Observation and Analysis of Strong Oscillating Electric Fields in a ps and fs Laser Plasma by High-resolution X-ray Spectroscopy Measurements.

The results of measurements of the multicharged ions spectra in plasma produced by moderately intense (up to 4×1017 W/cm-2) laser pulses are presented. In the first case, Nd glass laser radiation with energy 1 – 2 J and pulse duration 1.5 ps was focused on the flat solid Teflon target. In the second case, Ti:Sa laser radiation with energy ∼70 mJ and pulse duration 70 and 500 fs was focused onto an N2O gas jet, created by a high-pressure liquid nitrogen cooled, pulsed (10 Hz) nozzle. Profiles of the Lyα lines of F IX ions in Teflon target and O VIII ions in N2O clusters were recorded with a very high spectral resolution λ/δλ ∼ 3000 – 5000. The features (peaks and dips) in the Lyα line profiles were observed. They suggest the existence of intense plasma oscillations with a frequency appreciably lower than the frequency of the laser radiation. The observed X-ray spectra for the plasma satellites of the Lyα doublet of the hydrogen F IX and O VIII ions in plasma were modeled theoretically. It was shown that the resulting doublet profile have a complex structure, which depends nontrivially both on the plasma temperature, density and on the frequency and amplitude of the plasma oscillations. Experimental positions of the satellites and their separations allowed them to be associated with intense electrostatic oscillations with an amplitude E0 ∼ (4–6)×108 Vcm-1 and a frequency ω ∼ (0.7–1)×1015 s-1 for F IX ions and E0 ∼ (0.5 – 1.0)×10 9 V/cm and frequency ω ∼ (0.7–2.4)×1015 s-1 for O VIII, simultaneously. Assuming the oscillation frequency to be determined by the strength of the magnetic field B generated in the plasma, we obtained an estimate of B that is in reasonable agreement with other measurements. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]