High-pressure-ratio vacuum sealing by a perturbed free-vortex aerodynamic window

In this study, we investigated a perturbed free-vortex operating concept for enhancing the sealing pressure ratio phigh/plow of a free-vortex aerodynamic window by intentionally shifting the flow away from the conventional matched free-vortex condition. A compact free-vortex aerodynamic-window assembly with a 15-mm-diameter open beam passage was designed, fabricated, and tested using air under low-gas-flow-rate conditions while keeping the supply-plenum pressure below atmospheric pressure. Under the conventional matched condition, the pressure ratio was limited to about 5 at phigh = 9.1 kPa, whereas perturbed free-vortex operation increased it to about 70 and, in a lower-flow configuration with stronger low-pressure-side evacuation, to about 200. The obtained pressure ratios, the response to diffuser-geometry changes, and the internal pressure distributions showed good agreement with computational fluid dynamics simulations, suggesting that the performance enhancement was accompanied by reorganization of the internal flow structure. Measurements with Ar and He and a design-map evaluation of two free-vortex aerodynamic windows connected in series further indicated the extendability of the concept to other gases and multistage configurations, with an estimated overall pressure ratio of about 4550 for the two-stage configuration. These results show that, in a low-gas-flow-rate regime while keeping the supply-plenum pressure below atmospheric pressure, perturbed free-vortex operation can realize a high sealing pressure ratio that cannot be obtained under the conventional matched condition. This approach is promising for accelerator gas targets, gas strippers, and other high-pressure-ratio vacuum-sealing applications.