Low Order Modes of an Ion Cloud in a Penning Trap*

The electrostatic modes of a cloud of ions confined in a Penning trap are discussed in the limit that the Debye length is small compared to the cloud dimensions and the cloud dimensions are small compared to the trap dimensions. Experimental measurements of some of these mode frequencies on plasmas of laser-cooled Be+ ions agree well with calculations. Observa-tion of the modes provides a nondestructive method for obtaining informa-tion on the ion density and cloud shape. In addition, excitation of the modes by static field asymmetries may provide a practical limit to the density and number of charged particles that can be stored in a Penning trap. A cloud of charged particles stored in a Penning trap can be considered a plasma, in particular a nonneutral plasma, under conditions where the Debye length A, is small com-pared to the cloud dimensions [l, 23. The Debye length i