Reliable measurements of low-density plasmas using a novel Langmuir probe with a guard tube

A novel cylindrical Langmuir probe with an optimized probe structure and an additional guard tube is developed to obtain exact plasma electron densities. Using both this novel Langmuir probe and a conventional cylindrical Langmuir probe, a comparative measurement of low-density hot-filament discharge plasmas is performed. Although the plasma potentials and electron temperatures determined by the two probes are almost identical, the electron densities obtained using the conventional Langmuir probe are grossly underestimated by more than 10% compared to those from the novel Langmuir probe. The experimental results demonstrate that optimization of the structure of such cylindrical probes is very important, especially for reliable measurements of low-density plasmas using the Langmuir probe.