Speaker
Description
To achieve higher acceleration efficiency, carbon foils will be utilized to strip the pre-accelerated ions from iLinac and then the stripped ions will be injected into the Booster Ring (BRing) at the High Intensity heavy ion Accelera-tor Facility (HIAF) [1]. Considering the impacts of high energy deposition of heavy ions and extremely high beam intensity, the foils face severe lifetime challenges. Previous studies identified two types of radiation resistant multi-layer graphite (MLG) foils, those prepared by the methods of vacuum-assisted self-assembly (VASA) and graphitization of polyimide (PI), through irradiation experiments conducted at the Sector-Focused Cyclotron (SFC) of the Heavy Ion Research Facility in Lanzhou (HIRFL), but did not determine their lifetime limits [2]. In this study, lifetime measurement experiments were carried out using high-intensity Xe26+ and Bi35+ beams with varying beam duty cycles at the Low Energy High Intensity Heavy Ion Accelerator Facility (LEAF). Real-time monitoring of foil appearance, effective thickness, and stripped beam intensity was achieved by employing charge-coupled device (CCD) imaging, post-stripping charge-state distribution spectrum analysis, and time-of-flight measurements. Monitoring results reveal the failure mechanism of MLG foils originates from an elastic-to-plastic deformation transition, during which radial wrinkles radiating from the bombardment center cause cracking, increasing effective thickness and reducing the beam intensity after stripping. Moreover, the estimations of foil lifetimes based on the theory proposed by Lebedev [3], using measured elongation at break and lateral lattice size, aligned closely with experimental results, validating this estimation approach for guiding the lifetime assessments and process improvements of MLG foils under HIAF conditions. Consistent with previous studies, the experiments verified that the determinants of foil lifetime shift from radiation damage to evaporation effects at an average target temperature of approximately 2500 K. Finally, characterization results from X-ray diffraction (XRD) and Raman spectroscopy indicate that despite exhibiting macroscopic failure and reaching stripping performance limits, the MLG foils retain their fundamental internal microstructures and a high degree of graphitization.