Subhendu Kahaly

Enhanced hard x-ray emission from microdroplet preplasma

We perform a comparative study of hard x-ray emission from femtosecond laser plasmas in 15μm methanol microdroplets and Perspex target. The hard x-ray yield from droplet plasmas is ≃68 times more than that obtained from solid plasmas at 2×1015Wcm−2. A 10ns prepulse at about 5% of the main pulse appears to be essential for hard x-ray generation from droplets. Hot electron temperature of 36keV is measured from the droplets at 8×1014Wcm−2, whereas a three times higher intensity is needed to obtain similar hot electron temperatures from Perspex plasmas. Particle-in-cell simulations with very long scale-length density profiles support experimental observations.

Polarimetric detection of laser induced ultrashort magnetic pulses in overdense plasma

The interaction of intense (∼1016 W cm−2), subpicosecond pulses with solid targets can generate highly directional jets of hot electrons. These electrons can propagate in the solid along with the counterpropagating return shielding currents. The spontaneous magnetic field that is generated by these currents, captures in its time evolution, important information about the dynamics of the complex transport processes. By using a two pulse pump-probe polarimetric technique the temporal evolution of multimegagauss magnetic fields is measured for optically polished BK7 glass targets, each coated with a thin layer of either copper or silver. A simple model is then used for explaining the observations and for deducing quantitative information about the transport of hot electrons.

Measurements of high energy density electrons via observation of Cherenkov radiation

Direct measurement of extremely high energy density electrons created in ultraintense laser-plasma interactions is crucial issue for fast ignition. Recently Cherenkov radiation has been studied to obtain the energy distribution of electrons because the emission angle depends on the electron energy. However in the previous studies [F. Brandl et al., Europhys. Lett. 61, 632 (2003); M. Manclossi et al., Phys. Rev. Lett. 96, 125002 (2006)], the experimental configurations using a planar target raised issues of spatial overlapping among the light from the different energy electrons as well as from the other emissions, such as transition radiation. A novel prism shaped target is developed in which Cherenkov lights emitted from different energy electrons are spatially separated, realizing an absolute measurement of the energy spectrum by counting the light intensities in each observed position. The observed image clearly shows the horseshoe pattern as expected in fully three-dimensional ray-trace calculations, and the image is successfully converted into the electron spectrum inside the target. In addition, it is found from the blur of the outer edge of the Cherenkov pattern that the electrons have a small beam divergence. The calibrated energy spectrum well agrees with particle simulations.

Mapping giant magnetic fields around dense solid plasmas by high-resolution magneto-optical microscopy

We investigate the distribution of magnetic fields around dense solid plasmas generated by intense p-polarized laser approximately 10(16) W cm(-2), 100 fs) irradiation of magnetic tapes, using high sensitivity magneto-optical microscopy. By investigating the effect of irradiation on the magnetic tape, we present evidence for axial magnetic fields and map out the spatial distribution of these fields around the laser generated plasma. By using the axial magnetic field distribution as a diagnostic tool we uncover evidence for angular momentum associated with the plasma.

High resolution magneto optical microscopy of megagauss axial magnetic fields generated in laser plasma interaction

We present clear evidence for giant axial magnetic fields under P-polarised laser irradiation and map out the spatial distribution of these fields around the plasma by capturing the remnant magnetization distribution in a magnetic recordable media (magnetic tape) after laser irradiation. We postulate vortex like instabilities in the plasma to explain our results.

HOT ELECTRON GENERATION AND MANIPULATION ON 'STRUCTURED' SURFACES

We examine ways of generating hotter electrons by coupling more light into structured surfaces (nanoparticle coated surfaces and sub‐lambda gratings). It is known that such surfaces produce enhanced x‐ray yields. We study the effect of laser prepulse conditions on the enhancement of hard x‐ray emission (20 – 200 keV) from plasmas produced on nanoparticle (NP)‐coated optically polished copper surface, under different prepulse conditions and observe that enhancement reduces with increasing prepulse intensity. The dynamics of the process is seen to be in the ps regime. We attribute this to preplasma formation on nanoparticles and subsequent modification/destruction of the nanostructure layer before the arrival of the main pulse. We suggest that high‐contrast ultrashort pulses are essential for nanoparticles to function as yield enhancers. We exploit surface plasmon coupling of light into sub‐lambda grating to switch ‘ON’ a hotter component in hard x‐ray spectra from plasma.

Surface plasma attosource beamlines at ELI-ALPS

ELI-ALPS, one of the three pillars of the Extreme Light Infrastructure (ELI) project, will be in a unique position to offer dedicated experimental platforms for ultrashort time-resolved investigations of strongly excited dynamical systems. The state-of-the-art surface plasma attosource (SPA) beamlines at ELI-ALPS are being designed and developed to enable new directions in plasma-based attoscience research. The SPA beamlines will be driven by ultrashort, high peak power, high repetition rate lasers based on the latest technology and are aimed to develop previously unavailable attoscience experimental platforms employing surface high-harmonic generation process. This endeavor involves research and development challenges and careful considerations. Here we discuss the physics of plasma attosources and their characteristics under such extreme conditions and the beamline functionalities that would facilitate these objectives. Finally, we delineate the initial research possibilities with these sophisticated instruments

The extreme light infrastructure—attosecond light pulse source (ELI-ALPS) project

Globally, large international research infrastructures have over many decades promoted excellence in science and technology. Aligned with the international practice, the Europe Strategy Forum for Research Infrastructures (ESFRI) has developed and keeps updating a roadmap for research infrastructures. The Extreme Light Infrastructure (ELI) is one of the two large scale Laser Research Infrastructures (RI) proposed in the ESFRI Roadmap published in 2006. ELI aims to provide access to some of the most intense world-wide lasers for the international scientific user community, as well as secondary radiation and particle sources driven by them, offering to the users new interdisciplinary research opportunities. ELI is currently implemented as a distributed infrastructure in three pillars: ELI-Beamlines (ELI-BL) in Dolni Břežany, Czech Republic, ELI-Attosecond Light Pulse Source (ELI-ALPS) in Szeged, Hungary and ELI-Nuclear Physics (ELI-NP) in Magurele, Romania. This chapter is devoted to introduce the Hungarian pillar, ELI-ALPS, which will be operational in Szeged in 2018, with the primary mission to provide to the users the highest laboratory spatiotemporal resolution and a secondary mission to contribute to the technological development towards 200 petawatt (PW) lasers for high-field science, which is the ultimate goal of the ELI project. The chapter includes descriptions of the primary and secondary sources, while emphasis is given to selected examples of the scientific case of ELI-ALPS, presenting unique access offered by the technologies to be hosted in the infrastructure.

Micronscale spatially resolved and femtosecond time resolved megagauss magnetic pulse in hot, dense plasmas

We report the measurement of spatially and temporally resolved evolution of the self generated magnetic field in laser generated solid density plasma on A1 coated glass target. In a two pulse experiment we capture the magnetic field generated by the main pulse in the change in ellipticity of polarisation of the probe pulse, which is incident at various time delays. The observed multi megagauss magnetic field has a rise time which is very short. The decay of the field occurs over a much longer time scale.

Hot electron generation by highly efficient absorption of high intensity femtosecond laser light in plasma generated on sub-λ gratings

We report near total absorption of light in the interaction of intense, p-polarized ultrashort laser pulse with solid density plasma formed on a gold coated glass sub-λ grating structure aided by surface plasmon resonance (SPR). We measure absorption over a wide intensity range (2 × 1012W cm−2−2 × 1015Wcm−2). We compare the data with those obtained from highly polished (λ/10) Au mirror target under identical conditions. The hard X-ray spectrum shows a hotter electron component under the SPR condition.