  Fluid simulations show that when a sinusoidal density modulation is
superimposed on a linear density profile, convective instabilities can become
absolutely unstable. This conversion can occur for two-plasmon-decay and
stimulated Raman Scattering instabilities under realistic direct-drive inertial
confinement fusion conditions and can affect hot electron generation and laser
energy deposition. Analysis of the three-wave model shows that a sufficiently
large change of the density gradient in a linear density profile can turn
convective instabilities into absolute ones. An analytical expression is given
for the threshold of the gradient change, which depends on the convective gain
only.
