A study of ion spectra deduced from a hydro-dynamic code is undertaken for a 50-ps laser pulse irradiating a glass microballoon. For the thermal ions of the spectrum the best agreement between code and experiment occurs when the heat flux is limited to the flux brought about by the velocity distribution at the threshold of the ion acoustic instability. Fast ions are correctly reproduced if a suprathermal zone is assumed to be present at the outside edge of the plasma.