Magnetoplasmons in two-dimensional circular sheets of {sup 4}He{sup +} ions

We report the results of recent experiments on magnetoplasma modes in circular sheets of {sup 4}He{sup +} ions trapped below the surface of superfluid helium at a low temperature. The modes we observe include bulk modes, conventional edge modes, multipole edge modes, and extra satellites of unknown origin. The results are compared with earlier observations of bulk and conventional edge modes. Theories of the modes are reviewed and extended, and a detailed comparison with experiment is carried out. {copyright} {ital 1997} {ital The American Physical Society}