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I've seen two separate reasons given for why electrons are released from the cathode in a discharge tube.

  1. Thermionic emissions from heating up the filament
  2. Collisions with gas ions that are drawn to the cathode

So which is it that actually causes these emissions?

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2 Answers 2

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Both processes are present but depend sensitively on the quality of the vacuum. In a high vacuum, there are essentially no gas ion collisions with the electrodes and so this mechanism for electron emission is absent, or nearly so.

For a vacuum which has a significant number of gas molecules still in it, the kinetic emission of electrons upon gas ion impingement is an important contributor to the insulator-to-conductor breakdown process.

For more on these phenomena search on dielectric breakdown of air, Townsend, and Paschen.

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Normally heating up the fillament is not sufficient to ionize the atoms and release the electrons. However it helps because the electrons are is exited states and therefore has lower ionization energy.

Neither is it collisions with gas ions.

The electrons are released due to a strong electical field applied, pulling negative electrons in one direction and the positive core in the opposite direction.

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