Detailed Density and Ion Energy Profiles Downstream of a Small ECR Plasma Source

Abstract

The exhaust ion stream from an electron cyclotron resonance (ECR) plasma source is investigated in detail. Two sets of coils allow for different shapes of the external field under which the plasma is created. Parameters in the form of electron density $n_{e}$ , electron temperature $T_{e}$ , and ion energy distribution function (IEDF) are evaluated using Langmuir probes and retarding field energy analyzers (RFEAs). Parameters are evaluated under 10 and 20 W of microwave energy at pressures in argon between $1\times 10^{-3}$ and $2\times 10^{-3}$ mbar. In the middle of the plasma, we see electron temperatures in the range of 4–5 eV for most measurements and electron densities of up to $5\times 10^{16}/\text{m}^{3}$ in agreement with particle and power balance models. For a purely expanding magnetic field, the detailed IEDFs indicate a directed ion speed within the plasma column of up to $3.1\times 10^{3}$ m/s with respect to the low-energy background. With increasing downstream magnetic field, the low-energy part of the IEDFs vanishes downstream of the magnetic mirror. Structures with energies increasing in the radial direction appear as radial confinement improves.