The diameter and length of the component marked with arrow is:

Cathode: The cathode in an x-ray tube consists of a filament and a focusing cup. The filament is the source of electrons within the x-ray tube. It is a coil of tungsten wire approximately 2 mm in diameter and 1 cm or less in length. Filaments typically contain approximately 1% thorium, which greatly increases the release of electrons from the heated wire. The filament is heated to incandescence with a low-voltage source and emits electrons at a rate proportional to the temperature of the filament.