Citizendia

Avalanche effect between two electrodes
Avalanche effect between two electrodes

The Townsend discharge is a gas ionization process where an initially very small amount of free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication: when the number of free charges drops or the electric field weakens, the phenomena ceases. Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can It is a process characterized by very low current densities: in common gas filled tubes, typical magnitude of currents flowing during this process range from about 10 − 18A to about 10 - 5A, while applied voltages are almost constant. A gas-filled tube, also known as a discharge tube, is an arrangement of Electrodes in a Gas within an insulating, temperature-resistant Envelope Subsequent transition to ionisation processes of dark discharge, glow discharge, and finally to arc discharge are driven by increasing current densities: in all these discharge regimes, the basic mechanism of conduction is avalanche breakdown. Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas usually Argon or another Noble An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials Townsend discharge is named after John Sealy Townsend. John Sealy Edward Townsend (7 June 1868 - 16 February 1957 was a Mathematical Physicist who conducted various studies concerning the Electrical conduction

Contents

Quantitative description of the phenomenon

The basic setup of the experiments investigating ionization discharges in gases consist of a planar parallel plate capacitor filled with a gas and a continuous current high voltage source connected between its terminals: the terminal at the lower voltage potential is named cathode while the other is named anode. Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors This page is about the physical properties of gas as a state of matter Direct current ( DC) is the unidirectional flow of Electric charge. A voltage source is any device or system that produces an Electromotive force between its terminals OR derives a secondary voltage from a primary A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device An anode is an Electrode through which Electric current flows into a polarized electrical device Forcing the cathode to emit electrons (eg. by irradiating it with a X-ray source), Townsend found that the current I flowing into the capacitor depends on the electric field between the plates in such a way that gas ions seems to multilply as they moved between them. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. John Sealy Edward Townsend (7 June 1868 - 16 February 1957 was a Mathematical Physicist who conducted various studies concerning the Electrical conduction In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge He observed currents varying over ten or more orders of magnitude while the applied voltage was virtually constant: the experimental data obtained from his (and his school's) first experiments are described by the following formula

\frac{I}{I_0}=e^{\alpha_n d},

where

The almost constant voltage between the plates is equal to the breakdown voltage needed to create a self-sustaining avalanche: it decreases when the current reaches the glow discharge regime. Breakdown Voltage of an Insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas usually Argon or another Noble Subsequent experiments revealed that the current I rises faster than predicted by the above formula as the distance d increases: two different effects were considered in order to explain the physics of the phenomenon and to be able to do a precise quantitative calculation.

Gas ionisation caused by motion of positive ions

Townsend put forward the natural hypothesis that also positive ions produce ion pairs, introducing a coefficient αp expressing the number of ion pairs generated per unit length by a positive ion (cation) moving from cathode to anode. John Sealy Edward Townsend (7 June 1868 - 16 February 1957 was a Mathematical Physicist who conducted various studies concerning the Electrical conduction An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device An anode is an Electrode through which Electric current flows into a polarized electrical device The following formula was found

\frac{I}{I_0}=\frac{(\alpha_n-\alpha_p)e^{(\alpha_n-\alpha_p)d}}{\alpha_n-\alpha_p e^{(\alpha_n-\alpha_p)d}} \qquad\Longrightarrow\qquad \frac{I}{I_0}\cong\frac{e^{\alpha_n d}}{1 - {\alpha_p/\alpha_n} e^{\alpha_n d}}

since β < < α, in very good agreement with experiments.

Cathode emission caused by impact of ions

Townsend and Holst and Oosterhuis also put forward an alternative hypothesis, considering augmented emission of electrons by cathode caused by positive ions impact, introducing Townsends second ionization coefficient εi, the average number of electrons released from a surface by an incident positive ion, and working out the following formula:

\frac{I}{I_0}=\frac{e^{\alpha_n d}}{1 - {\epsilon_i}\left(e^{\alpha_n d}-1\right)}.

These two formulas may be thought as describing limiting cases of the effective behavior of the process: note that they can be used to well describe the same experimental results. John Sealy Edward Townsend (7 June 1868 - 16 February 1957 was a Mathematical Physicist who conducted various studies concerning the Electrical conduction A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Mathematics, specifically in Topology, a surface is a Two-dimensional Manifold. Other formulas describing, various intermediate behaviors, are found in the literature, particularly in reference 1 and citations therein.

Applications

Neon lamp/cold-cathode gas diode relaxation oscillator
Neon lamp/cold-cathode gas diode relaxation oscillator
f\cong\frac{1}{R_1C_1\ln\frac{V_1-V_{GLOW}}{V_1-V_{TWN}}},
where
  • VGLOW is the glow discharge breakdown voltage,
  • VTWN is the Townsend discharge breakdown voltage,
  • C1, R1 and V1 are respectively the capacitance, the resistance and the supply voltage of the circuit. Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas usually Argon or another Noble Breakdown Voltage of an Insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. Breakdown Voltage of an Insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical
Since temperature and time stability of the characteristics of gas diodes and neon lamps is low, and also the statistical dispersion of breakdown voltages is high, the above formula can only give a qualitative indication of what the real frequency of oscillation is. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of A neon lamp is a Gas discharge Lamp containing primarily Neon gas at low Pressure. In Statistics, (statistical dispersion (also called statistical variability or variation) is variability or spread in a Variable or a Probability

See also

References

© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
 Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic