This is my first post here so hello to anybody who reads this! I just finished calculus based physics II this semester and there are a few things I want to design and make for myself to help better my understanding of what I learned in physics. One is device I have seen that generates an electron beam and uses a magnetic field to give the electrons circular motion. The part of it that I am the least comfortable with is the vacuum. The question I have is this: What sort of pressure do I need in the vacuum so that the tube is evacuated enough to allow the electrons freedom to move? Also, I understand that helium gas will fluoresce when struck by the moving electrons. But how much helium per unit volume is needed? Currently I am considering the use of mercury vapor instead because it would be very easy for me to add a bit of mercury before I evacuate the tube/housing. I would appreciate any opinions on this because I intend to do a complete design before I attempt any assembly. Thanks in advance.