Sharp Density Transition Research Articles

Laser wake field acceleration with controlled self-injection by sharp density transition

Laser Wake Field Acceleration of relativistic electron bunches is a promising method to produce a large amount of energetic particles with table top equipment. One of the possible methods to inject particles in the appropriate acceleration phase of the wake behind the pulse takes advantage of the partial longitudinal breaking of the wake crests across a density downramp. In this paper results of 2.5D PIC simulations, showing the production of an electron bunch with reduced energy spread, are reported. Also, a possible method to produce the required plasma density transition by laser explosion of a suitable couple of thin foils is discussed.

Generation of high-energy electrons by a femtosecond terawatt laser propagating through a sharp downward density transition

We demonstrate by computer simulations the generation of high-energy electrons by using a femtosecond terawatt laser pulse propagating in a plasma with a sharp downward density transition. In the two-dimensional simulation, a 20-TW laser pulse with a pulse duration of 60 fs and a wavelength of 800 nm propagates through a plasma with a sharp density transition consisting of n0I=5×1018 cm−3 and n0II=0.75n0I. The simulation result demonstrates that a significant amount of electrons can be self-trapped and accelerated to an energy of 117 MeV over a distance of 0.62 mm by the ultrastrong nonlinear laser wake field. In addition, it has been found that the trapping dynamics is much different from the electron-beam-driven dynamics and that the energy spread of the trapped electrons can be reduced significantly by use of the density tapering method.

Compaction of Density-Stratified Cumulates: Effect on Trapped-Liquid Distribution

Igneous rocks formed by the accumulation of crystals from a cooling magma body compose a significant fraction of both the terrestrial oceanic crust and the lunar crust and mantle. Here we report the results of numerical simulations of compaction of accumulating crystals for which the bulk density is not constant. The simulations are designed to provide insight into the behavior of interstitial liquid during compaction when a sharp drop in the density of the cumulus minerals is encountered (e.g., at the ultramafic-mafic boundary in a layered intrusion). The simulations provide an explanation for trapped-liquid distributions associated with sharp mineral density transitions. Density transition in the mineral assemblage is a feature common to many layered intrusions and the oceanic crust. This process does not require external mechanisms, such as injections of new and different magmas, and has important implications for the formation of Pt-ores found at ultramafic-mafic transitions. The model also has implic...

Mechanism of RNA polymerase action: Formation of DNA-RNA hybrids with single-stranded templates

RNA polymerase converts single-stranded DNA templates to a DNA-RNA hybrid. Free RNA does not appear until the composition of the hybrid reaches a llimiting value of equal quantities of DNA and RNA. This is in contrast to the situation with double-stranded DNA templates where the double helix is conserved and only free RNA is produced. The bacteriophage φ X174 DNA-RNA hybrid undergoes a relatively sharp optical density transition with increasing temperature ; the T m of DNA-RNA hybrid is slightly less than that of the corresponding double-stranded φ X DNA although the dependence of T m on counter-ion concentration of the two helices is quite similar. After dissociation of the hybrid at high temperature, the helical structure can be reformed by annealing at high ionic strength. The RNA of the hybrid is less susceptible to low levels of pancreatic ribonuclease than the free RNA. φ hybrid can serve as template for RNA synthesis; in doing so a major portion of the RNA of the hybrid template is replaced by newly-synthesized RNA; i.e. the reaction is predominantly semi-conservative. These results have been discussed in terms of possible models of the copying process.