Experimental Study of Hard X-Ray Production at Sub- Relativistic Intensities: Effect of Polarization and Nanosecond Pre-Pulse

For the plasma formed under interaction of femtosecond laser radiation with the surface of a target electron velocity distribution has fundamentally non-maxwellian shape: besides thermal electrons, taking their origin from classical collisional mechanisms, hot electron component is formed. In particular such important effects as generation of hard X-rays with high spectral intensity and small pulse duration, protons and multicharged ions acceleration to high energies, nuclear reactions in plasma and so on, are closely linked with hot electrons. Generation of the hot electron component is related to additional, non-collisional mechanisms of electrons heating; its mean energy exceeds mean energy of thermal component by 1-2 orders [1]. Orientation of linear polarization of radiation with respect to plasma’s electron density gradient plays important role at moderate intensities (10-10W/cm), because all the basic mechanisms (anomalous skin-effect [2, 3], resonant absorption [4–6] and vacuum heating [6, 7]) do not work for s-polarized radiation and hot electrons are not generated. With intensities higher than relativistic threshold (10 W/cm), most important role in electrons acceleration play such phenomena as ponderomotive potential [8], [v ×B] component of the Lorentz force [9, 10] and wakefield acceleration [11, 12]. In particular, above-mentioned mechanisms are not sensitive to linear polarization orientation of laser radiation, but differ depending linear or circular polarization of laser radiation is used [13]. Intermediate range of subrelativistic intensities, when impact from all the mentioned mechanisms of hot electron component generation is possible, is investigated in much less details. Generation of hot electrons with s-polarized radiation becomes possible in this range, while hot electrons energy distribution may contain several components due to effect of different physical mechanisms. Starting with moderate intensities, important role in interaction of femtosecond laser pulse with supercritical dense plasma plays presence of a pre-pulse, which intensity may exceed target’s surface breakdown threshold (∼10-10 W/cm depending on the target material). In such conditions main pulse interacts with diffused plasma-vacuum boundary and spatial scale of electron density essentially defines main mechanism of hot electrons generation. With laser radiation intensity increase pre-pulse’s influence increases as well. In this work we present results of hot electrons generation study during interaction of femtosecond laser radiation with subrelativistic intensity (from 10 to 210 W/cm), having different linear polarization and nanosecond contrast, with surfaces of transparent (quartz glass) and absorbing (silicon) targets. Also, it is experimentally shown, that irradiating a bulk target with pair of femtosecond laser pulses (delay between pulses equal to 13 ns),