what type of equipotential surface is formed between the pre accelerating anode and focussing anode and why. And how focussing anode focus the beam of electron to the desire point.

It is made up of three anodes: the pre accelerating anode, focusing anode and ... at equipotential surfaces between pre-accelerating anode and focusing anode form ... knob controls the focal length of two convex lenses, created between the anodes