  Using _in situ_ transmission electron microscopy (TEM), we have observed
nanometre scale dislocation loops formed when an ultra-high-purity tungsten
foil is irradiated with a very low fluence of self-ions. Analysis of the TEM
images has revealed the largest loops to be predominantly of prismatic 1/2<111>
type and of vacancy character. The formation of such dislocation loops is
surprising since isolated loops are expected to be highly mobile, and should
escape from the foil. In this work we show that the observed size and number
density of loops can be explained by the fact that the loops are _not_ isolated
- the loops formed in close proximity in the cascades interact with each other
and with vacancy clusters, also formed in cascades, through long-range elastic
fields, which prevent the escape of loops from the foil. We find that
experimental observations are well reproduced by object Kinetic Monte Carlo
simulations of evolution of cascades _only_ if elastic interaction between the
loops is taken into account. Our analysis highlights the profound effect of
elastic interaction between defects on the microstructural evolution of
irradiated materials.
