Origin of hot ions observed in a modified Penning discharge

Abstract

Ions with a Maxwellian energy distribution and kinetic temperatures ranging from below 100 eV to several keV have been observed in a steady-state modified Penning discharge. Observations in the plasma, with capacitive probes at several azimuthal locations, are consistent with the existence of two distinct spokes rotating with different velocities in the sheath between the plasma and the anode ring. The faster (0.3−10 MHz) spoke consists of electrons rotating with the E/B drift velocity. The slow 0.06−0.8 MHz) spoke consists of ions, the measured thermal velocity of which is directly proportional to the spoke velocity. The interaction of the two spokes is apparently responsible for the observed electrostatic “turbulence” and ion thermalization. The anode sheath thickness is smaller than the ion gyrodiameter in this plasma. Thus, the ions are in the electric field of the sheath for only a fraction of their orbit, and their E / B drift (spoke) velocity is smaller than that of the electrons.