Ion energy distribution functions in a multipole confined argon plasma diffusing from a 13.56-MHz helicon source

Abstract

Ion energy distribution functions (IEDFs) are measured in a low-pressure multipole confined diffusion plasma produced by a 13.56-MHz helicon source and compared with the nonconfined case. The plasma confinement is efficient below 1-μbar pressure and induces a plasma density increase of a factor of 2 in the diffusion region. Uniform radial profiles over 15 cm diam are obtained at the bottom of the diffusion chamber. The presence of the magnetized wall has a major effect on the IEDFs at 0.5- and 1-μbar pressure conditions at distances over 10 cm from the source exit. The tail of the IEDFs, which corresponds to hot ions coming direct from the source to the bottom of the diffusion chamber, is not affected by the confinement. However, the peak of the IEDFs, which corresponds to the background density, is drastically increased due to the reflection of the ions on the magnetized wall. The ion collisionality, involving resonant charge exchange with neutrals, is enhanced by the confinement: the first reflection on the magnetized wall induces a longer path length in the diffusion area before recombination on the nonmagnetized bottom wall resulting in a better thermalization of the hot ions diverging from the source at very low pressures.