Study of heavy ion beam induced damage in tungsten for high power target applications

The spallation material at ESS is pure tungsten, which is cooled by gaseous helium flow. To study the behaviour of tungsten under dynamic beam conditions at ESS, pure tungsten specimens have been irradiated at the M3-beamline of the UNILAC facility at GSI Helmholtz Centre for Heavy Ion Research. Tungsten specimens of two thicknesses, 26 μm and 3 mm, were exposed to pulsed uranium and gold ion beams for fluences up to 7.5 · 10¹³ ions·cm₋₂ at 4.8 MeV/nucleon. Nanoindentation tests were performed on the cross section of the irradiated 3 mm sample, and microhardness was measured on the top surface. The measured data are compared with the calculated damage values, and a correlation between the radiation induced damage and the observed mechanical property is presented. Thermal diffusivities of foil samples irradiated up to four different fluences were measured with a Laser Flash Apparatus (LFA). The observed changes in the mechanical and thermal properties of irradiated tungsten were used to estimate the changes of operational temperature and mechanical stresses in the ESS target material with the progress of radiation damage, using coupled thermal and mechanical simulations. From the pulsed beam induced dynamic oscillations of thin tungsten specimens, information on fatigue properties of tungsten under irradiation was drawn. In addition to pure tungsten, oxidised tungsten samples were irradiated. This is to investigate the stability of the adhesive oxide layer under pulsed beam conditions, which would be formed due to oxygen impurities in the helium cooling loop. The irradiated oxide scale was examined using Auger Electron Spectroscopy (AES) and Scanning Electron Microscopy (SEM).