  A method is proposed for producing monoergetic, high-quality ion beams in
vacuum, via direct acceleration by the electromagnetic field of two
counterpropagating, variable-frequency lasers: ions are trapped and accelerated
by a beat-wave structure with variable phase velocity, allowing for fine
control over the energy and the charge of the beam via tuning of the frequency
variation. The physical mechanism is described with a one-dimensional theory,
providing the general conditions for trapping and scaling laws for the relevant
features of the ion beam. Two-dimensional, electromagnetic particle-in-cell
simulations, in which hydrogen gas is considered as an ion source, confirm the
validity and the robustness of the method.
