Effects of Plasma-Wall Sheath on Transport

July 1, 1987

J. Vac. Sci. Technol., A 5, 2315-2318 (1987).

Mandrekas, J., Singer, C. E., Ruzic, D. N.

The 1 (1)/(2) -dimensional plasma transport code baldur, along with the neutral transport code degas are used to study the effects of secondary and photoelectron emission from the wall and limiters of a tokamak on the plasma transport. Since different electron emission mechanisms appear to be dominant in different parts of the scrapeoff, the two-region scrapeoff model included in baldur is particularly useful. The effect that secondary and photoelectron emission have on recycling through the reduction of the sheath potential is taken into account in degas by prescribing appropriate values of the sheath potential on each recycling surface, according to the dominant electron emission mechanism present. Results indicate that when photoelectron emission from the first wall increases, the ion and electron temperatures increase in the scrapeoff, while they remain unaffected near the center. When the secondary electron coefficient of the limiters is increased, the ion and electron temperatures drop in the scrapeoff while they increase near the center. In both cases the neutral density decreases slightly throughout the device.