Electron Gun Grounding: Best Practices for Photoemission Experiments

[mtanti]

Will an electron gun firing electrons into the air end up having no more electrons to fire eventually?

 

[josh_einsle]

Will an electron gun firing electrons into the air end up having no more electrons to fire eventually?

eventually any emitter will not be able to excite electons off... but i think it is because something in the shape of the source changes not that you run out of electrons... I am consulting with my SEM designer friends at the moment...

 

[ZapperZ]

Will an electron gun firing electrons into the air end up having no more electrons to fire eventually?

Er.. no. Most electron "guns" are thermionic guns, and to operate something like that, the filament must be grounded (or else you'll end up with nothing very quickly).

So unless you want to argue that the "ground" will run out of electrons, it'll never happen. The filament might break, etc.. but it won't stop running due to lack of electrons.

Zz.

 

[mtanti]

so the electrons don't come from the mains supply? the ground could eventually run out of electrons if the electrons are fired into space... but i guess that when that happens we'll have bigger problems to worry about than electron guns... :P

 

[josh_einsle]

eventually any emitter will not be able to excite electons off... but i think it is because something in the shape of the source changes not that you run out of electrons... I am consulting with my SEM designer friends at the moment...

so an old friend of mine got back to me and here is an edited reply on this topic...

Firstly I'd point out to manti that electron guns are never "fired
into the air" because 1) the mean free path of electrons in air is so
short the gun would be useless and 2) your typical electron extraction
method usually involves static electric field strengths which would
easily cause breakdown in air. A vacuum is used instead. It seems
that the other posters and you already knew this so it went
unmentioned.

Some end of life symptoms for electron guns are typically either that
the source stops emitting (mechanisms below) or that the emission
becomes prohibitively noisy for imaging or that the emission parameters
such as chromatic spread or brightness deteriorate to the point that
image quality is unacceptable. Mind you these are only EOL symptoms
for the source, not the column as a whole, nor the detection chain,
etc.

Sources can stop emitting from slow degredation due to poisioning or
they can suffer from sudden shape changes due to arcing/breakdown in
the source region. For a Schottky emitter, emission can stop
due to depletion of the reservoir which will effectively
raise the work function at the tip and the electron current density
will drop steeply. This happens after some time, usually years,
depending on vacuum conditions and operator error.

 

[cesiumfrog]

...the filament must be grounded (or else you'll end up with nothing very quickly).

So unless you want to argue that the "ground" will run out of electrons, it'll never happen. The filament might break, etc.. but it won't stop running due to lack of electrons.

That sounds confused. It doesn't matter where "ground" is.

It's not that you run out of electrons, but you might get into the situation where the electrons left in the cathode are all too tightly bound or the electrons in the anode/target create too repulsive an electric field. This is avoided by connecting a high voltage power supply (or battery) between the cathode and anode. If you google for a cross-sectional diagram of your CRT monitor, you'll see how the electrons are being collected back from the screen/anode and pumped hard toward the cathode gun again.

 

[ZapperZ]

That sounds confused. It doesn't matter where "ground" is.

It's not that you run out of electrons, but you might get into the situation where the electrons left in the cathode are all too tightly bound or the electrons in the anode/target create too repulsive an electric field. This is avoided by connecting a high voltage power supply (or battery) between the cathode and anode. If you google for a cross-sectional diagram of your CRT monitor, you'll see how the electrons are being collected back from the screen/anode and pumped hard toward the gun/cathode again.

Er... and my post sounded confusing?

If you do any amount of photoemission experiment, for example, and your cathode or sample isn't not well grounded, or if you are using something that isn't a good electrical conductor such as a semiconductor, then you will see something called the "charging effect". This is where your cathode could not replenish itself as fast as the electrons leave its surface. What will occur is that the effective workfunction will start increasing and at some point, you will not get anymore photoelectrons!

Furthermore, you can't simply connect the cathode and the anode to a power supply that is floating. This is because you are not guaranteed that all the electrons leaving the surface of the cathode will get to the anode. The electrons are attracted to anything at a higher potential than the cathode, and the grounded walls of the vacuum chamber is such a thing. Again, look at most setup for thermionic and photoelectron guns. The source is always grounded.

Zz.

 

[cesiumfrog]

If you do any amount of photoemission experiment[...] Again, look at most setup for thermionic and photoelectron guns. The source is always grounded.

I would have thought it safer and more practical to ground the target and the surrounding apparatus (whilst typically making the source -ve with respect to "ground", at least for CRTs - the photoemission experiment I've done uses more than two electrodes..), but I'll defer to your experience. My confusion was as to whether your phrasing implied charge accumulates anywhere ad infinitum.