Formation and Control of Coulomb Crystals in Trapped Ion Plasmas

Abstract

Trapped non-neutral plasmas consisting of one charged particle species provide an experimental realization of a classical one-component plasma (OCP). I In Penning traps, which use static electric and magnetic fields for confinement, trapped plasmas can relax to a global thermal equilibrium which undergoes a rigid-body rotation about the magnetic field axis2 In a frame rotating with the plasma, there arises an induced electric field which takes the place of the field from the uniform neutralizing background in the OCP model. Active control of the rotation frequency prevents plasmas from spinning down under the ambient drag from static field errors and background neutral molecules, and allows variation of the plasma density and With Doppler laser cooling, pure ion plasmas with density no greater than 1 O8 cmP3 and temperature T less than 5 mK can be routinely obtained,2 resulting in a Coulomb coupling parameter r z ( e 2 / 4 ~ ~ o u w s ) ( k ~ T ) 1 greater than 200. Here, e is the ion charge and uws is the Wigner-Seitz radius defined by 47ru&/3 I/no. A classical, infinite OCP freezes into a bcc lattice at x 172.4 However, this result does not strictly apply to the trapped plasmas because of the surface effects associated with their finite size. Both simulations' and experiments6 show that a structure of concentric shells forms for nearly spherical plasmas with lo3 to lo4 ions. For plasmas with 2 2 x lo5 ions or 2 30 shells, time-averaged Bragg scattering patterns are consistent with bcc crystals (presumably located near the plasma enter),^ in agreement with a theoretical estimate.' But this measurement can not determine whether the Bragg pattems come from single crystals or polycrystals. In this report, we demonstrate that azimuthally asymmetric electric fields rotating in the same sense as the plasma can phase-lock the rotation of crystallized plasmas without slip, therefore precisely controlling the plasma rotation frequency, density, and surface shape.' We synchronize the detection of Bragg-scattered light either with this active rotation control or using the time dependence of the scattered light itself measured by a fast photomultiplier tube. Time-resolved (stroboscopic) Bragg diffraction pattems are obtained, effectively removing