Gas-filled tube

The gas discharge tube - sometimes just called discharge tube - is an array of cathode and anode in a gas-filled glass tube in which there is a gas discharge with the emission of light when a type- specific minimum voltage. The technical development for the light sources is generally referred to as gas discharge lamp.

• 4.2.1 Special features
• 4.2.2 Examples

History

In the 19th century, developed and built Heinrich Geissler named after him Geissler tubes. They exhibit, depending on the gas filling different spectral composition of the emitted light. With them, the pressure-dependent diameter and the structure of the gas discharge can be easily observed.

Philipp Lenard experimented with discharge tubes and thus contributed significantly to the development of atomic physics at. In his experiments, he joined the piston-like structured discharge tube to a vacuum pump, which greatly reduces the air pressure in the tube. There was a so-called Lenard window, which was ( approximately 0.5 microns thick) of aluminum foil on the side of the tube. This is thick enough to withstand the pressure difference, but still allows cathode rays (electrons) pass, which is an investigation outside the tube thus accessible. To this end was next to the window, for example, a fluorescent screen.

Construction

Gas discharge tubes consist of an approximately tubular discharge vessel made of glass ( low-pressure lamps), fused silica ( high and very high pressure lamps) or alumina ceramic ( high-pressure lamps). The housing contains two electrodes between which an electric field is established, and a gas discharge occurs. The electrical connections are passed through a pinch in a gastight manner to the outside. The electrodes can be "cold" or glowing.

All gas discharge lamps excluding flashbulbs need an operating current limit, otherwise the charge carrier density and the current due to impact ionization increase rapidly, which is desired in surge arresters and Nulloden, in lamps, however, leads to their destruction (see curve). The current limit is (engl TOE. Ballast ) through a resistor ( neon lamps ), a choke or an electronic ballast achieved.

Flashbulbs other hand, often work without current limitation of a storage capacitor. They are usually filled with xenon ( for pumping solid -state lasers with krypton ) and produce within about 0.1 to 5 milliseconds very high light output similar to daylight quality.

There are designs with heated or unheated with ( cold ) electrodes. Furthermore, there are also gas discharge tubes without electrodes as Nulloden and microwave lamps (such as the sulfur lamp). Recently there is also electrodeless energy saving lamps which work with high frequency.

Filling

The discharge bulb is basically filled with a gas or gas mixture, but can also contain substances that are active only at a later time due to evaporation.

At room temperature, these gases have in the piston to a low pressure, which promotes the ignition of the discharge by impact ionization and thereby the creation of the plasma. The filling with gas mixtures determines certain characteristics of the discharge or is solely for the purpose of providing enough heat to vaporize the imaginary for the real plasma substance available. By the formation of a temperature properties under storage piston of the plasma-forming material is vaporized, thereby increasing the pressure in the discharge space.

In the plasma-forming substances are metals or their vapors (sodium, mercury, see also metal halide lamp) which ignition noble gases are always included, or pure noble gases (xenon, krypton, neon) or mixtures of halogens and metals ( metal halide lamp). Spectral lamps use other gases.

High-pressure gas discharge lamps are also referred to as HID lamps (of English High Intensity Discharge. ); Current and luminance are here significantly higher than in the low-pressure plasmas, discharge working in the field of an arc or an arc discharge.

Categorization

A key distinguishing feature of the gas discharge tubes, the pressure in the discharge vessel and the burner. We distinguish:

• Low-pressure discharge lamps ( low pressure plasma, glow discharge, for example, fluorescent tubes, fluorescent lamps, fluorescent tubes ),
• High intensity discharge lamps ( pressures up to about 1 MPa, for example, mercury vapor lamps, krypton arc lamps for laser excitation ),
• High pressure lamps ( pressures up to about 10 MPa, for example, mercury vapor high pressure lamps in photolithography, xenon short arc lamps for projection purposes ).

For mercury vapor lamps, a distinction so-called medium pressure lamps; these are manufactured in long lengths with great achievements and are used for UV curing of paints and resins about.

The typical current - voltage characteristic of a high-pressure gas discharge (right) is closely related with the luminous phenomena. In the area of ​​non-independent discharge ( 1) a small current ( fractions of a second ignition) flows. In (2) the glow discharge begins ( beginning of discharge; fractions of a second ). Which is necessary for maintaining the glow discharge, the operating voltage lower than the ignition voltage. An increase of the current leads (3) to the arc discharge. It is characterized by low operating voltage and high current or high current density; the electrodes start to glow.

In the area of the dashed transitions in the picture, all gas discharge lamps act as negative differential resistance. With increasing voltage, the current decreases or the current intensity increases with decreasing voltage. To limit the current, gas discharge lamps need to be always operated with a bias resistor. At high powers and power AC chokes, are usually employed to keep the heat low.

A negative differential resistance can be used for exciting ( undamping ) of resonant circuits in the transmitter or arc to produce relaxation oscillations.

Low-pressure discharge lamp

Special

The low-pressure discharge lamp is characterized in that the electron and gas temperatures are hardly coupled. There is no thermal equilibrium. The discharge form is also referred to as glow discharge. A typical application is the fluorescent lamp, which is a special form of the mercury vapor lamp.

In a partially evacuated glass tube forming at opposite electrodes at sufficiently high voltage ( operating voltage of about one hundred to several 100 V) from a glow discharge. In the vicinity of the cathode (-) occurs in the negative glow in the center to the anode ( ) the so-called positive column.

Low pressure lamps operate

• With cold electrodes ( cold cathode fluorescent lamp, neon tubes, neon lamps )

Or

• With directly heated thermionic cathodes ( in fluorescent lamps and low pressure sodium vapor lamps). The thermionic cathodes are heated before ignition, and then mostly retain their temperature by itself by back heating.

Examples

Surge Protection

Isolate gas filled surge arresters, as long as the voltage drops below about 450 V remains, and do not interfere because of their low capacity of only about 2 pF. If the ignition voltage is exceeded, the resistance falls within microseconds to very low levels, with current peaks can be derived up to 20,000 A. In continuous operation, they will be thermally overloaded.

Glow lamps

The glow lamp uses the glow discharge and is usually used to Signalisierungsszwecken in various applications. The glow is produced at the cathode when operated with AC voltage light up alternately both electrodes. Since glow lamps are usually filled with the inert gas neon, they count in this design to the neon lights.

The glow is inexpensive to produce, but is replaced in its function as signaling display increasingly by light emitting diodes ( LED).

Tubes

In light tubes as cold cathode tubes and neon lamps ( then with red-emitting neon fill ), the electrodes are far apart in a glass tube. Here, the positive column is lit when a voltage of several 100 volts is applied. Other colors can be achieved by other fillings and phosphors. Fluorescent tubes, neon and cold cathode tubes have cold cathodes and require higher voltages than glow lamps or fluorescent lamps.

Fluorescent lamps

Sometimes tubes are referred to as fluorescent lamps, because they often contain, as well as a fluorescent phosphor. Genuine fluorescent lamps ( also known as fluorescent lamps) as well as compact fluorescent lamps and so-called energy saving light bulbs but have Thermionic ( directly heated oxide cathodes ). By thermionic emission, the required operating voltage drops to values ​​that allow operation over a series reactor to mains voltage.

The point in the circuit symbol of the tube symbolizes the filling with a gas or vapor. Without the point it would be a vacuum tube.

Sodium vapor low-pressure lamps

These are also known as LS-, Na or SOX lamps. They are among the most efficient bulbs and see for bad color reproduction but mostly in street lighting use.

High intensity discharge lamps ( HID) lamps

This is generally to metal halide lamps, which can be further divided into mercury, sodium and metal halide lamps.

Special

The two electrodes in the quartz glass or ceramic discharge vessel made ​​of tungsten (solid or wire wrap ) and have a peak in new condition. These burns during the life back slightly.

The current density is so high that the low-pressure discharge at start immediately goes into an arc so that the internal pressure is greatly increased by increasing temperature and evaporating filling components. The electrodes reach depending on the design temperatures of about 1000 to 1000 ° C and several are not preheated. For mercury vapor lamps are located next to the main electrodes still ignition electrodes so that an ignition device, as is required in other high-pressure gas discharge lamps, can be omitted.

Due to the higher density and resulting smaller mean free path of the particles are in the high- pressure discharge lamp ( p> 0.1 bar and> 10000 Pa), the electron and gas temperatures approximately in equilibrium. In contrast to tubes low voltages (50 ... 200 V) and the discharge current ( typically 1 ... 10 Amps) are significantly higher.

High-pressure gas discharge lamps are also called HID lights (of English. High intensity discharge).

In high-pressure gas discharge lamps shall assign a line broadening due to thermal motion, which is why these bulbs without fluorescent already have a slightly better color reproduction than low-pressure discharge lamps without phosphor.

High -pressure gas discharge lamps are often provided with additional protection glass flask, which also serves the thermal insulation and partly supports a phosphor.

Examples

• Mercury vapor high- pressure lamp, often called Carbide lamp (industrial and street lighting); Image of a discharge vessel see at construction
• Sodium vapor high pressure lamp, often called HS- lamp ( street lighting)
• Metal halide lamp, often called HI- lamp (C- HI: metal halide lamp with ceramic discharge vessel ) (business expenses, stadium lighting, architecture, beamer )

Highest -pressure gas discharge lamps,

In these lamps, the mercury - inert gas mixture ( argon, xenon ) has pressure at room temperature. In operation, the pressure rises to 100 to 400 bar. The lamps have a squat, thick-walled quartz glass vessel and massive tungsten electrodes. Are usual lamps with an input power of 50 watts to 24 kilowatts.

These lamps are produced as projection light source or sunlight simulator xenon filling and for the purpose of photolithography with mercury fill.

These lamps must be ignited before their continuous operation, since there is a non-conductive gas path between the two electrodes. This is done by a high voltage pulse (up to 50 kV) is applied that exceeds the breakdown voltage of the gas line. The resulting rollover ionizes the gas path between the electrodes and makes it conductive. So that the ionized path terminates immediately again, now a smoothed direct current to be supplied in a continuous operation at a voltage of 20 V to 30 V. The higher the operating voltage is smoothed, the higher is the lifetime of the lamp, and the higher the quality of the emitted light.

High -pressure gas discharge lamps have a luminous center of only a few millimeters expansion, the point of the highest luminance ( hot spot ) is located directly on the cathode; they are therefore also referred to as short-arc lamps. The small size of the light source and high intensity allows effective focusing or collimation at lighthouses, lights and projectors.

High -pressure gas discharge lamps have a strong thermal line broadening its emission spectrum and type in the case of xenon high -pressure lamps an almost continuous daylight- like spectrum.

Have high pressure lamps due to the danger of explosion handled with caution ( gloves, goggles ) and operated (no free -mountable, operating position often prescribed ) are.

Flash tubes

Flash tubes have cold cathode and work exclusively in pulsed operation. Power is provided by a storage capacitor, a peak electrical power to the range of megawatts ( photo flash ) or even gigawatts ( pumping of pulsed lasers) can deliver. The gas filling is Xenon for daylight- like color temperature, as well as krypton, in particular for the effective excitation pulse solid-state lasers. Other applications include strobes and the short-term heating of surfaces.

Are flash tubes as well as high pressure lamps with a high voltage ignition pulse (about 4 to 20 kV) is ignited or triggered. To do have an externally mounted on the bulb electrode. This is possible because the operating temperature of the glass bulb is small.

Areas of application

Gas discharge tubes are next demonstration purposes today, especially as a gas discharge lamp, known as fluorescent lamps, fluorescent tubes. Glow lamps are used as signal lamp; However digits ads on this principle ( Nixie tubes ) are outdated.

In a broader sense, to gas discharge tubes and gas discharge tubes, gas lasers, as well as the now-obsolete mercury vapor rectifiers, thyratrons or stabilizer tubes.

Plasma displays operate also in each pixel with a low-pressure gas discharge, the ultraviolet emissions are used for color display by means of fluorescent dyes.

Fast switch for high performance in the radar range using gas discharge tubes to using a duplexer for coupling the antenna to either the transmitter or the receiver ( Nullode ).

For street and industrial lighting sodium vapor lamps are often used. Sodium has a double line at 589.0 and 589.6 nm (sodium D line ), which cause the dominant yellow- orange color reproduction. The lower level of these lines is the ground state, so that the radiation density of resonance lines is very high.

In floodlighting high-pressure gas discharge lamps are used with mercury, metal halide or sodium vapor charge. The best color rendering Xenon lamps. A relatively good color rendering have metal halide gas discharge lamps (including metal halide lamps, halogen lamps not! ), Which also contain mercury vapor. They are often used in business expenses.

In cinema and video projectors Xenon gas discharge lamps are used in the power range from 100 watts to 15 kilowatts.

Manufacturers such as Osram HMI lamps provide up to 24 kW, which are used as well in cinema projectors, or in daylight headlights. These lamps have a luminous efficacy of 95 lm / W and achieve a luminous flux of up to 2,300,000 lm.

For motor vehicles, the so-called Xenon light is used in the headlights since 1991. This is to metal halide lamps with xenon as the filling and start gas. The exact name of the lamps is difficult, however, since it is mostly xenon are present as a gas and in addition to a small extent mercury and metal halides. Do you have over incandescent with less power consumption a higher luminous flux ( H7 halogen bulb with 1500 lumens compared to HID lamp with about 3000 lumens).