  The effect of the expansion of powerful FR-II radio sources into a cluster
environment is discussed. The analysis considers both the thermal and temporal
evolution of the ICM which has passed through the bow shock of the radio source
and the effect of this swept-up gas on the dynamics of the radio source itself.
The final state of the swept-up ICM is critcally dependent on the thermal
conductivity of the gas. If the gas behind the bow shock expands adiabatically,
and the source is expanding into a steeply falling atmosphere, then a narrow
dense layer will form as the radio source lifts gas out of the cluster
potential. This layer has a cooling time very much less than that of the gas
just ahead of the radio source. This effect does not occur if the thermal
conductivity of the gas is high, or if the cluster atmosphere is shallow. The
swept-up gas also affects the dyamics of the radio source especially as it
slows towards sub-sonic expansion. The preferential accumulation of the
swept-up gas to the sides of the cocoon leads to the aspect ratio of the source
increasing. Eventually the contact surface must become Rayleigh-Taylor unstable
leading both to inflow of the swept-up ICM into the cavity created by the
cocoon, but also substantial mixing of the cooler denser swept-up gas with the
ambient ICM thereby creating a multi-phase ICM. The radio source is likely to
have a marked effect on the cluster on timescales long compared to the age of
the source.
