Cold cathode
From Wikipedia, the free encyclopedia
- 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.
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[edit] Electron emission
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 ionized. A few cold cathode devices contain a vacuum.
[edit] Details
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.)
[edit] See also
[edit] Patents
- U.S. Patent 2184910 - Philo Farnsworth - Cold cathode electron discharge tube
- U.S. Patent 2263032 - Philo Farnsworth - Cold cathode electron discharge tube
- U.S. Patent 2448527 - Clarence W. Hansell - Cold cathode electron discharge device and circuits therefore
