Hot cathode

A hot cathode is a heated cathode (negatively charged electrode) in electron tubes and partly also in gas discharge tubes. It works on the principle of Edison Richardson effect and provides free electrons. It is often referred to as "filament " refers to.

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:

Function and materials

To maintain the required temperature of the hot cathode low, materials are used on the cathode surface, which have a low work function, such as rhenium, or thorium doped tungsten. Mostly, however, so-called oxide cathodes are used, for example, by means of a barium oxide layer particularly low cathode temperatures allow ( 700-800 ° C).

Importance for electron sources and plasmatron has also single-crystal or ceramic lanthanum hexaboride ( LaB6, work function < 4 eV ) or Ceriumhexaborid ( CeB6 ).

The electrons in the hot cathode have a Fermi velocity distribution. With increase in cathode temperature while the electrons in the medium are faster. The particularly fast electrons from the so-called " Fermi - tail" of the velocity distribution have sufficient energy to make the work function in the vacuum can. As opposed to the cold cathode in which the electrons are pulled by very strong fields from the cathode, with a thermionic cathode, the maximum amount of the emitted electrons is only dependent on the temperature and the material properties. It must distinguish two cases:

• In " noise diode " ( previously used noise generators ) all ejected electrons are sucked toward the anode, which is called " saturation ". The current intensity in this case depends only on the temperature of the cathode, but not from the anode voltage if it exceeds a minimum value of about 100 volts. To ensure a long life of the cathode, it must consist of pure tungsten. Is the saturation current of the oxide cathodes is so high that the surface is rapidly destroyed.
• For all other cathode significantly more electrons leave the cathode as needed. If none are aspirated, all fall very short " flight time " (several nanoseconds) back to the cathode, because opposite charges attract. The electrons form a space charge cloud around the cathode. By a sufficiently positively charged anode, a small fraction of all herumschwirrenden electrons can be drawn off.

Usually has the cathode current, that is, the quantity of electrons which are attracted against the anode can be regulated. Therefore, the cathode of a negatively charged electrode ( grid or Wehnelt ) is surrounded what is referred to as space-charge- limited operation. The very blurred edge of the space charge cloud is called a "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 cathode thoriated tungsten cathode, for example, in the magnetron and transmitting tubes
• So-called "hairpin cathode " in the electron microscope and in Kathodolumineszenzmikroskop

Thermionic represent an essential, the life -limiting factor in electron tubes and fluorescent lamps dar. Has a hot cathode its ability to emit electrons at the intended temperature, lost, she is " deaf".

Oxide cathodes, however, can often redo " perk " by being highly electrically charged under extreme heating. Dirt, the surface of the cathode " poisoned ", ie increases the work function is thereby torn from the cathode down.