Equilibrium orbit analysis in a free-electron laser with a coaxial wiggler

An analysis of single-electron orbits in combined coaxial wiggler and axial guide magnetic fields is presented. Solutions of the equations of motion are developed in a form convenient for computing orbital velocity components and trajectories in the radially dependent wiggler. Simple analytical solutions are obtained in the radially-uniform-wiggler approximation and a formula for the derivative of the axial velocity $v_{\|}$ with respect to Lorentz factor $\gamma$ is derived. Results of numerical computations are presented and the characteristics of the equilibrium orbits are discussed. The third spatial harmonic of the coaxial wiggler field gives rise to group $III$ orbits which are characterized by a strong negative mass regime.Comment: 13 pages, 9 figures, to appear in phys. rev.

Experimental Full-field Analysis of Size Effects in Miniaturized Cellular Elastomeric Metamaterials

Cellular elastomeric metamaterials are interesting for various applications, e.g. soft robotics, as they may exhibit multiple microstructural pattern transformations, each with its characteristic mechanical behavior. Numerical literature studies revealed that pattern formation is restricted in (thick) boundary layers causing significant mechanical size effects. This paper aims to experimentally validate these findings on miniaturized specimens, relevant for real applications, and to investigate the effect of increased geometrical and material imperfections resulting from specimen miniaturization. To this end, miniaturized cellular metamaterial specimens are manufactured with different scale ratios, subjected to in-situ micro-compression tests combined with digital image correlation yielding full-field kinematics, and compared to complementary numerical simulations. The specimens' global behavior agrees well with the numerical predictions, in terms of pre-buckling stiffness, buckling strain and post-buckling stress. Their local behavior, i.e. pattern transformation and boundary layer formation, is also consistent between experiments and simulations. Comparison of these results with idealized numerical studies from literature reveals the influence of the boundary conditions in real cellular metamaterial applications, e.g. lateral confinement, on the mechanical response in terms of size effects and boundary layer formation.Comment: 20 pages, 6 figures, Materials & Design, 11 May 202

Three-dimensional simulation of harmonic up-conversion in a prebunched two-beam free-electron laser

Three-dimensional simulation of harmonic up-conversion in a free-electron laser amplifier operating simultaneously with two cold and relativistic electron beams with different energy is presented in the steady-state regime. The fundamental resonance of the higher energy beam is adjusted at the third harmonic of the lower energy beam. By using slowly varying envelope approximation, the hyperbolic wave equations can be transformed into parabolic diffusion equations. By applying the source-dependent expansion to these equations, electromagnetic fields are represented in terms of the Hermite Gaussian modes which are well suited for the planar wiggler configuration. The electron dynamics is described by the fully three-dimensional Lorentz force equation in the presence of the realistic planar magnetostatic wiggler and electromagnetic fields. A set of coupled nonlinear first-order differential equations is derived and solved numerically. This set of equations describes self-consistently the longitudinal spatial dependence of radiation waists, curvatures, and amplitudes together with the evaluation of the electron beam. The evolutions of the transverse modes, in this system, are investigated for the fundamental resonance and its harmonic up-conversion. In addition to uniform beam, prebunched electron beam has also been studied. The effect of sinusoidal distribution of entry times for low energy electron beam on the evolution of radiation is compared with water bag distribution. It is shown that prebunching reduces the saturation length substantially. The analysis is related to extreme ultraviolet and x-ray emission where by seeding the lower frequency of the fundamental resonance of the lower energy beam substantial power is obtained at its third harmonic

Chloride Transport Behavior of LC3 Binders

Corrosion of metal reinforcement is the most important durability concern of concrete infrastructures. In this research, chloride transport in mortars prepared using ternary blends of OPC-calcined clay-limestone (LC3) as the cementitious materials is assessed and compared to a reference OPC system. Three clays with varying kaolinite content (17%, 50% and 80%) were employed. Through XRD analysis, transformation of carboaluminate phases - the stable form of AFm in LC3 binders - to Friedels salt, as a consequence of exposure to NaCl solution was observed. In addition, porosity of LC3 binders, with kaolinite content of 50% and higher was found to be significantly refined, which helps reduction of the depth of chloride penetration. The findings support excellent behavior of LC3 cement, prepared using low-grade clays, in corrosive environments

Performance of limestone calcined clay cement (LC3) with various kaolinite contents with respect to chloride transport

The durability of mortar and paste mixtures with respect to chloride ion ingress was investigated for binary blends of Portland Cement Calcined Clay, and ternary systems of Limestone Calcined Clay Cement (LC3). Five clays from various sources with different kaolinite content (17–95%) were studied. The main factor controlling the diffusivity of LC3 systems was found to be the kaolinite content of the clay. Resistance to chloride ingress increased to intermediate levels of kaolinite content and then stabilized. An intermediate kaolinite content of around 50% resulted in two orders of magnitude reduction in diffusivity compared to PC, indicating that the use of high grade (expensive) clays is not necessary to obtain good durability. The chloride binding capacity and distribution of bound chloride between Friedel’s salt and C–A–S–H were quantified for the different systems at fixed water to binder ratio of 0.5. The chloride binding capacity appeared to be a minor factor compared to the porosity refinement in the improved durability of LC3 systems

MRI quality control for low-field MR-IGRT systems: Lessons learned

PURPOSE: To present lessons learned from magnetic resonance imaging (MRI) quality control (QC) tests for low-field MRI-guided radiation therapy (MR-IGRT) systems. METHODS: MRI QC programs were established for low-field MRI- RESULTS: Image noise and artifacts were attributed to room noise sources, unsatisfactory system cabling, and broken RF receiver coils. Gantry angle-dependent magnetic field inhomogeneities were more prominent on the MRI-Linac due to the high volume of steel shielding in the gantry. B CONCLUSIONS: There are significant technological challenges associated with implementing and maintaining MR-IGRT systems. Most of the performance issues were identified and resolved during commissioning