Positive column

Faraday dark space. The positive column extends to tire anode, which may be covered by tire anode glow. 

The positive column is a region in which atoms, electrons, and ions are all present together in similar numbers, and it is referred to as a plasma. Again, as with the corona discharge, in mass spectrometry, plasmas are usually operated in gases at or near atmospheric pressure. 

Positive ions formed near the positive electrode (anode) are repelled by it and move into the positive column. Electrons that reach proximity to the anode are accelerated somewhat because... 

If the electrodes are moved closer together, the positive column begins to shorten as it moves through the Faraday dark space because the ions and electrons within it have a shorter distance through which to diffuse. Near the cathode, however, the electric-field gradient becomes steeper and electrons from the cathode are accelerated more quickly. Thus atom excitation through collision with electrons occurs nearer and nearer to the cathode, and the cathode glow moves down toward the electrode. 

There is also a potential difference between the positive column and tube wall. This potential difference is created because the electrons are much more mobile than heavy ions and tend to flow rapidly out toward any bounding surface. Since the tube wall is an insulator, they tend to collect there causing the insulator to assume a negative potential relative to the plasma. This creates an electric field close to the tube wall which hinders further electron flow towards it. A deficit of electrons forms in a sheath close to the surface, and this sheath assumes a net positive charge. Ions in the plasma, however, see the tube wall potential which is negative compared to the plasma and are attracted to it. This is the diffusion to the tube walls mentioned in the previous paragraph, and is often referred to as "ambipolar" diffusion. 

We begin by assuming that the ion current density to all internal surfaces is equal. Then, if we assume a collision-free sheath with the field being zero at the edge of the positive column, we can express the space charge-limited ion current as16... 

Cathode region Positive column Anode glow... 

A plausible estimate for maximum attainable value of rj for ozone formation in the positive column in oxygen is thus 20 X 10 molecule per e.v. however, a higher value may hold for other forms of the discharge, including that realized in ozonizers. 

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. 

The data of Henry (3/) are the simplest to analyze they relate to initial rates of ozone synthesis at p = 2.4 mm. Hg and mainly to positive column reaction because the rate of synthesis in the negative zones (cathode surface to Faraday dark space-positive column boundary) was relatively negligible. The cross section of the tube was about 0.8 cm.2, and hence the current density was about 1.25 I ma. cm. 2. Judged by the rough sketch given, the positive column was about 10 cm. long. 

Reaction assumed to occur only in positive column of estimated length z = 10 cm. I = total current, ma. pz = 24 mm. Hg X cm. 

Vpc = voltage acro positive column assuming cathode fall to be 400 volts Wa = power supplied to positive column = XI mw. per cm. 

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