  We study the tail of $p(U)$, the probability distribution of
$U=\vert\psi(0,L)\vert^2$, for $\ln U\gg 1$, $\psi(x,z)$ being the solution to
$\partial_z\psi -\frac{i}{2m}\nabla_{\perp}^2 \psi =g\vert S\vert^2\, \psi$,
where $S(x,z)$ is a complex Gaussian random field, $z$ and $x$ respectively are
the axial and transverse coordinates, with $0\le z\le L$, and both $m\ne 0$ and
$g>0$ are real parameters. We perform the first instanton analysis of the
corresponding Martin-Siggia-Rose action, from which it is found that the
realizations of $S$ concentrate onto long filamentary instantons, as $\ln U\to
+\infty$. The tail of $p(U)$ is deduced from the statistics of the instantons.
The value of $g$ above which $\langle U\rangle$ diverges coincides with the one
obtained by the completely different approach developed in Mounaix et al. 2006
{\it Commun. Math. Phys.} {\bf 264}~741. Numerical simulations clearly show a
statistical bias of $S$ towards the instanton for the largest sampled values of
$\ln U$. The high maxima -- or `hot spots' -- of $\vert S(x,z)\vert^2$ for the
biased realizations of $S$ tend to cluster in the instanton region.
