68 research outputs found
Long-term evolution of broken wakefields in finite radius plasmas
A novel effect of fast heating and charging a finite-radius plasma is
discovered in the context of plasma wakefield acceleration. As the plasma wave
breaks, the most of its energy is transferred to plasma electrons which create
strong charge-separation electric field and azimuthal magnetic field around the
plasma. The slowly varying field structure is preserved for hundreds of
wakefield periods and contains (together with hot electrons) up to 80% of the
initial wakefield energy.Comment: 5 pages, 6 figure
Numerical Studies of Electron Acceleration Behind Self-Modulating Proton Beam in Plasma with a Density Gradient
Presently available high-energy proton beams in circular accelerators carry
enough momentum to accelerate high-intensity electron and positron beams to the
TeV energy scale over several hundred meters of the plasma with a density of
about 1e15 1/cm^3. However, the plasma wavelength at this density is 100-1000
times shorter than the typical longitudinal size of the high-energy proton
beam. Therefore the self-modulation instability (SMI) of a long (~10 cm) proton
beam in the plasma should be used to create the train of micro-bunches which
would then drive the plasma wake resonantly. Changing the plasma density
profile offers a simple way to control the development of the SMI and the
acceleration of particles during this process. We present simulations of the
possible use of a plasma density gradient as a way to control the acceleration
of the electron beam during the development of the SMI of a 400 GeV proton beam
in a 10 m long plasma. This work is done in the context of the AWAKE project
--- the proof-of-principle experiment on proton driven plasma wakefield
acceleration at CERN.Comment: 4 pages, 5 figures
Antiferroelectricity and related chirality induced frustrations in smectic liquid crystals : effect of molecular structure
This work was devoted to synthesis of new antiferroelectric liquid crystalline materials in series of chiral I-methylheptyI4'-(4-n-alkyloxybenzoyloxy)biphenyl-4-carboxylates. 9 new types of materials were synthesised and their chemical, mesomorphic and electrooptical properties were investigated in the homologous series.The chemical structure of each material synthesised was proved through IHNMR and IR spectroscopy studies, mass spectrometry, optical rotation measurements and analytical HPLC technique.The mesomorphic properties of the final products were investigated by the DSC method and textural observations in polarising light microscope. The electrooptical experiments were done in thin planar cells by applying AC triangular wave field of various voltages and frequencies. The electrooptical response measured was plotted as a function of the external voltage and the resulting curves were analysed in detail. Apart from the investigation into mesomorphic behaviour of the compounds prepared, the effect of V -shaped switching was described for some of the homologues. The switching parameters were determined and conclusions were reached on the origins of the V -shaped electrooptical response.The studies on a new type of the twist grain boundary phase with local anti ferroelectric structure were described in separate chapter
Proton Beam Defocusing as a Result of Self-Modulation in Plasma
The AWAKE experiment will use a \SI{400}{GeV/c} proton beam with a
longitudinal bunch length of to create and sustain
GV/m plasma wakefields over 10 meters . A 12 cm long bunch can only drive
strong wakefields in a plasma with after the self-modulation instability (SMI)
developed and microbunches formed, spaced at the plasma wavelength. The fields
present during SMI focus and defocus the protons in the transverse plane
\cite{SMI}. We show that by inserting two imaging screens downstream the
plasma, we can measure the maximum defocusing angle of the defocused protons
for plasma densities above .
Measuring maximum defocusing angles around 1 mrad indirectly proves that SMI
developed successfully and that GV/m plasma wakefields were created. In this
paper we present numerical studies on how and when the wakefields defocus
protons in plasma, the expected measurement results of the two screen
diagnostics and the physics we can deduce from it.Comment: 3 pages, 2 figures, Conference Proceedings of NAPAC 201
Control of energy density inside disordered medium by coupling to open or closed channels
We demonstrate experimentally an efficient control of light intensity
distribution inside a random scattering system. The adaptive wavefront shaping
technique is applied to a silicon waveguide containing scattering
nanostructures, and the on-chip coupling scheme enables access to all input
spatial modes. By selectively coupling the incident light to open or closed
channels of the disordered system, we not only vary the total energy stored
inside the system by 7.4 times, but also change the energy density distribution
from an exponential decay to a linear decay and to a profile peaked near the
center. This work provides an on-chip platform for controlling light-matter
interactions in turbid media
Shape Dependence of Transmission, Reflection, and Absorption Eigenvalue Densities in Disordered Waveguides with Dissipation
The universal bimodal distribution of transmission eigenvalues in lossless diffusive systems underpins such celebrated phenomena as universal conductance fluctuations, quantum shot noise in condensed matter physics, and enhanced transmission in optics and acoustics. Here, we show that in the presence of absorption, the density of the transmission eigenvalues depends on the confinement geometry of the scattering media. Furthermore, in an asymmetric waveguide, the densities of the reflection and absorption eigenvalues also depend on the side from which the waves are incident. With increasing absorption, the density of absorption eigenvalues transforms from a single-peak to a double-peak function. Our findings open an additional avenue for coherent control of wave transmission, reflection, and absorption in random media
Control of Energy Density inside a Disordered Medium by Coupling to Open or Closed Channels
We demonstrate experimentally the efficient control of light intensity distribution inside a random scattering system. The adaptive wave front shaping technique is applied to a silicon waveguide containing scattering nanostructures, and the on-chip coupling scheme enables access to all input spatial modes. By selectively coupling the incident light to the open or closed channels of the disordered system, we not only vary the total energy stored inside the system by a factor of 7.4, but also change the energy density distribution from an exponential decay to a linear decay and to a profile peaked near the center. This work provides an on-chip platform for controlling light-matter interactions in turbid media
Model-Based Testing of Safety Critical Real-Time Control Logic Software
The paper presents the experience of the authors in model based testing of
safety critical real-time control logic software. It describes specifics of the
corresponding industrial settings and discusses technical details of usage of
UniTESK model based testing technology in these settings. Finally, we discuss
possible future directions of safety critical software development processes
and a place of model based testing techniques in it.Comment: In Proceedings MBT 2012, arXiv:1202.582
Diet and subsistence in Bronze Age pastoral communities from the southern Russian steppes and the North Caucasus
The flanks of the Caucasus Mountains and the steppe landscape to their north offered highly productive grasslands for Bronze Age herders and their flocks of sheep, goat, and cattle. While the archaeological evidence points to a largely pastoral lifestyle, knowledge regarding the general composition of human diets and their variation across landscapes and during the different phases of the Bronze Age is still restricted. Human and animal skeletal remains from the burial mounds that dominate the archaeological landscape and their stable isotope compositions are major sources of dietary information. Here, we present stable carbon and nitrogen isotope data of bone collagen of 105 human and 50 animal individuals from the 5th millennium BC to the Sarmatian period, with a strong focus on the Bronze Age and its cultural units including Maykop, Yamnaya, Novotitorovskaya, North Caucasian, Catacomb, post-Catacomb and late Bronze Age groups. The samples comprise all inhumations with sufficient bone preservation from five burial mound sites and a flat grave cemetery as well as subsamples from three further sites. They represent the Caucasus Mountains in the south, the piedmont zone and Kuban steppe with humid steppe and forest vegetation to its north, and more arid regions in the Caspian steppe. The stable isotope compositions of the bone collagen of humans and animals varied across the study area and reflect regional diversity in environmental conditions and diets. The data agree with meat, milk, and/or dairy products from domesticated herbivores, especially from sheep and goats having contributed substantially to human diets, as it is common for a largely pastoral economy. This observation is also in correspondence with the faunal remains observed in the graves and offerings of animals in the mound shells. In addition, foodstuffs with elevated carbon and nitrogen isotope values, such as meat of unweaned animals, fish, or plants, also contributed to human diets, especially among communities living in the more arid landscapes. The regional distinction of the animal and human data with few outliers points to mobility radii that were largely concentrated within the environmental zones in which the respective sites are located. In general, dietary variation among the cultural entities as well as regarding age, sex and archaeologically indicated social status is only weakly reflected. There is, however, some indication for a dietary shift during the Early Bronze Age Maykop period
Ancient human genome-wide data from a 3000-year interval in the Caucasus corresponds with eco-geographic regions
Archaeogenetic studies have described the formation of Eurasian 'steppe ancestry' as a mixture of Eastern and Caucasus hunter-gatherers. However, it remains unclear when and where this ancestry arose and whether it was related to a horizon of cultural innovations in the 4 th millennium BCE that subsequently facilitated the advance of pastoral societies in Eurasia. Here we generated genome-wide SNP data from 45 prehistoric individuals along a 3000-year temporal transect in the North Caucasus. We observe a genetic separation between the groups of the Caucasus and those of the adjacent steppe. The northern Caucasus groups are genetically similar to contemporaneous populations south of it, suggesting human movement across the mountain range during the Bronze Age. The steppe groups from Yamnaya and subsequent pastoralist cultures show evidence for previously undetected farmer-related ancestry from different contact zones, while Steppe Maykop individuals harbour additional Upper Palaeolithic Siberian and Native American related ancestry
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