SESAME, a third generation synchrotron light source for the Middle East region

Developed under the auspices of UNESCO, SESAME is being established as an autonomous international research centre in the Middle East/Mediterranean region. It will have as its centrepiece a 2.5 GeV third Generation synchrotron light source with 13 straight sections for insertion devices and an emittance of 26.6 nm-rad. It will provide intense radiation from the IR to hard X-rays to a community that is expected to exceed 1000 users a few years after the start of operation in 2008

Relativistic Particle Acceleration in a Folded Current Sheet

Two-dimensional particle simulations of a relativistic Harris current sheet of pair plasmashave demonstrated that the system is unstable to the relativistic drift kink instability (RDKI) and that a new kind of acceleration process takes place in the deformed current sheet. This process contributes to the generation of non-thermal particles and contributes to the fast magnetic dissipation in the current sheet structure. The acceleration mechanism and a brief comparison with relativistic magnetic reconnection are presented.Comment: 11 preprint pages, including 3 .eps figure

Particle Acceleration and Magnetic Dissipation in Relativistic Current Sheet of Pair Plasmas

We study linear and nonlinear development of relativistic and ultrarelativistic current sheets of pair plasmas with antiparallel magnetic fields. Two types of two-dimensional problems are investigated by particle-in-cell simulations. First, we present the development of relativistic magnetic reconnection, whose outflow speed is an order of the light speed c. It is demonstrated that particles are strongly accelerated in and around the reconnection region, and that most of magnetic energy is converted into "nonthermal" part of plasma kinetic energy. Second, we present another two-dimensional problem of a current sheet in a cross-field plane. In this case, the relativistic drift kink instability (RDKI) occurs. Particle acceleration also takes place, but the RDKI fast dissipates the magnetic energy into plasma heat. We discuss the mechanism of particle acceleration and the theory of the RDKI in detail. It is important that properties of these two processes are similar in the relativistic regime of T > mc^2, as long as we consider the kinetics. Comparison of the two processes indicates that magnetic dissipation by the RDKI is more favorable process in the relativistic current sheet. Therefore the striped pulsar wind scenario should be reconsidered by the RDKI.Comment: To appear in ApJ vol. 670; 60 pages, 27 figures; References and typos are fixe

Particle Acceleration in three dimensional Reconnection Regions: A New Test Particle Approach

Magnetic Reconnection is an efficient and fast acceleration mechanism by means of direct electric field acceleration parallel to the magnetic field. Thus, acceleration of particles in reconnection regions is a very important topic in plasma astrophysics. This paper shows that the conventional analytical models and numerical test particle investigations can be misleading concerning the energy distribution of the accelerated particles, since they oversimplify the electric field structure by the assumption that the field is homogeneous. These investigations of the acceleration of charged test particles are extended by considering three-dimensional field configurations characterized by localized field-aligned electric fields. Moreover, effects of radiative losses are discussed. The comparison between homogeneous and inhomogeneous electric field acceleration in reconnection regions shows dramatic differences concerning both, the maximum particle energy and the form of the energy distribution.Comment: 11 pages, 21 figure

Dissipation in Poynting-flux Dominated Flows: the Sigma-Problem of the Crab Pulsar Wind

Flows in which energy is transported predominantly as Poynting flux are thought to occur in pulsars, gamma-ray bursts and relativistic jets from compact objects. The fluctuating component of the magnetic field in such a flow can in principle be dissipated by magnetic reconnection, and used to accelerate the flow. We investigate how rapidly this transition can take place, by implementing into a global MHD model, that uses a thermodynamic description of the plasma, explicit, physically motivated prescriptions for the dissipation rate: a lower limit on this rate is given by limiting the maximum drift speed of the current carriers to that of light, an upper limit follows from demanding that the dissipation zone expand only subsonically in the comoving frame and a further prescription is obtained by assuming that the expansion speed is limited by the growth rate of the relativistic tearing mode. In each case, solutions are presented which give the Lorentz factor of a spherical wind containing a transverse, oscillating magnetic field component as a function of radius. In the case of the Crab pulsar, we find that the Poynting flux can be dissipated before the wind reaches the inner edge of the Nebula if the pulsar emits electron positron pairs at a rate >1.E40 per second, thus providing a possible solution to the sigma-problem.Comment: Accepted for publication in Ap

Simulation studies of CZT Detectors as Gamma-Ray Calorimeter

We describe the results of detailed 3-D Monte Carlo simulations of a "CZT calorimeter" that can be used to detect photons in the keV to several MeV range. Several astrophysics applications require the detection of photons in the energy range of keV up to several MeV with good position and energy resolution. For certain applications Cadmium Zinc Telluride (CZT) detectors might be the detector option of choice. Up to now, CZT detectors have mainly been used in the energy range between a few keV to ~1 MeV, we describe the results of detailed 3-D Monte Carlo simulations of a "CZT calorimeter" that can be used to detect photons in the keV to several MeV range. The main objective of these studies is to evaluate the feasibility of CZT calorimeters, to study their performance and detect and understand performance limiting factors. Such a calorimeter consists of many layers of closely packed pixellated CZT detector units. Our simulations of single detector units reproduce experimental results, indicating that our simulations capture the main factors that limit the performance of a detector unit. Overall the conclusion of our simulation study is that between 1 cm and 1.5 cm thick detector units can be used to build a calorimeter with good performance over the energy range from ~20 keV to ~10 MeV.Comment: Accepted for publication in Astroparticle Physics, 20 pages, 14 figure

Position-sensitive Si pad detectors for electron emission channeling experiments

Position-sensitive detector systems, initially developed for the detection of X-rays, have been adapted for their use in electron emission channeling experiments. Each detection system consists of a 30.8x30.8 mm$^{2}$ 22x22 -pad Si detector, either of 0.3 mm, 0.5 mm or 1 mm thickness, four 128$-$channel preamplifier chips, a backplane trigger circuit, a sampling analog to digital converter, a digital signal processor, and a personal computer for data display and storage. The operational principle of these detection systems is described, and characteristic features such as energy and position resolution and maximum count rate, which have been determined from tests with conversion electrons and $\beta^-\!$ -particles in the energy range 40--600 keV, are presented