# Voltage drop along electron beam

A focused electron beam represents a current and unless the charges (electrons) meet no resistance to their movement there should be a voltage drop along the length of the beam. So, assuming the beam is not traveling in total vacuum, is there such a resistance and associated voltage drop - and can it be measured, or is my understanding wrong?

• Neither an electron/ion source or a beamline is really an Ohmic element. Consider the transport of an electron (or ion) beam through a beam line - there are no potential gradients present. Interactions with residual gas will scatter the electron/ion out of the beam, so while you lose some beam current, the rest continues to propagate at full kinetic energy. It is not an Ohmic 'circuit' element. Now, in the gun (ion source) itself the electron/ion mean free path really needs to be much longer than the source dimensions (or you just get a plasma discharge). – Jon Custer Feb 17 '16 at 14:55
• I understand your viewpoint. However the scattering could be compared to the gas exerting a sort of "resistance" to the free passage of electrons resulting in a weaker main beam (and less current). In order to maintain the same current the field (anode katode voltage) would have to be greater than if no gas was present. In this sense it might be possible to regard the gas as something with a certain ohm per meter property. Just a thought. – Jens Feb 17 '16 at 19:29
• My hesitation is that, while current is lost the remaining beam energy is constant. To maintain the same current you just have to inject more current - it does not scale with voltage. If I want more electrons out of an e-gun I turn up the filament current to boil off more electrons. – Jon Custer Feb 18 '16 at 15:23
• Ok thank you for the answer. Is the current really voltage indepent? After all the ekectrons need to be accellerated but maybe I should study ekectron beam generation more. However it is not easy to find on the internet since most articles only deal with applications. Perhaps you could point me in the right direction. – Jens Feb 18 '16 at 18:34
• You need an electron source (hot filament, LaB6, whatever), and a "high" voltage power supply. As long as the total current (column current to maintain control on the voltage, and beam current) does not exceed the limit of the power supply, you can provide any amount of current. – Jon Custer Feb 18 '16 at 19:45