Spectral characteristics of laser-plasma instabilities with a broadband laser

Recent experimental progresses regarding broadband laser-plasma instabilities(LPIs)show that a 0.6%laser bandwidth can reduce backscatters of the stimulated Brillouin scattering(SBS)and the stimulated Raman scattering(SRS)at normal incidence[Phys.Rev.Lett.132035102(2024)].In this paper,we present a further discussion of the spectral distributions of the scatters developed by broadband LPIs,in addition to a brief validation of the effectiveness of bandwidth on LPIs mitigation at oblique incidence.SBS backscatter has a small redshift in the broadband case contrary to the blueshift with narrowband laser,which may be explained by the self-cross beam energy transfer between the various frequency components within the bandwidth.SRS backscatter spectrum presents a peak at a longer wavelength in the broadband case compared to the short one in the narrowband case,which is possibly attributed to the mitigation effect of bandwidth on filaments at underdense plasmas.The three-halves harmonic emission(3ω/2)has a one-peak spectral distribution under the broadband condition,which is different from the two-peak distribution under the narrowband condition,and may be related to the spectral mixing of different frequency components within the bandwidth if the main sources of the two are both two-plasmon decays.

Shock temperature and reflectivity of precompressed H2O up to 350 GPa:Approaching the interior of planets

Using a combination of static precompression and laser-driven shock compression, shock temperature and reflectivity of H2O have been measured up to 350 GPa and 2.1×10~4 K. Here, two calibration standards were applied to enhance temperature measurement reliability. Additionally, in temperature calculations, the discrepancy in reflectivity between active probe beam wavelength and self-emission wavelength has been taken into account to improve the data’s precision.Precompressed water’s temperature–pressure data are in very good agreement with our quantum molecular dynamics model,suggesting a superionic conductor of H2O in the icy planets’ deep interior. A sluggish slope gradually approaching Dulong–Petit limit at high temperature was found at a specific heat capacity. Also, high reflectivity and conductivity were observed at the same state. By analyzing the temperature–pressure diagram, reflectivity, conductivity and specific heat comprehensively at conditions simulating the interior of planets in this work, we found that as the pressure rises, a change in ionization appears; it is supposedly attributed to energetics of bond-breaking in the H2O as it transforms from a bonded molecular fluid to an ionic state. Such molecular dissociation in H2O is associated with the conducting transition because the dissociated hydrogen atoms contribute to electrical properties.

Convective amplification of stimulated Raman rescattering in a picosecond laser plasma interaction regime

We present particle-in-cell(PIC)simulations of laser plasma instabilities(LPIs)with a laser pulse duration of a few picoseconds.The simulation parameters are appropriate to the planar-target LPI experimental conditions on SG-II.In this regime,the plasmas are characterized by a long electron density scale length and a large electron density range.It is found that when the incident laser intensity is well above its backward stimulated Raman scattering(backward SRS,BSRS)threshold,the backscattered light via the primary BSRS is intense enough to excite secondary SRS(Re-SRS)in the region below one-ninth of the critical density of the incident laser.The daughter light wave via the secondary BSRS(Re-BSRS)is amplified as it propagates toward the higher-density region in the bath of broadband light generated through the primary BSRS process.A higher intensity of the incident laser not only increases the amplitude of the BSRS light but also increases the convective amplification lengths of the Re-BSRS modes by broadening the spectrum of the BSRS light.Convective amplification of Re-BSRS causes pump depletion of the primary BSRS light and may lead to an underestimate of the primary BSRS level in SP-LPI experiments.Asignificant fraction of the generation of energetic electrons is strongly correlated with the Re-BSRS modes and should be considered as a significant energy loss.