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

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

Magnetic instability in a dilute circular rarefaction wave

The growth of magnetic fields in the density gradient of a rarefaction wave has been observed in simulations and in laboratory experiments. The thermal anisotropy of the electrons, which gives rise to the magnetic instability, is maintained by the ambipolar electric field. This simple mechanism could be important for the magnetic field amplification in astrophysical jets or in the interstellar medium ahead of supernova remnant shocks. The acceleration of protons and the generation of a magnetic field by the rarefaction wave, which is fed by an expanding circular plasma cloud, is examined here in form of a 2D particle-in-cell simulation. The core of the plasma cloud is modeled by immobile charges, and the mobile protons form a small ring close to the cloud's surface. The number density of mobile protons is thus less than that of the electrons. The protons of the rarefaction wave are accelerated to 1/10 of the electron thermal speed, and the acceleration results in a thermal anisotropy of the electron distribution in the entire plasma cloud. The instability in the rarefaction wave is outrun by a TM wave, which grows in the dense core distribution, and its magnetic field expands into the rarefaction wave. This expansion drives a secondary TE wave. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769128]</p

One-dimensional thermal pressure-driven expansion of a pair cloud into an electron-proton plasma

Recently a filamentation instability was observed when a laser-generated pair cloud interacted with an ambient plasma. The magnetic field it drove was strong enough to magnetize and accelerate the ambient electrons. It is of interest to determine if and how pair cloud-driven instabilities can accelerate ions in the laboratory or in astrophysical plasma. For this purpose, the expansion of a localized pair cloud with the temperature 400 keV into a cooler ambient electron-proton plasma is studied by means of one-dimensional particle-in-cell (PIC) simulations. The cloud's expansion triggers the formation of electron phase space holes that accelerate some protons to MeV energies. Forthcoming lasers might provide the energy needed to create a cloud that can accelerate protons.Comment: 5 pages 4 figures, accepted for publication in Physics of Plasma

Particle-in-cell simulation study of a lower-hybrid shock

The expansion of a magnetized high-pressure plasma into a low-pressure ambient medium is examined with particle-in-cell simulations. The magnetic field points perpendicular to the plasmas expansion direction and binary collisions between particles are absent. The expanding plasma steepens into a quasi-electrostatic shock that is sustained by the lower-hybrid (LH) wave. The ambipolar electric field points in the expansion direction and it induces together with the background magnetic field a fast E cross B drift of electrons. The drifting electrons modify the background magnetic field, resulting in its pile-up by the LH shock. The magnetic pressure gradient force accelerates the ambient ions ahead of the LH shock, reducing the relative velocity between the ambient plasma and the LH shock to about the phase speed of the shocked LH wave, transforming the LH shock into a nonlinear LH wave. The oscillations of the electrostatic potential have a larger amplitude and wavelength in the magnetized plasma than in an unmagnetized one with otherwise identical conditions. The energy loss to the drifting electrons leads to a noticeable slowdown of the LH shock compared to that in an unmagnetized plasma. Published by AIP Publishing.Funding Agencies|EPSRC [EP/N022696/1]</p

Modification of the formation of high-Mach number electrostatic shock-like structures by the ion acoustic instability

The formation of unmagnetized electrostatic shock-like structures with a high Mach number is examined with one- and two-dimensional particle-in-cell (PIC) simulations. The structures are generated through the collision of two identical plasma clouds, which consist of equally hot electrons and ions with a mass ratio of 250. The Mach number of the collision speed with respect to the initial ion acoustic speed of the plasma is set to 4.6. This high Mach number delays the formation of such structures by tens of inverse ion plasma frequencies. A pair of stable shock-like structures is observed after this time in the 1D simulation, which gradually evolves into electrostatic shocks. The ion acoustic instability, which can develop in the 2D simulation but not in the 1D one, competes with the nonlinear process that gives rise to these structures. The oblique ion acoustic waves fragment their electric field. The transition layer, across which the bulk of the ions change their speed, widens and their speed change is reduced. Double layer-shock hybrid structures develop

Effects of Greek orthodox christian church fasting on serum lipids and obesity

BACKGROUND: No study to date has focused on the impact of Greek Orthodox Christian fasting on serum lipoproteins and obesity yet. METHODS: 120 Greek adults were followed longitudinally for one year. Sixty fasted regularly in all fasting periods (fasters) and 60 did not fast at all (controls). The three major fasting periods under study were: Christmas (40 days), Lent (48 days) and Assumption (August, 15 days). A total of 6 measurements were made during one year including pre- and end-fasting blood collection, serum lipoprotein analyses and anthropometric measurements. RESULTS: Statistically significant end-fasting total and LDL cholesterol differences were found in fasters. Fasters compared to controls presented 12.5% lower end-total cholesterol (p < 0.001), 15.9% lower end-LDL cholesterol (p < 0.001) and 1.5% lower end-BMI (p < 0.001). The end- LDL/HDL ratio was lower in fasters (6.5%, p < 0.05) while the change in end- HDL cholesterol in fasters (4.6% decline) was not significant. Similar results were found when the pre- and end-fasting values of fasters were compared. No change was found in control subjects. CONCLUSIONS: Adherence to Greek Orthodox fasting periods contributes to a reduction in the blood lipid profile including a non-significant reduction in HDL cholesterol and possible impact on obesity

Establishing a Community of Conversation: Creating a Context for Self-Reflection Among Teacher Scholars

This paper will discuss how the Teacher Scholars Project was created to encourage thoughtful conversations about teaching at the university, how portfolio activities such as videotape sessions and the sharing of narratives about teaching were integrated into project activities, and how faculty were encouraged to seriously look at their own practice and to reflect on it in conversations with a group of peers over the course of an entire academic year. It concludes by considering the importance of the creation of a community of conversation across disciplines in establishing conditions for more meaningful discussion and self-reflection on campus

Déconvolution robuste pour les systèmes lentement variables

Une méthode de déconvolution pour les systèmes linéaires à paramètres lentement variables dans le temps est présentée. Pour cela, nous utilisons la structure développée par Sekko & al [8] où on est amené à calculer un filtre et une loi de commande optimale. Il est possible dans ce cadre de déterminer un filtre et une loi de commande variables dans le temps. L'inconvénient de cette approche est la capacité mémoire qu'elle nécessite. Afin de résoudre ce problème, nous proposons une méthodologie permettant d'utiliser un "déconvolueur" fixe dans le temps. A cet effet, nous utilisons les outils développés pour les systèmes incertains. Ce travail permet d'effectuer une déconvolution pour des systèmes temps variant