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Tube questions

Hello satellite amateurs,

For some days I am trying to find out something more about tube technology 
(because I am not that old that I had to learn these things in school) but 
there is one thing I don't understand.

I several tubes, like CRT,TWT,Klystron etc they have an accelerator grid 
with a little gap. This grid attracts the electrons from the heated kathode, 
and the accelerated electrons escape through the gap with a high speed.

What I was wondering is that this grid is actually an anode. When I am right 
this means that only a small number of electrons will pass through the gap, 
and the rest will hit the accelerator grid, like an anode plate in a diode 
tube. This means less power efficiency. Or this is not true ?

Then my next question is why an collector grid with high voltage is used ? 
The electron is already accelerated towards the collector plate, so why 
another anode is needed to atract it ? Is it to guide the electron towards 
the collector and make it less sensitive to outer electrical/magnetical 
forces ?

With an electron gun an electron beam can be made in vaccuum, so I was 
wondering if in space such gun (in theory) can be used as a electron 

73 de PE1RAH, William

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