Current limitation and formation of plasma double layers in a non-uniform magnetic field

Formation of strong double layers has been observed experimentally in a magnetised plasma column maintained by a plasma source. The magnetic field is approximately axially homogeneous except in a region at the anode where the electric current flows into a converging magnetic field. For field strengths larger than about 200 G the double layer has a stationary position only in the region of non-uniform magnetic field or at the aperture separating the source and the plasma column. It is characterised by a negative differential resistance in the current-voltage characteristic of the device. The parameter space, where the double layer exists, has been studied as well as the corresponding potential profiles and fluctuation spectra. The electric current and the axial electric field are oppositely directed between the plasma source and a potential minimum which is formed in the region of inhomogeneous magnetic field. Electron reflection by the resulting potential barrier is found to explain the essential features of the current-voltage characteristics observed, but further investigations of waves and fluctuations are required to clarify the current transport.