Step 5: What Is a Coolidge Tube?
Essentially, a Coolidge tube is a thermionic diode optimized for both high voltages and high powers. Like a thermionic diode, all elements are contained in a glass envelope which has been evacuated to the hardest vacuum reasonable.
A Coolidge tube’s method of operation deviates not too far from that of a diode. The heater is given a bit of current to warm it to incandescence, where the now hot tungsten cathode boils off a cloud of electrons while simultaneously focusing them into a beam. These electrons are then attracted to the positively biased anode and move towards it at a very high speed. Upon arrival at the anode, the high energy electrons lose energy through collisions with the metal atoms. Most of these electrons will do little more than heat the anode, but about 2% will generate x-rays in a process called bremsstrahlung.