Plasma-Neutral Interaction Modeling for Advanced Limiter Test-I Operation on Textor.

The plasma flow properties along a magnetic field line intercepting a limiter or other neutralizing surface are significantly altered through charge exchange and reionization and dissociation of neutrals produced at the surface. The absorption and retention characteristics of the material surfaces also affect the plasma near the surface. This problem was investigated using a one-dimensional kinetic strip model linked with the Monte Carlo code, DEGAS, for two-dimensional neutral transport. The collisional Boltzman equation is solved for the ion and electron particle distribution functions in one velocity variable and one spatial variable. Effects of plasma-neutral interactions alter the particle distribution functions and are incorporated through use of interaction rates obtained from DEGAS. Experimental results from ALT-I limiter operation on TEXTOR were modeled. Spatial profiles for the various plasma flow properties are presented, such as density, temperature, flux, interaction rates, etc. Results indicate that significant absorption exists in the duct portion of the ALT-I limiter, part of which may be due to co-depositional processes. It is also indicated that a modest ion accelerating pre-sheath electric field exists in the throat region. The various profiles demonstrate that ALT-I's designed throat length of 28 cm could have been as short as 9 cm and still maintained excellent operating characteristics. Future limiter design is also examined. A low temperature edge plasma is best for operation of a pumped limiter. It was also seen that ballistic scattering is not important. A steeper angled deflector plate that reduces the distance from the point of neutralization to the duct entrance, would produce the best results.