The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called hot cathode tube, is the most widely used. It works with a very good quality vacuum (about 10−4 Pa, or 10−6 Torr).
In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, and then hit the anode.
There are two designs: end-window tubes and side-window tubes.
End window tubes usually have "transmission target" which is thin enough to allow X-rays to pass through the target (X-rays are emitted in the same direction as the electrons are moving.) In one common type of end-window tube, the filament is around the anode ("annular" or ring-shaped), the electrons have a curved path.(half of a toroid)
What is special about side-window tubes is:
- An Electrostatic Lens to focus the beam onto a very small spot on the anode
- The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons. Some anodes are:
- Mechanically spun to increase the area heated by the beam.(Medical "rotating anode")
- Cooled by circulating coolant. (indirectly on most rotating anodes)
- The anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as to allow escape of some of the X-ray photons which are emitted essentially perpendicular to the direction of the electron current.
- The anode is usually made out of tungsten or molybdenum.
- The tube has a window designed for escape of the generated X-ray photons.
The power of a Coolidge tube usually ranges from 0.1 to 18 kW.
Read more about this topic: X-ray Tube
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