The cathode is connected to the negative of the high voltage supply. It is partially shielded by a surrounding grid which has a small aperture to let some electrons pass through. The anode is mounted below the grid and is connected to the high voltage positive. Electrons escaping through the grid hole are rapidly accelerated toward the anode by the potential between them. Most of the electrons hit the anode, but some of them pass through the hole in the anode and form the electron beam in the tube.
V is eV, where V is the anode potential and e is the charge on the electron.
As you have learned in the lecture part of this course, the electron beam is traveling in the transverse magnetic field B of the Helmholtz coils. The magnetic field is provided by the Helmholtz coils surrounding the vacuum tube. They are mounted so that their field is transverse to the tube axis and the electron's motion. These coils produce a nearly uniform magnetic field everywhere inside of the volume they enclose. This is a very useful property, since it guarantees that there is a constant force on the moving electrons.