Low cost, mechanically refrigerated diffusion pump baffle for ultrahigh vacuum chambers

Abstract

In the course of constructing an ultrahigh vacuum apparatus employed in molecular beam scattering from crystal surfaces, we have designed, tested, and now continuously employ a simple diffusion pump cryobaffle that is mechanically refrigerated to approximately −50 °C. Modifications were made to a commercial water baffle for this purpose. After baking the chamber at 140–150 °C for three or four days, we routinely achieve a base pressure of 1–2×10−10 Torr. Relative to a conventional liquid nitrogen trap, the cost and maintenance of the cryobaffle are minimal. The surprising feature of the baffle is its usefulness in helping generate such a high vacuum, despite the relatively high temperature of −50 °C. As might be expected, the baffle operates by eliminating backstreaming of the diffusion and mechanical pump fluids. However, water vapor is not cryopumped; rather, it passes through the baffle and is removed by the diffusion pump.