What is an X-ray tube?

The X-ray tube, see figure 1-4, consists ofconsists of a glass (or ceramic) envelope containing a positive electrode (the anode) and a negative electrode (the cathode) evacuated to an ultra high vacuum [10 - 9 hPa (hectoPascal)]. The cathode comprises a filament that generates electrons. Under the effect of the electrical tension set up between the anode and the cathode (the tube voltage) the electrons from the cathode are attracted to the anode, which accelerates their speed.
This stream of electrons is concentrated into a beam by a “cylinder” or “focusing cup”. When the accelerated electrons collide with a target on the anode, part of their energy is
converted to X-radiation, know as X-rays.

What is the relationship between tube voltage and tube current?