Plasma parameters profiles from Langmuir probe measurements in low-density, low-temperature plasmas in an axial magnetic field

Determining plasma parameters in low-density, low-temperature, weakly ionized, magnetised plasmas can be challenging, especially when using in-house-built Langmuir probes that may perturb the plasma. Radial profiles along the plasma column in such conditions present further challenges, not only because the densities evanesce at the edges, but also because of the presence of magnetic fields that may significantly affect the measurements. The analysis is presented here of radial profiles in pure argon and pure helium plasmas obtained using in-house-built Langmuir probes. The study was done in detail not only to obtain plasma parameter profiles, but also to gain an insight into the relevant physical mechanisms in the operating conditions of interest. Results from a plasma model were used to complement the analysis together with qualitative observations from resulting electron energy distribution functions. The main conclusions are: profiles obtained with the plasma model closely represent those obtained from Langmuir probe data, with irregularities qualitatively explained using electron density profiles obtained with Druyvesteyn and first derivative methods; plasma densities and electron currents are sufficiently small that no diamagnetic effects are evident; the strong magnetisation of the electrons is evident from the resulting characteristic Bessel-type density profiles.