PROPUESTA DE REFORMA

propagation of the bending lags behind the zone of detachment, vibration of detached part of a film are excited.

( )

I

h E₂

1 1

2 2

≅ −

≅λ π

λ ⁽⁸⁾

These vibration results in the rupture of the film, when the energy of vibration, equal to about Wp, is sufficient for rupture along the perimeter of the separated film part, Wp > Ws⋅h/λ. We assume that the relationship (8) determines the maximal flake size, λ¹. Hence, it follows that λ¹ ∞ T^-1/2, and the maximal flake size is reduced with temperature rise. It was really observed at transition from C-C composite target to combined C-C composite + W target (higher radiation of shielding plasma with W ions), and at transition to higher pulse power in MK-200 facility. The particle acceleration in shielding plasma with a pressure gradient, ∆P, is equal:

dv/ dt = ∆Ɋ/ρ (9)

For an estimation, let us assume that ∆P ≅ P/l, where l is the shielding plasma width, and P=n⋅T1 (T1 is the ion temperature in the plasma). Hence, one obtains the particle velocity by the end of the pulse ( τ = 5 ɯ 10^-5 s) v ≈ 10 cm/ s. Thus, the particles are able to move from the surface during the pulse to distance, v⋅t ≈ 5 µm, only. It means that the electron temperature near the target surface determines particles surviving. For longer pulses, as it is expected under plasma disruptions, all other parameters being equal, the particles fly through the shielding plasma for the time, t = 10^-3 s, attain the velocity, v ≈ 10³ cm/s and move the distance ~ 0.5 µm. Increase in the power and duration of the pulse can also result in an increase in minimal particle size and in a reduction of the maximal size, and, at a rather high parameter of shielding plasma, in the complete disappearance of surviving graphite particles.

Thus, the main results of erosion products investigation for graphite, W, C-C composite are:

1. Micron and sub-micron size particles, balls and flakes were observed to be removed from graphite surface under both MKT and MK-200 plasma pulses.

2. Electron diffraction shows the graphite crystal structure of the particles.

3. Joined C-C composite and W target increases graphite particle emission as compared with C-C composite target.

4. Increase of deuterium plasma flow or shielding plasma radiation results in an increase in minimal collected particle size and in decrease maximal flake size.

5. Theoretical estimation shows that at high enough power density of deuterium plasma flux and at long pulse all the graphite particles can be exploded in shielding plasma.

5. SURFACE MODIFICATION AND EROSION OF CARBON BASED MATERIALS

For imitation of the expected ITER-operating conditions the samples of C-C composite were initially exposed to high doses in a stationary deuterium plasma, then irradiated by the pulsed high power D plasma and then again exposed in a stationary deuterium plasma.

The Russian composite C-C composite, UAM-92-5D, was taken for investigation. This C-C composite contains 0.15% by wieght Ti. Samples of RGT-type graphite were used in experiments.

The exposition in steady state deuterium plasma was performed in the LENTA facility with a beam plasma discharge at irradiation temperatures of 1000⁰C for C-C composite and 770⁰ C or 1150⁰ C for RGT-type graphite. The energy of deuterium ions was 200 eV. The current densities to the target were equal to 2.4⋅10¹⁷ ion/cm²s and 5⋅10¹⁷ ion/cm²s for C-C composite and RGT-type graphite, correspondingly. The doses of deuterium ions were 5⋅ 10²¹ ion/cm^-2 and ⋅10²² ion/cm², correspondingly. The erosion rate was measured by weight loss method. Several measurements were performed and the average yield was taken as a result.

Later the samples were irradiated in the MKT- plasma accelerator at the deuterium plasma density of about 10¹⁵ cm^-3 and at the maximal ion energy of 1–2 keV. The energy flux density was equal to 0.25 MJ m^-2. The pulse duration was 60 µs, the number of pulses was equal to six.

5.1. C-C composite erosion and surface modification

Sputtering yield of C-C composite by 200 eV deuterium ions measured by weight loss after exposition in LENTA facility is 0.5 atom/ion. The temperature of C-C composite at exposition was in the range of radiation enhanced sublimation, therefore the measured sputtering yield is higher than that for physical sputtering, calculated to be 0.0144 atom/ion [7].

The SEM pictures of various parts C-C composite surface after exposition in steady state plasma are shown in the Figure 13 (a) and (b). One can see rough eroded surfaces in each case.

The Figures 14 (a) and (b) show pictures of various parts of theC-C composite surface after exposition in steady state plasma and followed irradiation in MKT plasma accelerator.

Droplets and rather large melted pieces are seen on each part of the surface. Between the melted parts one can see the surface which was not melted. The melted parts can be identified as titanium carbide, which has a lower melting point than graphite. The thickness of the melted parts is not higher than ~ 1µm.

The Figures 15 (a) and (b) show the SEM pictures of various parts of theC-C composite surface after exposition in steady state plasma, then irradiation in MKT plasma accelerator and then again exposition in steady state plasma. The area of the melted surface decreases as compared to Figures 14 (a, b), but nevertheless some melted droplets are seen on the surface.

The surface between the melted parts is like that after steady state plasma exposition.

a b

Figure 13. SEM photographs of the various parts C-C composite surface after irradiation by steady state (200 eV, 5·10²⁵ m^-2, 1000 C) deuterium plasma.

The calculated thickness of the sputtered layer at steady state plasma exposition is ~ 100 µm. It is much higher than the thickness of the layer modified by irradiation in the MKT plasma accelerator. Therefore the irradiation in the plasma accelerator does not influence sputtering in a steady state plasma. The melted droplets of titanium carbide have lower sputtering yield and part of them remain on the surface even after removing the much thicker layer of pure graphite. In ITER the dose at the graphite part of the divertor during one pulse will be even higher then in our experiment and the disruption will not affect sputtering during the normal regime operation.

a b

Figure 14. SEM photographs of the various parts C-C composite surface after irradiation by steady state (200 eV, 5·10²⁵ m^-2, 1000 C) plasma and followed irradiation by pulsed (0.25 MJ/m² per pulse, 60 µs, 6 pulses) deuterium plasma.

a b

Figure 15. SEM photographs of the various parts C-C composite surface after irradiation by steady state (200 eV, 5·10²⁵ m^-2, 1000⁰ C ) plasma, followed irradiation by pulsed (0.25 MJ/m² per pulse, 60 µs, 6 pulses) deuterium plasma and then again irradiation by steady state (200 eV, 5·10²⁵ m^-2, 1000⁰ C ) plasma

Thus, main results of C-C composite investigation are:

1. The exposition of C-C composite in steady state plasma with parameters close to that in ITER divertor shows all the known features of graphite erosion: physical sputtering, chemical sputtering and radiation enhanced sublimation.

2. The sputtering yield of C-C composite by deuterium ions with the energy 200 eV at the temperature 1000⁰ C is 0.5 atom/ion.

3. Disruption simulation in the MKT plasma accelerator results in erosion of about 0.1 µm layer and in the appearance of a melted layer on part of the C-C composite surface. The melted materials is identified as titanium carbide. The thickness of the melted layer is ~ 1µm.

4. The exposition of C-C composite in steady state plasma after plasma accelerator irradiation gives the same sputtering yield as before plasma accelerator irradiation if the sputtered layer exceeds the thickness of the layer modified by plasma accelerator irradiation.

5. Taking into account normal operation pulse duration of 400 s, one can expect sputtered layer thickness much higher than depth of the layer modified at disruption and hence neglect the influence of disruption on C-C composite sputtering during the normal operation.

5.2. RGT-type graphite erosion and surface modification

A relief characteristic for the growth of whiskers is developed upon the RGT graphite surface sputtered by extra high deuterium ion doses. Figure 16a illustrates a characteristic structure of the RGT graphite surface after its bombardment by 200 eV D⁺- ions at 770⁰C.

The condition for the emergence of whiskers is the presence of an impurity in the material under sputtering. In this case, it is related with different sputtering yields for graphite together with the particles of TiC present in it. The size of the produced whiskers is about 10 µm. Slits are seen between whiskers. The formation of such a developed structure upon the graphite surface is a result of interaction among the processes of sputtering, redeposition and surface diffusion. Relief of the surface is brush-like.

The subsequent effect of six high power deuterium plasma pulses results in an inessential change in the surface structure (Figure 17a). Upon the surface the configurations known as “balls with the legs” are produced. From the comparison of the surface structures, which have been developed under the influence of steady state (Figure 16a) and successive steady state and pulsed plasma (Figure 17a), we can conclude that the surface formed by deuterium ions with high dose sputtering is thermally stable. The developed structure is considerably different than that formed after irradiation only by the pulsed plasma flux [2].

Figure 16b illustrates the surface structures of the graphite sample irradiated by deuterium steady state plasma at the temperature 1150⁰C. The surface microstructure differs from that which is being formed upon the surface of the same graphite exposed in the LENTA deuterium plasma at the temperature of 770⁰ C, all other conditions being equal. Such an effect is provided by different erosion processes at different temperatures of the target:

physical sputtering takes place at 770⁰C and the radiation-enhanced sublimation of graphite takes place at 1150⁰ C. As a result of an effect by high deuterium ion fluence on the graphite surface, the periodic structure with a honey-comb morphology of a terrace-type was produced. These terrace-type structures are being developed across the whole surface of the grain. The size of terraces varied in the range from 0.5 to 2.5 µm .

Along with the formation of a recurring structure, somewhere on the surface one can see separate pieces of a “husk” which are probably produced as a result of the sublimed graphite back-diffusion and of its condensation upon the target.

After the joint effect of steady state and pulsed plasmas the surface topography is essentially changed (Figure 17b). The periodic relief produced after an effect of the steady state plasma is destroyed. One can see the traces of chipping and brittle destruction.

Thus, the graphite erosion during disruption is determined by the temperature of preliminary steady state plasma irradiation. At the temperature when the physical sputtering takes place the brush-like structure is developed on the surface. This structure is stable under thermal shock. At the temperature T > 1000 C when the radiation enhanced sublimation takes place the periodic terrace-type structure with lentil-like grains appears after steady state plasma exposure. Most of the lentil-like grains are weakly bounded one with another and can be easily removed under disruption.

a b

Figure 16. SEM photographs of the RGT graphite surface after irradiation by steady state (200 eV, 10²⁶ m^-2) deuterium plasma at 770⁰C (a) and 1150⁰C (b).

a b

Figure 17. SEM photographs of the RGT graphite surface: (a) - after irradiation by steady state (200 eV, 10²⁶ m^-2, 770⁰C ) + pulsed (0.25 MJ/m² per pulse, 60 µs, 6 pulses) deuterium plasmas; (b) - after irradiation by steady state (200 eV, 10²⁶ m^-2, 1150⁰C ) + pulsed (0.25 MJ/m² per pulse, 60 µs, 6 pulses) deuterium plasmas.

6. DEUTERIUM BEHAVIOUR IN TUNGSTEN AND GRAPHITE AT COMBINED