Turbomolecular pump structure and working principle

Turbo molecular pumps are advanced vacuum devices that utilize the high-speed rotation of an impeller to transfer momentum to gas molecules, creating a directional flow that results in a vacuum. These pumps are known for their fast startup times, resistance to radiation, and ability to withstand atmospheric pressure fluctuations. They do not store or release gas, nor do they produce oil vapor, making them ideal for achieving clean ultra-high vacuum conditions. As a result, they are widely used in applications such as high-energy accelerators, controlled nuclear fusion reactors, heavy ion accelerators, and advanced semiconductor manufacturing. The concept of the turbomolecular pump was first introduced in 1958 by the Federal Republic of Germany with the development of the Baker turbocharger. Since then, various designs have emerged, including vertical and horizontal configurations. A typical vertical turbomolecular pump consists of a casing, a rotor with moving blades (rotor impellers), stationary impellers, and a drive system. The outer edge of the rotating impeller moves at speeds between 150 to 400 meters per second—fast enough to match the thermal motion of gas molecules. A single impeller has a very low compression ratio, so multiple stages of moving and stationary impellers are required to achieve effective pumping. These impellers are alternately arranged, with each moving blade followed by a static one. The geometry of both types is similar, but their blade angles are opposite. A monolithic rotor with 20 blades is often used, with static impellers placed between each pair of moving ones. The outer edges of the impellers are fixed with a ring, maintaining a gap of about 1 mm. The moving impeller rotates freely between the static ones, ensuring smooth operation. When gas molecules interact with the moving blades, they are scattered in different directions. On one side of the blade, some molecules reflect back, while others pass through to the other side. Due to the blade’s tilt, more molecules are directed from left to right than vice versa. This continuous motion creates a net flow of gas from the inlet to the outlet, resulting in the pumping effect. In terms of performance, the compression ratio—the ratio of the discharge pressure to the intake pressure—varies depending on the gas type. Heavier gases like oil vapor have much higher compression ratios compared to lighter ones like hydrogen. For nitrogen or air, the ratio can reach up to 10⁹, while for hydrogen it is around 10⁴. The ultimate pressure achievable is as low as 10⁻⁹ Pa, with a working range between 10⁻¹ and 10⁻⁸ Pa. Pumping speeds can range from tens to thousands of liters per second. These pumps operate effectively in the molecular flow regime, where the mean free path of gas molecules is much greater than the cross-sectional dimensions of the ducts. To achieve this, the operating pressure must be between 1 and 10⁻² Pa. Most turbomolecular pumps are driven by motors spinning at 10,000 to 60,000 revolutions per minute, ensuring efficient and reliable performance.

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