On the validity of a hydrodynamic description of laser-driven fusion

The validity of hydrodynamic approximations for non-equilibrium plasmas is examined, with emphasis on applications to laser-driven fusion pellets. Typical density-temperature trajectories of such pellets, as predicted by hydrodynamic calculations in the published literature, are shown to lie sometimes outside the region of validity of hydrodynamic theory, which is an unsatisfactory situation. In view of this, we discuss certain criteria which can be easily used to test the self- consistent validity of any hydrodynamic result. Finally, it is noted that in the low-density, low-temperature plasma region, hydrodynamics initially breaks down because of a breakdown in conventional microscopic linear response theory. A modified microscopic theory is proposed which continues to be valid in this region. This may correspondingly extend the validity of hydrodynamics to plasmas of somewhat lower temperatures and densities.