Seals & Materials

You seal a joint you can reopen one of two ways. Elastomer (Viton, O-rings): cheap, reusable, demountable in seconds, good to high vacuum, but it permeates, outgasses, and cannot take a hot bakeout. KF and ISO flanges use it, and stop at the high-vacuum range. [C] Metal (copper gasket, ConFlat): a knife-edge bites into a soft oxygen-free copper gasket; the copper flows, fills every machining mark, and work-hardens into a spring. All metal, no polymer. It holds from atmosphere to below 1e-13 Torr and bakes to 450°C. [B]

The price of UHV. Copper gaskets are single-use. You replace the gasket every time you open the joint. The third way, a joint you never reopen, is Brazing.

The ConFlat was invented by Varian in the early 1960s and is now ISO 3669: all-metal, bakeable to 450°C. [A] What to keep:

Detail	Choice
Body	stainless 304 / 304L / 316L / 316LN; 316L costs more but welds better
Gasket	OFHC copper standard; silver-plated for hot, frequent bakes; Viton CF for low-temp
Assembly	tighten bolts diagonally, not around the circle, for an even seal

Body [A][B]; gaskets and assembly [B].

In UHV the pump fights the walls. They release gas, water first, then hydrogen from the bulk metal, and that outgassing rate sets your base pressure.

• Water dominates early. It sits on the surface and comes off slowly at room temperature; this slow release is the main obstacle to low pressure. [A]
• Hydrogen dominates late. Once water is gone, hydrogen diffusing out of the steel bulk sets the floor. Vacuum firing at ~950°C for ~24 h drives it out and pushes the rate toward 1e-13 mbar·l/s/cm² and below. [A]
• The diffusion energy is measurable. CERN measured ~0.5 eV for a 316LN chamber and ~0.4 eV for OFS copper, both after a 200°C / 20 h bakeout. [A]

Material shortlist for UHV: stainless 304L/316L/316LN, OFHC copper, titanium, surface-hardened aluminium. Avoid mild steel and brass. [A]

Bakeout is the biggest lever. Heat the whole system during pumpdown and water leaves faster; the pressure rises while hot, then drops far lower once cooled. Effective above ~120°C for 12 h or more (200°C typical on stainless, lower for aluminium). It cuts the outgassing rate, and the base pressure, by roughly 100×. [A]

The heart of the matter, and the physics behind the connector story.

A triple junction is where three things meet: metal, dielectric, and vacuum. Under voltage the field there is far higher than the average field between the electrodes, because the dielectric polarizes and piles surface charge on the metal at the corner. Electrons are emitted there, by field emission or the Schottky effect, and that emission starts the surface flashover that destroys the part. The enhancement is set by the ratio of the angles the dielectric and the vacuum subtend at the corner: geometry, not just voltage. [A]

Three lines of defence, all from primary work:

Fix	What it does
Shape the dielectric	a sloped (obtuse-angle) insulator raises breakdown voltage, accelerators settled on 45°
Screen the junction	grow the insulator area, lengthen the flashover path, shield the corner
Recess the junction	pull the anode triple junction away from the cathode to improve HV hold-off

[A] for all three.

The lesson behind the connector. A high-voltage feedthrough does not fail because the insulator is too thin. It fails because nobody asked what the field does at the corner where metal, ceramic, and vacuum meet. Under steady DC the geometry decides, not the datasheet rating.

Seal	Range	Bakeout	Reusable	Use
Viton O-ring (KF/ISO)	atm to high vacuum	low temp only	yes	quick, demountable, HV
Viton CF gasket	down to ~UHV edge	limited	yes	low-temp UHV-ish
OFHC copper (CF)	atm to < 1e-13 Torr	to 450°C	no, single use	true UHV/XHV
Silver-plated copper (CF)	atm to < 1e-13 Torr	hot, frequent	no	repeated hot bakeout