Abstract: Understanding how ion beam modes influence irradiation responses is essential for accurately simulating neutron damage. In this study, in-situ 30 keV He⁺ irradiations were performed on high-purity Fe using both defocused and scanning beams at varying frequencies to investigate their effects on dislocation loop evolution. Compared to defocused beams, scanning beam irradiation produced larger loops with lower number densities and reduced overall damage. This effect grows more pronounced at higher scanning frequencies. However, the scanning effect gradually diminishes when the frequency exceeds a threshold value. The intermittent beam-off periods act as annealing phases. They facilitate defect recombination and disrupt the establishment of a high nucleation rate. In contrast, high frequencies result in increasingly short beam-off periods that are insufficient for long-range defect migration, thus hindering effective defect recovery. These findings reveal significant differences between defocused and scanning beam irradiation results, and fill the knowledge gap regarding the trend of the scanning effect at high frequency. Moreover, the results suggest that high-frequency scanning improves consistency with defocused beam experiments when simulating steady-state irradiation systems.

Key words: In-situ TEM irradiation, Radiation effect, Scanning beam irradiation, Rastering beam irradiation, Dislocation loop