Electron gun design

Question

Hello,
I have a question about electron guns. I have read a lot, and all designs use high NEGATIVE voltage (-1000v for example). Is possible to use high POSITIVE voltage like this drawing?. Or do you think it will not work this way?
Considering that all the chamber walls are at ground voltage (0v) in both cases

Comments

not graduate+ level. Users with 500+ reputation may vote here to close. 

PhysicsOverflow is a site for advanced physics. Please ask elementary 
questions on other online platforms that value such questions. 

Answer 1

Yes, it will work, but not in a permanent way. When your system emits electrons for a long period, it gets charged positively, so the emitted elecrtons are more and more get attracted to the system. This phenomenon is called a vitrual cathode. You must make a closed loop for emitted electrons in order to keep the system neutral.