Precise control of the global rotation of strongly coupled ion plasmas in a Penning trap

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Abstract
Rotating asymmetric electric fields have been applied to control the rotation frequency (and hence the density) of non-neutral plasmas, which are confined in Penning-type traps and have relaxed close to thermal equilibrium characterized by a global rigid-body rotation. “Infinite” confinement times and density compression were first reported for uncorrelated plasmas of ∼10 8 Mg + ions with temperatures ranging from 1 K to 5×10 4 K (4 eV) [Huang et al., Phys. Rev. Lett. 78, 875 (1997)]. In this paper, the rotating field technique has been applied to control strongly coupled plasmas of ∼10 5 9 Be + ions which are laser-cooled to millikelvin temperatures so that the plasma freezes into a solid with a crystalline lattice. Here, Bragg diffraction peaks from crystals provide an accurate way of measuring the rotation frequency, and it is observed that the plasma rotation can be phase locked to the applied rotating field without any slip. In essence, these corotating plasmas have reached thermal equilibrium with the rotating field, and the azimuthally asymmetric boundaries of the equilibrium states have been measured experimentally. Both rotating dipole and quadrupole fields have been used to provide this precise control of the plasma rotation. However, the effectiveness of the dipole field depends on the presence of multiple ion species. With the rotating dipole field, density compression to near the Brillouin limit and increase of the rotation frequency to near the cyclotron frequency have been achieved.