Collisionless laser-energy conversion by anharmonic resonance.

After two decades of experiments with intense fs laser pulses the physical mechanism of collisionless absorption in overdense matter is still not understood. We show that anharmonic resonance in the self-generated plasma potential at a steep ion density profile may represent the leading physical absorption mechanism. Resonance provides for the phase shift of the free electron current which is compulsory for laser beam energy transfer to any medium and is capable of explaining the prompt generation of fast electrons with maximum energies exceeding many times their quiver energy, and the polarization dependence.

Absorption of ultrashort laser pulses in strongly overdense targets.

Absorption measurements on solid conducting targets have been performed in s and p polarization with ultrashort, high-contrast Ti:sapphire laser pulses at intensities up to 5x10{16}W/cm{2} and pulse duration of 8 fs. The particular relevance of the reported absorption measurements lies in the fact that the extremely short laser pulse interacts with matter close to solid density during the entire pulse duration. A pronounced increase of absorption for p polarization at increasing angles is observed reaching 77% for an incidence angle of 80 degrees . Simulations performed using a 2D particle in cell code show a very good agreement with the experimental data for a plasma profile of L/lambda approximately 0.01.

Two very efficient nonlinear laser absorption mechanisms in clusters.

Experiments show strongly enhanced absorption of ultrashort superintense laser beams in clustered matter in the so-called collisionless regime. Despite numerous particle in cell simulations confirming this behavior, the underlying physical processes are not sufficiently clear. The familiar linear resonance absorption does not apply as long as the plasma frequency exceeds that of the laser. However, we show here that with increasing laser intensity the oscillations become nonlinear and can enter into resonance with the laser frequency because of restoring force lowering in Coulomb systems. Excellent absorption already at moderate intensities is the consequence. The other absorption mechanism we analyze explicitly consists in the coherent superposition of electron-ion collisions in ionized clusters. Collisional absorption enhancement factors of several orders of magnitude are found.

Time-dependent electron-ion collision frequency at arbitrary laser intensity-temperature ratio.

In superintense laser beams collisional absorption exhibits a large amplitude modulation over a laser cycle. In this paper formulas for the time-dependent electron-ion collision frequency nu(ei)(t) are presented. On the basis of a ballistic interaction model we deduce an expression for nu(ei)(t) which holds for an arbitrary isotropic distribution function and arbitrary anharmonic oscillatory electron motion [Eq. (4)]. For a Maxwellian we present compact formulas for the various ratios v(os)(t)/v(th). It is shown that the strong time dependence over one laser cycle leads to the generation of intense odd harmonics. The cycle-averaged collision frequency nu(ei);(t) is compared with expressions derived from the more complex dielectric model. It is shown that the correct choice of cutoffs as a consequence of dynamical screening is essential.

Fast ignition without hole boring.

A fast-ignitor scheme for inertial confinement fusion is proposed which works without hole boring. It is shown that a thermonuclear burn wave starts from the pellet corona when an adequate amount of energy (typically 10 kJ) is deposited in the critical layer by a petawatt laser ("coronal ignition"). Burn efficiencies as high as predicted for standard central spark ignition are achieved. In addition, the scheme is surprisingly insensitive to large deviations from spherical precompression symmetry. It may open a new prospect for direct drive.