10,196 research outputs found
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 with respect to Lorentz factor
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 orbits which
are characterized by a strong negative mass regime.Comment: 13 pages, 9 figures, to appear in phys. rev.
Studies of a Terawatt X-Ray Free-Electron Laser
The possibility of constructing terawatt (TW) x-ray free-electron lasers
(FELs) has been discussed using novel superconducting helical undulators [5].
In this paper, we consider the conditions necessary for achieving powers in
excess of 1 TW in a 1.5 {\AA} FEL using simulations with the MINERVA simulation
code [7]. Steady-state simulations have been conducted using a variety of
undulator and focusing configurations. In particular, strong focusing using
FODO lattices is compared with the natural, weak focusing inherent in helical
undulators. It is found that the most important requirement to reach TW powers
is extreme transverse compression of the electron beam in a strong FODO
lattice. The importance of extreme focusing of the electron beam in the
production of TW power levels means that the undulator is not the prime driver
for a TW FEL, and simulations are also described using planar undulators that
reach near-TW power levels. In addition, TW power levels can be reached using
pure self-amplified spontaneous emission (SASE) or with novel self-seeding
configurations when such extreme focusing of the electron beam is applied.Comment: 10 pages, 12 figure
Two-Dimensional Ultrathin Silica Films
Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO2 stoichiometry. It consists of precisely two layers of tetrahedral [SiO4] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials
Branching Instabilities in Rapid Fracture: Dynamics and Geometry
We propose a theoretical model for branching instabilities in 2-dimensional
fracture, offering predictions for when crack branching occurs, how multiple
cracks develop, and what is the geometry of multiple branches. The model is
based on equations of motion for crack tips which depend only on the time
dependent stress intensity factors. The latter are obtained by invoking an
approximate relation between static and dynamic stress intensity factors,
together with an essentially exact calculation of the static ones. The results
of this model are in good agreement with a sizeable quantity of experimental
data.Comment: 9 pages, 11 figure
Elucidating Surface Structure with Action Spectroscopy
Surface Action Spectroscopy, a vibrational spectroscopy method developed in recent years at the Fritz Haber Institute is employed for structure determination of clean and H2O-dosed (111) magnetite surfaces. Surface structural information is revealed by using the microscopic surface vibrations as a fingerprint of the surface structure. Such vibrations involve just the topmost atomic layers, and therefore the structural information is truly surface related. Our results strongly support the view that regular Fe3O4(111)/Pt(111) is terminated by the so-called Fetet1 termination, that the biphase termination of Fe3O4(111)/Pt(111) consists of FeO and Fe3O4(111) terminated areas, and we show that the method can differentiate between different water structures in H2O-derived adsorbate layers on Fe3O4(111)/Pt(111). With this, we conclude that the method is a capable new member in the set of techniques providing crucial information to elucidate surface structures. The method does not rely on translational symmetry and can therefore also be applied to systems which are not well ordered. Even an application to rough surfaces is possible
Learning from Minimum Entropy Queries in a Large Committee Machine
In supervised learning, the redundancy contained in random examples can be
avoided by learning from queries. Using statistical mechanics, we study
learning from minimum entropy queries in a large tree-committee machine. The
generalization error decreases exponentially with the number of training
examples, providing a significant improvement over the algebraic decay for
random examples. The connection between entropy and generalization error in
multi-layer networks is discussed, and a computationally cheap algorithm for
constructing queries is suggested and analysed.Comment: 4 pages, REVTeX, multicol, epsf, two postscript figures. To appear in
Physical Review E (Rapid Communications
The Effect of High-Intensity Intermittent Exercise on Body Composition of Overweight Young Males
To determine the effect of a 12-week high intensity intermittent exercise (HIIE) intervention on total body, abdominal, trunk, visceral fat mass, and fat free mass of young overweight males. Participants were randomly assigned to either exercise or control group. The intervention group received HIIE three times per week, 20 min per session, for 12 weeks. Aerobic power improved significantly (P < 0.001) by 15% for the exercising group. Exercisers compared to controls experienced significant weight loss of 1.5 kg (P < 0.005) and a significant reduction in total fat mass of 2 kg (P < 0.001). Abdominal and trunk adiposity was also significantly reduced in the exercising group by 0.1 kg (P < 0.05) and 1.5 kg (P < 0.001). Also the exercise group had a significant (P < 0.01) 17% reduction in visceral fat after 12 weeks of HIIE, whereas waist circumference was significantly decreased by week six (P < 0.001). Fat free mass was significantly increased (P < 0.05) in the exercising group by 0.4 kg for the leg and 0.7 kg for the trunk. No significant change (P > 0.05) occurred in levels of insulin, HOMA-IR, and blood lipids. Twelve weeks of HIIE resulted in significant reductions
in total, abdominal, trunk, and visceral fat and significant increases in fat free mass and aerobic power
- …