Electron acceleration in high-amplitude surface plasma waves excited by ultrashort laser

Abstract

We have investigated a new mechanism for creation of relativistic electrons via the acceleration by the resonant field of laser excited surface plasma waves in sharp-edged overdense plasmas. This mechanism consists in a generalization to high-intensity laser fields of an effect recently observed in the context of short-pulse laser metal interaction. As it is well known, a p-polarized laser impinging onto a structured metal surface creates a plasma during the rise time of the laser pulse, which can reach temperatures of several hundreds of eV. If the pulse duration (&lt;~ 100 <i>fs</i>) is such that the interaction of the electrons with the surface plasma wave occurs before the hydrodynamic expansion has time to smooth the plasma density sharp edge, the conditions for resonant excitation of surface plasma waves by the laser can be fulfilled. We show that in this case the strongly inhomogeneous enhanced electric field located near the plasma surface may accelerate the electrons toward the vacuum, the efficiency of this mechanism depending on the ratio <i>R_L</i> between two characteristic lengths: the extension length of the surface wave field in the vacuum and the typical distance covered by the particles in the high-frequency high-amplitude field. We find an optimum regime for <i>R_L</i> of the order of unity, in which case the electrons can be accelerated up to a momentum of the order of magnitude of the high-frequency momentum <i>p_osc</i> in the enhanced field of the surface plasma wave. The results of a 1D relativistic test-particle simulation modeling the interaction of the electrons with the plasma wave field are presented. In particular, we show that electron energies of some MeV may be reached for laser intensities of the order of 10¹⁸<i>W/cm</i>². The resulting electron energy distribution function is numerically calculated for the optimum case. The spectrum shows a well-defined peaked structure due to the dependence on the phase of the plasma wave field experienced by the accelerated electrons. This study suggests a novel possibility of high-current energetic pulsed electron sources.