Development of Rotating Target with Ferrofluid Seal for ILC Electron-Driven Positron Source

In the ILC electron-driven positron source, acceleration tubes are located immediately downstream of the positron generation target to capture the generated positrons, necessitating high vacuum for their operation. We designed a rotating target using a ferrofluid rotating vacuum seal that can stably maintain a vacuum of $10^{-6}$ Pa and a tangential velocity of 5 m/sec, which meets the requirements of the ILC positron source. We fabricated a full-scale prototype and confirmed the compatibility of rotation and stable vacuum maintenance. Various tests on the radiation resistance of the ferrofluid demonstrated its ability to withstand six years of radiation exposure in normal ILC operation without degradation and to continue exhibiting vacuum sealing performance. Radiation exposure tests on a small-scale target with a structure and material similar to the full-scale target confirmed its operability after two years of radiation exposure in normal ILC operation. The design, including thermal load, stress analysis, and cooling, led to the conclusion of an estimated lifespan of the target of over one year, based on comparison with the SLC target. These results demonstrate and establish the technology of positron generation targets in the ILC electron-driven positron source.