Conclusiones desde lo analizado 1 Cambio de época

The NERD method was applied to the study of hydrogen isotopes concentration depth profiles in plasma-facing components of thermonuclear reactors [5]. The capability of the technique can be demonstrated on JET carbon specimens investigation. It was devoted to determination of hydrogen isotopes amount in the specimen. The experiments were performed assuming the homogenous distribution of hydrogen isotopes in the redeposited layer. The results are shown in Table 1.

TABLE I. HYDROGEN AND DEUTERIUM CONTENTS FOR JET SPECIMENS

Specimen CH,

10¹⁸cm^-2

CD, 10¹⁸cm^-2 1 8.8± 0.5 1.5 ± 0.2

2 9.3± 0.4 1.1 ± 0.1

5 0.7± 0.1 0.17 ± 0.03

2 (2) 1.9± 0.3 0.62± 0.08

3 (2) 7.2± 0.5 0.80 ± 0.11

4 (2) 6.5± 0.3 0.85 ± 0.09

5 (2) 8.7± 0.4 0.53 ± 0.07

control (C_D=0.6×10¹⁸cm)

- 0.75± 0.14

TiD1.97

(CD=26×10¹⁸cm^-2)

- 23±2

However, these data do not correspond to the data obtained by using ion methods.

Deuterium content in 100 mkm thick layer obtained with our technique only slightly exceeds one obtained by using of ion methods for the thickness of 1 mkm. This fact made us to refuse of our supposition of homogenous distribution and we had to resolve the problem of deep depth concentration profiling. We had tried to attain the deep depth distribution in these specimens by the use of computer simulation. A part of the calculations is shown in fig.2, hence it follows that neither uniform distribution of deuterium (fig.2a) nor its accumulation in 1 µm layer near surface (fig.2b) fits the experimental data. A linear slope of deuterium contents from maximum value in surface to 0 in 70 µm depth, fig 2c, is also unsatisfactory.

The best description was achieved for the profile shape shown in fig. 2e.The sensitivity of DRIN is very well from the comparison of fig.2e with fig.2d: one can see that a small Assumption of the identity of deuterium and protium distributions, fig. 3a, or the uniform hydrogen distribution, fig.3b, does not lead to satisfactory description of experimental data. A good fitting is obtained here for a hypothesis that the most amount of protium is concentrated in 50 µm layer near surface while the total analysable thickness of specimen is 120 µm (fig. 3c). The availability of hydrogen may be explained by absorption of a water steam in specimens after removal it from reactor.

Figure 2. The comparison of experimental deuteron spectrum for the carbon JET sample with spectra simulated by the DRIN for different assumption on the depth profile of concentration shapes.

The supposed shape of the deuterium concentration profile can be defined more precisely by using 2.5 MeV dD neutrons. Such experiment have been carried out. It confirmed validity of profile choice in fig.2e. Variant of the NERD method on dD neutrons (total flux 10⁸ n/s) allows to carry out the investigation of hydrogen distribution in thin (≤20 µm on carbon) hydrogen containing layers.

4.2 DIII-D specimens

Five different carbon specimens were investigated. Four of them had been previously exposed to plasma: 3 with boron coating and one carbon specimen without covering. The fifth boronized specimen had no contact with plasma. Two dimensional spectrum obtained for pure carbon specimen is shown at the right of figure 4. The left part of this figure shows the spectrum measured for boron coated graphite specimen taken from divertor of DIII-D tokamak. Visual analysis of these two spectra shows that the presence of boron atoms is displayed by the ¹⁰B - neutron interaction with protons, deuterons and tritons production. The background is practically absent in region of deuteron localisation on the two dimensional energy spectra and the presence of boron atom does not affect the D isotope determinations.

Figure 3. The comparison of experimental proton spectrum for the carbon JET sample with spectra simulated by the DRIN for different assumption on the depth profile of concentration.

Boron content is most simply determined from deuteron spectrum from reaction n+¹⁰B=d+⁹Be which leads to excitation of two levels of final nucleus ⁹Be: ground (right peak in fig. 5) and first excited with energy E*=2.43 MeV (left peak in fig. 5). The deuteron energy spectrum and boron concentration profile, which is restored by using of the right peak of this spectra, are shown in the figure. The "true" concentration profile of a boron restored by DRIN code is shown too. The concentration profile obtained by DRIN code for triton spectra is analogous (fig. 6). An agreement between the boron depth distributions restored from deuteron and triton energy spectra is shown in these figures.

After the boron concentration profile has been restored, we calculate the shape of proton energy spectrum for n+¹⁰B=p+¹⁰Be reaction by DRIN code. The concentration profile of hydrogen in a specimen is obtained by subtraction of calculated spectrum from an experimental one. The triton concentration profile can be obtained by a similar way.

Figure 4. Two-dimensional ∆E-E spectra from carbon sample coated with B4C layer (at the top) and from carbon (at the bottom)

Figure 5. Deuteron energy spectra taken from a carbon specimen coated B4C and its depth profiles both calculated by DRIN code and extracted from experiment.

Figure 6. Triton energy spectra taken from a carbon specimen coated B4C and its depth profiles both calculated by DRIN code and extracted from experiment.

TABLE II. HYDROGEN AND DEUTERIUM CONCENTRATION AND LAYER THICKNESS [6]

Specimen Concentration, H_×10¹⁸ñm^-2

Relative

concentration B B thickness, µm

Control < 0.3 1 ∼ 80

6410 1.4± .4 1.5 ± 0.1 ∼ 115

6420 2.5± 0.3 0.15 ± 0.03

-6440 < 0.3 1± 0.08 ∼ 80

6250 2.8± 0.3 0.79 ± 0.08 ∼ 80

4.3 RGT and tungsten specimens

We used the technique described above for the analysis of RGT specimens (from TRINITI) coated by boron carbide also. We expected a presence of hydrogen isotopes inside the specimen which was in touch with the plasma. The traces of plasma are clearly seen on the specimen surface. The front and back specimen sides were studied visually. The surface hydrogen only is found out for both sides of RGT-graphite specimen. However the hydrogen contents on the surface covered by B4C is nearly twice as low as on the reverse. The layer of a B4C coating has a thickness 115 ± 5 microns and the volume concentration of boron ~ 0.75 ± 0.05 relative to the control specimen. The probable reason of a change of boron concentration is the burn-out of a boron at some microplots on all area of plasma influence.

The tungsten specimens from RRC "Kurchatov Institute" irradiated by deuterium ions with the energy 200 eV. One of them (#02) has been irradiated by stationary plasma at the temperature 800 K. Another tungsten specimen (#4) has been additionally treated by the irradiation of pulsed plasma flux. The availability of hydrogen isotopes inside of material of tungsten specimens is not revealed. The surface content is a little bit higher for specimen which was subjected to pulse plasma processing then for another one (treated by stationary plasma flux). It may be explained by a change of adsorbing properties of a surface after pulse treating by plasma.

TABLE III. AMOUNT CONTENT OF HYDROGEN ISOTOPE AND BORON

Hydrogen Deuterium Boron Specimen CH, atH/cm² Thickness,

µm

CD, atD/cm² CB, cm^-3 Thickness, µm

Control (C coating B4C) 1 95± 5 RGT-graphite front 0.3×10²⁰ < 20 - 0.75± 0.05 115 ± 5

coating B4C back 0.6×10²⁰ <20 - -

W(N02) front 0.2×10²⁰ < 1⋅10¹⁷

-W (N4) front 0.4×10²⁰ < 1⋅10¹⁷

-back 0.5×10²⁰ - -

CONCLUSIONS

The NERD method combined with computer simulation model is fairly suitable for the simultaneous non-destructive determination of overall absolute content of hydrogen isotopes and boron as well as their deep depth concentration profile in plasma-facing components of tokamak. The detection limit values are ~10¹⁷ cm^-2 for H and ~ 10¹⁶ cm^-2 for other isotopes.

The proposed method can be used for the definition of the coating thickness without specimen destruction. The presented results should be considered as preliminary demonstration of the NERD method possibility which may be substantially expanded, if higher neutron flux intensity be used.

REFERENCES

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