How a Turbomolecular Pump Works
A turbomolecular pump looks like a jet engine compressor, but it works in a regime where gas molecules hardly meet each other. Fast-spinning blades hit individual molecules and knock them toward the outlet, stage after stage.
Step by step
- Molecular flow. Below about 10⁻³ mbar the mean free path of the gas is longer than the gaps in the pump. Each molecule travels on its own, bouncing between surfaces rather than colliding with other molecules.
- Rotor stage. Angled rotor blades turning at 20,000–90,000 rpm reach tip speeds of a few hundred m/s, similar to the molecules themselves. A molecule that hits a blade leaves with a strong bias downward.
- Stator stage. Fixed stator blades, angled the opposite way, pass the molecules on to the next rotor stage and make it unlikely that they bounce back up.
- Many stages in series. Ten or more rotor–stator pairs multiply the compression. Upper stages are open for high pumping speed; lower stages are tighter for high compression.
- Backing pump. At the outlet the pressure is still around 10⁻² to 10 mbar, so a backing pump such as a scroll, diaphragm or rotary vane pump has to take the gas away.
Light gases are the weak spot: hydrogen and helium move so fast that the blades give them relatively little push, so their compression ratio is much lower than for nitrogen. In UHV systems the residual gas is therefore often mostly hydrogen.
Pressure range and performance
- Ultimate pressure
- ≈ 10⁻⁹ – 10⁻¹⁰ mbar (lower in baked UHV systems)
- Pumping speed
- 10 – 4,000 L/s
- Working range
- ≈ 10⁻² mbar to UHV, with a backing pump
- Dry?
- Yes — no oil in the gas path
Advantages
- Clean, oil-free high vacuum
- Ready within minutes
- Wide range from 10⁻² mbar to UHV
- No cryogens or regeneration
Limitations
- Always needs a backing pump
- Sensitive to shock, sudden venting and particles
- Lower compression for hydrogen and helium
- Higher purchase price
Typical applications
- Mass spectrometers and analytical instruments
- Electron microscopes (SEM/TEM)
- Thin-film deposition and coating
- Helium leak detectors
- Research and UHV systems
Useful calculators
References & further reading
- K. Jousten (ed.). Handbook of Vacuum Technology, 2nd ed.. Wiley-VCH (2016). Standard reference on gas flow, conductance, pumps and gauges.
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
- CERN Accelerator School. CAS Vacuum for Particle Accelerators, Glumslöv, Sweden, 2017 — proceedings. CERN, free to read. Up-to-date lectures on materials, pumps, gauges and leak detection.
Frequently asked questions
Why does a turbomolecular pump need a backing pump?
It only works in molecular flow and has a limited compression ratio, so it cannot push gas all the way to atmospheric pressure. A backing pump holds the outlet at about 10⁻² to 10 mbar.
How fast does a turbomolecular pump spin?
Typically 20,000 to 90,000 rpm, depending on size: small pumps spin faster. The blade tips reach a few hundred metres per second.
Can a turbo pump start at atmospheric pressure?
No. The chamber must first be roughed down by the backing pump, typically below about 1 mbar, before the turbo pump runs up to full speed. Venting at full speed can damage it.
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