Potential, plasma-floating

When a probe is inserted into a plasma, it will experience electrons and ions colliding with its tip. Due to the high mean speed of electrons, the flow of electrons is higher than the flow of ions. Consequently, the tip will charge up negatively until the electrons are repelled, and the net current then is zero. The probe potential then is the floating potential, Vfl. The electron current density Je then balances the ion current density 7,. At potentials lower than Vfl the ion current cannot increase further—in fact, only ions are collected from the plasma—and the ion saturation current /,s is measured. The plasma potential Vpi is defined as the potential at which all electrons arriving near the probe are collected and the probe current equals the electron current. Note that the plasma assumes the plasma potential in the absence of a probe hence probe perturbation at Vpi is... 

Figure 31. Current-voltage characteristics of an electric probe. The ion and electron current are f and 1, and the floating and plasma potentials are Vf and Vp, respectively.

In earlier work no reaction was observed to occur when metallic samples at floating potential were exposed to nitrogen discharge. It is difficult to elucidate the details of the plasma conditions used in work180 on plasma nitriding reported in the literature but two effects appear to be the probable cause of the contradictory results that have been obtained 1) impurities and 2) a low degree of nitrogen dissociation. 

The sample at floating potential is charged negatively with respect to the surrounding plasma and the surface is bombarded by ions with energy of a few eV. This, as well as further exothermic processes such as recombination and de-excitation help to keep the surface free of impurities. 

Fig. 10.1 Positive space charge layer at the interface between a plasma and (a) a dielectric, (b) a metallic, (c) an electrolytic wall with floating potential w- Due to the negative surface charge mainly neutral, positive and only high energetic negative plasma particles reach the wall (je- flow of electrons, j <+ flow of cations, ji< flow of neutralized particles, jA- flow of anions). The potential difference between the zero poten-...

It, rather, represents an ion-conducting wall of the plasma at a floating potential and reactions are motivated by the plasma-wall interactions described earlier. It is feasible to introduce a third electrode to the system, placing it in contact with the electrolyte, but not with the plasma, and therefore gaining some control over the potential difference between the electrolyte and the plasma. In the case of purely ion-conducting electrodes, the electric current offers information about the reaction rate at the plasma/electrolyte interfaces. 

Floating Potential. Micro-particles or aerosols (1-10 xm) are usually negatively charged in a steady-state non-thermal plasma and have floating potential (3-237) with respect to the plasma. Estimate the typical negative charge of such particles as a function of their radius. Assume that micro-particles are spherical and located in non-thermal plasma with electron temperature about 1 eV... 

The plasma and metal vapor in the positive column extend out to the shield or container walls encompassing the arc. The metal vapor and metal droplets from the jets will collect on the relatively cool walls. In addition, if the shield is not electrically connected to the arc circuit, it will assume a floating potential that is slightly negative with respect to the plasma and collect ions and electrons at the same rate. If the shield is connected to the cathode, a positive ion sheath is formed over its inner surface adjacent to the plasma. This is in contrast to the flaming sheath that surrounds a high-pressure arc operating in open air. 

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