NOVEL EDGE MAGNETOPLASMONS IN A 2-DIMENSIONAL SHEET OF HE-4(+) IONS

We report the experimental observation of novel low-frequency edge mag netoplasma waves in circular pools of positive ions trapped below the surface of superfluid He-4. The modes were detected through a nonlinea r coupling with an axisymmetric plasma mode of the pool, when both typ es of modes were driven simultaneously. The observed frequencies of th e new modes and their dependence on magnetic field and pool radius all ow us to identify them with those predicted to exist by Nazin and Shik in [Sov. Phys. JETP 67, 288 (1988)] when account is taken of the corre ct density profile at the edge of the pool.