Cathode rays

In 1854, experiments with high voltage in rarefied air began. And it was noticed that sparks run a noticeably greater distance under vacuum, compared to normal conditions.

Cathode rays consist of electrons accelerated in vacuum by the potential difference between the cathode and anode, i.e. electrodes located respectively under negative and positive potential relative to each other. Cathode rays possess kinetic energy and are capable of imparting mechanical motion to, for example, vanes. Cathode rays are deflected by magnetic and / or electric fields. Cathode rays can cause luminosity of phosphors . Therefore, when applying phosphors to the inner surface of a transparent tube, the glow can be seen on the outer surface of the tube. This effect is used in vacuum electronic devices , such as cathode ray tubes , electron microscopes , x-ray tubes and radio tubes .

The kinetic energy E of the cathode rays near the anode (if there are no obstacles between the cathode and the anode) is equal to the product of the electron charge e and the electrode potential difference U : E = eU . For example, if the potential difference is 12 kV , the electrons acquire an energy of 12 kilo electron volts (keV).

For the appearance of cathode rays, the exit of electrons from the cathode into the interelectrode space, which is called electron emission, is necessary. It can occur as a result of heating the cathode ( thermionic emission ), its illumination ( photoelectron emission ), electron impact ( secondary electron emission ), etc.

Although the cathode ray electrons quickly lose energy in a dense substance, they can penetrate through a rather thin wall (fractions of mm) from a vacuum tube into the air if the accelerating potential is high enough (tens of kilovolts). The mileage of cathode rays in the air with energies of tens of kiloelectron-volts is limited to a few centimeters.

In a vacuum, cathode rays are not visible, but when interacting with a substance, they cause its radioluminescence due to the excitation of atomic shells and the emission of energy by an atom through photons, including visible light. In particular, in the presence of residual gas in the vacuum tube, its glow can be observed (see the pink glow in the tube in the photo below). Radioluminescence is also observed in the substance of the anode or other objects falling under the beam (for example, glass at the end of the Crookes tube), and in air when cathode rays are removed from the tube.

Cathode rays are used in electron beam technologies ^[1] , for example, the universal electron beam evaporator UELI-1 ^[2] , created for spraying film coatings , as well as in electronic lithography . Electron beam technologies are more environmentally friendly, less energy-intensive and practically waste-free ^[3] . They are also used in 3D printers ( Electron-beam melting, EBM , Layer-by-layer synthesis by electron beam ).

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  Crookes tube

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  Crookes tube at work

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  Cathode ray in a magnetic field

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  Cathode ray in a magnetic field

• Cathode-ray tube
• Electron beam melting
• Electron beam welding
• Electron gun
• Charged particle accelerator
• Anode rays

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• Vinogradov M.I., Maishev Yu.P. Vacuum processes and equipment of ion - and electron-beam technology. - M .: Engineering, 1989 .-- 56 p. - ISBN 5-217-00726-5 .

• Electron beam in intersecting fields en
• Intersecting Electron Beam en
• Animation of the above photo on the FizKapu portal (Hungarian)