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3. Glow Discharge

Table of Contents:

1. Introduction
2. Gaseous Conduction
3. Glow Discharge
4. Sputter Coating
5. Operating Characteristics
6. Specification

Once the condition for a sustained discharge is met, the tube exhibits the characteristic glow discharge, so called because of the associated luminous glow. It has been established that free ions and electrons are attracted to opposite electrodes producing a discharge; however, for a discharge to be self-sustaining requires regeneration of the electrons by the positive ion bombardment of the cathode. This produces secondary electrons and enhances ionisation. The resulting positive ion excess creates a positive space charge near the cathode. The voltage drop experienced is termed the cathode fall. If the discharge is established in a long narrow tube it exhibits the characteristics indicated.

Physical Appearance of Glow Discharge

Figure 2.

The positive ion density in the Crookes dark space is very high; as a result a significant voltage drop is experienced between it and the cathode. The resulting electric field accelerates the positive ions, which produce secondary electron emission from the cathode.

These electrons are accelerated in the direction of the anode and cause ionisation, generating positive ions to sustain discharge. Subsequently, excitation of the gas results in intense illumination in the negative glow region. From this stage the electrons have insufficient exciting or ionising energy, resulting in the Faraday dark space. Towards the anode, a small accelerating field can produce ionisation and excitation, the gas again becoming luminous.

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