A theory of reflectivity oscillations in high-power laser interaction with inhomogeneous expanding plasmas for the case of strong anomalous absorption near critical plasma density. 1

It is shown at first that the ponderomotive laser force appreciably may enhance the values of the dielectric function epsilon(x) near the critical plasma density. If in some spatial region near epsilon = 0 a large anomalous collision frequency is assumed, sharply rising at some point, pump field depletion may also lead to strong changes in the density scale length. As a first approach we have calculated the laser reflectivity in the case when the anomalous collision frequency and the density scale length jump at some point near the critical one. We obtain temporal oscillations of the reflectivity if due to thermal plasma expansion the density scale length increases with time. The calculated frequency and amplitude of the reflectivity oscillations are shown to be in good agreement with measured values in the case of Nd-laser heating of solid targets. (author)