Turbomolecular Pump

A jet-engine rotor spinning at 90,000 rpm that flings individual gas molecules toward the exhaust one collision at a time.

A turbomolecular pump removes gas from a chamber to create high or ultra-high vacuum — pressures from about 10⁻³ down to below 10⁻¹¹ mbar. It does this continuously, without oil contaminating the vacuum space, and without a cold trap. Once a rotary or diaphragm backing pump has roughed the chamber down to a few mbar, the turbo takes over and drives the pressure down by ten or more additional orders of magnitude. You'll find them behind the ion source of every mass spectrometer, on the process chambers of semiconductor fabs, and bolted to the dilution refrigerators used in quantum-computing research.

At low enough pressures — roughly below 10⁻³ mbar — gas molecules travel in straight lines without colliding with each other (molecular flow). The turbopump exploits this: a stack of rotor discs, each carrying dozens of angled blades, alternates with identical stationary stator discs. The rotor spins at 24,000–90,000 rpm depending on rotor diameter, driving blade-tip speeds of 300–500 m/s — comparable to the thermal speed of nitrogen molecules (about 470 m/s at room temperature). A molecule entering the pump inlet is far more likely to collide with the underside of a rotor blade and receive a kick toward the exhaust than to drift back toward the chamber. Each rotor-stator pair adds a compression factor of roughly 10–30 for nitrogen; the cascade of 12–20 stages multiplies these to an overall N₂ compression ratio exceeding 10¹¹. Because lighter molecules (H₂, He) move faster, their thermal velocity is harder to outrun, so compression ratios for hydrogen are only around 10³–10⁵. A downstream Holweck or Siegbahn drag stage handles the transition from molecular to viscous flow and allows a smaller, quieter backing pump.

In plain terms

Imagine a long hallway with many spinning paddle wheels tilted at an angle. If a ball rolls in, the paddles swat it toward the far door. By the time the ball reaches the end it has been smacked so many times it can only go one way — out. A turbomolecular pump does exactly that, but with gas molecules instead of balls, and the paddles spin faster than a dentist's drill.

Also called: turbo pump, TMP, turbopump, turbo-molecular pump.

Why it matters

Where it lives

Key numbers

Surprising facts

A short history

The concept of imparting directional momentum to gas molecules via a moving surface dates to Wolfgang Gaede's molecular drag pump of 1912–1913 at the University of Freiburg, and to Manne Siegbahn's disc-type variant of 1943. The modern turbomolecular pump — with its alternating angled rotor and stator blades — was invented and patented in 1957 by Dr. Willi Becker at Pfeiffer Vacuum in Asslar, Germany, and reached the market in 1958. Becker's design borrowed the axial-turbine stage geometry from jet-engine compressors but scaled the tip velocity to match molecular thermal speeds rather than bulk flow. Pfeiffer's first commercial turbopump displaced the oil diffusion pump as the workhorse of high-vacuum science, and the basic geometry remains largely unchanged today. Magnetic-bearing variants, introduced from the 1980s onward, eliminated the lubricated ball bearings that limited pump lifetime and introduced hydrocarbon vapour; modern all-magnetic pumps run for 40,000 hours or more without maintenance.

Sources

Related