Nonneutral ion plasmas and crystals in Penning traps

Experimental work which uses laser-cooled ions in Penning traps is reviewed. With laser-cooling the ions are strongly coupled and exhibit spatial correlations characteristic of a liquid or crystal. In plasmas with dimensions less than 10–15 interparticle spacings, the observed correlations are strongly affected by the finite size and shape of the trapping potential. Plasmas with greater than 60 interparticle spacings should exhibit correlations characteristic of an infinite one-component plasma. Radiation pressure from a laser is also used to apply a torque to the plasma and change the plasma density. This permits access to all possible thermal equilibria, including the maximum density state where the plasma undergoes Brillouin flow. If the size of the plasma is small compared to the trap dimensions, Penning traps produce plasmas with simple shapes whose normal modes can be calculated exactly. The modes provide a nondestructive diagnostic technique for obtaining information on the plasma density and shape.