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Discharge Regimes

All discharges have in common that free ions and electrons in a gaseous atmosphere are involved. We can distinguish different kinds of discharges:

Arcs frequently observed during thunder storms,
Glow discharges as in Neon tubes
Dark dischages less frequently observed in day to day live

Discharges are distinguished not only by their luminescence but also by their Current-Voltage characteristics, the current density and breakdown voltage. These main characteristics depend on the geometry of the electrodes and the vessel, the gas used, the electrode material. By changing the discharge current, we move from one discharge type to the next. The observed voltage current characteristic will be highly non-linear.

GD Regime

Dark Discharges

The regime between A and E on the voltage-current characteristic is termed a dark discharge because, except for corona discharges and the breakdown itself, the discharge remains invisible to the eye.

During the background ionisation stage of the process (A-B ) The charge density is extremely low. The electric field applied along the axis of the discharge tube only sweeps out the ions and electrons created by ionisation from background radiation. The background radiation originates from cosmic rays, radioactive minerals, or other sources. It produces a constant and measurable degree of ionisation in air at atmospheric pressure. Under the force of the electrical field the ions and electrons migrate to the electrodes producing a weak electric current. When voltage is increased more charged particles are drawn to the electrodes and the current increases. An avalanche reaction does not occur.

If the potential difference between the electrodes is further increased, eventually all the available electrons and ions are swept away, the current consequently saturates. In this saturation region (B - C), the current remains constant while the voltage is increased. This amplitude of the current depends, however, linearly on the radiation source strength This property makes the regime useful for the design of radiation counters.

When further increasing the voltage across the low pressure discharge tube; beyond point C, the current rises exponentially. The electric field is now high enough for the electrons initially present in the gas to acquire sufficient energy. They are now able to ionise a neutral atom creating more free charged particles. The avalanche process has started as these newly generated secondary electrons may gain enough energy to take their turn and ionise yet another neutral gas atom. This region of exponentially increasing current is called the Townsend discharge (C-E).

Corona discharge

Corona Discharge

Corona discharge occurs in Townsend dark discharges in regions of high electric field near sharp points, edges, or wires in gases prior to electrical breakdown. It is important to point out that the electrical field is here the dominant parameter, rather then the potential difference between the electrodes . If the coronal currents are high (D-E) enough, corona discharges can be visible to the eye and resemble a glow discharge For low currents, the entire corona is dark, as appropriate for the dark discharges. Corona discharges are also called partial discharges as they do not occupy the entire distance between the two electrodes, but are present only in the region of high electrical field. Corona discharges in air emit light mainly in the UV spectral region with a small portion in the blue. They are therefore not always visible to the eye. Related phenomena include the silent electrical discharge. They are called silent because they are inaudible form of filamentary discharges. A Brush discharge, a luminous discharge in a non-uniform electric field, consists of many corona discharges which are active at the same time and form streamers through the gas.

Brush Discharge
Brush Discharge

The electrical breakdown occurs in Townsend regime when the ions reaching the cathode have sufficient energy to generate secondary electrons(E). Photon impact is a different possible process for generating secondary electrons. At the breakdown, or sparking potential VB, the current might increase by a factor of 10to 108. It is usually limited only by the internal resistance of the power supply connecting the two electrodes. If the current supply is to low, discharge tube cannot draw enough current to break down the gas, and the tube will remain in the corona regime with small corona points or brush discharges being visible at the electrodes. If power supply delivers enough current the gas will break down at the voltage VB, avalanche processes weill ocurre and the discharge will move into the normal glow discharge regime. The breakdown voltage for a particular gas and electrode material depends on the product of the pressure and the distance between the electrodes, as expressed in Paschenís law (1889).

Glow Discharge

The glow discharge regime owes its name to the typical luminous glow. The plasma gas emits light because the electron energy and number density are high enough to generate excited gas atoms by collisions. These excited gas atoms will eventually relax to their ground state by emission of photons. The applications of glow discharge include fluorescent lights (Neon tubes), dc parallel plate plasma reactors, used for depositing thin films. Glow discharges are also extensively used in plasma chemistry.

After a discontinuous transition from E to F, the gas enters the normal glow region (F - G), in which the voltage is almost independent of the current over several orders of magnitude in the discharge current. This current-voltage behaviour is very different from a normal Ohm type resistance. The electrode current density does not change with the total current in this regime. Only a small part of the cathode surface at low currents is in contact with the plasma. As the current increases from F to G, the fraction of the cathode occupied by the plasma increases, until plasma covers the entire cathode surface at point G. The discharge voltage remains constant over a large range of current variation( 2 or 3 magnitudes).
Once the whole surface of the cathode is covered by the discharge, the only way the total current can increase further is to drive more current through the cathode by increasing the current density. This requires more energy, applying more voltage moving away from the Paschen minimum. This regime where the voltage increases significantly with the increasing total current (G-H) is named the abnormal glow regime. The discharge now behaves here more like a normal resistance. Starting at point G and decreasing the current, a form of hysteresis is observed in the voltage-current characteristic. The discharge maintains itself at considerably lower currents and current densities than at point F and only then makes a transition back to Townsend regime.

In the abnormal discharge the cathode fall potential increases rapidly, and the dark space shrinks. Except for being brighter, the abnormal glow discharge resembles the normal discharge. The structures near the cathode may blend into one another and a rather uniform glow can be observed. At the same time as voltage and cathode current density increase the average ion energy bombarding the cathode surface also increases. Due to the high current density abnormal discharges are commonly used as sputter sources. The bombardment with ions ultimately heats the cathode causing thermionic emission. Once the cathode is hot enough to emit electrons thermionically, the discharge will change to an arc regime.

Arc Discharges

At point H, the electrodes become sufficiently hot that the cathode emits electrons thermionically. If sufficient current is supplied to the discharge it will undergo a glow-to-arc transition, (H-I). The arc regime, from I through K is one where the discharge voltage decreases as the current increases, until large currents are achieved at point J, and after that the voltage increases again slowly with increasing current. In spectro chemistry arc discharges are used in Spark OES and DC arc spectroscopy.