Monte-Carlo simulation of vacuum system for advanced charge selector

To intercept unwanted charge states from stripped beams with higher power densities, an advanced charge selector is currently under development at the Facility for Rare Isotope Beams (FRIB). This upgraded charge selector is designed to intercept beam spots that have a power of up to 5 kW and an rms size as small as 0.7 mm × 1.25 mm. To enhance heat dissipation and mitigate thermal stress, rotating graphite wheels are employed as the beam-intercepting medium. An essential aspect of this design is the development of a robust vacuum system to ensure reliable and efficient operation while minimize beam losses in downstream sections. The high graphite temperature, maintained over 1000 °C for radiation damage annealing, raised concerns about gas load. To aid the vacuum system design, vacuum simulations were carried out using a Monte Carlo-based simulation code, MolFlow. The sublimation of graphite, outgassing from the vacuum chamber’s inner wall and the effect of pumping speed on vacuum performance are considered. The results demonstrate that the proposed vacuum system can maintain a pressure under 1×10⁻⁷ mbar, ensuring adequate vacuum conditions for beam operations.