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Contribution Oral

X-ray structural analysis

Dynamical observation of X-ray Laue diffraction on single-crystal tungsten during pulsed heat load

Speakers

  • Dr. Aleksey ARAKCHEEV

Primary authors

Co-authors

Content

The normal operation of fusion reactor based on tokamak involves periodical transient heat loads to divertor plates. The currently the divertor plates of ITER is supposed to be covered by tungsten. The tungsten tends to cracking as a result of the pulsed heating. The reasons of the crack formation are deformation and mechanical stresses caused by the raise of temperature of thin surface layer. The diagnostic of the dynamic of the deformations and stresses in tungsten under pulsed heat load is under developments at the VEPP-4M beamline 8 (scattering station “Plasma”). The observation of the material deformation is based on the change in the X-ray diffraction scattering angle as a result of the non-uniform material extension. Previous experiments demonstrated measurements of residual stress and measurements of the depth-average deformation dynamics. Currently the scattering station “Plasma” was upgraded. Namely, a the energy of the Nd:YAG laser pulsed was increased from 1J to 50J. The pulse duration remains ~140µs. The number of frames of the one-dimensional gas X-ray detector DIMEX was increased from 30 to 100. And the intensity of the synchrotron radiation with energy 69keV (K-edge of tungsten) increased with a factor about 50 due to the new wiggler. Also a fast pyrometer was developed to measure the temporal dependence of the temperature of the heated surface. The combination of the improvements resulted in the measurements of the diffraction peak shape dynamics during the heating of the 500µm thick single crystal tungsten. The change in scattering angle was measured to be about 2°. The three stages of the diffraction peak evolution can be clearly distinguished here: the heating of the surface, the equalizing of the heat distribution transversely to surface and cooling to room temperature. During the first stage approximately from 0µs and to 140µs (the heating of the surface) the surface temperature increases. The scattering angle of X-ray grows coupled with the temperature. Consequently the large angle side of the diffraction peak moves to the large angles. The equalizing of the temperature distribution transversely to the surface occurs at the second stage (approximately from 140µs and to 240µs. The equalizing of the temperature means that the cooler layers warm up and hotter layers cool down. Consequently the change in scattering angle is positive at the lesser angles and is negative at the larger angles. As a result the diffraction peak becomes narrower. The long term evolution during the third stage (cooling to the room temperature) was not measured. However, the final state of the diffraction peak shape was measured after several seconds. Note that the initial and final stats are static. So the diffractograms were measured significantly more accurately. According to the relation between temperature and scattering angle the latter decreases after cooling. The difference between initial and final positions of diffraction peaks means that the plastic deformation was happened.