Re: [amsat-bb] Tube questions

["John M. Franke" <j.m.franke@xxxxxxxxxxxxx>]

The accelerator grid is somewhere between an anode and a grid.  The grid 
accelerates the electrons and provides some focusing action.  Yes there is 
a loss of efficiency, but a larger area, lower temperature, cathode can be 
used.  It is hard to get the same current from a near-point source.

The collector has to operate a voltage higher than the helix or last 
electrode in order to attract the electrons in the beam.  If the electrons 
just impacted on the collector, the collector would eventually charge up 
and repel the beam.  Also, because secondary electrons are emitted when the 
main electron beam hits the collector, the field between the collector and 
the last electrode has to be high enough to attract these high speed 
secondary electrons.  Too low a voltage and the electrons escape and go 
places where they are not wanted.  Some TWT's have depressed collectors to 
reduce improve the power efficiency.

Yes, such an electron gun can be used in space to shoot electrons.  A 
variation on this are the ion propulsion system that ionize heavy atoms and 
accelerate them into space.  Small thrusts are produced, but the efficiency 
in terms of ISP is higher than chemical rockets.  A problem with electron 
or ion guns is charge balance.  As the gun fires more and more 
electrons,  the gun develops a higher and higher positive 
charge.  Eventually, the charge becomes high enough to attract the 
electrons back and there is no net thrust.  To counteract this effect in 
ion propulsion systems, there is an electron gun that fires electrons in 
the same direct as the ions and at a rate to neutralize the charge.  The 
electrons are moving with the ions and are attracted by the ions, which are 
then neutralized.

Hope this helps,

John WA4WDL

At 11:06 PM 1/15/2003 +0100, you wrote:
>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 
>transmiter.
>
>73 de PE1RAH, William
>---
>
>
>
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