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Various methods have been proposed to condition an electron beam in order to reduce its emittance effect and to improve the short-wavelength free electron laser (FEL) performance. In this paper, we show that beam conditioning does not result in a complete elimination of the emittance effect in an alternating-gradient focusing FEL undulator. Using a one-dimensional model and a three-dimensional simulation code, we derive a criteria for the emittance limitation of a perfectly conditioned beam that depends on the focusing structure.

The initial multi-mode interfacial velocity and density perturbations present at the onset of a small Atwood number, incompressible, miscible, Rayleigh-Taylor instability-driven mixing layer have been quantified using a combination of experimental techniques. The streamwise interfacial and spanwise interfacial perturbations were measured using high-resolution thermocouples and planar laser-induced fluorescence (PLIF), respectively. The initial multi-mode streamwise velocity perturbations at the two-fluid density interface were measured using particle-image velocimetry (PIV). It was found that the measured initial conditions describe an initially anisotropic state, in which the perturbations in the streamwise and spanwise directions are independent of one another. The evolution of various fluctuating velocity and density statistics, together with velocity and density variance spectra, were measured using PIV and high-resolution thermocouple data. The evolution of the velocity and density statistics is used to investigate the early-time evolution and the onset of strongly-nonlinear, transitional dynamics within the mixing layer. The early-time evolution of the density and vertical velocity variance spectra indicate that velocity fluctuations are the dominant mechanism driving the instability development. The implications of the present experimental measurements on the initialization of Reynolds-averaged turbulent transport and mixing models and of direct and large-eddy simulations of Rayleigh-Taylor instability-induced turbulence are discussed.

Preventing and protecting against catastrophic terrorism is a complex and dynamic challenge. Small groups or individuals can use advanced technology to cause massive destruction, and the rapid pace of technology and ease of information dissemination continually gives terrorists new tools. A 100% defense is not possible. It's a numbers problem--there are simply too many possible targets to protect and too many potential attack scenarios and adversaries to defend against. However, science and technology (S&T) is a powerful force multiplier for defense. We must use S&T to get ahead of the game by making terrorist attacks more difficult to execute, more likely to be interdicted, and less devastating in terms of casualties, economic damage, or lasting disruption. Several S&T areas have potential to significantly enhance homeland security efforts with regard to detecting radiation, pathogens, explosives, and chemical signatures of weapons activities. All of these areas require interdisciplinary research and development (R&D), and many critically depend on advances in materials science. For example, the science of nuclear signatures lies at the core of efforts to develop enhanced radiation detection and nuclear attribution capabilities. Current radiation detectors require cryogenic cooling and are too bulky and expensive. Novel signatures of nuclear decay, new detector …