LOWER-BOUND IN ENERGY FOR CHAOTIC DYNAMICS OF IONS

When a charged particle is acted upon by an electrostatic wave propaga ting across a uniform magnetic field, its dynamics is chaotic provided that its initial Larmor radius is larger than a threshold value rho(m ). We analytically compute rho(m) when the wave Frequency is an intege r multiple of the particle cyclotron frequency, and show that rho(m) i ncreases with the wave amplitude. This leads to the counter-intuitive result that the dynamics of a low energy particle can be made stochast ic by decreasing the amplitude of the wave. (C) 1997 Elsevier Science B.V.