Trapping and reflection coefficients for deuterium in graphite at low energy and oblique incidence

Nuclear Instruments and Methods in Physics Research B 85 (1994) 560-565 North-Holland

NIOMI B

Beam Interactions with Materials 6i Atoms

Trapping and reflection coefficients for deuterium in graphite at low energy and oblique incidence M. Mayer *, B.M.U. Scherzer, W. Eckstein Max-Planck-Institut fiir Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching /Miinchen,

Germany

Trapping and particle reflection coefficients for 33-500 eV D on graphite have been measured for angles of incidence 0” I (YI 75” by determining the area1 density of implanted D as a function of the implanted fluence using the D(3He, p)(~ nuclear reaction. The experimental data are compared to computer calculations with the TRIMSP-program. The particle reflection coefficients obtained from a smooth and a rough surface are compared. Surface roughness accounts for a strong decrease of particle reflection at oblique incidence and low energies.

1. Introduction

Data on particle reflection coefficients of plasma particles (H, D, T, He) with energies between several eV and several keV at first wall materials in thermonuclear fusion devices are required for the assessment of the contribution of particle reflection in the particle recycling process [l]. The particle bombardment of walls and divertor plates in present days machines takes place predominantly at energies of some 100 eV and below [2]. Data on particle reflection are available for many light ion-target combinations at normal incidence [3-61, but for oblique incidence experimental data are very scarce [5]. A major problem in evaluating low energy particle reflection is the assessment of the influence of surface roughness. Due to the low penetration depth of the particles at low energies (about 25 A for 100 eV deuterium on carbon) a strong influence of surface roughness on the reflection coefficients can be expected. Computer simulation using fractal surfaces as a model for a rough surface predict a decrease of the reflection coefficients up to a factor of 2 for oblique incidence [7]. In previous studies [8,9] it has been shown that the particle reflection coefficient R, can be determined by measuring the particle trapping coefficient 17 in such particle-target combinations in which thermal diffusion of the implanted species is negligible. This condition is fulfilled for hydrogen in carbon and many carbides up to 500°C [lO,ll].

* Corresponding author, phone +49 89 3299 2619. fax +49 89 3299 2591.

In the present work we have chosen the combination deuterium-graphite. With the experimental setup absolute measurements can be made. The measurements were performed in the energy range 30 eV to 500 eV at angles of incidence 0” I (YI 75” for a smooth and a rough target. The experimental results are compared with computer simulation using the TRIM.SPprogram.

2. Experimental The experimental setup is shown in Fig. 1. The target is implanted with Dz-ions from a colutron ion source through an implantation diaphragm of 1.5 mm diameter. D:-ions are chosen because the particle flux of this ion species is about 100 times higher than the flux of D+-ions. The ions are extracted with an energy of 2 keV from the ion source, magnetically massanalysed and decelerated to the desired energy with a deceleration lens [12]. The area1 density of the trapped deuterium is measured by the D(3He, p)cw nuclear reaction by bombarding the center of the target spot with 650 keV 3He through a 1 mm diameter diaphragm. The protons from the nuclear reaction are counted with a wide angle detector covered with a 0.1 mm SS-foil to restrain backscattered particles. The detector is located at 135” and has a solid angle of 0.11 sr 52.7% as determined with a calibrated target. The analyzing current was kept below 15 nA, yielding a maximum power density of 1.2 W/cm2 at the sample, to prevent local heating of the target surface and thermal release of implanted hydrogen. In order to obtain a homogeneous implantation fluence the beam is swept over the implantation di-

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M. Mayer et al. / Nucl. Instr. and Meth. in Phys. Rex B 85 (1994) 3X-565

aphragm in horizontal and vertical directions by means of two pairs of electrostatic deflection plates located between the last deceleration stage of the deceleration lens and the target. The beam sweep was adjusted so that on each side at maximum deflection the beam current on the target was reduced to zero. By this method the average beam current is reduced by a factor of more than 10. The uniformity of implantation over the beam spot was found to be better than 5% using a 3He beam collimated through a 0.1 mm diameter diaphragm. The energy spread of the Dl beam is about 3 eV [13] and the angular spread due to the beam sweep is 2”. However, the major contribution to the energy and angular spread accounts from the dissociation of the molecular ions. The experimentally observed energy spread obtained for 240 eV Dl was about 20 eV [13]. An energy spread of the same order is expected for the present experiments. The target materials are a fine grain graphite (EK98 from Ringsdorff) and highly oriented pyrolytic graphite (HOPG) from Le Carbone-Lorraine. EK98 is a polycrystalline graphite with a density of 1.85 g/cm3 and metal impurities below 1 ppm each. The target surface was mechanically polished with diamond paste with decreasing grain size down to 1.0 pm. The mean surface roughness was determined with a profilometer and was about 1000 A. HOPG is a pseudo single crystal with preferential orientation along the c-axis and a mosaik-spread below 0.5”. The HOPG surface was prepared by stripping with adhesive tape. The HOPG-surface is smooth on atomic scale over areas of some 1000 k [14]. The basic pressure in the target chamber was 5 X lo-’ mbar. The oxygen coverage of the HOPG surface was measured using RBS and was between 1 X 1014 to 5 x 1014 oxygen atoms/cm2. This is most likely due to a water coverage of the surface between 0.1 to 0.5 monolayer. No other impurities were detected.

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tial, energy loss model and surface binding energy as given above were used.

4. Results and discussion

Area1 densities of trapped D-atoms as a function of the incident fluence are shown in Fig. 2 for HOPG for primary energies E, = 50, 100 and 500 eV, respectively, and in Fig. 3 for EK98 for primary energies E, = 33, 50, 100 and 500 eV, respectively. For each E, a set of curves is shown for angles of incidence 0” I (Y I 70”. Solid lines are drawn to guide the eyes. The initial linear part of these curves has been used to determine n (the trapping coefficient) with linear regression. Between 7 and 12 data points in the linear part of each curve were measured. At larger fluences the trapping curve levels off, because the implanted D reaches local saturation. Deuterium implanted in graphite at room temperature saturates at a local concentration of 0.4 D-atoms/C-atom [20]. Up to the saturation concentration the implanted deuterium does not diffuse, but after reaching saturation any additionally implanted deuterium is reemitted as D,. For inci-

650 keV 3He+ t ionsource 2keVD; magnet

eiuzel lens

3. Computer simulation A vectorized version, TRVMC, of the Monte Carlo program TRIM.SP was used to calculate reflection coefficients. The program uses an amorphous target structure and is based on the binary collision approximation [15]. The Kr-C interaction potential [16], which is a good mean potential for many ion-target combinations [15,17], was used. The inelastic energy loss was taken as an equipartition of the nonlocal LindhardScharff [18] and the local Oen-Robinson [19] models. A surface binding energy for hydrogen of 1 eV was assumed. For simulation of rough surfaces the program VFTRIM [7] was applied. The same interaction poten-

decekration lens movable . .

0.1 mm S-foil

Faraday cup

Fig. 1. Experimental setup. IX. IBA THEORY

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dent energies below 100 eV also the release of deuterium through the formation of CD, has to be taken into account. This effect, known as chemical sputtering, gives a chemical erosion yield of 0.02 CD,-molecules per incident D-ion for particle energies below 100 eV [21]. Therefore 8% of the deuterium leaves the target as methane. Like the formation of D,, the formation of methane starts when the target reaches local saturation [22] and therefore does not affect the linear part of the trapping curves at low fluences. The particle reflection coefficient R, is obtained from

The measured values of R, for HOPG are plotted in Fig. 4 as a function of the angle of incidence. Lines result from computer simulation using the TRIM.SP program. The relative experimental error of n has been estimated to + 10%. The relative uncertainty of R, is therefore largest for small values of R,. As in a previous measurement [9], there is good agreement between measurement and calculation for E, = 500 eV. For 50 and 100 eV however, the experimental reflection coefficients are 25% higher than the calculated ones for 0” I (YI 70”. This may be due to an additional loss mechanism of deuterium in the linear

part of the trapping curves. Such mechanism could be thermal diffusion to the surface, sputtering or ion-induced desorption. (1) Thermal diffusion: Thermal diffusion coefficients of hydrogen in carbon at room temperature are not available. Taking the empirical formula of Causey [23], the diffusion coefficient is 8.5 x 1O-48 cm2/,s at room temperature. Diffusion perpendicular to the graphite planes is even lower. Saeki [24] denied diffusion of hydrogen perpendicular to the graphite planes even at a temperature of 1000 K. Therefore no loss of implanted deuterium caused by thermal diffusion is expected. (2) Sputtering: The self-sputtering yield of deuterium in carbon at saturation concentration (composition C,.,D,.,) for 100 eV is calculated to be 0.03 sputtered D-atoms per incident ion. For the low fluences used in the experiment the self-sputtering yield is lower due to the lower concentration of deuterium in the target. The loss of implanted deuterium caused by self-sputtering is therefore < 1%. The sputtering yield for 650 keV 3He is below 0.001 sputtered D-atoms per incident ion and therefore negligible. (3) Zon-induced desorption: Ion-induced desorption caused by the analyzing beam of 790 keV 3He was observed in earlier experiments [20] for carbon targets

E

Incident

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Fluence (1015D-ions/cm3

I:~isi 0 E b

Incident

Fluence (10” D-ions/cm2)

E h

10

20

30

Incident Fluence (10” D-ions/cm’)

Fig. 2. Area1 densities of trapped D implanted in HOPG at 50, 100 and 500 eV, respectively, at angles of incidence 0”~ CY I 70” as a function of incident fluence. Lines are drawn to guide the eye.

M. Mayer et al. / Nucl. instr. and Meth. in Phys. Res. B 85 (I 994) 560-565

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Incident Fluence (10’” D-ions/cm’)

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Fig. 3. Areal densities of trapped D implanted in EK98 at 33, 50, 100 and 500 eV, respectively, at angles of incidence 0” i (YI 70” as a function of incident fluence. Lines are drawn to guide the eye.

to saturation with 1 keV deuterium. A decrease of the area1 density of implanted deuterium of 10% was observed for a fluence of 1 X 1Or7 3He/cm2 in pyrolytic graphite. The maximum analyzing fluence used in the present e~e~ments was 3 X 101’ Heions/cm’. For the present measurements, the targets are far from saturation, thus reducing the release of hydrogen additionally because of the larger availability of vacant trapping sites. The influence of ion-induced implanted

D -> HOPG

Angie of Incidence a (deg)

Fig. 4. Particle retlection coefficients for HOPG as functions of the angle of incidence for 50, 100 and 500 eV incident energy. Lines represent calculated values.

desorption for 1 keV D by the analyzing beam therefore would be < 1%. For low energy implantation (E, < 1 keV), the influence of ion-induced desorption is unknown but may be higher due to the smaller distance to the surface. Desorption of already implanted deuterium by the implantation beam was observed in [25,26]. Carbon targets preimplanted with 300 eV D until saturation were bombarded with H or He ions with energies between 3 and 30 keV. For a bombardment with 1 X 10” H-ions at 3 keV 30% of implanted deuterium were reIeased [ZS]. For the low fluences used in the present experiments the loss of implanted deuterium due to this mechanism therefore would be < 1%. For the low energies used here this may be higher, as mentioned above. Since no data on ion-induced desorption for low energy implantation are available, a loss of deuterium from the target cannot be excluded completely. Water coverage of the carbon surface, as mentioned in the experimental section, reduces the particle reflection coefficients of deuterium, mainly at low energies. For 100 eV incident deuterium and one monolayer of water (about 1 x lo*’ water molecules/cm2) on carbon the calculated reduction of the particle reflection coefficients is about 15%. For the water coverages present IX. IBA THEORY

M. Mayer et al. / Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 560-565

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in this experiment, the coverage of the carbon surface with water accounts for a decrease of the experimental values between 2% and 8%. Due to the angular spread caused by the dissociation of the molecular ions, channeling in HOPG is assumed to be negligible. The influence of the interaction potential used in the computer simulation was studied extensively in [27] for the bombardment of silicon by silicon. The particle reflection coefficients showed a strong dependence up to a factor of 2 of the potential, mainly for low energies. The interaction potential and the inelastic energy loss model used in the present TRIM.SP calculations turned out to be a good approximation in previous calculations of particle reflection coefficients and sputtering yields [9,15,28]. For the low energy range the nuclear energy loss exceeds the electronic energy loss (for 100 eV deuterium in carbon the ratio of nuclear to electronic energy loss is about 2.6: 1). Therefore the inelastic energy loss model has only minor influence on the computed results. The measured values of Ii, for EK98 are plotted in Fig. 5 as a function of the angle of incidence. Lines represent the computer simulation using a smooth surface. For normal incidence no difference is observed between the reflection coefficients from HOPG and EK98 in the limit of the experimental error. For incident energies Eb I 100 eV and CYI 30” the reflection coefficients are about 25% higher than the calculated values. For oblique incidence the reflection coefficients are significantly lower than the calculated values obtained for a smooth surface. The discrepancy is most predominant for the lowest energies. The decrease of reflection coefficients from a rough surface at oblique incidence has several reasons: (1) On a rough surface the angle of incidence of the ions differs locally from that on a flat surface. Espe-

D .> EK98 z 3 g t ” I 3 x 5 ti 0 3 iz * PI

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Angle OFIncidence a (deg) Fig. 6. Comparison of particle reflection coefficients for EK98 with calculation simulating surface roughness with a fractal surface. The fractal dimension is 2.05. Symbols with error bars are experimental values. The experimental data are the same as in Fig. 5.

cially at grazing incidence the mean local angle of incidence is much smaller, For smaller angles of incidence the reflection coefficient decreases. (2) For oblique incidence the angular distribution of the reflected particles is peaked into the forward direction. Particles being reflected in a first collision with the surface can hit the surface a second time. This can lower the reflection coefficient even in those cases where the initial local angle of incidence is higher than that on a flat surface. (3) Only surface roughness with an amplitude in the range of or higher than the penetration depth of the particles does affect particle reflection noticeable. The influence of surface roughness is therefore smaller for higher energies. This leads to the lower deviation of the angular dependence of the reflection coefficient for 500 eV in Fig. 5 and explains the good agreement of experimentally measured reflection coefficients at rough surfaces with calculations using a smooth surface for particle energies in the keV-range [9]. Fig. 6 shows a comparison between the experimentally determined reflection coefficients and computer simulation including surface roughness for 33, 50 and 100 eV, respectively. The surface roughness was simulated by fractals using the program VFTRIM [7]. The fractal dimension of the surface used for the calculation was 2.05, which corresponds to a medium rough surface. The experimental values and the simulation show good agreement for oblique incidence. For normal incidence the experimental values are higher than the calculated values, as mentioned before.

80

Angle of Incidence a (deg)

Fig. 5. Particle reflection coefficients for EK98 as functions of the angle of incidence for 33, 50, 100 and 500 eV incident energy. Data points for 50 eV are shifted to the Ieft and data points for 100 eV are shifted to the right for better visibility. Lines represent calculated values.

5. Conclusions Absolute trapping and particle reflection coefficients of D in graphite have been measured in the energy range 33 eV I E, I 500 eV and angles of inci-

M. Mayer et al. / Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 560-565

dence 0” I Q I 75” for a smooth (HOPG) and a rough sample (EK98). The experimentally determined particle reflection coefficients agree well with calculations for 500 eV for HOPG. For incident energies. I 100 eV the experimental values lie 25% above the calculated values. This is not yet fully understood. Particle reflection from a rough surface is not affected by surface roughness for normal incidence. Surface roughness accounts for a strong decrease of reflection at oblique incidence. This effect is predominant at low energies. Surface roughness can be simulated using fractal surfaces. With a fractal dimension of 2.05 good agreement between experiment and calculation is obtained for oblique incidence.

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