Abstract
There is an increasing need to systematically investigate the performance of commercial Cryo–TMP [in-line Cryogenic Water Pump (CWP) + Turbo Molecular Pump (TMP)] combination pumps. In this study, the factors affecting the pumping speed of water in a Cryo–TMP, such as the number of cold rings, dimensions, shape, etc., are systematically analyzed by Monte Carlo simulation. We constructed a test dome and a Cryo–TMP500 based on the simulation results to measure the strict effective pumping performance of the Cryo–TMP. In addition, we established a systematic measurement method for pumping performance using the Cryo–TMP500 in three experimental modes: TMP-only mode, CWP-off mode, and CWP-on mode. The results show that the water pumping speed of the Cryo–TMP500 with only one cold ring is ∼76% of that of a standalone cryopump (∼30 000 L/s) with the same opening inlet diameter; this speed is 22 550 L/s, which is in good agreement with the simulation result of 23 276 L/s. The pumping speeds for non-condensable gases (H2, N2) are 8%–19% higher than expected.
Highlights
When air is evacuated from a vacuum chamber with atmospheric pressure using a vacuum pumping system, the pressure rapidly falls in the initial stage of volumetric pumping
Cryogenic water pump (CWP) + turbo molecular pump (TMP) multi-pump systems have become more widely used as they maximize TMP pumping performance for normal gases while improving the water pumping speed
The Cryo–TMP500 is constructed based on the simulation results; it consists of an in-line cryogenic water pump (CWP) with d1 = 400 mm, L = 160 mm, a 6 mm-thick cold ring, and a TMP (X4304LMF TMP, Shimadzu, Japan) with an inlet diameter of 350 mm
Summary
When air is evacuated from a vacuum chamber with atmospheric pressure using a vacuum pumping system, the pressure rapidly falls in the initial stage of volumetric pumping. The tedious drop is due to the slow desorption rate of water molecules adsorbed on the inner wall of the vacuum chamber at room temperature.. In high-vacuum systems, most gas loads are due to water molecules and high water pumping speed is essential to minimize pumping time. Cryogenic water pump (CWP) + turbo molecular pump (TMP) multi-pump systems have become more widely used as they maximize TMP pumping performance for normal gases while improving the water pumping speed. A so-called Cryo–TMP is composed of an in-line CWP with a Gifford–McMahon (G–M) single-stage cryocooler and a TMP; these devices exhibit excellent water pumping performance. There has been no systematic evaluation of the water pumping performance of a Cryo–TMP because adsorption and desorption of water from the surface of the vacuum system makes measurement quite difficult.
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