Double layer in an expanding plasma: Simultaneous upstream and downstream measurements

Abstract

Ion energy measurements were taken simultaneously using one retarding field energy analyzer placed at the open end of the plasma source, and one in the plasma diffusion region of an expanding low pressure argon plasma. An electric double layer was found, which is well separated from the region of high magnetic field and which is downstream of the maximum in the magnetic field gradient. An axially movable analyzer was used to determine the position of the double layer. It appears to be more closely connected to the rapid change in diameter from the source to the diffusion chamber, but still has a radial dimension close to that of the source diameter. These results suggest that the double layer forms, not as much as a result of a magnetic nozzle, but rather as a reaction to a dramatic change in boundary conditions. Still, a magnetic field of at least a few tens of Gauss in the double layer region is necessary for its spontaneous formation.