Convective Cells in a Quiescent Plasma Region

Abstract

Periodic spatial variations of density and potential were observed in the quiescent region of a plasma, in the magnetohydrodynamically stable region of a linear multipole. The variations were of the order of 50%, and were both radial and along the axial direction z, along which the magnetic field configuration was approximately invariant. Measurements of E/B velocities, as well as of particle flux, revealed drift velocities in closed vortices or convective cells, with the drift kinetic energy as high as 30% of the electron thermal energy. The cells persisted in the afterglow for at least a millisecond, roughly the plasma containment time. The axial dimension of the cells depended on the spatial variation of the input power density during ionization and heating; radially they extended from the stagnation point to near ψc [where ∂(§ dl/B)/∂ψ = 0], where there were large fluctuations of density and potential with time. The density in the quiescent region decayed at about the same rate as the density in the region of temporal fluctuations. The effect of convective cells on plasma loss is discussed.