  The interaction of optically emitting clouds with warm X-ray gas and hot,
tenuous radio plasma in radio jet cocoons is modelled by 2D compressible
hydrodynamic simulations. The initial setup is the Kelvin-Helmholtz instability
at a contact surface of density contrast 10,000. The denser medium contains
clouds of higher density. Optically thin radiation is realised via a cooling
source term. The cool phase effectively extracts energy from the other gas
which is both, radiated away and used for acceleration of the cold phase. This
increases the system's cooling rate substantially and leads to a massively
amplified cold mass dropout. We show that it is feasible, given small seed
clouds of order 100 solar masses, that all of the optically emitting gas in a
radio jet cocoon may be produced by this mechanism on the propagation timescale
of the jet. The mass is generally distributed as T^-1/2 with temperature, with
a prominent peak at 14,000 K. This peak is likely to be related to the
counteracting effects of shock heating and a strong rise in the cooling
function. The volume filling factor of cold gas in this peak is of the order
10^-5 to 10^-3 and generally increases during the simulation time. The
simulations tend towards an isotropic scale free Kolmogorov-type energy
spectrum over the simulation timescale. We find the same Mach-number density
relation as Kritsuk & Norman (2004) and show that this relation may explain the
velocity widths of emission lines associated with high redshift radio galaxies,
if the environmental temperature is lower, or the jet-ambient density ratio is
less extreme than in their low redshift counterparts.
