Electric discharge in gases
Of a gas discharge occurs when electric current flows through a gas and it is thereby ionized. This can result in visible light. The gas discharge can be " fired " in different ways, maintaining the impact ionization avalanche effect requires a certain minimum current.
If the gas discharge with unheated electrodes, the characteristic line at low gas pressure in three different areas:
- At currents below about 1 uA no visible light is generated. One speaks of dark discharge. The current sets in, when individual gas atoms are ionized. For example, sufficient natural radioactive radiation. From an operating voltages above 100 V, the current is amplified by the avalanche effect in which each electron can ionize other atoms, which release additional electrons ( impact ionization ). This is desirable in the counter tube, because it replaces a million-fold amplification of the signal.
- If the current is between 1 mA and 50 mA, is formed by glow discharge weak, visible light, whose color is determined by the gas composition. It is characterized by the so-called cathode fall, a low-light zone around the cathode and the negative differential resistance in the range D to G, which allows the construction of a simple relaxation oscillator. Since the recombination rate is very high, the avalanche effect disappears when falling below the minimum current.
- For currents above about 500 mA is called an arc discharge, in addition to very intense light and high temperatures, especially at the electrodes, arise. The additional electrons emerging from glowing cathode, increase the Number of available free electrons huge. Pincheffektes result of the current flowing in a relatively thin channel, as can be seen also in flash.
Plasma formation and gas discharges are also possible by means of a electrodeless high-frequency field. This possibility is used in induction lamps and some lasers.
If the spontaneous flow of current is started by the gas or only has to be initiated, depends primarily on the gas pressure, because this will affect the mean free path of the electrons. On this " racetrack " free electrons are accelerated by the electric field between the electrodes and gain kinetic energy. Only when these before the next collision with an atom of the minimum value of the ionization energy ( Größenenordnung: 20 eV ) exceeds, another electron is released and the avalanche effect starts.
- If the pressure is too low ( for example, in a vacuum chamber ), free electrons can indeed exceed these minimum energy, but hardly find a collision partner and the amperage is very low. Since no avalanche effect is caused, the current can not increase indefinitely as well.
- Glow lamps in gas pressure and electrode spacing are selected such that safe from a total voltage of approximately 80 V, the avalanche effect starts as soon as a "start electron" is present. This is knocked out, for example, by the natural radioactivity of an atom. Without adequate bias resistor, the current increases indefinitely.
- In xenon gas discharge lamps, there is a very high gas pressure at low electrode spacing; in flash tubes, the gas pressure but the electrode spacing is less, but larger. In both cases, the avalanche effect does not start at voltages below 500 V, because too many free electrons are bonded again by recombination. Only at starting voltages of several thousand volts is sufficient initial number. As soon as a minimum current is exceeded by a few milliamperes, and the main power supply those sufficient number of electrons, is an arc and the lamp voltage drops to approximately 30 V.
- In fluorescent lamps is due to the large electrode spacing the accelerating field strength is so low that when the ignition Thermionic need to increase the number of free electrons sufficient to initiate the avalanche effect on ignition voltage to 600V.
- For strongly curved, negatively charged surfaces can spontaneously due to field emission electron escape and make the surrounding gas electrically conductive. Since the field strength decreases drastically with increasing distance, it usually comes at no avalanche effect and the current remains low. This gas discharge is desirable in electrostatic precipitators and is prevented by the corona rings in high voltage power lines.
- Electric arcs, such as for welding, and high-pressure gas discharge lamps,
- Glow discharges in fluorescent tubes, neon lamps, plasma screens, plasma lamps
- Spark discharges, for example, for the ignition in internal combustion engines
- Photo flashes in or on the camera
- Plasmatron for cutting and welding
- Pump discharge of gas lasers such as He-Ne laser, nitrogen laser, CO2 laser, argon ion laser, excimer laser
- Geiger- Müller counter
- Ionization gauge
- Mercury vapor rectifier and thyratron
- Geißlersche tube
- Corona discharge causes energy losses in high-voltage lines
- Silent electric discharge to produce ozone
- Sputtering, for example, for producing thin layers