Flux threshold measurements of nano-fuzz formation by He-ion beam impact on hot tungsten surfaces

We report measurements of the energy dependence of flux thresholds and incubation fluences for He-ion induced nano-fuzz formation on hot tungsten surfaces using real-time sample imaging of tungsten target emissivity change together with accurate ion-beam flux-profile measurements. The measurements were carried out at the Multicharged Ion Research Facility (MIRF) at ion energies from 218 eV to 8.5 keV, using a high-flux deceleration module and beam flux monitor for optimizing the decel optics on the low energy MIRF beamline. The measurements suggest that nano-fuzz formation proceeds only if a critical rate of change of trapped He density in the W target is exceeded. The energy dependence of three factors contributing to the overall energy dependence, ion reflection, ion range and target damage creation, were determined using the SRIM simulation code. The observed energy dependence can be well reproduced by the combined energy dependences of these three factors. The incubation fluences deduced from first visual appearance of surface emissivity change were 2-4x10$^{23}$/m$^{2}$ at 218 eV, and roughly a factor of 10 less at the higher energies, which were all at or above the displacement energy threshold. Additional measurements at 100 and 200 keV, using beams from the MIRF HV-platform-based ECR source will be presented.