Glow discharge

A glow discharge is a gas discharge that occurs autonomously between two on a DC or AC voltage source lying cold electrodes at low gas pressure. The color of the glow depends on the gas, for neon, for example, it is reddish.

Application

The costs arising from the discharge light emissions are used in fluorescent lamps and neon lamps for illumination or display.

In vacuum technology, a glow discharge for the cleaning of surfaces to be coated is used in Beglimmen. For this purpose, in the recipient a vacuum of 10-2 to 10-3 mbar is set and configured with a variable high voltage source, a glow discharge between a ring electrode and ground. The recipient in the whole sprawling gas discharge (plasma ) sublimated and oxidized adhering to the surfaces of impurities such as organic compounds with other cleaning methods are difficult to remove.

Spatial structure of the discharge levels

From the cathode to the anode border, the glow discharge can be divided into eight successive layers:

• Closest to the cathode is the Aston dark space. It is quite thin, but visible when inert gases or hydrogen are in the discharge tube.
• It is followed by a thin, reddish light skin, referred to as the first cathode layer or Kathodenglimmhaut.
• This is followed by a fainter zone closes at which Hittorf or Crookes dark space or cathodic dark space is called.
• The brightest part of the discharge process is the negative glow, which is clearly separated from the Hittorf dark space and is weaker toward the other side. The large voltage drop between the cathode and the onset of Glimmlichts called cathode fall. This voltage drop is due to the ionization of the gas particles. The resulting positive ions ( they drift 'slow' to the cathode ) are responsible for the sharp voltage drop ( = cathode drop ) between the cathode and the negative glow. Due to the larger inertia ( Masse! ) of the positive ions produced an excess of positive charge carriers. The field strength between the negative glow and the anode decreases. The electrons are therefore less strongly accelerated and it decreases their ionization capacity. In this area there is a negative space charge.
• The subsequent lightless zone called Faraday dark space.
• Further, the positive column joins, which comes into appearance depending on pressure and gas fill as a continuous ribbon of light or in the form of separated layers.
• In the vicinity of the anode, the anodic glow occurs.
• Directly at the anode is the anode dark space.

Properties

In contrast to other forms of gas discharge in the glow discharge, the temperature of the electrodes and walls is low, since only little heat is released through the low current density and the associated impact of charge carriers.

By the typical glow discharges low gas pressure, the mean free path of the electrons is higher than that of atmospheric discharges. Thereby, the exchange of energy between electrons and the heavier gas particles (atoms, molecules and ions) is reduced by glow discharge, since the number of particle collisions to decrease. Therefore, the temperatures of the individual gas components differ considerably. Is the average energy of electrons in a temperature equivalent, then temperatures from 103 to 105 K. The resulting temperature of the ions and neutral particles, however, remains in the vicinity of room temperature. One speaks in this case of a non-thermal plasma.

The negative glow and the stratification of the positive column described above arises from the fact that the electrons between the individual layers are in each case unless accelerated until they have built up the necessary for the excitation of the gas energy. At the cathode, electrons are released by means of thermal emission or secondary ions or electrons by photon and accelerated by the electric field. As long as their energy is below the excitation energy of the gas, the collision between the electrons and the neutral particles of the gas are substantially elastic. In an elastic collision between two bodies with very different mass, the kinetic energy of the lighter collision partner is almost maintained (in this case the electron). If the energy of the electrons by the acceleration in the field so large that the excitation energy of the gas is reached, the gas particles are excited and the excitation electrons lose most of their kinetic energy ( inelastic collision). The excited gas particles lose their excited state by optical radiation. The first light-emitting layer is, therefore, the portion at which the electrons have built up by the acceleration in an electric field required for the type of gas excitation energy for the first time. The decelerated electrons are accelerated again from this area by the electric field until they lose one more time their energy by exciting the gas particles. Through this mechanism, the different light layers are built.

The expression of the positive column depends on the interaction of the electrons with the glass tube. The glass tube decelerates the electrons, thereby increasing the recombination rate and the reduced electron density. This will give the remaining electrons have enough energy to ionize other atoms. Is the diameter of the tube relative to the electrode is too large, no positive column is formed. For this reason, there is no spherical fluorescent tubes. Tubes with a smaller diameter allow higher gas pressures and at the same operating voltage lower lengths.

A clear expression of the luminescent layers is only possible when there is very defined excitation states in the gas. For this reason, no gas mixtures should be used for observation of the phenomenon in an experiment, and the gas should have a simple structure of electrical excitation, as is the case for example with rare gases.