Nonlinear physics of laser-irradiated microclusters

A nonlinear theory has been developed to describe electron response and ion acceleration in dense clusters that are smaller in size than the laser wavelength. This work is motivated by high-intensity laser-cluster interaction experiments. The theory reveals that the breakdown of quasineutrality affects the cluster dynamics in a dramatic way: the laser can create a positively charged ion shell that expands due to its own space charge much faster than the central part of the cluster. The developed theory also shows a trend for the electron population to have a two-component distribution function: a cold core that responds to the laser field coherently and a hot halo that undergoes stochastic heating. The hot electrons expand together with the equal number of ions that are accelerated to supersonic velocities in a double layer at the cluster edge. This mechanism produces fast ions with energies much greater than the ponderomotive potential and it suggests that larger deuterium clusters can significantly enhance the neutron yield in future experiments.