Pressure-driven instabilities in astrophysical jets

Astrophysical jets are widely believed to be self-collimated by the hoop-stress due to the azimuthal component of their magnetic field. However this implies that the magnetic field is largely dominated by its azimuthal component in the outer jet region. In the fusion context, it is well-known that such configurations are highly unstable in static columns, leading to plasma disruption. It has long been pointed out that a similar outcome may follow for MHD jets, and the reasons preventing disruption are still not elucidated, although some progress has been accomplished in the recent years. In these notes, I review the present status of this open problem for pressure-driven instabilities, one of the two major sources of ideal MHD instability in static columns (the other one being current-driven instabilities). I first discuss in a heuristic way the origin of these instabilities. Magnetic resonances and magnetic shear are introduced, and their role in pressure-driven instabilities discussed in relation to Suydam's criterion. A dispersion relation is derived for pressure-driven modes in the limit of large azimuthal magnetic fields, which gives back the two criteria derived by Kadomtsev for this instability. The growth rates of these instabilities are expected to be short in comparison with the jet propagation time. What is known about the potential stabilizing role of the axial velocity of jets is then reviewed. In particular, a nonlinear stabilization mechanism recently identified in the fusion literature is discussed. Key words: Ideal MHD: stability, pressure-driven modes; Jets: stabilityComment: 20 pages, 3 figures. Lecture given at the JETSET European school "Numerical MHD and Instabilities". To be published by Springer in the "Lectures notes in physics" serie

Ultra fast cycleaccurate compiled emulation of inorder pipelined architectures

Emulation of one architecture on another is useful when the architecture is under design, when software must be ported to a new platform or is being developed for systems which are still under development, or for embedded systems that have insufficient resources to support the software development process. Emulation using an interpreter is typically slower than normal execution by up to 3 orders of magnitude. Our approach instead translates the program from the original architecture to another architecture while faithfully preserving its semantics at the lowest level. The emulation speeds are comparable to, and often faster than, programs running on the original architecture. Partial evaluation of architectural features is used to achieve such impressive performance, while permitting accurate statistics collection. Accuracy is at the level of the number of clock cycles spent executing each instruction (hence the description cycle-accurate). Key words: instruction set emulator, interpreter, compiled emulation, pipelined VLIW architectur

Impediments to clinical research in the United States

Item does not contain fulltextClinical trials are essential to the evaluation of promising scientific discoveries, but they are becoming unsustainably burdensome, threatening to deprive patients and health-care providers of new therapies and new evidence to guide the use of existing treatments. Regulations are often blamed for impeding clinical research, but there are other elements of the clinical trials enterprise that also have the potential to add burdens, through either imposed requirements or incentives that do not favor clinical research (Figure 1)