Electron Gun Grounding: Best Practices for Photoemission Experiments

In summary, the conversation discusses the possibility of an electron gun running out of electrons to fire eventually. However, it is explained that most electron guns are thermionic and operate in a vacuum, so they do not "fire into the air." The end-of-life symptoms for these electron guns include the source stopping emitting or becoming too noisy or deteriorating in quality. These issues can be caused by factors such as depletion of the electron reservoir or arcing/breakdown in the source region. It is also mentioned that the source must be grounded for proper operation. Ultimately, it is concluded that the gun will not run out of electrons as long as it is properly operated and maintained.
  • #1
mtanti
172
0
Will an electron gun firing electrons into the air end up having no more electrons to fire eventually?
 
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  • #2
mtanti said:
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...
 
  • #3
mtanti said:
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.
 
  • #4
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
 
  • #5
josh_einsle said:
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.
 
  • #6
ZapperZ said:
...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.
 
  • #7
cesiumfrog said:
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.
 
  • #8
ZapperZ said:
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.
 
Last edited:

Related to Electron Gun Grounding: Best Practices for Photoemission Experiments

1. What is an electron gun?

An electron gun is a device that generates a beam of electrons by accelerating them using an electric field. It is commonly used in cathode ray tubes, electron microscopes, and particle accelerators.

2. How does an electron gun work?

An electron gun consists of a cathode, which emits electrons, and an anode, which accelerates the electrons towards a target. The cathode is heated to release electrons, which are then focused into a narrow beam by an electric field created by the anode.

3. What is the purpose of electron gun drains electrons?

The purpose of an electron gun draining electrons is to remove any excess electrons from the electron beam. This ensures that the beam remains focused and does not scatter, providing a clear and accurate image or measurement.

4. Can an electron gun be used to create a positive ion beam?

No, an electron gun is specifically designed to generate a beam of electrons. To create a positive ion beam, a different type of device called an ion gun is used, which uses a magnetic field to accelerate positively charged particles.

5. What are the common applications of electron guns?

Electron guns have a wide range of applications, including in television and computer displays, electron microscopes for imaging and analysis, and particle accelerators for scientific research. They are also used in welding, semiconductor manufacturing, and medical equipment such as X-ray machines.

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