Characterization of a magnetized plasma in cylindrical geometry

Abstract

Njord is a novel plasma device in which a low-temperature plasma is produced by RF waves inductively coupled into a cylindrical, magnetized vacuum. The main part of this project is to carry out an experimental characterization of the plasma in this device. The plasma expansion and acceleration of the source plasma into the main chamber will be of specific interest. The ultimate aim will then be to find operational regimes of the controlled parameters (e.g. magnetic field configuration and pressure), for which a so-called current-free double layer may be set up spontaneously and an ion beam formed. Such double layers have earlier been demonstrated in devices with a similar plasma source, and the formation of such double layers in Njord will be of importance for the feasibility of studying instabilities and turbulence set up by ion beams in this particular configuration. We performed this characterization using RF-compensated Langmuir and double probes. In particular, we investigated the region between the RF source and the expansion chamber, performing measurements along axial and radial positions. The results of the two probes were in agreements with what we were expecting, except for an unexpected behaviour of plasma density as a function of the magnetic field. We also found evidence of a large potential drop (40V) inside the source, which is a primary element in the formation of double layers.