Measurements of long-range enhanced collisional velocity drag through plasma wave damping

Abstract

We present damping measurements of axial plasma waves in magnetized, multispecies ion plasmas. At high temperatures T≳10−2 eV, collisionless Landau damping dominates, whereas, at lower temperatures T≲10−2 eV, the damping arises from interspecies collisional drag, which is dependent on the plasma composition and scales roughly as T−3/2. This drag damping is proportional to the rate of parallel collisional slowing, and is found to exceed classical predictions of collisional drag damping by as much as an order of magnitude, but agrees with a new collision theory that includes long-range collisions. Centrifugal mass separation and collisional locking of the species occur at ultra-low temperatures T≲10−3 eV, which reduce the drag damping from the T−3/2 collisional scaling. These mechanisms are investigated by measuring the damping of higher frequency axial modes, and by measuring the damping in plasmas with a non-equilibrium species profile.