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Register a new userMetal nanoplasmas as bright sources of hard x-ray pulses
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Added by
Paramel Pattathil Rajeev
Publication date
2002
fields
Physics
and research's language is
English
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We demonstrate a 13-fold increase in hard x-ray bremsstrahlung (10 - 200 keV) emitted by a copper plasma created by 100 fs, 806 nm pulses at $10^{14}-10^{15}$ Wcm$^{-2}$. This enhancement is achieved by roughening the target surface with copper nanoparticles of ~15 nm size. A simple model that invokes local field modifications by surface plasmon excitation and `lightning rod effects explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing the emission.
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The development of a directional, small-divergence, and short-duration picosecond x-ray probe beam with an energy greater than 50 keV is desirable for high energy density science experiments. We therefore explore through particle-in-cell (PIC) computer simulations the possibility of using x-rays radiated by betatron-like motion of electrons from a self-modulated laser wakefield accelerator as a possible candidate to meet this need. Two OSIRIS 2D PIC simulations with mobile ions are presented, one with a normalized vector potential a0 = 1.5 and the other with an a0 = 3. We find that in both cases direct laser acceleration (DLA) is an important additional acceleration mechanism in addition to the longitudinal electric field of the plasma wave. Together these mechanisms produce electrons with a continuous energy spectrum with a maximum energy of 300 MeV for a0 = 3 case and 180 MeV in the a0 = 1.5 case. Forward-directed x-ray radiation with a photon energy up to 100 keV was calculated for the a0 = 3 case and up to 12 keV for the a0 = 1.5 case. The x-ray spectrum can be fitted with a sum of two synchrotron spectra with critical photon energy of 13 and 45 keV for the a0 of 3 and critical photon energy of 0.3 and 1.4 keV for a0 of 1.5 in the plane of polarization of the laser. The full width at half maximum divergence angle of the x-rays was 62 x 1.9 mrad for a0 = 3 and 77 x 3.8 mrad for a0 = 1.5.
Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of ${}^{57}$Fe Mossbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.
High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest attosecond (as) pulses have been produced only in the extreme ultraviolet (EUV) region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we use advanced experiment and theory to demonstrate a remarkable convergence of physics: when mid-infrared lasers are used to drive the high harmonic generation process, the conditions for optimal bright soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2 mu m driving lasers. Harnessing this realization, we demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, that emerge as linearly chirped 300 as pulses with a transform limit of 35 as. Most surprisingly, we find that in contrast to as pulse generation in the EUV, long-duration, multi-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright attosecond pulses of electromagnetic radiation throughout the soft X ray region of the spectrum.
Magnetic reconnection is a process whereby magnetic field lines in different directions reconnect with each other, resulting in the rearrangement of magnetic field topology together with the conversion of magnetic field energy into the kinetic energy (K.E.) of energetic particles. This process occurs in magnetized astronomical plasmas, such as those in the solar corona, Earths magnetosphere, and active galactic nuclei, and accounts for various phenomena, such as solar flares, energetic particle acceleration, and powering of photon emission. In the present study, we report the experimental demonstration of magnetic reconnection under relativistic electron magnetization situation, along with the observation of power-law distributed outflow in both electron and proton energy spectra. Through irradiation of an intense laser on a micro-coil, relativistically magnetized plasma was produced and magnetic reconnection was performed with maximum magnetic field 3 kT. In the downstream outflow direction, the non-thermal component is observed in the high-energy part of both electron and proton spectra, with a significantly harder power-law slope of the electron spectrum (p = 1.535 +/- 0.015) that is similar to the electron injection model proposed to explain a hard emission tail of Cygnus X-1, a galactic X-ray source with the same order of magnetization. The obtained result showed experimentally that the magnetization condition in the emitting region of a galactic X-ray source is sufficient to build a hard electron population through magnetic reconnection.
We investigate molecular dynamics of multiple ionization in N2 through multiple core-level photoabsorption and subsequent Auger decay processes induced by intense, short X-ray free electron laser pulses. The timing dynamics of the photoabsorption and dissociation processes is mapped onto the kinetic energy of the fragments. Measurements of the latter allow us to map out the average internuclear separation for every molecular photoionization sequence step and obtain the average time interval between the photoabsorption events. Using multiphoton ionization as a tool of multiple-pulse pump-probe scheme, we demonstrate the modification of the ionization dynamics as we vary the x-ray laser pulse duration.
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