Quantification of Hydrogen-Helium Retention in Tungsten Using Laser Induced Breakdown Spectroscopy Coupled with Laser Ablation Mass Spectrometry

Tungsten (W) has been selected for the ITER divertor because of its high melting temperature, low sputtering yield, and high thermal conductivity. During deuterium/tritium (D/T) plasma exposure in ITER, a large flux (10²⁴m^-2s^-1) of relatively low-energy (100 eV) of D/T plus helium (He) will strike the divertor. The resulting plasma surface interactions (PSI) will lead to surface damage and deformation such as bubble formation, surface blistering and/or erosion, and tritium retention. Experiments have shown that the formation of helium bubbles can have a direct effect on hydrogen retention, although the extent is not fully known. This dissertation has developed and demonstrated two new, complementary laser-based characterization techniques (LIBS and LAMS) for assessing gas concentrations in nuclear materials as a function of spatial position (depth below the surface), with an emphasis on assessing the He-H interaction synergies in tungsten that are expected to impact tritium retention in the ITER divertor and future fusion reactors. In the newly established ultrahigh vacuum setup, the LIBS capability is coupled with the ability to simultaneously pump the ablated gases into a quadrupole mass spectrometer in an existing thermal desorption system (TDS) to simultaneously (although with a small time delay) measure the ion current of the detected gas species in a QMS, this capability we define as LAMS. The gas fluxes measured in LAMS are converted to an absolute quantity of measured gas per laser ablation pulse through calibration with known leak volumes in the TDS. Results of gas concentration as a function of depth in tungsten are shown following exposure to various fluences and plasma configurations, as well as compared to other surface gas evaluation techniques. Altogether this dissertation provides significant new results that: demonstrate the ability of LIBS and LAMS to perform depth dependent gaseous species concentration measurements in nuclear materials; offer new data that comprehensively reflects the complex He-H synergistic interactions and the role of He on tritium retention expected in the ITER tungsten divertor; and provide depth dependent concentration measurements (in addition to integrated retention values) for validation of multiscale models.

Portions of this document were previously published in journal Applied Surface Science. Shaw, G., Bannister, M., Biewer, T. M., Martin, M. Z., Meyer, F., & Wirth, B. D. (2018). The detection of He in tungsten following ion implantation by laser-induced breakdown spectroscopy. Applied Surface Science, 427, 695-703.