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A laser-cooled ion plasma in a Penning trap provides a rigorous realization of a strongly coupled one-component plasma. After a brief review of the crystal structures that have been observed in Penning traps, we summarize two recent experiments. First we describe careful measurements of the stability of the plasma rotation which is controlled by a rotating electric field. We then discuss the excitation of plasma wakes produced by radiation pressure from a laser.

Experiments and theory on collisional diffusion and viscosity in quiescent single-species plasmas demonstrate enhanced transport in the two-dimensional (2D) bounce-averaged regime, limited by shear in the plasma rotation. For long plasma columns, the measured diffusion agrees quantitatively with recent theories of three-dimensional long-range E×B drift collisions, and is substantially larger than predicted for classical velocity-scattering collisions. For short plasmas, diffusion is observed to be enhanced by Nb, the number of times a thermal particle bounces axially before being separated by shear. Equivalently, recent theory in the 2D bounce-averaged regime shows how diffusion decreases with increasing shear, generalizing the zero-shear perspective which gives Bohm diffusion. Viscosity is similarly enhanced in the 2D regime, but there is presently only qualitative agreement with theory. These results apply to both non-neutral and neutral plasmas, and provide the first rigorous analysis of shear reductio...

A simple one-dimensional model of Maxwellian injection into a cylindrical Penning–Malmberg trap is presented. This model is used to predict the radial density profile of an electron column produced by a biased cathode with an r2 potential variation. The column density n(r) is assumed to depend upon the cathode potential voltage Vk(r) and the self-consistent space-charge potential φ(r) as n(r)∝exp{e[φ(r)−Vk(r)]/T}. A one-parameter family of theoretical solutions describes the radial density profiles. The model’s predictions agree well with electron density profiles resulting from a spiral tungsten filament measured over a wide range in cathode voltages.

Locally excited plasma waves are generated in a Coulomb crystal by ``pushing'' with radiation pressure on a rotating cloud of laser-cooled ${}^{9}{\mathrm{Be}}^{+}$ ions. The waves form a stationary wake that is directly imaged through the dependence of the ion fluorescence on Doppler shifts, and theoretical calculations in a slab geometry are shown to accurately reproduce these images. The technique demonstrates a new method of exciting and studying waves in cold ion clouds.

Wakes in a Coulomb crystal are produced by “pushing” with radiation pressure on a rotating spheroidal cloud of laser-cooled 9Be+ ions. The wakes are stationary in the lab frame and are caused by the interference of “drum-head” type oscillations. Velocity images of these wakes are obtained directly through the dependence of the ion fluorescence on Doppler shifts, and new analytical calculations accurately reproduce these experimental wake images. The technique demonstrates a way to excite and study modes, that were not accessible with previous techniques.

Viscous transport in pure-electron plasmas is a rearrangement of particles due to like-particle interactions, eventually leading to a confined global thermal equilibrium state. The measured transport is observed to be proportional to the shear in the total (E×B+diamagnetic) fluid rotation of the plasma, for both hollow and monotonic rotation profiles. We determine the local kinematic viscosity, κ, from measurements of the local flux of electrons. The measured viscosity is 50−104 times larger than expected from classical transport due to short-range velocity-scattering collisions, but is within a factor of 10 of recent theories by O’Neil and Dubin of transport due to long-range drift collisions. The measured viscosity scales with magnetic field and plasma length roughly as κ∝B/L. This scaling suggests a finite-length transport enhancement caused by particles interacting multiple times as they bounce axially between the ends of the plasma.

In a cylindrical trap, azimuthally asymmetric electric or magnetic fields (such as inherent trap asymmetries) cause the cross-magnetic-field transport of particles, leading to bulk radial expansion and eventually to particle loss at the trap walls. Experiments with applied electrostatic asymmetries identify two different transport regimes, “slightly-rigid” and “highly-rigid.” Here the plasma rigidity, R≡fb/fE, is the ratio of the axial bounce frequency to the azimuthal E×B rotation frequency. In the slightly-rigid regime (1 20), the transport scales as Va2R0.

Penning traps use static magnetic and electric fields to confine charged atomic particles. Static confinement fields enable large numbers of ions to be routinely laser-cooled to low temperatures. We have laser-cooled up to ~106 ions in spherically shaped plasmas to temperatures less than ~10 mK. These cold plasmas provide a rigorous realization of a strongly coupled one-component plasma (OCP). After reviewing the ion crystal structures that have been observed in Penning traps and comparing them with theoretical predictions, we summarize two recent experiments. First, we describe careful measurements of the stability of the plasma rotation, which is controlled by a rotating electric field. The application of a shearing force is observed to produce sudden angular jumps or 'slips' of the crystal orientation spaced by intervals where the crystal was 'stuck' relative to the rotating field. We observed power-law distributions of the slip frequency versus slip amplitude. Similar stick–slip dynamics and power-law distributions are also seen with earthquakes, avalanches, and many other stressed systems in nature. We then discuss plasma wakes produced by the radiation pressure of a laser beam directed along the magnetic field but offset from the plasma rotation axis. This pressure excites localized plasma waves which interfere to form a stationary wake that is directly imaged through the dependence of the ion fluorescence on Doppler shifts. Theoretical calculations accurately reproduce these images. The above work has recently been discussed in detail and is not repeated below. For a detailed discussion, see Bollinger et al (2002 Trapped Charged Particles and Fundamental Interactions I. Invited papers from the conference (Wildbad Kreuth, Germany, 25–30 Aug. 2002)), to be published in J. Phys. B: At. Mol. Opt. Phys. 36 499–510

We describe a technique and present results for imaging wakes and modes excited in a laser-cooled plasma of /sup 9/Be/sup +/ ions in a cylindrical Penning trap. Wakes are created by an off-axis laser "push" beam, while individual modes are excited by sinusoidally time-varying potentials applied to the trap endcaps. Variations in ion velocities are imaged by changes in the ion fluorescence due to Doppler shifts. A comparison between theory and experiment shows good agreement.