When a Coolidge tube is operated for some time it becomes hot. Where does the heat come from?

William Coolidge refurbished the hot cathode ray tube to generate X-rays, which is now known as Coolidge tube. It contains a hot filament which generates electrons by thermionic emission and a metal anode where electrons loose the kinetic energy to produce x-rays. There exists an electric field between the filament and the metal to accelerate the electrons to high energies. As the electrons strike the metal, some of their energy is utilized in created in x-rays, and the leftover energy (due to collisions) is absorbed by the metal as heat. This energy along with the hot filament, raises the temperature of the Coolidge tube. So, water .

vents are provided to cool off the metal