Steady-State Confinement of Non-neutral Plasmas by Rotating Electric Fields

Abstract

We apply "rotating wall" electric fields to spin up a non-neutral plasma in a Penning-Malmberg trap, resulting in steady-state confinement (weeks) of up to 10 9 Mg 1 ions. The resulting ion columns can be near global thermal equilibrium, with near-uniform temperature and rotation frequency. The equilibrated plasma E 3 B rotation rate f E is observed to be somewhat less than the drive frequency f w , with slip Df ϵ f w 2 f E depending on temperature as Df~T 1͞2 for 0.05 & T & 5 eV. Dynamic measurements of applied torque versus slip frequency show plasma spin up and compression for Df. 0 and plasma slowing and expansion for Df , 0. By gradually increasing f w , density compression up to 20% of the Brillouin density limit has been achieved. Heating resonances and hysteresis in plasma parameters are also observed.

Collisional transport in non-neutral plasmas

“…The test particle diffusion data is obtained from steady-state ion plasmas maintained near thermal equilibrium by the Òrotating wallÓ drive [19]. A localized density n t of ÒtestÓ particles is created by placing some Mg + ions in a different atomic spin state than the rest of the plasma, using laser light.…”

Shear-limited test particle diffusion in 2-dimensional plasmas

“…Here we report measurements of test particle diffusion in a quiescent magnesium ion plasma for direct comparison to a new theory of collisional diffusion in a 2D point-vortex gas with background shear. The ions are kept in steady state with a rotating wall [3] drive superimposed on the confining potential at one end. We observe that for short plasmas (2D) the diffusion rate is up to 100 times larger than for long plasmas (3D).…”

Trapped-Particle-Mediated Damping and Transport

“…If the asymmetry is not static, the sign of the torque can be positive or negative. The "rotating wall" confinement technique utilizes wall voltages rotating faster than f E to obtain plasma compression [15,16]. For the present experiments, the θ -asymmetries are static in the lab frame and exert a negative torque on the electrons, resulting in bulk radial expansion.…”