1.2 Brief history of electric discharge research

Today new conductive materials are of great interest. For example, only gaseous medium conductivity is being investigated in such countries as Japan, Germany, Russia, USA, and here, in Kazakhstan.

So here some lyrical digression. Leaving lightning aside, man’s first acquaintance with electric discharges eas the observation, dating back to 1600, that friction-charged insulated conductors lose their charge. Coulomb proved experimentally in 1785 that charge leaks through air, not through imperfect insulation. We understand now that the cause of leakage is the non-self-sustaining discharge.

Occasional experiments were conducted in the 18^th century with sparks produced by charging a body by an electrostatic generator, and with atmospheric electricity, experiments with lightning sometimes having tragic consequences.

Sufficiently powerful electric batteries were developed at the 19^th century to allow the discovery of the arc discharge. V.V.Petrov. who worked in the Saint Peterburg Medical Surgery Academy in Russia, reported the discovery in 1803. The arc was obtained by bringing two carbon electrodes connected to battery terminals into contact and then separating them. Several years later Humphrey Davy in Britain produced and studied the arc in air. This type of discharge became known as “arc” because it’s bright horizontal column between two electrodes bends up and arches the middle owing to the Archimede’s force. In 1831-1835, Faraday discovered and studied the glow discharge.

Beginning in 1900 J.S.E. Townsend, a student of J.J.Thomson and the creator of a school in a physics of gas discharges discovered the laws governing ionization and the gaseous discharge in uniform electric field. The concept of plasma was introduced by I.Lengmuir and L.Tonks in 1928 [3].

As we interested in electric current in gases we need to understand how this process leads. The operative part of this conception is to understand ionization process proceeding.

3. Ionization

Ionization (or ionisation) is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons. Ionization, often, results from the interaction of an atom or a molecule with an ionizing particle, including charged particles with sufficient energies and energetic photons. A rare case of ionization in the absence of an external particle is the internal conversion process, through which an excited nucleolus transfers its energy to one of the inner-shell electrons and ejects it with high kinetic energy.

The ionization process is of particular interest in fundamental science. It is also encountered in many fields of practical interest, ranging from mass spectroscopy to radiotherapy for eliminating cancer cells. Obviously, all aspects of the ionization process cannot be addressed in a single encyclopedic article. Here, ionization is described by invoking the simple case of an atom in strong laser field. The case is amenable to analytic treatment and represents the main characteristics of the process.

Due to the nature of initializing source ionization divides into some kinds.

Ionization by electron impact. Ionization contained in the collision, the electrons emitted by the surface of the negative electrode pole (cathode), moved generally in its own way collide with neutral gas particles. And so, in inelastic collisions the released energy is used partly to excitation or ionization of neutral particles.

Produced by ionization of secondary electrons while moving in a gaseous environment has the potential to cause further excitation or ionization of other neutral particles. By the way, the excitation and ionization of neutral particles really likely only if the kinetic energy of the particles become enough in the ionization or excitation work [3, 5].

Picture 1. Ionization by electron impact.

Photoionization - the formation of gases of electrically charged particles through absorption of radiation photons by atoms. Perhaps photoionization likely the case if a photon energy will become more than ionization of the gas.

Visible light does not cause photoionization of gases because of very great wavelengths. Ultraviolet photoionization may appear in gases of small ionization potential, for example vapors of alkali and alkaline earth metals. It was said, that strong ionizers reasonably considered just the same X-ray and gamma- rays having simply small wavelengths and ionizing all the gases in the absence of exceptions.

Picture 2. Photoionization.

Heat (thermal) ionization takes place at high temperatures. In the end, the higher the temperature of the gas heating, the more calmly move its particles ( atoms, molecules ), and more they might appear collisions. In the general case of inelastic collisions ionization of neutral particles may take place. Probably , the formation of electrically charged particles in the gases is already noticeable at a temperature of 2000 ° C.

At a temperature of the arc column about 6000 ° K thermal ionization become solid .

Appeared as a consequence of collisions independent electrons among others will participate in the actions of the excitation and ionization of neutral particles. Fortunately, once the gas happens quite the reverse process of reunification particles other symbol to form neutral particles (recombination). In fact the gas ionization stage determines the ratio of any decent electrically charged particles to the total number of neutral particles that existed in this medium. Apparently quite at atmospheric pressure ionization stage gas varies with configuration and temperature is dependence on the ionization potential [2, 4].

In the criteria of welding of arc is being found not one gas, but the mixture of vapors and gases, so that the ionization of any included in the mixture gas passes differently from individual especially gas. Valid for evaluation stages of ionization of the gas mixture is introduced the concept of the effectiveness of ionization potential. Apparently a successful ionization potential of the gas mixture have an ionization potential of a homogeneous gas, which at the same temperature and the same pressure, the same selects the ionization step , which part of the gas mixture composition.

If in the atmosphere of the arc will be induced drugs with a small ionization potential, the effective ionization potential of the gas mixture significantly decreases that highly improve the stability of the arc.

Picture 3. Thermal ionization.

Electric field ionization shows that the electric field rather calmly acting on electrically charged particles of gas directs and accelerates their displacement. Moreover in a collision with a neutral particle of case or excitation or ionization final [2, 4].

Picture 4. Electric field ionization.