A turbomolecular pump (TMP) is a kinetic vacuum pump that achieves high and ultra-high vacuum by transferring linear momentum from fast-moving rotor blades to gas molecules. It works only in the molecular-flow regime (high Knudsen number), so it must be backed by a roughing pump.

Working principle

• Rotor–stator stages: Alternating rows of angled rotor and stator blades. A molecule entering the inlet is struck by a rotor blade moving with tip speed u = ω·r (typically 300–600 m/s), gaining a downstream axial velocity component. The stator redirects the molecule toward the next rotor. Repeating this across tens of stages creates a directional drift from inlet to exhaust → compression.
• Drag stages (Holweck/Gaede): Many TMPs add viscous-drag stages at the bottom to raise the discharge pressure and improve compression for light gases (H₂, He).
• Operating range: Efficient when the mean free path λ ≫ blade spacing (molecular flow). Above ~1 mbar, intermolecular collisions dominate and pumping becomes ineffective.

Typical performance

• Pumping speed (S, for N₂): 50–4,000 L/s depending on inlet diameter (DN63–DN320 and larger).
• Compression ratio (K): N₂/Ar 10⁷–10¹¹; H₂/He 10³–10⁷ (species-dependent).
• Ultimate pressure: 10⁻⁸…10⁻¹⁰ mbar (with clean system, low outgassing, good conductance).
• Backing/foreline pressure: Requires 0.1–1 mbar (dry scroll/diaphragm/roots+scroll).
• Speed: 20,000–90,000 rpm (large rotors run slower, small rotors faster).

Construction & subsystems

• Rotor/stator materials: Al, Ti, or stainless; high stiffness, low mass; precision-balanced.
• Bearings:
  • Magnetic (active/passive): oil-free, low vibration, any orientation, needs controller.
  • Ceramic ball bearings: compact, cheaper; require periodic vent/grease service and preferred vertical orientation.
• Cooling: Air or water; water recommended for high gas loads or hot environments.
• Controller: Manages spin-up, speed regulation, bearing control, vent valve, purge, interlocks.
• Valving: Inlet gate valve (protects during roughing and power loss), vent valve (N₂), purge for corrosives, foreline check valve to prevent backstreaming.

Key equations & sizing hints

• Throughput: Q = S · P (mbar·L/s). At a given gas load Q, choose S so inlet pressure P meets your target.
• Conductance matters: chamber-to-pump duct conductance C limits effective speed S_eff via 1/S_eff = 1/S_pump + 1/C → keep inlet line wide and short.
• Gas dependence: Effective speed and compression drop for light gases; check the H₂ curves in datasheets if you run UHV or hydrogen-rich processes.

Integration & operation

1. Pump-down sequence: Rough to ~1 mbar → open gate → start TMP → regulate backing pressure.
2. Cleanliness: Oil-free backing recommended; avoid condensables/particulates (install traps/filters if needed).
3. Bake-out: Pump body typically 80–120 °C; rotor not baked while spinning. Use gate valve during chamber bake.
4. Safety: High stored kinetic energy—never vent or expose to atmosphere at speed; mount on rigid base, use vibration isolators, follow run-down interlocks.

Selection checklist

• Required ultimate pressure and gas load (species & throughput).
• Pumping speed at the flange size you can accommodate; review speed curves for N₂ and H₂.
• Bearing type (magnetic for low vibration/clean UHV; ball for cost/compactness).
• Cooling method, orientation, and environmental heat.
• Backing pump capacity and cleanliness; foreline pressure control.
• Process compatibility: corrosives, halogens, water vapor—use purge, coatings, or select corrosion-resistant models.
• Controls & I/O: vent/purge, speed setpoints, fail-safe on power loss.

Advantages vs. alternatives

• Compared to diffusion pumps: no hydrocarbon backstreaming, faster start/stop, lower maintenance; more tolerant of small air in-leaks but less tolerant of large condensable loads.
• Compared to cryopumps: continuous operation without regeneration but generally higher base pressure and sensitivity to light gases.

Bottom line: A turbomolecular pump is a high-speed momentum-transfer device that, with a suitable backing pump and clean, high-conductance plumbing, delivers high-/ultra-high-vacuum with excellent cleanliness, fast response, and precise controllability for semiconductor, surface-science, coating, and analytical systems.