什么是辉光放电？（What is a glow discharge and what is it？）

1帖  Tracy95排长 199十2010-06-02 14:51 A glow discharge is a kind of plasma. It is an ionized gas consisting of equal concentrations of

positive and negative charges and a large number of neutral species.

In the simplest case, it is formed by applying a potential difference (of a few 100 V to a few kV)

between two electrodes that are inserted in a cell (or that form the walls of the cell). The cell is filled

with a gas (an inert gas or a reactive gas) at a pressure ranging from a few mTorr to atmospheric

pressure.

Due to the potential difference, electrons that are emitted from the cathode by the omnipresent

cosmic radiation, are accelerated away from the cathode, and give rise to collisions with the gas

atoms or molecules (excitation, ionization, dissociation, …). The excitation collisions give rise to

excited species, which can decay to lower levels by the emission of light. This process is responsible

for the characteristic name of the “glow” discharge. The ionisation collisions create ion‐electron

pairs. The ions are accelerated toward the cathode, where they release secondary electrons. These

electrons are accelerated away from the cathode and can give rise to more ionization collisions. In its

simplest way, the combination of secondary electron emission at the cathode and ionization in the

gas, gives rise to a self‐sustained plasma.

 Due to the various collision processes in the plasma, a large number of different plasma species can

be present: electrons, atoms, molecules, several kinds of radicals, several kinds of (positive and

negative) ions, excited species, etc. These different species can all be in interaction with each other,

making the glow discharge plasma a complicated gas mixture. The aim of our work is to obtain a better insight in the complex processes occurring in glow discharges and related plasmas, and we try

to do that by numerical simulations.

    

Different variants to glow discharge plasmas

The glow discharge described above can be called the “basic version”. In this direct current (dc) glow

discharge, a continuous potential difference is applied between cathode and anode, giving rise to a

constant current. However, this set‐up gives problems when one of the electrodes is non‐conducting,

as is the case in some applications (see below). Indeed, due to the constant current, the electrodes will

be charged up, leading to burn‐out of the glow discharge.

This problem is overcome by applying an alternating voltage between the two electrodes, as in the

capacitively coupled radio‐frequency (cc rf) glow discharge. Indeed, the charge accumulated during

one half of the cycle, will be neutralized by the opposite charge accumulated during the next halfcycle.

Beside a time‐dependent rf voltage, an alternating voltage can also be applied in a lower frequency

range, giving rise to an alternating current (ac) glow discharge. This can be considered as a

consecution of short discharges, in which the two electrodes alternatingly play the role of cathode and

anode. An important type of ac glow discharge, operating at atmospheric pressure, is the dielectric

barrier discharge (DBD), where the electrodes are typically covered by a dielectric barrier.

A variation to the ac discharge is the pulsed glow discharge, which also consists of short glow

discharges (with lengths typically in the milli‐ or microsecond range), followed by an afterglow,

which is generally characterized by a longer time‐period. The advantage is that high peak electrical

powers can be reached for a low average power, resulting in high peak efficiencies for various

applications.

In addition to applying an electric field (or potential difference), a magnetic field can also be applied

to a glow discharge. The most well‐known discharge type with crossed magnetic and electric fields is

the magnetron discharge. The electrons circulate in helices around the magnetic field lines and give

rise to more ionisation. Hence, magnetron discharges are typically operated at lower pressures and

higher currents than conventional glow discharges.

Recently, some new discharge types have been developed, which are also characterized by low

pressure and high plasma densities, and which have their main application in the semiconductor

industry and for materials technology. The major difference with the conventional glow discharge is

that the electrical power is not applied through a potential difference between two electrodes, but

through a dielectric window. The two most important “high‐density sources”, are the inductively

coupled discharge, where the rf power is inductively coupled to the plasma, and the electron

cyclotron resonance reactor, where microwave power and a magnetic field are applied.

Microwave power can also be applied in so‐called microwave induced plasmas. Various discharge

types can be classified under this name, among others the resonance cavity plasmas, free expanding

plasma torches and surface wave discharges.

In fact, the list of discharge plasmas related to glow discharges is longer than this. But because they

are at this moment beyond the research interest of our group, we will not go into detail here. More

information can be found in: A. Bogaerts, E. Neyts, R. Gijbels and J.J.A.M. van der Mullen, Gas

discharge plasmas and their applications, Spectrochimica Acta B, 57, 609‐658 (2002).

Applications of glow discharges and related plasmas

Glow discharges and related plasmas are used in a large number of application fields. The most

important application is probably in the microelectronics industry and in materials technology, for

surface treatment, etching of surfaces (e.g., for the fabrication of integrated circuits), deposition of

thin protective coatings, plasma polymerisation, plasma modification of polymers and other surfaces.

The exciting and light emitting character of discharge plasmas is also used for several interesting

applications, such as in the light industry (e.g., fluorescence lamps, neon advertisements), as gas

lasers, and as flat plasma display panels for the new generation of flat, large area television screens.

Because a lot of chemical reactions take place in the plasma, several types of discharges (mainly

atmospheric pressure glow discharges and dielectric barrier discharges) find also increasing interest

for environmental applications (e.g., the destruction of volatile organic compounds) and biomedical

applications (e.g., the sterilisation of materials).

Finally, an application of glow discharges that is of special interest to our group is its use in

analytical chemistry, for the spectrochemical trace analysis of (mainly solid) materials