Differences of Kinds of Cathodes

Each cathode has advantages and disadvantages, and is used for different applications. In choosing a cathode for a Kimball Physics electron gun system, consider the following:

- Vacuum pressures

- Lifetime

- Spot size

- Beam current

- Amount of photons emitted in addition to electrons

- Cost

The general purpose cathode is the Ta disk cathode, and it is also the least expensive. It tolerates 1e-5 torr and will reach hundreds of hours of lifetime at medium heating currents.

The Y2O3 coated disk cathode has the advantage of tolerating 1e-4 torr, because of the Iridium heater and disk, which are very unreactive at operating temperatures. It also provides more beam current at lower temperatures compared to the Tantalum, but has similar lifetimes to Ta.

The BaO coated disk cathode has a very low work function, and so runs at a lower temperature with lower photon brightness. It has more strict vacuum limitations, requiring 1e-7torr or better. It requires in-vacuum activation, after which is sensitive to water vapor, and must remain in vacuum as much as possible. Its lifetime can be thousands of hours at medium heating currents with proper handling.

LaB6 is not a refractory metal disk cathode like the others, but is a more complicated structure made from carbon and single crystal LaB6. It has a smaller emission surface than the disk cathodes, which results in smaller spot size. Along with smaller spot sizes come lower beam currents in most gun models. It requires 1e-7torr or better, and is the most expensive cathode. Its lifetime is also thousands of hours with medium heating currents.