The Vacuum Stack
Block 5 — The Big Machines
Everything in the first four blocks meets here. Pumps, gauges, seals, materials, tubes. A particle accelerator is all of it at once, stretched across kilometers, held to pressures that would be hard in a coffee cup and are absurd over 27 km. This is the scale-up, from the gun to the largest machine ever built.
Source rating: [A] primary only in this block. Lab publications (SLAC, LBNL, CERN), conference proceedings (JACoW), peer-reviewed journals, theses, patents. No encyclopedia, no trade page.
Pressure in Torr unless noted (1 Torr is about 1.33 mbar; 1 Pa is 7.5e-3 Torr).
1. The start: the photoinjector
A modern light source starts with a photoinjector. A laser pulse hits a photocathode inside an RF gun, and the freed electrons are accelerated at once by a strong RF field. The vacuum here is brutal, because the cathode is fragile.
The LCLS-II injector, built by LBNL out of their decade-long APEX program: a one-cell normal-conducting CW gun at 185.7 MHz, plus a two-cell 1.3 GHz buncher, and a gun load-lock that changes photocathodes under vacuum. [A] SLAC/LBNL, LCLS-II commissioning (eScholarship), https://escholarship.org/content/qt4g93w33q/qt4g93w33q.pdf
The numbers, from the bench:
- The APEX/VHF gun ran at high gradient (20 MV/m), high gun voltage (~750 kV), and low pressure (~3e-10 Torr). [A] JACoW, alkali cathode testing for LCLS-II at APEX, https://proceedings.jacow.org/FEL2015/papers/mod03.pdf
- The gun needs low 1e-9 Torr overall, and a partial pressure below 1e-11 Torr for harmful molecules like oxygen, to keep the Cs2Te photocathode alive. Good vacuum here means ~1e-10 Torr. [A] Frontiers in Physics, CW guns and LCLS-II RF gun performance, https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2023.1150809/full
- During commissioning, slow NEG activation cut the hydrocarbon partial pressure, improved the gun vacuum, and eliminated multipacting, which then allowed full 80 kW of RF. The remaining outgassing came from the coaxial waveguides. Goal: below 1e-9 Torr. [A] InspireHEP, first commissioning of LCLS-II CW injector, https://inspirehep.net/files/2de1c4766f7c631e9ee5f9b4f4b2ec54
Cathode life is measured in Langmuirs, the exposure unit (1 Langmuir = 1e-6 Torr·s). At APEX the oxygen 1/e lifetime was about 14.5 Langmuirs, about 402 hours of operation. That is the whole point of UHV here: every decade of pressure you win buys cathode life. [A] JACoW APEX, same paper.
The recurring failure: cathodes are damaged by electrical arcing and intense laser fields, and must be removed and replaced. The RF tuning and vacuum integrity depend on how the cathode is mounted. [A] US Patent 11,031,206, electron photoinjector, https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/11031206
This is Block 4 again, at a higher voltage. The gun arc, the cathode poisoning, the mounting that controls both vacuum and RF. Same physics, bigger machine.
2. The middle: the FEL linac
From the gun, the beam enters the linac. LCLS-II uses a 4 GeV linac built on 1.3 GHz superconducting RF, producing 100 MeV, 100 pC, 12 A bunches at the injector with a normalized emittance around 0.4 µm, at beam rates up to 1 MHz. [A] SLAC/LBNL, LCLS-II commissioning (eScholarship), https://escholarship.org/content/qt4g93w33q/qt4g93w33q.pdf
SRF means the cavities run at 2.0 K. Cold is not only for superconductivity; cold walls are also near-perfect cryopumps. Cryogenics and vacuum become the same engineering. The LCLS-II-HE upgrade pushes this further with an SRF low-emittance gun. [A] arXiv, thermal emittance isolation, LCLS-II-HE LEI, https://arxiv.org/pdf/2409.03499
3. The limit: the LHC, largest of all
The LHC is the largest vacuum system in the world. It is the whole stack at maximum scale. Numbers, all from CERN-side primary sources.
The architecture: two proton storage rings, 7+7 TeV, in the 26.7 km LEP tunnel, 8.3 T superconducting magnets at 1.9 K. The beam vacuum runs inside the magnet cold bore, which acts as a near-perfect cryopump. [A] CERN, overview of the LHC vacuum system (ScienceDirect abstract), https://www.sciencedirect.com/science/article/abs/pii/S0042207X00002402
Two separate vacuum systems, very different requirements:
- Insulation vacuum: around the cryomagnets and helium lines. Only needs about 1e-6 mbar to stop heat conduction. Huge: 50 km, 15,000 m3. Once cold, cryopumping reaches the 1e-4 Pa range. [A] CERN, LHC vacuum systems (ScienceDirect), https://www.sciencedirect.com/science/article/abs/pii/S0042207X09002887
- Beam vacuum: must give a beam lifetime of days, so at least three orders of magnitude better. 54 km of UHV. 48 km of it is cryogenic at 1.9 K; the remaining 6 km is at room temperature and uses NEG coating. [A] same CERN source.
The beam screen, the elegant part: a perforated screen (4% transparency) inside the 1.9 K cold bore, running at 5 to 20 K. It intercepts synchrotron radiation (critical energy ~45 eV at 7 TeV) and image-current heat so they do not load the 1.9 K system, while the slots still let gas pump through to the cold bore. In situ heating up to 90 K flushes condensed gas. [A] arXiv 1705.09499, HL-LHC Vacuum System chapter, https://arxiv.org/pdf/1705.09499 ; radiation/heat detail [A] CERN (ScienceDirect, S0042207X00002402).
The instrument count, the scale made concrete: 780 ion pumps, 170 Bayard-Alpert gauges, 1084 Pirani and cold-cathode Penning gauges. NEG coating was born and industrialized at CERN. [A] CERN, LHC world's largest vacuum systems (academia copy of the CERN paper), https://www.academia.edu/11146952/LHC_World_Largest_Vacuum_Systems_Being_Commissioned_at_CERN
The standard: all beam vacuum elements leak-tight to better than 1e-11 mbar·l/s helium equivalent, clean to CERN vacuum standards, free of contamination. [A] arXiv 1705.09499.
Read that last line against Block 3. The largest machine in the world comes down to the same rule as a single ConFlat joint: a leak-tight, clean, contamination-free surface. Scale does not change the physics. It multiplies it by 54 km.
4. The scale ladder
| Machine | Vacuum | Cold? | Pumping | Vacuum is for |
|---|---|---|---|---|
| RF photoinjector (LCLS-II gun) | ~3e-10 Torr, O2 partial < 1e-11 | no (NC gun) | ion + NEG, load-lock | photocathode life |
| SRF linac (LCLS-II) | UHV, cavities at 2.0 K | yes | cold walls cryopump | superconductivity + beam |
| Synchrotron / FEL beamline | UHV, 1e-9 to 1e-10 | partly | ion, NEG, sublimation | beam lifetime, clean light |
| Hadron collider (LHC beam) | UHV over 54 km, beam screen 5-20 K | 48 km at 1.9 K | cold bore + 780 ion pumps + NEG | beam lifetime in days |
Sources: LCLS-II rows [A] eScholarship, JACoW, Frontiers, InspireHEP; LHC row [A] CERN/ScienceDirect and arXiv 1705.09499.
5. The thread, from Block 1 to here
A vacuum career is one idea at five scales. The mean free path sets the regime. The pump fights the wall. The gauge reads what you cannot see. The seal and the material decide whether you ever reach base pressure. The tube turns a beam into power, and dies at the triple junction if you ignore the field at the corner. And the machine is all of it, leak-tight and clean, multiplied by kilometers.
The physics does not get easier as the machine gets bigger. It gets repeated, more times, with less room for error. That is the job.
Method note
Primary sources only: SLAC/LBNL commissioning papers, JACoW proceedings, a Frontiers journal article, CERN vacuum-system papers, an arXiv HL-LHC chapter, and two US patents. Figures are quoted as published. Where a number was only in a figure and not the text, it was left out. No internal lab data is used.
This closes the first pass of The Vacuum Stack: five blocks, regimes to machines. Open items flagged earlier (titanium sublimation and NEG detail in Block 1) remain for a later sourcing pass.