Dark space

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

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. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

An automobila that runs on nntural gas. (a) The vehicle pulled up to a CNG (compressed natural gas) pump. The gas is 90% methane, according to the label on the pump, (b) The car s gas tank (which occupies the dark space inside the back of the trunk) is being filled. 

Elastic scattering within the dark space is generally assumed (7) not to affect the energy distribution of the ions seriously. This is apparently caused by the low scattering angles involved which result in little or no momentum transfer. 

Theory. The requirement of a threshold energy for an endothermic ion-molecule reaction can be used together with the described model to calculate the exact position within the dark space where an ion can undergo reaction having gained from the field at least the necessary... 

If at any point x (Figure 2) away from the cathode but within the dark space 8N(E,X) denotes the number of ions per unit volume with energy between E and E + dE, such that E > E0, the threshold energy for the reaction, then as the ions move toward the cathode, the total amount of a bimolecular reaction they will undergo with neutral reactant species of density p, to yield Ns secondary ions per cc. at the cathode is given by ... 

Figure 2. Schematic diagram of the model of the cathode dark space. Concentration of the ions is denoted by N0...

Figure 4. Calculated energy distribution of the argon ions at the cathode of a glow discharge in a mixture of 99% argon and 1 % nitrogen for two different optically measured cathode dark space lengths...

---