Coherent ion acceleration using two electrostatic waves

We analyze a recently discovered coherent ion acceleration mechanism that relies on the nonlinear interaction of a magnetized ion with multiple electrostatic waves, at least two of which differ in frequency by an integer multiple of the cyclotron frequency. The mechanism does not require the ion to be in resonance and can accelerate an ion with an arbitrarily low initial energy, hence its basic importance to practical applications. We illustrate the fundamental features of the mechanism through a parametric numerical study of the ion's nonlinear interaction with two electrostatic waves. Compared to the wave frequencies, the wave amplitudes were found to have a weak effect on setting the energy bound of the coherent portion of the acceleration but have a strong impact on the nature of the acceleration as they control the connectivity between the coherent and stochastic domains of phase space. The lack of requirements on the initial ion energy and other fundamental properties of the mechanism point to its promise as an ion energization method for plasma propulsion.