01 Why Vacuum?
The physics of the invisible enemy — and 400 years of fighting it
The fundamental problem: background gas collisions
A laser-cooled atom or molecule confined in an optical tweezer is cold — maybe 5 μK — but the residual gas in your chamber is at room temperature (~300 K). When a hot N₂ molecule (moving at ~470 m/s) collides with your precious cold atom, the energy transfer easily exceeds the trap depth, and the atom is ejected in microseconds. The collision rate per trapped atom is:
$$\Gamma_{\rm bg} = n_{\rm bg}\,\sigma_{\rm eff}\,\bar{v} \qquad \Rightarrow \qquad \tau_{\rm trap} = \frac{1}{\Gamma_{\rm bg}} = \frac{k_BT}{P\,\sigma_{\rm eff}\,\bar{v}}$$
where $n_{\rm bg} = P/k_BT$ is the background number density, $\sigma_{\rm eff} \approx 5 \times 10^{-15}$ cm² is the effective loss cross-section for a hot atom on a cold atom, and $\bar{v} \approx 470$ m/s for N₂. At $P = 10^{-8}$ mbar the trap lifetime is only ~10 s. At $10^{-10}$ mbar it reaches ~1000 s — long enough for a multi-hour experiment.
A very brief history
Trap lifetime vs pressure
The difference between 10⁻⁸ mbar and 10⁻¹⁰ mbar is 100× in pressure — and 100× in atom lifetime. Most experiments need at least several minutes of trap lifetime to run a meaningful protocol, so UHV is non-negotiable.
02 The Pressure Landscape
Drag the slider to explore number densities, mean free paths, and what lives at each pressure
Interactive Pressure Explorer
| Regime | Pressure range (mbar) | n (cm⁻³) | Mean free path | Example applications |
|---|---|---|---|---|
| Atmosphere | ~10³ | 2.7×10¹⁹ | ~68 nm | Ambient lab air |
| Low Vacuum | 10³ – 1 | 10¹⁹ – 10¹⁶ | nm – μm | Rough pumping, CVD |
| Medium Vacuum | 1 – 10⁻³ | 10¹⁶ – 10¹³ | μm – mm | Freeze-drying, vacuum ovens |
| High Vacuum | 10⁻³ – 10⁻⁶ | 10¹³ – 10¹⁰ | mm – km | Thin-film deposition, SEMs |
| Ultra-High Vacuum | 10⁻⁶ – 10⁻⁹ | 10¹⁰ – 10⁷ | 10 – 10⁴ km | MOTs, surface science, particle accelerators |
| XHV / AMO | < 10⁻⁹ | < 10⁷ | > 10⁴ km | Optical tweezers, BEC, Rydberg arrays |
Mean free path for N₂ ($d \approx 3.7$ Å, $T = 293$ K). At $10^{-10}$ mbar: $\lambda \approx 500$ km — longer than the flight from Purdue to New York.
03 The Pumping Chain
No single pump spans 15 decades of pressure — you need a staged cascade
Animated gas flow (→) shows the pumping direction. GV = gate valve (isolates the turbo from the chamber during venting or pump failure). Ion pump and NEG/TSP are always-on; turbo + rough are the initial pumpdown path.
The rough pump is first in line. It operates by mechanical compression — gas is drawn in, physically compressed, and pushed out to atmosphere. Two designs dominate AMO labs:
- Scroll pump (recommended): Oil-free, two interleaved spiral scrolls orbit to compress gas. No oil mist, no vibration from pistons. Leybold, Agilent, and Edwards make reliable versions used in most tweezer labs.
- Diaphragm pump: Even simpler, no sliding contacts — a flexing membrane compresses gas. Lowest ultimate pressure (~5 mbar), best for clean low-throughput applications.
- Rotary vane (legacy): Uses oil-lubricated vanes. High throughput but oil backstreaming can contaminate your chamber. Always use a foreline trap if you have one.
A turbo pump is essentially a very precisely machined turbine. Blades spin at 30,000–90,000 RPM — fast enough that the blade speed ($\sim$300 m/s) is comparable to the mean thermal velocity of residual gas molecules. Molecules that hit the blades are imparted a directed momentum toward the backing port; molecules trying to re-enter from the backing side are physically swept away.
The pumping mechanism is purely kinetic (molecular flow regime). This is why a turbo needs the rough pump to first bring the pressure below ~10 mbar — at higher pressures the mean free path is shorter than the blade spacing and viscous flow prevents the turbo from working.
- Compression ratio for N₂: ~10⁹; for H₂: ~10³ (lighter molecules are harder to pump).
- Tip seals: Modern turbos use magnetic bearings (vibration-free, no oil). Ideal for optical experiments where vibration couples to trap displacement.
- Gate valve: Always place a gate valve between turbo and chamber. If power fails, the gate valve closes and protects your UHV from the backstreaming turbo.
Ion pumps have no mechanical parts. They work by ionizing residual gas atoms and then burying the ions inside the pump material. A strong magnetic field ($\sim$0.1 T) traps electrons in long helical orbits, massively increasing their path length and ionization cross-section. The resulting ions are accelerated into titanium cathode plates where they are implanted and chemically bound (chemisorption).
- Pumping mechanism: Penning discharge. Residual gas is ionized; ions slam into Ti cathodes and are embedded. Reactive gases (O₂, N₂, CO) pump well; noble gases (Ar) pump poorly and can cause "argon instability" — avoid noble gas leaks.
- Current readout: The ion pump current is proportional to pressure — it doubles as a gauge. A 20 L/s pump at 10⁻¹⁰ mbar draws ~1 nA of current.
- Magnetic field: The external magnet stacks make ion pumps heavy (~5–10 kg for a 20 L/s pump). The stray field ($\sim$1 mT at 10 cm) can affect magnetic-field sensitive experiments — account for this in your layout.
Ion pumps have poor compression ratios for H₂ (the dominant residual gas at UHV). NEG (Non-Evaporable Getter) pumps and TSP (Titanium Sublimation Pumps) address this through chemisorption — gas molecules stick permanently to a reactive surface.
NEG cartridges (e.g., SAES St707: Zr-V-Fe alloy) adsorb H₂, CO, N₂, and O₂ at elevated temperatures or passively near room temperature after activation at ~400°C. They have zero magnetic field, zero vibration, and zero power in passive mode — ideal for sensitivity-limited experiments.
TSP: An electrical current heats a Ti filament until Ti evaporates and coats the chamber walls. The fresh Ti film aggressively chemisorbs nearly all reactive gases. A single filament firing lasts ~30 min; TSPs are typically operated every few days to maintain XHV. They can achieve transient pumping speeds of thousands of L/s.
04 Baking Out — Cleaning from the Inside
The single most important ritual before first pump-down
Why does every surface hate you?
A freshly machined stainless steel surface has roughly one monolayer of water adsorbed on every exposed face — about $10^{15}$ H₂O molecules/cm². A typical AMO chamber has perhaps 5,000 cm² of internal surface. That's $5 \times 10^{18}$ water molecules slowly evaporating into your vacuum over days and weeks. At room temperature, the thermal desorption rate is so slow that it would take years to pump all of that water out through the turbo. Baking dramatically accelerates desorption: the outgassing rate scales roughly as $e^{-E_a/k_BT}$ where $E_a \approx 0.5$ eV for physisorbed water. A modest 150°C bake speeds outgassing by a factor of ~10⁶.
The bakeout pressure curve
Bakeout protocol
Typical schedule
1. Preparation: Remove all heat-sensitive parts: viton O-rings (max ~200°C), elastomers, electronics boards, optical coatings on unprotected elements. Loosen flange bolts slightly to allow thermal expansion (re-torque during cool-down).
2. Ramp up: Heat to 150–250°C at <5°C/min. Fast ramps crack CF gaskets. Wrap the chamber in aluminium foil + fiberglass heating tape. Thermocouples on flange bodies, not heating tape.
3. Hold: 24–72 hours at temperature. Pressure will spike, then slowly decrease. The turbo (or a dedicated bakeout turbo) should run the whole time. Do not close the gate valve during bake.
4. Cool down: <3°C/min. The biggest pressure drop occurs as you cool below 100°C — remaining water re-adsorbs on any cold surface outside the chamber, leaving your inner surfaces clean.
05 Materials & Connections
Not everything belongs inside a UHV system — material choice is irreversible
CF vs KF flanges
| Property | CF (ConFlat) | KF (ISO-KF) |
|---|---|---|
| Seal type | Cu knife-edge gasket | Viton O-ring + centering ring |
| Pressure range | 10⁻¹² mbar → 10 bar | 10⁻⁸ mbar (practical) |
| Bakeout | ✓ up to 450°C | ✗ max ~120°C (viton) |
| Reuse | Gasket single-use | Reusable if clean |
| Use in AMO | Main chamber, sources | Foreline, RGA ports |
Outgassing rate comparison
| Material | Outgassing rate* | AMO suitability |
|---|---|---|
| 304/316L SS (electropolished) | ~10⁻¹² mbar·L/s·cm² | Excellent |
| OFHC copper | ~10⁻¹² mbar·L/s·cm² | Excellent (gaskets) |
| Fused silica / quartz | ~10⁻¹² mbar·L/s·cm² | Excellent (windows) |
| Aluminium (bare) | ~10⁻¹⁰ mbar·L/s·cm² | Acceptable; avoid bake |
| Viton | ~10⁻⁸ mbar·L/s·cm² | KF only; not baked |
| Kapton (wires) | ~10⁻⁹ mbar·L/s·cm² | Acceptable in low flux |
| Regular rubber / PVC | >10⁻⁶ mbar·L/s·cm² | Never use in UHV |
| Zinc / cadmium | High vapor pressure | Never use |
*After 24h bakeout at 200°C. Values highly dependent on surface preparation.
Copper gaskets — handle with care
CF gaskets are OFHC (oxygen-free, high-conductivity) copper — soft enough for the SS knife edges to bite into. Key rules:
- Never reuse a gasket that has been fully torqued. The knife edges work-harden and deform the copper permanently.
- If a flange was slightly loose, inspect the gasket for uneven bite marks — replace if in doubt.
- Nickel-plated copper gaskets exist for higher corrosion resistance; use standard OFHC for baking to 400°C+.
- Store gaskets in sealed bags — oxidised copper is harder and seals less reliably.
What never goes inside a UHV chamber
- Zinc or cadmium — vapor pressure too high; will plate out on optics and destroy the vacuum.
- Regular brass — contains zinc; dezincifies under vacuum and outgasses.
- PVC, regular rubber, silicone (RTV) — extreme outgassing.
- Fingerprints — the oils from your skin outgas for weeks. Always use nitrile gloves inside the chamber and clean parts with methanol + acetone before installation.
- Unprocessed plastics — PEEK and PTFE are tolerable in small amounts; avoid everything else.
06 Reading Your Vacuum
Gauges, RGA, and the art of finding invisible leaks
Gauge types
| Gauge | Pressure range | Principle | Notes |
|---|---|---|---|
| Pirani | 10³ – 10⁻³ mbar | Thermal conductivity of gas (heated wire) | Gas-species dependent; great for rough/medium vacuum readout |
| Penning (cold cathode) | 10⁻² – 10⁻⁹ mbar | Ion current in crossed E+B fields | No hot filament = no contamination; reads falsely high in strong B-fields |
| Bayard-Alpert (hot-ion) | 10⁻² – 10⁻¹¹ mbar | Electron emission → gas ionization → ion current collected | Gold-plated collector reduces X-ray limit; standard UHV gauge |
| Ion pump current | 10⁻⁵ – 10⁻¹¹ mbar | I ∝ P × pumping speed | Free readout from your existing ion pump supply; calibrate vs gauge |
| Spinning rotor gauge | 10⁻² – 10⁻⁷ mbar | Drag on magnetically suspended rotor | Absolute, gas-independent; excellent for calibration |
Residual Gas Analyzer (RGA)
Reading the mass spectrum
An RGA is a miniature quadrupole mass spectrometer that ionizes residual gas and sorts ions by mass-to-charge ratio. The peak heights tell you exactly what's in your vacuum. Key signatures:
- m/z = 2 (H₂): Dominant peak in UHV — always. H₂ permeates stainless steel from the bulk. Normal, expected.
- m/z = 12, 16, 28, 44 (C, O, CO, CO₂): Hydrocarbon contamination or outgassing from surfaces. Drop significantly after baking.
- m/z = 18 (H₂O): High before bakeout, should be much smaller than H₂ after. If water stays high post-bake, suspect a small leak.
- m/z = 14, 28 (N, N₂): Elevated N₂ usually means a real air leak. N₂/Ar ratio ≈ 3.7 confirms atmospheric air.
- m/z = 40 (Ar): Unambiguous real leak indicator — Ar is not outgassed from steel.
Helium Leak Detection
Finding leaks down to 10⁻¹⁰ mbar·L/s
The He leak detector is a mass spectrometer tuned to m/z = 4, connected to your system's roughing line or directly to the pump port. He is small (0.26 nm kinetic diameter), inert, and rare in lab air (~5 ppm) — it threads through any crack the atmosphere leaks through, but faster.
Procedure:
- Pump your system to the best vacuum you can achieve.
- Spray He gas gently with a wand (or medical-grade He tubing) around suspect joints — CF flanges, viewports, electrical feedthroughs.
- Wait 5–10 s (He transit time through a small leak).
- A rise in the He signal identifies the leaking joint.
- Start from the top and work down so He (lighter than air) doesn't drift upward into non-leaking regions.
07 Practical Checklist
From assembly to first UHV — a condensed field guide
- 1Clean all parts with acetone then methanol; handle only with nitrile gloves from this point forward.
- 2Verify all materials: no zinc, cadmium, PVC, or standard rubber inside the system.
- 3Use fresh OFHC Cu gaskets on every CF flange. Torque in star pattern: 3 passes from 3 → 8 → 14 N·m.
- 4Install a gate valve between chamber and turbo pump. Verify it closes cleanly.
- 5Install an ion gauge on the chamber. Label its filament voltage and emission current settings.
- 6Leak check all connections with a He leak detector at rough vacuum (10⁻² mbar) before proceeding.
- 7Start scroll/diaphragm pump. Rough pump to <5×10⁻² mbar before starting the turbo.
- 8Bring turbo to full speed. Open gate valve slowly. Monitor ion gauge.
- 9Once below 10⁻⁶ mbar, begin bakeout ramp: <5°C/min up to 150–200°C.
- 10Bake for 24–72h. Run ion gauge intermittently. Monitor RGA: water peak should decrease.
- 11Cool at <3°C/min. Below 50°C: close gate valve, start ion pump, turn off turbo.
- 12Fire TSP once (if installed). Ion pump current should read <1 nA for a 20 L/s pump at UHV.
| Stage | Start P (mbar) | Target P (mbar) | Time | Key action |
|---|---|---|---|---|
| Rough pump only | 1013 | ~10⁻² | 10–30 min | Verify no gross leaks |
| Turbo pump on | 10⁻² | ~10⁻⁶ | 1–4 hours | He leak test |
| Bakeout ramp | 10⁻⁶ | ~10⁻⁴ (spike) | 2–5 hours | Remove heat-sensitive parts |
| Bakeout hold | 10⁻⁴ | ~10⁻⁷ | 24–72 hours | Monitor RGA, run turbo |
| Cool + ion pump | 10⁻⁷ | ~10⁻¹⁰ | 12–24 hours | TSP fire, NEG activate |
Foundational texts, key experimental papers, and online resources for vacuum technology in AMO labs.