Hot cathode

A hot cathode is a heated cathode (negatively charged electrode ) in electron tubes and sometimes also in gas discharge tubes . It works on the principle of the Edison-Richardson effect and supplies free electrons . It is often referred to as filament (English for filament ).

Glowing cathodes in the small tubes of an amateur radio transmitter

Tube with partially detached cathode layer

Features are the work function of the materials used as well as the life and behavior at different current densities .

Heating

There are two types of heating:

Indirect heating : In this process, the hot cathodes are heated by a separate heating circuit with a tungsten filament, which is electrically isolated from the cathode . The ceramic- insulated coil is located in a metal tube (often made of nickel), which carries the oxide cathode layer.
Direct heating : The cathode is formed by the heating wire itself. The heating conductor can be a wire or a tape. It can be stretched between springs or coiled (self-supporting).

Function and materials

Current-voltage characteristic of a diode. Dashed: saturation currents for different cathode temperatures

In order to keep the required temperature of the hot cathode low, materials are used on the cathode surface that have a low work function, e.g. B. rhenium or thorium - doped tungsten. Usually, however, so-called oxide cathodes are used, which, for example, enable particularly low cathode temperatures (approx. 700-800 ° C) by means of a barium oxide layer.

Importance for electron sources and plasmatron also has single-crystal or ceramic lanthanum hexaboride (LaB ₆ , work function <4 eV), or Ceriumhexaborid (CeB ₆ ).

The electrons in the hot cathode have a Fermi velocity distribution . As the cathode temperature increases, the electrons become faster on average. Ultra-fast electrons from the so-called "Fermi tail" of the velocity distribution have enough energy to the work function to be able to afford in the vacuum. In contrast to cold cathodes , in which the electrons are torn from the cathode by very strong fields, the maximum amount of electrons escaping from a hot cathode depends only on the temperature and the material properties. A distinction must be made between two cases:

In noise diodes (previously used noise generators ), all the electrons emitted are sucked off to the anode, which is known as saturation . The current strength only depends on the temperature of the cathode, but not on the anode voltage, provided that this exceeds a minimum value of about 100 V. To ensure a long service life for the cathode, it must be made of pure tungsten. The saturation current of oxide cathodes is so great that the surface is quickly destroyed.
With all other cathodes, considerably more electrons leave the cathode than are required. If none are extracted, they all fall back onto the cathode after a very short “flight time” (a few nanoseconds) because opposing charges attract each other. The electrons form a space charge cloud around the cathode. A sufficiently positively charged anode can suck up a small fraction of all electrons floating around.

Usually the cathode current, i.e. the amount of electrons that are attracted to an anode, has to be regulated. The cathode is therefore surrounded by a negatively charged electrode (grid or Wehnelt cylinder ), which is known as space-charge-restricted operation. The very fuzzy edge of the space charge cloud is called the virtual cathode.

application

Indirectly heated oxide cathodes in cathode ray tubes and other electron tubes
Directly heated oxide cathodes in fluorescent lamps , vacuum gauges, fluorescent displays and rectifier tubes
Directly heated metal cathodes
- z. B. thoriated tungsten cathodes in magnetrons and transmission tubes
- so-called “ hairpin cathodes ” in the electron microscope and in the cathodoluminescence microscope

Hot cathodes are an essential factor that limits the service life of electron tubes and fluorescent lamps. If a hot cathode has lost its ability to emit electrons at the intended temperature, it is "deaf".

Oxide cathodes can often be "regenerated" or reformed again by overheating them and placing a strong electrical load on them. Dirt that "poisons" the surface of the cathode, ie increases the work function, is torn off the cathode and new metallic barium is released. In the past, permanent overheating was also common in aged picture tubes , before long-life cathodes with no interlayer were used.

Individual evidence

↑ ^a ^b Hanno Krieger: Radiation sources for technology and medicine . Teubner, Wiesbaden 2005, ISBN 3-8351-0019-X , p. 49 .
^ Nagamitsu Yoshimura: Vacuum technology. Practice for scientific instruments . Springer, Berlin et al. 2008, ISBN 978-3-540-74432-0 , pp. 335 .
↑ PeroLan - Cathodes ( Memento from March 17, 2011 in the Internet Archive )
^ Christian Gerthsen : Gerthsen Physics . 22nd, completely revised edition. Springer, Berlin et al. 2004, ISBN 3-540-02622-3 , pp. 886 .

[Krieger-1] Hanno Krieger: Radiation sources for technology and medicine . Teubner, Wiesbaden 2005, ISBN 3-8351-0019-X , p. 49 .

[2] Nagamitsu Yoshimura: Vacuum technology. Practice for scientific instruments . Springer, Berlin et al. 2008, ISBN 978-3-540-74432-0 , pp. 335 .

[3] PeroLan - Cathodes ( Memento from March 17, 2011 in the Internet Archive )

[4] Christian Gerthsen : Gerthsen Physics . 22nd, completely revised edition. Springer, Berlin et al. 2004, ISBN 3-540-02622-3 , pp. 886 .