What angle should an elctron beam leave an electron gun?

[drmerby3737]

1. At what angle should a 1kV electron beam with a total current of 1mA leave an electron gun in order that the cross section of minimum diameter will occur on a screen 25cm away? The original diameter of this beam is 2mm.



2. Cannot find equations in book or notes anywhere.



3. No attempt, just looking for some help maybe in the form of equations please.

 

[berkeman]

1. At what angle should a 1kV electron beam with a total current of 1mA leave an electron gun in order that the cross section of minimum diameter will occur on a screen 25cm away? The original diameter of this beam is 2mm.



2. Cannot find equations in book or notes anywhere.



3. No attempt, just looking for some help maybe in the form of equations please.

Are there any more details or a diagram available? Why would the e-beam leave the gun at an angle? Are there some magnetic fields or other fields involved? What is doing the focusing?

 

[drmerby3737]

This is all the information that is stated in the question. I asked 5 classmates and no one knows where to begin. In the notes they talk about lenses but it does not specify that is what is doing the focusing.