  A unitary transformation of the N-ion Jaynes-Cummings hamiltonian is
proposed. It is shown that any approximate expression of the evolution operator
associated with the transformed hamiltonian retains its validity independently
from the intensity of the external driving field. In particular, using the
rotating wave approximation, one obtains a solution for the N-ion
Jaynes-Cummings model which improves the standard rotating wave approximation
solution.
