Study of second harmonic emissions for characterization of laser–plasma X-ray sources

Abstract

An investigation of second harmonic (SH) and X-ray emissions from Al plasmas produced by 3-ns, 1.064-μm laser pulses at 1014 W/cm2 is reported. The SH and X-ray yields are strongly correlated as a function of the target position with respect to the laser beam focus. The SH originates from the underdense coronal plasma and has a filamentary source, while the X-ray source is uniform. The results suggest that, although the X-ray emission is significantly enhanced by the filamentation of the laser light in the corona, there is a smoothing effect in the energy transport process toward the overdense region.

Volume effect of laser produced plasma on X-ray emissions

“…Thus supports the model of Tallents [3] even in nanosecond LPP, independently confirms the experimental result of Chvojka [8]. This is also in agreement with the experimental results Giulietti et al [9] for ns LPP, except that they see prominent enhancement of x-ray signal when laser beam is focused in the front side of the target and it is correlated with second harmonic emission and thus related to filamentation in the plasma corona region. A more comprehensive experiment is called for to understand the role of lateral energy transport, filamentation and plasma volume in order to optimize the x-ray yield from LPP of nanosecond duration at varying laser intensities.…”

“…The measurement of x-ray emission from the laser produced copper plasma formed by focussing of nsec laser beam with varying size of focal spot seems to confirm the basic assumption of model by Tallents et al [3] and also corroborates the experimental result of Chvojka et al [8] at sub-nanosecond laser pulse and Giulietti et al [9] at nsec laser pulse duration. The presence of two maxima in the x-ray signal plotted against the target placement distance with respect to the laser beam focal plane confirms the volume effect to be responsible for enhancement in the x-ray yield.…”

“…Thus in the intensity range of around 10 ½ W cm ¾ , results support the theoretical model of Tallents et al [3]. However they are different as per the results of A. Giulietti et al [9], showing x-ray and second harmonic yields to be correlated as a function of target position with respect to the laser beam focus. Although the x-ray emission is significantly enhanced but due to the filamentation of the laser light in the corona region, they rule out the possibility of any geometrical effect of focusing leading to the enhancement.…”

“…If we consider the fact that filamentation is the main reason for the x-ray enhancement then it should show itself by increase in the plasma temperature and fluctuation in the density in the off focus position. This has not been seen by them and as per Chvojka's result [8], which is at an intensity value one order higher compared with Giulietti [9] experiment, occurrence of filamentation leading to transient jump in temperature and density should have shown by itself in the temperature and density variations shown by Chvojka et al [8]. Further, how the filamentation is leading to smooth x-ray emission from LPP is not clear!…”

X-ray conversion of ultra-short laser pulses on a solid sample: Role of electron waves excited in the pre-plasma

“…5,6 Historically, all the nonlinear effects related to the laserplasma coupling, in particular all the parametric stimulated instabilities, including SRS (Stimulated Raman Scattering) and TPD (Two Plasmon decay), were investigated in detail in the long pulse regime (pulse duration longer than 100 ps), [7][8][9] during the 1980s and 1990s, in the framework of the Inertial Confinement Fusion (ICF). 10 In some cases, these studies were aimed at characterizing X-ray sources 11 and study hot electron generation. 12,13 However, little information on the role of polarization emerged from these studies, due to the long pre-plasma, which prevented significant interaction at the critical density.…”