Plasma Waves and Rotation in the Gas-Fed Hollow-Cathode Arc

Abstract

Mass rotation and azimuthally propagating waves have been observed in gas-fed hollow-cathode arcs having ne ≈ 1014cm−3, Te ≈ 1 eV, Ti = 3 to 30 eV, and Bz = 1 to 6 kG. The column rotates as a rigid body at an angular velocity of 12 ωci; it is shown theoretically that at this velocity the radial density gradient has the maximum value at which equilibrium is possible in the absence of a radial electric field. This maximum gradient corresponds to an exp [− r2/(2ρi)2] dependence of density on radius, where ρi is the ion Larmor radius. The observed waves propagate in the electron drift direction exactly perpendicular to the axis of the arc, but have nonzero k‖ because of the azimuthal field of the arc current. They are identified as resistively unstable drift waves, modified torsional Alfvén waves, and ion cyclotron waves. The drift and ion cyclotron waves occur as coherent oscillations with phase-locked harmonics, while the Alfvén waves are noncoherent. Because of the dependence of the density gradient on Bz, the drift wave frequency is proportional to Bz and the Alfvén wave frequency is independent of Bz. The Alfvén and ion cyclotron waves are excited by a feedback mechanism involving electron flow across the anode sheath.