DOI: 10.1051/jp4:2006133108 Directed inward beams of ions in laser produced plasma

Abstract. We investigated the ion characteristics in picosecond laser plasma at laser radiation intensity of up to 2 × 1018 W/cm2. Our experimental observations of X-ray spectral line profiles confirmed the presence of a great quantity of high-energy ions (∼1MeV) in laser plasma. Besides, by the Ly Doppler profile’s red shift we revealed the high-energy ion motion directed inward the target. For this phenomenon we suggest our theoretical model. The high energy “tail ” in the energy spectrum of ions in laser plasmas can be explained by the pinch effect. The effective way for production of beams of charged particles in laser plasmas is the electric drift of these particles in electromagnetic fields generated in the laser-produced plasmas. 1. EXPERIMENT We investigated properties of fluorine ions in laser plasmas produced at the irradiation of solid fluorocarbon target by laser radiation with the intensity of 2 × 1018W/cm2. Detailed description of experiments has been presented in Ref. [1]. Doppler spectra of hydrogenlike fluorine ions have been measured; one obtains that the significant “tail ” in the ion energy spectrum appears when the laser intensity is higher than 1018W/cm2. This tail corresponds to the generation of fast ions. Fluorine ions with the energy up to 1.4MeV are measured at the laser intensity of 2 × 1018W/cm2. The drift of fast ions inside the target has been also observed by means of direct measurements of the red Doppler shift for the Lyman line of the hydrogenlike fluorine ions. The value of the drift energy is in good agreement with the previously known data [2] for averaged ion energies at these laser intensities. 2. THEORY The high energy “tail ” in the energy spectrum of ions in laser plasmas can be explained by the pinch effect. The Bennett criterion is the threshold condition for pinch effect in laser plasma. According to it, the magnetic and kinetic pressures must be equa