Glow Discharge

The name of glow discharge is the phenomenon that plasma is luminous and depends on the geometry of the electrodes and the vessel, the gas used, the electrode material. When breakdown occurs, the gas suddenly becomes conductive. The current and voltage in the gas discharge vary resulting changing supply voltage. These parameters can be measured to obtain a voltage-current (V-I) characteristic for a discharge as illustrated in Fig. 2.5. In addition, current is limited through the discharge by using an external ballast resistor.

Fig. 2.5. Voltage-current (V-I) characteristic for a discharge.

Three general regions can be identified on the diagram above, the dark discharge (Townsend region), the glow discharge and the arc discharge. Because the discharge remains invisible to the eye except for corona discharge and the breakdown itself, the regime between A and E on the voltage-current characteristic is termed a dark

discharge. The electric field sweeps out the ions and electrons created by ionization from background radiation during the background ionization. The ions and electrons migrate to the electrodes in the applied electric field producing a weak electric current.

Increasing voltage results in an increasing fraction of these ions and electrons. If the voltage between the electrodes is increased far enough, eventually all the available electrons and ions are swept away, and the current saturates. The regime is named the saturation region because the current remain constant while the voltage is increased.

After the voltage across the saturation region, the current will rise exponentially. The electric field is now high enough so the electrons initially present in the gas can acquire enough energy before reaching the anode to ionize a neutral atom. As the electric field becomes even stronger, the secondary electron may also ionize another neutral atom leading to an avalanche of electron and ion production. The regime of exponentially increasing current is called the Townsend discharge. The Corona discharge occurs in the regime of Townsend discharge because of high electric field near sharp points, edges, or wires in gases prior to electrical breakdown. If the coronal currents are high enough, corona discharge can be technically “glow discharges”, visible to the eye. For low currents, the entire corona is dark, as appropriate for the dark discharge. After the breakdown, the gas enters the normal glow regime, which the voltage is almost independent of the current over several orders of magnitude in the discharge current. The electrode current density is independent of the total current in this regime. This means that the plasma is in contact with only a small part of the cathode surface at low currents. As the current is increased, the fraction of the cathode occupied by the plasma increases until plasma covers the entire cathode surface.

When the cathode is completely covered by the plasma, the current is increased by increasing the voltage in order to force the cathode current density above its natural value and provide the desired current. The regime which has the positive slope of

current-voltage curve is so-called the abnormal discharge. The discharge maintains itself at considerably lower currents and current densities and only then makes a transition back to Townsend discharge regime. Further increase in the current will result in a sudden drop in voltage and the discharge will undergo a glow-to-arc transition. After transition of the discharge is an arc discharge where the discharge voltage decreases as the current increases, until large currents are achieved at a low voltage, and after that the voltage increases slowly as the current increases.

The glow can be produced by applying a potential difference in a gas between two electrodes. The potential drops rapidly close to the cathode, vary slowly in the plasma, and change again close to the anode. The voltage distribution in a dc glow discharge process is shown in Fig. 2.6. The electric fields between the gaps are restricted to sheath at each of electrodes. The sheath fields are such as to repel electrons trying to reach either electrode. Electrons originating at the cathode will be accelerated, collide, transfer energy, leave by diffusion and recombination, slow by the anode and get transferred into the outside circuit. The luminous glow is produced because the electrons have enough energy to generate visible light by excitation collisions. Since there is a continuous loss of electrons, there must be an equal degree of ionization 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 also.

Simplistically, the electrons absorb energy from the field, accelerate, ionize some atoms, and the process becomes continuous. Additional electrons are produced by secondary emission from the cathode. These are very important to maintain a sustainable discharge.

Fig. 2.6. The potential distribution in a dc glow discharge process.

Fig. 2.7. The spatial distribution of (a) the luminous regions in a typical discharge and (b) the electric field.

Unlike Townsend or arc discharge, the glow discharge is characterized by distinct regions with large variations in electric field [34]. The approximate appearance of the different luminous regions is shown in Fig. 2.7(a). Also shown for reference is the associated potential distribution between cathode and anode in Fig.

2.7(b). Three basic regions are described below, the cathode region, the glow regions and the anode region. The length of the cathode region is from the cathode surface to the boundary of the negative glow. Most of the potential drop across the cathode and anode occur between the cathode and the negative glow. The energies of electrons accelerated in this region are high enough to produce ionization and avalanching in the regions to the right of the right of the negative glow. The electrons are accelerated from the cathode to Aston Dark Space which is a thin regime with a strong electrical field. This regime has a negative space charge, meaning that stray initial electrons together with the secondary electrons from the cathode outnumber the ions in this regime. The electrons are too low density and/or energy to excite the gas, so it appears dark. The next regime to right of Aston dark space is cathode glow. The electrons are energetic enough to excite the neutral atoms they collide with. In addition, the cathode glow has a relatively high ion density. The cathode dark space that is to the right of the cathode glow has moderate electric field, a positive space charge and a relatively high ion density. These fine-structured regions make up what is known as Crooke’s dark space which is commonly referred to as the cathode fall or sheath. The brightest intensity of the entire discharge names negative glow. The negative glow has relatively low electric field, long, compared to the cathode glow and is the most intense on the cathode side. Electrons carry almost the entire current in the negative glow region. Electrons that have been accelerated in the cathode region to high speeds produce ionization, and slower electrons that have had inelastic collisions already produce excitations. These slower electrons are responsible for the negative glow. The

electron number density in the negative glow is about 10¹⁶ electrons/m³. Before reaching the positive column, the electrons lose their energy through more collisions in the Faraday dark space. The electron density decreases by recombination and diffusion to the walls. Furthermore, the net space charge is very low and the electric field is small. The uniform electric field in the positive column continually accelerates the electrons, which, in turn, undergo decelerating collisions. In the regime, small electric field is typically 1 V/cm. The electric field is just large enough to maintain the degree of ionization at its cathode end. The electron number density is about 10¹⁵ to 10¹⁶ electrons/m³ in the positive column, and the electron temperature of 1 to 2 eV. A thin sheath also exists near the anode with a similar structure to the cathode fall made-up of the anode glow and anode dark space. The voltage drop in this region is significantly smaller than near the cathode and plays little role in the discharge dynamics. Anode glow is slightly brighter than positive column. This is the boundary of the anode sheath. The anode dark space between the anode glow and the anode is the anode sheath. It has negative space charge due to electrons traveling from the positive column to the anode. There is a higher electric field than the positive column.

The anode pulls electrons out of the positive column and acts like a Langmuir probe in electron saturation.