Strong laser field interaction with a sharp nonuniform overdense plasma

The interaction of ultrashort (≤1 psec) high-intensity laser pulses with matter has attracted considerable attention in recent years because of its potential as a source of picosecond x-ray pulses. Hard x rays associated with very energetic electrons have been measured in experiments. Ultrashort pulse lasers have also opened a regime representing an entirely new situation for description of laser-plasma interaction. Since the plasma has little time to expand during the laser pulse, it has density gradient length much shorter than the laser wavelength and allow the possibility of generating high-temperature near-solid density plasmas. Initially, experimental results for plasmas interacting with high-intensity contrast laser pulses at intensities in the range of 1015W/cm2 suggested that inverse bremsstrahlung is the dominant absorption mechanism. However, at higher intensities, the electrons acquire high energies for which his collisional absorption mechanism becomes ineffective. Evidence based on x-ray emission as well as theory has suggested that at intensities of laser radiation exceeding 1017Wcm-2 resonance absorption due to p-polarized component of the incoming laser field which describes the energy transfer from an electromagnetic wave to an electron plasma wave could play an important role even for plasmas with a scale length considerably shorter than the laser wavelength. It was found that the characteristics of the interaction were determined by the intensity of the p-polarized component rather than the total intensity and a significant fraction of the laser energy was absorbed in the suprathermal electrons. In the high-intensity regime, absorption involves nonlinear kinetic effects and the isothermal approximation, which was successfully applied for the description of plasma dynamics in nanosecond pulse laser-plasma interactions, becomes invalid