Shock creation and particle acceleration driven by plasma expansion into a rarefied medium

The expansion of a dense plasma through a more rarefied ionised medium is a phenomenon of interest in various physics environments ranging from astrophysics to high energy density laser- matter laboratory experiments. Here this situation is modeled via a 1D Particle-In-Cell simulation; a jump in the plasma density of a factor of 100 is introduced in the middle of an otherwise equally dense electron-proton plasma with an uniform proton and electron temperature of 10eV and 1keV respectively. The diffusion of the dense plasma, through the rarified one, triggers the onset of different nonlinear phenomena such as a strong ion-acoustic shock wave and a rarefaction wave. Secondary structures are detected, some of which are driven by a drift instability of the rarefaction wave. Efficient proton acceleration occurs ahead of the shock, bringing the maximum proton velocity up to 60 times the initial ion thermal speed

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, twostage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the hightemperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.This is the author accepted manuscript. The final version is available from Springer via http://dx.doi.org/10.1007/s10494-016-9710-

Ion dynamics and coherent structure formation following laser pulse self-channeling

The propagation of a superintense laser pulse in an underdense, inhomogeneous plasma has been studied numerically by two-dimensional particle-in-cell simulations on a time scale extending up to several picoseconds. The effects of the ion dynamics following the charge-displacement self-channeling of the laser pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of the plasma channel walls, causing an inversion of the radial space-charge field and the filamentation of the laser pulse. At later times a number of long-lived, quasi-periodic field structures are observed and their dynamics is characterized with high resolution. Inside the plasma channel, a pattern of electric and magnetic fields resembling both soliton- and vortex-like structures is observed.Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download a high-resolution version), to appear in Plasma Physics and Controlled Fusion (Dec. 2007), special issue containing invited papers from the 34th EPS Conference on Plasma Physics (Warsaw, July 2007

Shocks in unmagnetized plasma with a shear flow: Stability and magnetic field generation

A pair of curved shocks in a collisionless plasma is examined with a two-dimensional particle-in-cell (PIC) simulation. The shocks are created by the collision of two electron-ion clouds at a speed that exceeds everywhere the threshold speed for shock formation. A variation of the collision speed along the initially planar collision boundary, which is comparable to the ion acoustic speed, yields a curvature of the shock that increases with time. The spatially varying Mach number of the shocks results in a variation of the downstream density in the direction along the shock boundary. This variation is eventually equilibrated by the thermal diffusion of ions. The pair of shocks is stable for tens of inverse ion plasma frequencies. The angle between the mean flow velocity vector of the inflowing upstream plasma and the shock's electrostatic field increases steadily during this time. The disalignment of both vectors gives rise to a rotational electron flow, which yields the growth of magnetic field patches that are coherent over tens of electron skin depths.Comment: 10 pages, 10 figures accepted for publication in Physics of Plasma

Direct Numerical Simulations of premixed methane flame initiation by pilot n-heptane spray autoignition

Autoignition of n-heptane sprays in a methane/air mixture and the subsequent methane premixed flame ignition, a constant volume configuration relevant to pilot-ignited dual fuel engines, was investigated by DNS. It was found that reducing the pilot fuel quantity, increases its autoignition time. This is attributed to the faster disappearance of the most reactive mixture fraction (predicted from homogeneous reactor calculations) which is quite rich. Consequently, ignition of the n-heptane occurs at leaner mixtures. The premixed methane flame is eventually ignited due to heating gained by the pressure rise caused by the n-heptane oxidation, and heat and mass transfer of intermediates from the n-heptane autoignition kernels. For large amounts of the pilot fuel, the combustion of the n-heptane results in significant adiabatic compression of the methane–air mixture. Hence the slow methane oxidation is accelerated and is further promoted by the presence of species in the oxidizer stream originating from the already ignited regions. For small amounts of the pilot fuel intermediates reach the oxidizer stream faster due to the very lean mixtures surrounding the n-heptane ignition kernels. Therefore, the premixed methane oxidation is initiated at intermediate temperatures. Depending on the amount of n-heptane, different statistical behaviour of the methane oxidation is observed when this is investigated in a reaction progress variable space. In particular for large amounts of n-heptane the methane oxidation follows roughly an autoignition regime, whereas for small amounts of n-heptane methane oxidation is similar to a canonical premixed flame. The data can be used for validation of various turbulent combustion models for dual-fuel combustion.The computational costs for this work were covered by the EPSRC project ref. no. EP/J021997/1.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.combustflame.2015.09.01

Experimental evidence for amorphous carbon grains in comets

Amorphous carbon grains similar to those produced in the laboratory, but with a higher hydrogen content, appear to be good candidates to simulate both the IR continuum emission and the 3.4 micron band measured for P/Halley. The comparison of the cometary features with those detected in the laboratory for carbon grains characterized by various sp(exp 2)/sp(exp 3) ratios seems to indicate that a prevalent diamond-like (sp(exp 3)) structure should be present in cometary particles. These kinds of solid particles seem also suitable to explain the daily and monthly variations of the 3.4 micron band intensity, relative to the continuum, and, at the same time,- to fulfill the abundance constraints. The same grains appear to be able to reproduce the absorption bands detected in the IR galactic source IRS 7. This result may be considered as a first experimental evidence of a relation existing between interstellar dust and cometary materials

Should I stay or should I go? Carbon leakage and ETS in an evolutionary model

Emissions trading is gaining increasing importance around the world as a suitable instrument to address climate change. In the absence of a global carbon market, however, unilateral carbon policies may end up causing carbon leakage effects, the more so if carbon prices are to increase in the future to achieve more ambitious emissions abatement targets. This paper intends to explore the possible delocalization effects of an Emissions Trading System (ETS) by proposing an evolutionary theoretical model in which regulated firms decide whether to stay (keep their production activities in the domestic country) or leave (move production abroad where no ETS is in place) imitating what other firms do. We investigate how this decision is affected by some key ETS design features, such as the emissions cap, the number of allowances granted for free to ETS firms, the level of a floor price for allowances. Numerical simulations show that the firms' decision on whether to abate emissions or relocate abroad are more sensitive to policies that reduce the cost of green technologies than to changes in specific features of the ETS design such as the emissions cap, the floor price and the number of permits granted for free.Web of Science103art. no. 10556

Numerical investigation of kerosene single droplet ignition at high-altitude relight conditions

In this study, the fundamental problem of the ignition of a kerosene single droplet in a quiescent medium at engine high-altitude relight conditions is investigated using numerical simulations. The main objective is to improve the understanding of ignition phenomena with a focus on the effect of droplet evaporation in determining the growth of the ignition kernel and flame establishment. Results show that when the droplet is fully immersed in a high temperature region, ignition occurs when the scalar dissipation rate associated with evaporation decreases enough to allow the initiation of a flame. The ignition time depends on the droplet diameter and the far field temperature, i.e. the position of the droplet with respect to the spark location. As the fuel is consumed, the flame is found to move closer to the droplet surface until the flame cannot sustain itself any more due to increasing scalar dissipation rates. Furthermore, results show that at very low temperatures typical of high-altitude relight conditions no flammable mixture is available around the droplet. Therefore, the success of an ignition event mainly depends on the energy released by the spark and the rate at which this energy is diffused toward the droplet surface to enhance the evaporation rate and create a flammable mixture. The findings are analysed from the perspective of gas turbine applications

Raman properties of various carbonaceous materials and their astrophysical implications

It is well known that a large number of celestial objects exhibit, in the range 3 to 12 micron, a family of emission features called unidentified infrared bands (UIR). They usually appear together and are associated with UV sources. Recently various authors have suggested that these features could be attributed to solid carbonaceous materials. Following this interest, a systematic analysis was performed of various types of amorphous carbon grains and polycyclic aromatic hydrocarbons (PAH), produced in lab. Updating results of Raman measurements performed on several carbonaceous materials, chosen according to their astrophysical interest, are presented. The measurements were made by means of a Jobin-Yvon monochromator HG2S and standard DC electronic. The line at 5145 A of an Ar+ laser was used as excitation source