  We investigate signals of trapping states in the micromaser system in terms
of the average number of cavity photons as well as a suitably defined
correlation length of atoms leaving the cavity. In the description of
collective two-atom effects we allow the mean number of pump atoms inside the
cavity during the characteristic atomic cavity transit time to be as large as
of order one. The master equation we consider, which describes the micromaser
including collective two-atom effects, still exhibits trapping states for even
for a mean number of atoms inside the cavity close to one. We, however, argue
more importantly that the trapping states are more pronounced in terms of the
correlation length as compared to the average number of cavity photons, i.e. we
suggest that trapping states can be more clearly revealed experimentally in
terms of the atom correlation length. For axion detection in the micromaser
this observable may therefore be an essential ingredient.
