The emission mechanism of a barium tungsten cathode involves a process called thermionic emission. Here's how it works:
Barium Deposition: In a barium tungsten cathode, a thin layer of barium is deposited onto the surface of a tungsten substrate. This is typically achieved through a process called barium flashing, where the cathode is heated in a vacuum chamber containing a small amount of barium metal. The heat causes the barium to vaporize and deposit onto the tungsten surface.
Activation: Once the barium is deposited, the cathode is subjected to further heating, which activates the barium layer. During this activation process, some of the barium atoms lose electrons, becoming positively charged ions (Ba+), while others absorb electrons, becoming negatively charged (Ba-).
Electron Emission: When the cathode is heated to its operating temperature, typically several hundred to a few thousand degrees Celsius, the electrons in the tungsten lattice gain enough thermal energy to overcome the work ff of the material. The work ff is the minimum amount of energy required to liberate an electron from the surface of a material. In the case of barium tungsten cathodes, the presence of the barium layer lowers the work ff of the tungsten substrate, making it easier for electrons to escape.
Emission Current: Once emitted from the surface of the cathode, the electrons form a cloud or beam, depending on the application. This emission of electrons constitutes the cathode's emission current, which can be controlled and manipulated to generate the desired electron beam characteristics.
In summary, the emission mechanism of a barium tungsten cathode relies on the thermionic emission process, facilitated by the presence of a barium layer on the tungsten substrate. This process enables the efficient generation and emission of electrons from the cathode surface, making it suitable for various applications such as electron microscopy, X-ray tubes, and electron beam welding.
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