary from 0 to 75 Elcctropolished and vacuum-annealed W (99.9%) targets are used. Refcction
of
2. Experimental
T,
Two cxpcrimental procedures are used in this work for R, measurement. The first one is based on the measurement of a dcuterium pressure jump in a small volume round the target during switch-on of the ion beam. The registration of the pressure changes is performed in the ‘dynamic’ regime where the rate of gas input due to ion reflection is proportional to the gas pressure increase above the base equilibrium pressure. The absolute R, values are obtained using a calibration proccdure at high ion fluences, when z 100% of incoming ions rc-emit into the vacuum. so the net partial pressure increase corresponds to the number of primary particles. To prevent dcuterium trapping on Ni walls, surrounding the target, the wall is kept at z 500 K. This procedure was shown to give reasonable results at energies about 1 kcV. At ion energies as low as lOPlOO eV per deutcron the ion tlux is too small to ensure the quantitativc registration of the partial pressure jump due to reflection. Therefore an altcrnutivc procedure of R, measurement is used. The trapping coefficient ~7 is measured and the reflection cocficient is calculated as R, = I -PI.
500
450
400
procedures
K
Figure I. Deuteriuln thermodesorption
spectra from W after D; ion implication at 300 K, fluence C/I,,= 3.4 x IOIJ cm ‘. (I) 30 eV D:, 3,, = 60 : (2) 90 eV DI . 3,, = 60 ; and (3) 90 eV D: . S,,= 75 3. Results Typical trapping glancing reflection
thermodesorption spectra are shown in Figure I. The probability at an energy of 3U eV per deuteron and a angle of 75 is seen to be very small. Therefore, the coefficient at these parameters may be taken as R, 1 1 though the error (- 15%) in these measurements is rather high. The energy dependences in the reflection coefficient for different angles of incidence are shown in Figure 2. At glancing, 9,, = 60 and 75 incidence one may see that R, is close to unity over all
(1)
This procedure is valid only if all non-reflected ions are trapped in the target and no rc-emission from the target is observed. Thcrcforc. these experiments are performed at low ion fluences where expression ( I) still applies. The number of trapped particles representing the quantitative characteristics of ‘1 is measured using thcrmodesorption after ion bombardment together with calibration at high fluences. In all experiments the partial pressure of HD and Dz molecules is monitored to find the number of dcuterium atoms in the volume around the target.
0.6
0.4
t
1 0
I
I
I
I
20
40
60
80
I 100
E, (eV)
Figure 2. Reflection coefficient from W as a function of ion energy per deuteron (E,,) at different angles of incidence 3,,: 0’ (O), 60’ (O), 75 (A). The solid line represents calculations upon formula (2) for 9,) = 0 , the broken lines are those for 9, = 60 937
V V Bandurko
938
and VA
Kurnaev
Low
energy
deuterlum