Question

Let you compare gas discharges under high and low pressure and describe processes that determine each kind of discharge

Answer 1

Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma. Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, like mercury, sodium, and metal halides, which are vaporized during startup to become part of the gas mixture. In operation, some of the electrons are forced to leave the atoms of the gas near the anode by the electric field applied between the two electrodes, leaving these atoms positively ionized. The free electrons thus released flow onto the anode, while the cations thus formed are accelerated by the electric field and flow towards the cathode. Typically, after travelling a very short distance, the ions collide with neutral gas atoms, which transfer their electrons to the ions. The atoms, having lost an electron during the collisions, ionize and speed toward the cathode while the ions, having gained an electron during the collisions, return to a lower energy state while releasing energy in the form of photons. Light of a characteristic frequency is thus emitted. In this way, electrons are relayed through the gas from the cathode to the anode. The colour of the light produced depends on the emission spectra of the atoms making up the gas, as well as the pressure of the gas, current density, and other variables. Gas discharge lamps can produce a wide range of colours. Some lamps produce ultraviolet radiation which is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface. The fluorescent lamp is perhaps the best known gas-discharge lamp.

Compared to incandescent lamps, gas-discharge lamps offer higher efficiency but are more complicated to manufacture and most exhibit negative resistance, causing the resistance in the plasma to decrease as the current flow increases. Therefore, they usually require auxiliary electronic equipment such as ballasts to control current flow through the gas, preventing current runaway (arc flash). Some gas-discharge lamps also have a perceivable start-up time to achieve their full light output. Still, due to their greater efficiency, gas-discharge lamps were preferred over incandescent lights in many lighting applications, until recent improvements in LED lamp technology.

Each gas, depending on its atomic structure emits certain wavelengths, its emission spectrum, which determines the colour of the light from the lamp. As a way of evaluating the ability of a light source to reproduce the colours of various objects being lit by the source, the International Commission on Illumination (CIE) introduced the colour rendering index (CRI). Some gas-discharge lamps have a relatively low CRI, which means colours they illuminate appear substantially different from how they do under sunlight or other high-CRI illumination.

Lamps are divided into families based on the pressure of the gas, and whether or not the cathode is heated. Hot cathode lamps have electrodes that operate at a high temperature and are heated by the arc current in the lamp. The heat knocks electrons out of the electrodes by thermionic emission, which helps maintain the arc. In many types the electrodes consist of electrical filaments made of fine wire, which are heated by a separate current at startup, to get the arc started. Cold cathode lamps have electrodes that operate at room temperature. To start conduction in the lamp a high enough voltage (the striking voltage) must be applied to ionize the gas, so these lamps require a higher voltage to start.

Low-pressure lamps have working pressure much less than atmospheric pressure. For example, common fluorescent lamps operate at a pressure of about 0.3% of atmospheric pressure.

High-pressure lamps have a discharge that takes place in gas under slightly less to greater than atmospheric pressure. For example, a high pressure sodium lamp has an arc tube under 100 to 200 torr pressure, about 14% to 28% of atmospheric pressure; some automotive HID headlamps have up to 50 bar or fifty times atmospheric pressure.