Introduction to Glow Discharges
The glow discharge owes its name to the luminous glow of the plasma. When a sufficiently strong electric field reigns in a gaseous medium, atoms and molecules in the medium will break down electrically, permitting current to flow. The initial break down is created by free electrons generated by collisions with cosmic particles constantly bombarding the earth. These free electrons are accelerated in the electric field. If they gain sufficient energy to cause ionisation of neutral gas atoms, a chain reaction starts creating more and more free charges.
The simplest glow discharge configuration consists of two parallel elecetrode plates being held on different electrical potential. One electrode is called cathode and is negatively charged, the other is the anode; it is on positive potential.Once the glow discharge is established the potential drops rapidly close to the cathode, varies slowly in the plasma, and changes again close to the anode. Consequently, the electric field is strong in the vincinity of the cathode (Cathode Dark Space, CDS) and the anode (Anode Zone).
The plasma, or more precisely the negative glow (NG) is virutually field free. The electric fields in the system are restricted to sheaths adjacent to each of the electrodes. The sheath fields repel electrons, having a much higher mobilitw then the ions, trying to reach either electrode. In fact, the plasma potential is always higher then the adjacent walls, thus reducing the electron loss rate towards the walls. Electrons originating at the cathode will be accelerated, collide, ionise, transfer energy, "dissapear" by recombination with a positively charged particle. Some reach the anode and get transferred into the outside circuit.
The luminous glow is produced because the electrons have sufficient energy to generate visible light by excitation collisions with the plasma carrier gas. Since there is a continuous loss of electrons and ions there must be an equal degree of ionisation going on to maintain the steady state. The energy is being continuously transferred out of the discharge and hence the energy balance must be satisfied as well. Simplistically, the electrons absorb energy from the field by accelerating, ionise some atoms, and the process becomes continuous. Additional electrons must be produced by secondary electron emission from the cathode. These are very important to maintaining a sustainable discharge. Three major regions can be distinguished in the discharge: the cathode region, the glow regions, and the anode region.
The Aston Dark Space (A) is a thin region close to the cathode. The electrical field is strong in this region accelerating the electron away from the cathode. The Aston dark space has a negative space charge, meaning that electrons outnumber the positive ions in this region. The electron density and energy is too low to efficiently excite the gas, it consequently appears dark.
In the Cathodic Glow, (B) next to the Aston dark space, the electrons are energetic enough to excite the neutral atoms during collisions. The cathode glow has a relatively high ion density. The cathodic glow sometimes masks the Aston dark space as it approaches the cathode very closely. The axial length of the cathode glow depends on the carrier gas, the pressure and temperature.
The Cathode (Crooks, Hittorf) dark space (C) is a relatively dark region that has a strong electric field, a positive space charge and a relatively high ion density. Its axial extension depends on the pressure and the applied voltage. For discharges operating at a few hPa its length is about 0.5 mm. In this region the electrons are accelerated by the electric field. Positive ions are accelerated towards the cathode. They cause the pulverization of the cathode material and the emission of secondary electrons. These electrons will be accelerated and cause the creation of new ions through collision with neutrals. The majority potential difference between the two electrodes is across a narrow region surrounding the cathode. Hence, the CDS is also called 'cathode fall'.
The Negative Glow NG (D) is the brightest intensity of the entire discharge. It extends typically for about 2-3 mm away from the sample. Electrons carry almost the entire current in the negative glow region. Electrons that have been accelerated in the cathode region to high speeds produce ionisation, and slower electrons
that have had inelastic collisions already produce excitations. The negative glow is predominantly generated by the slow electrons, however other processes play a significant role. The NG is the region where most exciting and ionising collision processes occur because of the high density for both negative and positive charged particelsin this area. Hence, this zone is the source of light used in GD-OES and allows acquiring most analytical information. In the presence of argon ions, electrons can determine the space charge. Indeed, in this region, positive and negative space charges are equal to each other, resulting in charge neutrality. However, electrical current in NG is predominantly carried by the electrons, due to their high mobility. At the end of the negative glow, the electrons have lost most of their energy, exctitaion and ionisation proceses cease to exist. This is the start of the next dark region.
The Faraday dark space (E) separates the negative glow from the positive column. The electron energy is low in this region. The net space charge is very low, and the axial electric field is small.
The Positive Column (F) is a luminous region that prolongs the negative glow to the anode. It has a low net charge density, only a small electric field of typically 1 V/cm. The electric field is just large enough to maintain the degree of ionisation to reach the anode. As the length of the discharge tube is increased at constant pressure, the cathode structures do not change in size. It is the positive column that lengthens to form a long, uniform glow region. The uniformity can easily been perturbed and except when standing or moving striations observed.
The Anodic glow (G)is slightly brighter than the positive column. It is not always observed. The anode glow is the boundary of the anode sheath.
The Anode dark space (H) or anode sheath is the space between the anode glow and the anode itself.
It has a negative space net charge density due to electrons traveling towards the anode. The electric field is higher than in the positive column.
First published on the web: 17 October 2006.
Authors: Lydie Salsac & Thomas Nelis
This article is based on the master works of Lydie Salsac and Anouar Kanzari. Both have gained their master degree at INSTN, Saclay, France, after performing their master work at EMPA Material Science and Technology, Thun, Switzerland