Time-resolved study of holeboring in realistic experimental conditions

The evolution of dense plasmas prior to the arrival of the peak of the laser irradiation is critical to understanding relativistic laser plasma interactions. The spectral properties of a reflected laser pulse after the interaction with a plasma can be used to gain insights about the interaction itself, whereas the effect of holeboring has a predominant role. Here we developed an analytical model, describing the non-relativistic temporal evolution of the holeboring velocity in the presence of an arbitrary overdense plasma density and laser intensity profile. We verify this using two-dimensional particle-in-cell simulations, showing a major influence on the holeboring dynamic depending on the density profile. The influence on the reflected laser pulse has been verified during an experiment at the PHELIX laser. We show that this enables the possibility to determine the sub-micrometer scale length of the preplasma by measuring the maximum holeboring velocity and acceleration during the laser-plasma interaction.

How Does Transverse MHD Wave-driven Turbulence Influence the Density Filling Factor in the Solar Corona?

It is well established that transverse MHD waves are ubiquitous in the solar corona. One of the possible mechanisms for heating both open (e.g., coronal holes) and closed (e.g., coronal loops) magnetic field regions of the solar corona is MHD wave-driven turbulence. In this work, we study the variation of the filling factor of overdense structures in the solar corona due to the generation of transverse MHD wave-driven turbulence. Using 3D MHD simulations, we estimate the density filling factor of an open magnetic structure by calculating the fraction of the volume occupied by the overdense plasma structures relative to the entire volume of the simulation domain. Next, we perform forward modeling and generate synthetic spectra of Fe xiii 10749 Å and 10800 Å density-sensitive line pairs using FoMo. Using the synthetic images, we again estimate the filling factors. The estimated filling factors obtained from both methods are in reasonable agreement. Also, our results match fairly well with the observations of filling factors in coronal holes and loops. Our results show that the generation of turbulence increases the filling factor of the solar corona.

Harmonic generation in the interaction of laser with a magnetized overdense plasma

The mechanism of harmonic generation in both O- and X-mode configurations for a magnetized plasma has been explored here in detail with the help of particle-in-cell simulations. A detailed characterization of both the reflected and transmitted electromagnetic radiation propagating in the bulk of the plasma has been carried out for this purpose. The efficiency of harmonic generation is shown to increase with the incident laser intensity. A dependency of harmonic efficiency has also been found on magnetic field strength. This work demonstrates that there is an optimum value of the magnetic field at which the efficiency of harmonic generation maximizes. The observations are in agreement with theoretical analysis. For the O-mode configuration, this is compelling as the harmonic generation provides for a mechanism by which laser energy can propagate inside an overdense plasma region.

Spontaneous formation of coherent structures by an intense laser pulse interacting with overdense plasma

The formation and the dynamics of coherent magnetic field structures in the context of laser plasma interaction has attracted considerable attention. In the literature the formation of these structures has, however, mostly been reported in the wake of a laser pulse propagating in an underdense plasma medium (Bulanov et al., Phys. Rev. Lett., vol. 76, 1996, pp. 3562–3565; Nakamura & Mima Phys. Rev. Lett., vol. 100, 2008, 205006; Bulanov et al., Plasma Phys. Rep., vol. 31, no. 5, 2005, pp. 369–381; Naumova et al., Phys. Plasmas, vol. 8, no. 9, 2001, pp. 4149–4155; Nakamura et al., Phys. Rev. Lett., vol. 105, no. 13, 2010, 135002). The study here focuses on the formation of coherent structures by an intense laser pulse when it interacts with an overdense plasma medium. The laser in this case gets reflected and partially dumps its energy to the lighter electrons species. Particle-in-cell simulation studies have been carried out in two dimensions to show that the energetic electrons (generated at the critical layer and having relativistic energies), together with the background plasma electrons often self-organize to form distinct electron current vortices. These electron vortices have associated magnetic fields with monopolar or dipolar symmetries depending on the rotation profile of the electron current. The formation, stability and dynamics of these structures in the context of overdense plasma is of special importance as they provide a possibility of energy transport into those regions of plasma which are inaccessible to lasers. For such applications, higher energy content (involvement of relativistic electrons in their formation) of these structures is desirable. It is shown that their salient propagation characteristics even at relativistic energies follow the rules of electron magnetohydrodynamics (EMHD) (Isichenko & Marnachev, Sov. Phys. JETP, vol. 66, 1987, p. 702; Biskamp et al., Phys. Rev. Lett., vol. 76, 1996, p. 1264) (Generalized - EMHD Yadav et al., Phys. Plasmas, vol. 15, no. 6, 2008, 062308; Yadav et al., Phys. Plasmas, vol. 16, no. 4, 2009, 040701) for homogeneous (inhomogeneous) plasma medium, respectively.

Study of femtosecond laser pulse induced shockwave in aluminum-coated dielectric target

The influence of the preplasma on laser induced shockwave in the laser and aluminum-coated planar dielectric target interaction at vacuum has been investigated with the shadowgraphy method. While the laser irradiate on the aluminum-coated dielectric target at intensity of about 1017 W/cm2, the metallic layers absorb laser energy, evaporate and ionize into plasma, it is verified that the scale length of laser-produced plasma is dramatically dependent on the contrast ratio of femtosecond-laser while the main laser pulse energy is almost kept. The characteristics of laser induced shock wave in nanosecond time scale were studied. In the nanosecond time scale, shock wave is only observed in the case of relatively short plasma scale length. This result can be explained by the dissipation of the shock wave during its propagation in the preplasma. In addition, we performed numerical simulation with MULTI2D to get an insight into the propagation of shock wave in the overdense plasma [R. Ramis, J. Meyer-ter-Vehn, and J. Ramírez, Comput. Phys. Commun. 180, 977 (2009)].