G.R. Kumar

Role of surface roughness in hard-x-ray emission from femtosecond-laser-produced copper plasmas

Hard x-ray emission in the energy range of 30-300 keV from copper plasmas produced by 100-fs, 806-nm laser pulses at intensities in the range of 10 1 5 -10 1 6 W cm - 2 is investigated. We demonstrate that surface roughness of the targets overrides the role of polarization state in the coupling of light to the plasma. We further show that surface roughness has a significant role in enhancing the x-ray emission in the above mentioned energy range.

Short laser pulse induced generation of hot electrons and their anomalous stopping in overdense plasmas

In the fast ignition (FI) scheme of inertial confinement fusion, the igniter pulse falls on a precompressed overdense target and hence is unable to penetrate it. Thus, for the task of hot spot generation one has to rely on energetic electrons which are produced by the laser pulse at the critical surface. These electrons subsequently move towards the target core and deposit their energy in a sufficiently localized region. It is thus clear that the production of hot electrons by the incident sub-picosecond laser pulse at the critical surface and their subsequent transport in the overdense plasma region are the two main physics issues which are of relevance to the FI scheme. An experimental study and theoretical analysis which may be of relevance to these two issues are presented here. The study shows that the production of energetic electrons occurs through the wave breaking of plasma waves excited at the critical surface by the incident laser beam. Further, the propagation of hot electrons through the overdense region is influenced by electrostatically induced and/or by turbulence induced anomalous resistivity.

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.

Absolute keV X-ray yields from intense, ultrashort laser driven solid plasmas

Abstract Absolute yields of characteristic X-ray emission from solids irradiated by picosecond laser pulses are measured in a novel manner using a simple solid state detector. The crucial role of photon statistics in estimating X-ray yields is pointed out. We have obtained an efficiency of 1.0 × 10 −9 for energy conversion into the K X-ray of aluminium at 3.0 × 10 13 W cm −2 . The spectral resolution of the detector enables the measurement of the energy shift of the characteristic X-rays signifying the existence of highly charged ions.

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.

Multiple filamentation and supercontinuum emission from tightly focused femtosecond laser pulses in air

Filamentation characteristics of tightly focused 45 fs laser pulses in air are investigated using transverse imaging in the power range of 20-200 GW. The effect of linear and circular polarized light pulses on the onset and spatial evolution of filamentary substructures and the associated supercontinuum emission is studied.