Microfocus X-ray Tubes
Some X-ray examinations (such as, e.g., non-destructive testing and 3-D microtomography) need very high-resolution images and do therefore require X-ray tubes that can generate very small focal spot sizes, typically below 50 µm in diameter. These tubes are called microfocus X-ray tubes.
There are two basic types of microfocus X-ray tubes: solid-anode tubes and metal-jet-anode tubes.
Solid-anode microfocus X-ray tubes are in principle very similar to the Coolidge tube, but with the important distinction that care has been taken to be able to focus the electron beam into a very small spot on the anode. Many microfocus X-ray sources operate with focus spots in the range 5-20 µm, but in the extreme cases spots smaller than 1 µm may be produced.
The major drawback of solid-anode microfocus X-ray tubes is the very low power they operate at. In order to avoid melting of the anode the electron-beam power density must be below a maximum value. This value is somewhere in the range 0.4-0.8 W/µm depending on the anode material. This means that a solid-anode microfocus source with a 10 µm electron-beam focus can operate at a power in the range 4-8 W.
In metal-jet-anode microfocus X-ray tubes the solid metal anode is replaced with a jet of liquid metal, which acts as the electron-beam target. The advantage of the metal-jet anode is that the maximum electron-beam power density is significantly increased. Values in the range 3-6 W/µm have been reported for different anode materials (gallium and tin). In the case with a 10 µm electron-beam focus a metal-jet-anode microfocus X-ray source may operate at 30-60 W.
The major benefit of the increased power density level for the metal-jet X-ray tube is the possibility to operate with a smaller focal spot, say 5 µm, to increase image resolution and at the same time acquire the image faster, since the power is higher (15-30 W) than for solid-anode tubes with 10 µm focal spots.
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