Glow discharge

Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry.

When the current is increased above the level where the entire cathode surface is involved, the discharge is known as an abnormal glow.

A low pressure is used to increase the mean free path; for a fixed electric field, a longer mean free path allows a charged particle to gain more energy before colliding with another particle.

Ions strike the more numerous neutral gas atoms, transferring a portion of their energy to them.

Once free of the cathode, the electric field accelerates electrons into the bulk of the glow discharge.

Their mass identifies the type of atoms and their quantity reveals the amount of that element in the sample.

As the discharge becomes more extended (i.e., stretched horizontally in the geometry of the illustrations), the positive column may become striated.

The cathode layer begins with the Aston dark space, and ends with the negative glow region.

The cathode layer has a positive space charge and a strong electric field.

[3] With fewer ions, the electric field increases, resulting in electrons with energy of about 2 eV, which is enough to excite atoms and produce light.

There is no universal mechanism explaining the striations for all conditions of gas and pressure producing them, but recent theoretical and modelling studies, supported with experimental results, mention the importance of the Dufour effect.

For example, neon signs have hollow cathodes designed to minimize sputtering, and contain charcoal to continuously remove undesired ions and atoms.

Surrounding the cathode is a negative field, which slows electrons as they are ejected from the surface.

Only those electrons with the highest velocity are able to escape this field, and those without enough kinetic energy are pulled back into the cathode.

During this acceleration electrons are deflected and slowed down by positive ions speeding toward the cathode, which, in turn, produces bright blue-white bremsstrahlung radiation in the negative glow region.

This method is referred to as glow discharge mass spectrometry (GDMS) and it has detection limits down to the sub-ppb range for most elements that are nearly matrix-independent.

Bulk analysis assumes that the sample is fairly homogeneous and averages the emission or mass spectrometric signal over time.

In bulk measurement, a rough or rounded crater bottom would not adversely impact analysis.

In contrast, analysis of a non conductive cathode requires the use of a high frequency alternating current.

The power (product of voltage and current) may be held constant while the pressure is allowed to vary.

Both radio-frequency and direct-current glow discharges can be operated in pulsed mode, where the potential is turned on and off.

Analogously, in mass spectrometry, sample and background ions are created at different times.

An interesting application for using glow discharge was described in a 2002 scientific paper by Ryes, Ghanem et al.[9] According to a Nature news article describing the work,[10] researchers at Imperial College London demonstrated how they built a mini-map that glows along the shortest route between two points.

The Nature news article describes the system as follows: The approach itself provides a novel visible analog computing approach for solving a wide class of maze searching problems based on the properties of lighting up of a glow discharge in a microfluidic chip.

In the mid-20th century, prior to the development of solid state components such as Zener diodes, voltage regulation in circuits was often accomplished with voltage-regulator tubes, which used glow discharge.