Collisionless absorption. Physical mechanisms and related strengths

Complete test of publication follows. At laser intensities beyond 1017 W/cm2 dense matter, i.e. dense gases, liquids, solids, clusters, acquires the major fraction of its energy from the laser in the so-called collisionless regime where collisional electron-ion interaction becomes inefficient owing to drastic shortening of the mutual interaction time. Excellent absorption in this regime is well documented by experiments and a whole variety of particle-in-cell, Vlasov and molecular dynamics simulations. Significant enhancement of laser-matter coupling occurs in extended cluster media. Good absorption, typically 50%, is measured and calculated in solids for sub-ps pulses during the irradiation before substantial rarefaction of the dense matter has set in by the thermal pressure. However, neither the numerous experiments nor the many numerical simulations do tell what the physical mechanisms are by which the necessary phase shift for absorption is induced between current density j and laser electric field E in Poynting's theorem. Linear resonance cannot be invoked because of the missing matching between the plasma and laser frequencies. At least ten different collisionless absorption mechanisms have been proposed, as for example skin layer absorption, sheath inverse, bremsstrahlung, stochastic heating, relativistic ponderomotive heating, longitudinal and transverse, linear and nonlinear Landau damping, excitation of surface plasmons, j x B and vacuum heating. A discussion and ordering of the strength of the individual processes, however, is still missing. In addition, to the authors' view the most efficient absorption mechanism has not been proposed yet. In the presentation we show that at high laser intensities the most significant contribution to collisionless absorption is to be attributed to nonlinear resonance between the laser field and the collective field of the plasma layers. The efficiency of the mechanism is evaluated as a function of laser intensity and it is compared with the efficiency of the above mentioned mechanisms found in the literature.