Collisionless plasma expansion in the presence of a dipole magnetic field

The collisionless interaction of an expanding high-energy plasma cloud with a magnetized background plasma in the presence of a dipole magnetic field is examined in the framework of a 2D3V hybrid (kinetic ions and massless fluid electrons) model. The retardation of the plasma cloud and the dynamics of the perturbed electromagnetic fields and the background plasma are studied for high Alfven-Mach numbers using the particle-in-cell method. It is shown that the plasma cloud expands excluding the ambient magnetic field and the background plasma to form a diamagnetic cavity which is accompanied by the generation of a collisionless shock wave. The energy exchange between the plasma cloud and the background plasma is also studied and qualitative agreement with the analytical model suggested previously is obtained.Comment: 10 pages, 4 figure

A Self-Similar Solution for the Propagation of a Relativistic Shock in an Exponential Atmosphere

We derive a fully relativistic, self-similar solution to describe the propagation of a shock along an exponentially decreasing atmosphere, in the limit of very large Lorentz factor. We solve the problem in planar symmetry and compute the acceleration of the shock in terms of the density gradient crossed during its evolution. We apply our solution to the acceleration of shocks within the atmosphere of a HyperNova, and show that velocities consistent with the requirements of GRB models can be achieved with exponential atmospheres spanning a wide density range.Comment: ApJL in pres

High pressure operation of the triple-GEM detector in pure Ne, Ar and Xe

We study the performance of the triple-GEM (Gas Electron Multiplier) detector in pure noble gases Ne, Ar and Xe, at different pressures varying from 1 to 10 atm. In Ar and Xe, the maximum attainable gain of the detector abruptly drops down for pressures exceeding 3 atm. In contrast, the maximum gain in Ne was found to increase with pressure, reaching a value of 100,000 at 7 atm. The results obtained are of particular interest for developing noble gas-based cryogenic particle detectors for solar neutrino and dark matter search.Comment: 7 pages, 4 figures. Submitted to Nucl. Instr. and Meth. A as a letter to the Edito

Ionization fronts in negative corona discharges

In this paper we use a hydrodynamic minimal streamer model to study negative corona discharge. By reformulating the model in terms of a quantity called shielding factor, we deduce laws for the evolution in time of both the radius and the intensity of ionization fronts. We also compute the evolution of the front thickness under the conditions for which it diffuses due to the geometry of the problem and show its self-similar character.Comment: 4 pages, 4 figure

The onset of tree-like patterns in negative streamers

We present the first analytical and numerical studies of the initial stage of the branching process based on an interface dynamics streamer model in the fully 3-D case. This model follows from fundamental considerations on charge production by impact ionization and balance laws, and leads to an equation for the evolution of the interface between ionized and non-ionized regions. We compare some experimental patterns with the numerically simulated ones, and give an explicit expression for the growth rate of harmonic modes associated with the perturbation of a symmetrically expanding discharge. By means of full numerical simulation, the splitting and formation of characteristic tree-like patterns of electric discharges is observed and described

Secondary Education in Brazil : a system that persists in social reproduction

This empirical research analyzes the transformation and expansion of secondary education in Brazil over the last decades. Despite the rapid growth in the enrolment rate observed after 2000, factors such as income, ethnicity, urban-rural are still strongly related to educational inequity. Secondary education in Brazil is extremely selective and responsive to the students’ labor situation, with a low percentage of student workers accessing and succeeding at this level of education. Despite the policies and programs implemented in recent years, the crucial differences between public and private schools persist and reduce the role of education in social mobility, shaping a strongly reproductive educational system. Moreover, rather than being a propeller for reducing social inequality, secondary education is an important element in the process of formation and reproduction of the enormous inequality that marks the Brazilian society

Numerical Investigation of Power Transmission Efficiency in a RF Plasma

Capacitively coupled radio frequency discharges are used in a variety of applications in which the power transmission efficiency of the discharge is an important performance parameter. While previous research addressed the discharge properties and discharge modeling, little analysis has been done on the dependence of the power transmission efficiency on main discharge paremeters such as applied voltage, operating frequency and pressure. To investigate the effects of a dual frequency waveform on the power transmission efficiency, Particle-InCell/Monte-Carlo Collison (PIC/MCC) methods are used to simulate RF co-axial plasma discharge. Plasma characteristics are studied for a range of operating pressures and radii, as electrode voltages were varied between 0, 100 and 250V. The investigation concludes that the addition of a RF power source to the outer electrode increase power transmission efficiency by about 100%. Power transmission efficiency increases with a decrease in radius and an increase in pressure, in general. Low-frequency high-voltage power source combination is found to generate a more efficient discharge than a high-frequency high-voltage power source