Cold cathode
cold cathode ion sources (used in particle accelerators) operate using the same principles.A cold cathode''' is an element used within some
Nixie tubes,
gas discharge lamps,
gas filled tubes, and
vacuum tubes. Cold cathodes don't employ any
cathode heater.
Neon lamps are a very common example of a cold cathode lamp.
A
cathode is any
electrode that emits
electrons. When used in electrical and electronic devices (most
fluorescent lamps,
vacuum tubes, etc.), the cathode is explicitly heated, creating a
hot cathode. By taking advantage of
thermionic emission, this decreases the
work function of the cathode and makes it easier for an
electric field to strip electrons out of the cathode. But if sufficient voltage is present, electrons can still be stripped even out of a cathode operating at
ambient temperature. Because it is not deliberately heated, such a cathode is referred to as a cold cathode, although several mechanisms may eventually cause the cathode to become quite hot once it is operating. Most cold cathode devices are filled with a
gas which can be . A few cold cathode devices contain a vacuum.
Before looking at cathode ray tubes of any kind, a few basic terms must be defined.
#Cathode - The negative electrode in a
cathode ray tube.#Cathode rays - The positive ions created when some electrons are removed from the molecules of a gas at low pressure.
A
cold cathode is distinguished from a
hot cathode that is heated to induce
thermionic emission of
electrons. These electron discharge tubes have an envelope evacuated or filled with low pressure gas and containing a pair of cathodes, usually parallel to one another. The interior surface of the cathodes are capable of producing secondary electrons at a ratio greater than unity (amplification) upon electron impact. A third node is present in the device, acting as an accelerator between the two cathodes. These devices use
longitudinal magnetic fields and
electrostatic focusing.
Cold cathodes sometimes have
rare earth coating on them for enhancing electron emission. Some types contain a source of
beta radiation to start
ionization of the gas that fills the tube. In such a tube,
glow discharge is usually minimized, in favor of
arc discharge. The best example is the humble
neon lamp. Another good example is
Nixie tubes.
Nixie tubes too are cold cathode, neon displays that also happen to be in-line, but not in-plane display devices.
A common cold cathode application is in
neon signage. Other examples include the
thyratron,
krytron,
sprytron, and
ignitron tubes. Large-scale cold cathode
fluorescent lamps (CCFLs) have been produced in the past, and are still used today when shaped, long life linear light sources are required. Nowadays, miniature CCFLs are extensively used as
backlights for
computer liquid crystal displays. Additionally, CCFLs are directly used by
computer modders to light the insides of their highly customized transparent computer cases.
Despite their name, cold cathodes don't necessarily remain cold as they operate; they can get painfully hot. In systems using
alternating current but without separate
anode structures, the cathodes alternate as anodes and the impinging electrons can cause substantial localized heating, often to red heat. The cathode may or may not take advantage of this heating to facilitate the thermionic emission of electrons when it is acting as a cathode. (
Instant start fluorescent lamps definitely do employ this aspect; they start as cold-cathode devices but soon localized heating of the fine
tungsten wire cathodes causes them to operate as ordinary
hot cathode lamps.)
*
Field emission*
Ionization gauge*
Vacuum* -
Philo Farnsworth -
Cold cathode electron discharge tube* - Philo Farnsworth -
Cold cathode electron discharge tube* -
Clarence W. Hansell -
Cold cathode electron discharge device and circuits therefore*
Pulse Power Switching Devices - An Overview- by John Pasley
