filament

 
 

Filaments, mostly made of tungsten, are frequently used in UHV for heating and for generating electrons by thermal emission. The most simple electron gun, for example, works like in an cathode-ray television tube:


                                           


Here, a hot filament generates the electrons. These are accelerated by a positive potential on the anode and focussed by a negative potential on the Wehnelt cylinder. Hot filaments are almost ubiquitous in UHV because of their use in ion gauges like here:


                                                   


For high-end electron emission, tungsten filaments are not very good: The resistance and hence the voltage drop over the filament can be quite high (one volt or so), which is bad if you want to use the filament to generate electrons with a small energy distribution. Moreover, even when bent to a sharp angle, the area which is hot is fairly big. This means that the electron source is big and that it is hard to focus it on a small point again. Both problems are solved by using indirectly heated crystals of low work-function materials like LaB6. But these are much more expensive and sensitive than tungsten filaments.

 

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