Now showing 1 - 6 of 6
  • Publication
    Charge resolved electrostatic diagnostic of colliding copper laser plasma plumes
    (American Institute of Physics, 2011) ; ; ;
    The collision of two laser generated plasma plumes can result, under appropriate conditions, in the formation of ‘stagnation layer’. The processes underlying this phenomenon are complex and time dependent. The majority of experiments over the last few decades have focused upon spectroscopic diagnostic of colliding plasmas. We have performed electrostatic diagnosis of multiply charged copper ions (Cu+ to Cu5+) generated via Q-switched pulsed laser (λ=1.06 μm, τ=6 ns, EL=52-525 mJ) generation of copper plasma plumes from a planar target. Time dependent current traces, charge yields and kinetic energy (Ke) distributions are obtained for single plasma plumes (Sp) and colliding plasma plumes (Cp). The charge yield from a Cp relative to twice that from a Sp is characterized by a charge yield ratio (CYR) parameter. Superior ion yields for all charge states occur for a discrete range of fluences (F) from colliding plasma plumes leading to a CYR parameter exceeding unity. The kinetic energy distributions from colliding plasma plumes display well defined energy compression via narrowing of the distributions for all fluences and charge states. The extent of this energy compression is charge dependent. Space charge forces within the stagnation layer and the resulting charge dependent acceleration of ions is proposed to account for the transfer of ion kinetic energy in favour of collisional ionization mechanisms.
      794Scopus© Citations 26
  • Publication
    Emission characteristics and dynamics of the stagnation layer in colliding laser produced plasmas
    The expansion dynamics of ion and neutral species in laterally colliding laser produced aluminium plasmas have been investigated using time and space resolved optical emission spectroscopy and spectrally and angularly resolved fast imaging. The emission results highlight a difference in neutral atom and ion distributions in the stagnation layer where, at a time delay of 80 ns, the neutral atoms are localised in the vicinity of the target surface (< 1 mm from the target surface) while singly and doubly charged ions lie predominantly at larger distances, < 1.5 mm and < 2 mm respectively. The imaging results show that the ions were found to form a well defined, but compressed, stagnation layer at the collision front between the two seed plasmas at early times (Δt < 80 ns). On the other hand the excited neutrals were observed to form a V shaped emission feature at the outer regions of the collision front with enhanced neutral emission in the less dense, cooler regions of the stagnation layer.
      309Scopus© Citations 38
  • Publication
    Femtosecond X-ray pulse length characterization at the LCLS free electron laser
    Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linear Coherent Light Source (LCLS) to measure the laser-assisted Auger decay in the X-ray regime. This X-ray-optical cross correlation technique provides a straightforward, non-invasive and on-line means of determining the duration of femtosecond (> 40 fs) X-ray pulses. In combination with a theoretical model of the process based on the soft-photon approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-Sapphire laser and X-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75 fs yield X-ray pulse shapes of 120 ± 20 fs FWHM and an upper limit of 40 ± 20 fs FWHM, respectively. Simulations of the SASE process using the GENESIS code agree well with the experimental results.
      345Scopus© Citations 100
  • Publication
    Electron and ion stagnation at the collision front between two laser produced plasmas
    We report results from a combined optical interferometric and spectrally resolved imaging study on colliding laser produced aluminium plasmas. A Nomarski interferometer was used to probe the spatio-temporal distribution of the electron density at the collision front. Analysis of the resulting interferograms reveals the formation and evolution of a localised electron density feature with a well defined profile reminiscent of a stagnation layer. First signs of electron stagnation are observed at a time delay of 10 ns after the peak of the plasma generating laser pulse. The peak electron density was found to exceed 1019 cm−3 and the layer remained well defined up to a time delay of ca. 100 ns. Temporally and spectrally resolved optical imaging was also undertaken to compare the Al+ ion distribution, with that of the 2D electron density profile. This revealed nascent stagnation of singly charged ions at a delay time of 20 ns. We attribute these results to the effects of space charge separation in the seed plasma plumes.
      456Scopus© Citations 38
  • Publication
    Theory of AC-Stark splitting in core-resonant Auger decay under strong x-ray fields
    (American Physical Society, 2011-12-21) ; ;
    In this work we report the modification of the normal Auger line shape under the action of an intense x-ray radiation. Under strong Rabi-type coupling of the core, the Auger line profile develops into a doublet structure with an energy separation mainly determined by the relative strength of the Rabi coupling. In addition, we find that the charge resolved ion yields can be controlled by judicious choice of the x-ray frequency.
      217Scopus© Citations 28
  • Publication
    Ion emission in collisions between two laser-produced plasmas
    Measurements of the total ion emission from a pair of colliding laser-produced aluminium plasmas were obtained with the aid of a Faraday cup detector. The energy profile width at half height of the kinetic energy distribution for ions emitted normal to the target was found to be 30% narrower for colliding plasmas compared to a single plasma. Similar to ion emission from single plumes, the mean ion kinetic energy is observed to increase with the energy of the incident laser pulse. However, the width of the ion energy distribution increases at a significantly slower rate than in the single plume case.
      524Scopus© Citations 20