The experiments were done in the plasma accelerator MKT, at a 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 300 kJ/m2, pulse duration was 60 µs long. After 10 pulses, the total energy flux density to the target was at the level of 3 MJ/m2. For studying a plasma flux energy intensity effect on the erosion product morphology, some experiments were done in the MK-200 plasma accelerator at the energy flux density within the range 10–15 MJ/m2 per pulse at the pulse duration equal to 50 µs. Similar to the experiments in the MKT-accelerator, the number of discharges was equal to 10. As a result the energy flux intensity was (2–3) x 105 MW/m2, 30–50 times above the value in the experiments with the MKT-accelerator. Impurity-free graphite, MPG-8, was used in those experiments as a target, its morphological erosion product features under irradiation in the MKT-accelerator were studied previously in detail [2,14].
The samples of MPG-8 graphite, as well as tungsten and C-C composite, UAM-92-5-D trademark were used in the experiments.
The erosion, evaporation, and sublimation products were collected upon the plates of monocrystalline silicon and on the basalt filter located in a plasma flux shadow around the targets under exposure. In the MK-200 accelerator experiment the incident plasma interacted with a graphite lattice. In this case the conditions for the material erosion were, naturally, different from those which existed in the plasma interaction with a flat target. This scheme was chosen to protect the products of erosion and the collectors against the plasma flux effect.
In a given case, some fraction of the produced graphite particles passes through the lattice orifices and gets out of the plasma. Due to this circuit-diagram we managed to collect the graphite pieces produced under high (10–15 MJ/m2) fluxes.
For studying the erosion and sublimation products the techniques of transmission electron and scanning microscope, electron transmission diffraction, X ray spectral analysis
were used.
Studying the filter, some layers, including 2–3 fibres laying with the particles deposited on them, were traced with both the scanning microscope and with the transmission electron one. One should note that the erosion product morphology study with the transmission microscopy has a great advantage over the scanning one, since it gives an opportunity to study their nature by the electron transmission diffraction.
4.1. Results
4.1.1. Energy flux density effect on the erosion products sizes.
Some typical microphotographs of one of the basalt filter parts and of the MPG-8 graphite pieces captured on it after 10 deuterium plasma pulses, at energy flux density 0.25 MJ/cm2 per pulse in MKT accelerator, are given in Figure 7.
The particles and flakes of various shapes, from 0.01 µm to 40 µm in size, are registered on the filter. In Figure 8 the graphite erosion product size distribution is shown per unit size.
The size distribution has two maxima in the range 0.01–0.03 µm and 2–4 µm. The “tail” of the distribution is expanded up to 40 µm, that unambiguously confirms the brittle destruction of graphite.
a b
Figure 7. Microphotographs of a basalt filter with the erosion products of MPG-8 graphite after the irradiation by the pulsed plasma flux with the energy density 0.25 MJ/m2 per pulse, t
= 50µs, pulses number is 10, as flakes (a) and spheres (b).
Figure 8. Erosion product size distribution of the MPG-8 graphite per a size unit in the ranges: 0.01 µm≤ R1 ≤ 0.3 µm (a) and 1µm≤ R2 ≤ 40 µm (b) after an effect of the deuterium plasma pulse with the energy density 0.25 MJ/m2 per pulse, t= 50µs, in MKT accelerator.
0.0 0.1 0.2 0.3
d, m
0.0 0.5 1.0 1.5 2.0
N, 1 0 cm m
a
-2-19
0 10 20 30 40
d, m
0 100 200 300 400 500
N, cm m
b
-2-1
Figure 9 illustrates the electron microscope microphotographs of MPG-8 graphite erosion products deposited upon different parts of the basalt filter after 10 deuterium high power plasma pulses in the MK-200 accelerator. The erosion products are characterized by various shapes and by an essential difference in size. We managed to fix both large pieces (up to 4 µm) and sub-micron ones (> 0.25 µm) on the basalt fibers. Along with the graphite flakes of an irregular shape (Figure 9a) some graphite pieces of an almost spherical geometry are seen (Figure 9b).
a b
Figure 9. Electron-microscopic photographs of MPG-8 graphite erosion products after irradiation by the deuterium plasma in MK-200 (10–15 MJ/m2per pulse, t = 50µs,10 pulses).
Thus, in accordance with the results obtained in the MKT accelerator, confirming the presence of large flakes (up to 40 µm) and sub-micron particles (from 0.01 µm in size), in the experiment at MK-200 accelerator under an effect of the plasma fluxes of an essentially greater power, the micron (~ 4 µm) and sub-micron (>0.25 µm) particles were also registered.
4.1.2. Erosion products under simultaneous plasma effect on the W and C-C composite targets.
Studies of the samples with electron and scanning microscopes have allowed one to register the particles, < 0.1 µm up to 20 µm in size, upon the collectors after simultaneous irradiation of W and CFC. The basalt filter microphotograph with the captured laminated flakes, of uniform thickness, is shown in Figure 10a. According to the data of a transmission electron microscope, such flakes are about 1µm thick. On the photograph (Figure 10b) one can see, along with the places of laminated configuration, spherical drop phase particles, their diameters are in the range 0.1–1 µm.
The results of the spectral X ray analysis of round particles show that such particles are tungsten. The erosion products of irregular shape are pieces of graphite. Thus, elements W and C, belonging to both irradiated targets, were unambiguously registered among the erosion products.
The particle distribution in size for the case of simultaneous irradiation of W and CFC are represented in Figure 11.
1 µm 1 µm
a b
Figure 10. Microphotographs of W and CFC target erosion products collected on the basalt filter after simultaneous irradiation by the pulsed plasma flux (0.3 MJ/m2 per pulse, t = 50 µs,10 pulses).
Figure 11. Erosion product size distribution after an effect of the pulsed plasma flux with the energy density 0.3 MJ/m2 per pulse, t= 50µs, pulses number is 10 on the W+C-C composite target, on C-C composite target and that after the pulsed plasma flux with the energy density 0.45 MJ/m2 per pulse, t= 50µs, pulses number is 20 on W target.
0,01 0,1 1 10 100
1E-7 1E-6 1E-5 1E-4 1E-3 0,01 0,1 1 10 100 1000
ICFC = 5.5*107 cm-2, IW = 0.5*107 cm-2, ICFC+W = 17.5*107 cm-2.