PRECISE CONTROL OF THE GLOBAL ROTATION OF STRONGLY COUPLED ION PLASMAS IN A PENNING TRAP

Rotating asymmetric electric fields have been applied to control the r otation frequency (and hence the density) of non-neutral plasmas, whic h 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 u ncorrelated plasmas of similar to 10(8) Mg+ ions with temperatures ran ging from 1 K to 5 x 10(4) K (4 eV) [Huang er al., Phys. Rev. Lett. 78 , 875 (1997)]. In this paper, the rotating field technique has been ap plied to control strongly coupled plasmas of similar to 10(5) Be-9(+) ions which are laser-cooled to millikelvin temperatures so that the pl asma freezes into a solid with a crystalline lattice. Here, Bragg diff raction peaks from crystals provide an accurate way of measuring the r otation frequency, and it is observed that the plasma rotation can be phase locked to the applied rotating field without any slip. In essenc e, these corotating plasmas have reached thermal equilibrium with the rotating field, and the azimuthally asymmetric boundaries of the equil ibrium states have been measured experimentally. Both rotating dipole and quadrupole fields have been used to provide this precise control o f the plasma rotation. However, the effectiveness of the dipole field depends on the presence of multiple ion species. With the rotating dip ole field, density compression to near the Brillouin limit and increas e of the rotation frequency to near the cyclotron frequency have been achieved.