Inflow angular dependence of the capture coefficient in cryopumps

This study investigated the inflow angle and capture coefficient є using numerical simulation, and then validated the inferred relation by experimentation. The inflow angular dependent capture coefficient є(θ) of a cylindrical cryopump used to create a high-vacuum environment for electric propulsion tests was calculated from a three-dimensional particle trajectory simulation. Results show that є(θ) is 0.25–0.83 as a function of the inflow angle of particles to the pump surface. A total capture coefficient є_total is obtained by integration of the є(θ) multiplied by the flux distribution of particle to the pump surface. For the thermal inflow flux, є_total is calculated as 0.55 for the cylindrical cryopump alone and 0.70 for a cylindrical cryopump with a duct for a gate valve. The analytically estimated є_total was evaluated by pressure distribution measurements taken in a φ2.0 × 3.0 m vacuum chamber evacuated by two cylindrical cryopumps. The numerically simulated pressure shows good agreement with the measured pressure within ±6%. Further investigation of the improvement of the є_total when the particles flow directly to a pump demonstrates that є_total increases 16% when particles flow directly to a pump.