Transport and damping from rotational pumping in magnetized electron plasmas.

Abstract

Collisional cross-field transport due to electric or magnetic field asymmetries is important in many neutral and non-neutral plasma confinement devices. In magnetic mirrors, it has long been postulated that particles resonant with field asymmetries enhance radial diffusion [1], but experimental verification [2] is difficult. In tokamaks, “magnetic pumping” by a spatially varying magnetic field is thought to dissipate poloidal rotation [3]. In non-neutral traps, confinement times much greater than the rotation and transit times are important for a number of technologies and experiments [4–6]; but trap asymmetries can degrade confinement [7,8]. Non-neutral plasmas are often approximated as 2D guiding center fluids [9,10] on the rotational time scale, with 3D collisions causing dissipative or viscous effects [11,12]. Here, we present measurements of radial particle transport and resulting mode damping from “rotational pumping” of a magnetized electron column displaced from the axis of a cylindrical trap. Rotational pumping is the collisional dissipation of the axial compressions which are caused by E 3 B rotation of the column through asymmetric confinement potentials; here, the confinement potentials appear asymmetric only because of the displacement of the column away from the symmetry axis of the trap. We find that this transport conserves particle number, conserves angular momentum by moving the column back to the trap axis as the column expands, and conserves total energy by dissipating electrostatic energy into thermal energy. This dissipation is analogous to that caused by the “second” or “bulk” viscosity [13] in polyatomic gases, which causes weak absorption of sound waves [14]; here, it causes readily measured particle transport. The transport rate is proportional to the electron-electron collision rate, which drops precipitously in the cryogenic, strongly magnetized regime; surprisingly, the transport is otherwise independent of the magnetic field strength. The observed transport rates are in close agreement with a new theory by Crooks and O’Neil [15]. We confine the electron plasmas in a PenningMalmberg trap [16,17], shown schematically in Fig. 1. Electrons from a tungsten filament are confined in a series of conducting cylinders of radius Rw ­ 1.27 cm, enclosed in a vacuum can at 4.2 K. The electrons are confined axially by negative voltages Vc ­ 2200 V on cylinders 1 and 4; radial confinement is provided by a uniform axial magnetic field, with 10 , B , 60 kG. The trapped plasma typically has initial density 109 # n # 1010 cm23, rms radius Rp , 0.04 cm, and length Lp , 3 cm, with a characteristic expansion time 102 # tm , 103 sec. The apparatus is operated in an inject-manipulate-dump cycle and has a shot-to-shot reproducibility of dnyn , 1%.