Ionisation Gauge

Filament, grid and collector — the same architecture as a triode valve, doing a different job.

1. Mount a fine tungsten filament as the electron source.
2. Surround it with an open helical grid of wire, which will be held at a positive voltage.
3. Place a collector electrode outside the grid.
4. Bring three separate feedthroughs out of the envelope.
5. Check none of the electrodes can touch another.

This is a Fleming valve with an extra electrode and an opposite purpose. A valve is built to have as little gas inside as possible so the electron stream is undisturbed; an ionisation gauge is built to measure the small amount that remains by deliberately letting electrons collide with it. The same structure, optimised for opposite ends.

Each electrode has a job, and the voltages are what assign them.

1. Heat the filament until it emits — a low voltage at significant current.
2. Hold the grid strongly POSITIVE, so electrons are accelerated toward and through it.
3. Hold the collector slightly NEGATIVE, so it repels electrons but attracts positive ions.
4. Measure the collector current with a sensitive meter.

The collector must reject electrons and accept ions. Making it slightly negative does both: electrons are turned back, positive ions are drawn in. Without that discrimination the collector would receive a huge electron current that swamps the tiny ion current entirely — the signal is often a millionth of the electron current or less.

Run both gauges together across the whole pump-down and watch the handover.

1. Fit the Pirani and the ionisation gauge on the same system.
2. Start the backing pump and record both from atmosphere.
3. Note where the Pirani flattens and stops responding.
4. Start the diffusion pump and continue recording the ionisation gauge alone.
5. Plot both on one logarithmic pressure axis.

An instrument that changes what it measures — the classic instrumentation trap.

1. Isolate a small volume with the gauge running and the pumps valved off.
2. Record the indicated pressure over twenty minutes.
3. Note that it FALLS, with no pump connected.
4. Switch the gauge off for ten minutes, switch it back on and read again.

The hot-cathode ionisation gauge dates from around 1916, but its useful floor was limited by a phenomenon nobody understood until Robert Bayard and Daniel Alpert identified it in 1950. Electrons striking the grid produce soft X-rays; those X-rays strike the collector and eject photoelectrons; that current is indistinguishable from ion current and does not depend on pressure at all. It set a false floor.

Their fix was geometric and beautifully simple: make the collector a very fine wire rather than a large cylinder, so it presents almost no area for X-rays to strike, and move the filament outside the grid. The X-ray limit dropped by orders of magnitude, and pressures previously unmeasurable became routine. The Bayard-Alpert gauge is still the standard ultra-high-vacuum instrument seventy-five years later.

Where it sits in the chain: the McLeod gauge is a primary standard but slow, manual and blind to vapours. The Pirani reads continuously but saturates. The ionisation gauge reads far lower than either, continuously and electrically — but it is not a primary standard, it depends on the gas species, and it perturbs the system. Three gauges, three regions, and a real system carries at least two.

Its honest limits: gas-dependent sensitivity; the pumping effect from step 4; a filament that can burn out if exposed to air while hot; and contamination of the electrodes changing the calibration. It also cannot be used above a certain pressure at all, because the filament oxidises — which is why it must be interlocked to switch off if the vacuum is lost.