CAN A LOCAL REPULSIVE POTENTIAL TRAP AN ELECTRON

We study the classical dynamics of a charged particle in two dimension s, under the influence of a perpendicular magnetic and an in-plane ele ctric field. We prove the surprising fact that there is a finite regio n in phase space that corresponds to the otherwise drifting particle b eing trapped by a local repulsive potential. Our result is a direct co nsequence of Kolmogorov-Arnold-Moser theory and, in particular, of Mos er's theorem. We illustrate it by numerical phase portraits and by an analytic approximation to invariant curves.