Glow discharge positive column

An added complication that must be taken into account, especially in glow discharges, is that different portions of the discharge have different characteristics. Thus, the negative glow and positive column have quite different electric fields, the ions and electrons present have different energies and the distribution of ions in the two regions is different. 

Key C, Corona CDS, cathode dark space M, microwave discharge NG, negative glow PC, positive column rf, radiofrequency discharge. 

Devins (18) assumed that the ozonizer discharges in oxygen studied by him may be regarded as similar to the more or less ideal plasma of the uniform positive column of a glow discharge because, in other experiments, Manley (52) found that, at atmospheric pressure, X/p is nearly independent of the alternating potential applied to the electrodes. 

Fig. 71. Geometry (A) and potential distribution (B) of a dc electrical discharge under reduced pressure. (1) Aston dark space, (2) Hittorfdark space, (3) negative glow, (4) Faraday dark space, (5) positive column, (6) anode region.

Since the formation of He is an excitation process, it will occur over a relatively small energy range with electrons of energy close to the ionization potential of He. Thus, it would be expected to predominate in the positive column of a glow discharge. This has been observed by Morris (75) and by Pahl (82, 83). 

A quite different situation with respect to the spatial behavior of the electron component arises if the inhomogeneity of the plasma is caused by a plasma confinement. The electron kinetics established in the radial direction of the positive column of dc glow discharges, which usually operates in a cylindrical discharge tube with an isolated wall, presents a representative example (Kortsha-gen, 1995 Uhrlandt and Winkler, 1996 Pfau et al, 1996 Alves et al, 1997) of such a condition. From the point of the electron kinetics, this space-dependent problem is somewhat more complex than those considered above. 

The thermal conductivity coefficient X does not depend on pressure and can be estimated as A 3 W W/cm K. Therefore, the specific discharge power (4 1) also does not depend on pressure and can be estimated as 0.7 W/cm for i = 1 cm. Higher powers result in higher gas temperatures and glow discharge contraction. The typical current density in the positive column with the heat removal controlled by thermal conduction is inversely proportional to pressure ... 

Low-pressure are diseharges. The positive column of arc discharges at low pressures (10 -1 Torr) consists of non-equilibrium plasma. The ionization degree in the non-thermal arcs is higher than that in glow discharges because arc currents are much larger (see Table 4-7). 

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