  The amplitude equation for an unstable electrostatic wave in a multi-species
Vlasov plasma has been derived. The dynamics of the mode amplitude $\rho(t)$ is
studied using an expansion in $\rho$; in particular, in the limit
$\gamma\rightarrow0^+$, the singularities in the expansion coefficients are
analyzed to predict the asymptotic dependence of the electric field on the
linear growth rate $\gamma$. Generically $|E_k|\sim \gamma^{5/2}$, as
$\gamma\rightarrow0^+$, but in the limit of infinite ion mass or for
instabilities in reflection-symmetric systems due to real eigenvalues the more
familiar trapping scaling $|E_k|\sim \gamma^{2}$ is predicted.
