Non-local plasma generation in a magnetic nozzle

Abstract

Axial plasma density measurements in a 1.5 m long plasma chamber are presented for when the regions of high magnetic field and radio frequency heating are progressively separated using a movable solenoid pair. The results show that the operating regime changes based on the degree of ion magnetisation under the antenna. When ions are magnetized, electrons heated under the antenna are efficiently transported to the solenoids along a column defined by the magnetic field lines which connect to the antenna region. The cross section of this column decreases due to the converging magnetic field geometry, thereby increasing the density of electrons on the axis. This results in a density profile which is singly peaked and centered on the location of maximum magnetic field strength. When the ions are unmagnetised under the antenna, the flux of positive charges to the wall there is increased. Electrons streaming along field lines that intersect the radial wall in the antenna region are then more attracted to the antenna region to balance this flux. This affects the equilibrium conditions along the entire magnetic field line and results in less efficient transport of electrons heated by the antenna to the region of high magnetic field strength. In this regime, there is a global decrease in plasma density and the axial density profile is doubly peaked.