Gas discharge

from Wikipedia, the free encyclopedia

One speaks of a gas discharge when electrical current flows through a gas and this is ionized in the process. This can also result in visible light. The gas discharge can be "ignited" in different ways, maintaining the impact ionization with avalanche effect requires a certain minimum current .

Characteristic curve of an electrical discharge in neon gas over a wide current range. The pressure is 1 Torr, the electrode spacing 50 cm.

Classification

Gas discharge at a metal tip carrying high voltage. The pinch effect ensures thin strands of electricity.

If the gas discharge takes place with unheated electrodes, three areas of the characteristic curve shown can be distinguished at low gas pressure (far below atmospheric pressure):

  • With currents below about 1 µA, no visible light is generated. One speaks of a dark discharge . Electricity starts when individual gas atoms are ionized. For example, natural ionizing radiation is sufficient for this . From an operating voltage above about 100 V, the current is increased by the avalanche effect, in which each released electron ionizes additional atoms, which release additional electrons ( impact ionization ). This is desirable in the counter tube because it replaces a millionfold amplification of the signal.
  • If the current is between 1 mA and 50 mA, the glow discharge produces weak, visible light, the color of which is determined by the gas composition. Characteristic are the so-called cathode drop , a low-light zone around the cathode and the negative differential resistance in the range D to G, which enables the construction of simple tilting oscillators . Since the recombination rate is very high, the avalanche effect disappears when the current falls below the minimum value.
  • At currents above around 500 mA, one speaks of an arc discharge , in which, in addition to very intense light, high temperatures are generated, especially at the electrodes. The extra electrons emerging from glowing cathodes greatly increase the total number of free electrons. As a result of the pinch effect , the current flows in a relatively thin channel, as can also be seen in lightning .

Plasma formation and gas discharges are also possible without electrodes by means of a high-frequency field . This possibility is used in induction lamps and some lasers .

ignition

Whether the flow of current through the gas starts spontaneously or has to be initiated first depends primarily on the gas pressure, because this influences the mean free path of the electrons. On this “race track”, free electrons are accelerated by the electric field between the electrodes and gain kinetic energy. Only if this exceeds the minimum value of the ionization energy (order of magnitude: 20 eV) before the next collision with an atom , another electron is released and the avalanche effect begins.

  • If the pressure is too low (for example in a vacuum chamber), free electrons can exceed this minimum energy, but hardly find any collision partners and the current strength remains very low. Since there is no avalanche effect, the current cannot increase indefinitely.
  • In glow lamps , the gas pressure and the distance between the electrodes are selected in such a way that the avalanche effect starts reliably from a total voltage of around 80 V as soon as a “starting electron” is present. This is knocked out of an atom , for example, by natural radioactivity . Without a sufficient series resistor, the current increases indefinitely.
  • In xenon gas discharge lamps there is a very high gas pressure with a small distance between the electrodes; In flash tubes the gas pressure is lower, but the electrode distance is larger. In both cases, the avalanche effect does not start at voltages below 500 V because too many free electrons are bound again through recombination. The number of starts is only sufficient at ignition voltages of a few thousand volts. As soon as a minimum current of a few milliamperes is exceeded and the main power supply supplies a sufficient number of electrons, the arc starts and the operating voltage drops to around 30 V.
  • In fluorescent lamps , due to the large electrode spacing, the accelerating field strength is so low that glow cathodes first have to increase the number of free electrons sufficiently to initiate the avalanche effect at ignition voltages of around 600 V.
  • Electrons can spontaneously escape from strongly curved, negatively charged surfaces through field emission and ionize the surrounding gas. Since the field strength drops drastically with increasing distance, there is usually no avalanche effect and the current strength remains low. This gas discharge is desirable in electrostatic precipitators; it is undesirable on high-voltage lines and is prevented by corona rings .

Applications

literature

Experimental physics textbooks, e.g. B. Christian Gerthsen: Physik , 6th edition, Heidelberg 1960, pp. 300-301