139 research outputs found
Feedback Systems for Linear Colliders
Feedback systems are essential for stable operation of a linear collider,
providing a cost-effective method for relaxing tight tolerances. In the
Stanford Linear Collider (SLC), feedback controls beam parameters such as
trajectory, energy, and intensity throughout the accelerator. A novel dithering
optimization system which adjusts final focus parameters to maximize luminosity
contributed to achieving record performance in the 1997-98 run. Performance
limitations of the steering feedback have been investigated, and improvements
have been made. For the Next Linear Collider (NLC), extensive feedback systems
are planned as an intregal part of the design. Feedback requiremetns for JLC
(the Japanese Linear Collider) are essentially identical to NLC; some of the
TESLA requirements are similar but there are significant differences. For NLC,
algorithms which incorporate improvements upon the SLC implementation are being
prototyped. Specialized systems for the damping rings, rf and interaction point
will operate at high bandwidth and fast response. To correct for the motion of
individual bunches within a train, both feedforward and feedback systems are
planned. SLC experience has shown that feedback systems are an invaluable
operational tool for decoupling systems, allowing precision tuning, and
providing pulse-to-pulse diagnostics. Feedback systems for the NLC will
incorporate the key SLC features and the benefits of advancing technologies.Comment: Invited talk presented at IEEE Particle Accelerator Conference
(PAC99
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A prototype fast feedback system for energy lock at CEBAF
The beam energy at CEBAF must be controlled accurately against phase and gradient fluctuations in RF cavities in order to achieve a 2.5 {times} 10{sup {minus}5} relative energy spread. A prototype fast feedback system based on the concepts of Modern Control Theory has been implemented in the CEBAF control system to function as an energy lock. Measurements performed during the pulsed mode operations indicate presence of noise components at 4 Hz and 12 Hz on beam energy. This fast feedback prototype operates at 60 Hz rate and is integrated with EPICS. This paper describes the implementation of the fast feedback prototype, and operational experience with this system at CEBAF. 5 refs., 3 figs
Whats special about Y6; the working mechanism of neat Y6 organic solar cell
Non-fullerene acceptors (NFA) have delivered advance in bulk heterojunction
organic solar cell efficiencies, with the significant milestone of 20% now in
sight. However, these materials challenge the accepted wisdom of how organic
solar cells work. In this work we present neat Y6 device with efficiency above
4.5%. We thoroughly investigate mechanisms of charge generation and
recombination as well as transport in order to understand what is special about
Y6. Our data suggest Y6 generates bulk free charges, with ambipolar mobility,
which can be extracted in the presence of transport layersComment: 11 pages, 5 figure
Defining paleoclimatic routes and opportunities for hominin dispersals across Iran
Fossil and archaeological evidence indicates that hominin dispersals into Southwest Asia occurred throughout the Pleistocene, including the expansion of Homo sapiens populations out of Africa. While there is evidence for hominin occupations in the Pleistocene in Iran, as evidenced by the presence of Lower to Upper Paleolithic archaeological sites, the extent to which humid periods facilitated population expansions into western Asia has remained unclear. To test the role of humid periods on hominin dispersals here we assess Paleolithic site distributions and paleoenvironmental records across Iran. We developed the first spatially comprehensive, high-resolution paleohydrological model for Iran in order to assess water availability and its influence on hominin dispersals. We highlight environmentally mediated routes which likely played a key role in Late Pleistocene hominin dispersals, including the expansion of H. sapiens and Neanderthals eastwards into Asia. Our combined analyses indicate that, during MIS 5, there were opportunities for hominins to traverse a northern route through the Alborz and Kopet Dagh Mountains and the Dasht-I Kavir desert owing to the presence of activated fresh water sources. We recognize a new southern route along the Zagros Mountains and extending eastwards towards Pakistan and Afghanistan. We find evidence for a potential northern route during MIS 3, which would have permitted hominin movements and species interactions in Southwest Asia. Between humid periods, these interconnections would have waned, isolating populations in the Zagros and Alborz Mountains, where hominins may have continued to have had access to water
Orders of Recombination in Complete Perovskite Solar Cells â Linking Time-Resolved and Steady-State Measurements
Funder: EPSRC; Id: http://dx.doi.org/10.13039/501100000266Abstract: Ideally, the charge carrier lifetime in a solar cell is limited by the radiative free carrier recombination in the absorber which is a secondâorder process. Yet, realâlife cells suffer from severe nonradiative recombination in the bulk of the absorber, at interfaces, or within other functional layers. Here, the dynamics of photogenerated charge carriers are probed directly in pinâtype mixed halide perovskite solar cells with an efficiency >20%, using timeâresolved optical absorption spectroscopy and optoelectronic techniques. The charge carrier dynamics in complete devices is fully consistent with a superposition of firstâ, secondâ, and thirdâorder recombination processes, with no admixture of recombination pathways with nonâinteger order. Under solar illumination, recombination in the studied solar cells proceeds predominantly through nonradiative firstâorder recombination with a lifetime of 250 ns, which competes with secondâorder free charge recombination which is mostly if not entirely radiative. Results from the transient experiments are further employed to successfully explain the steadyâstate solar cell properties over a wide range of illumination intensities. It is concluded that improving carrier lifetimes to >3 ”s will take perovskite devices into the radiative regime, where their performance will benefit from photonârecycling
The role of fullerenes in the environmental stability of polymer:fullerene solar cells
Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device currentâvoltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices
Influence of nanostructure on the device performance and charge recombination dynamics of P3HT:PCBM solar cells
Crystallinity Effects in Sequentially Processed and Blend-Cast Bulk-Heterojunction Polymer/Fullerene Photovoltaics
Although most polymer/fullerene-based solar cells are cast from a blend of the components in solution, it is also possible to sequentially process the polymer and fullerene layers from quasi-orthogonal solvents. Sequential processing (SqP) not only produces photovoltaic devices with efficiencies comparable to the more traditional bulk heterojunction (BHJ) solar cells produced by blend casting (BC) but also offers the advantage that the polymer and fullerene layers can be optimized separately. In this paper, we explore the morphology produced when sequentially processing polymer/fullerene solar cells and compare it to the BC morphology. We find that increasing polymer regioregularity leads to the opposite effect in SqP and BC BHJ solar cells. We start by constructing a series of SqP and BC solar cells using different types of poly(3-hexylthiophene) (P3HT) that vary in regioregulary and polydispersity combined with [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM). We use grazing incidence wide-angle X-ray scattering to demonstrate how strongly changes in the P3HT and PCBM crystallinity upon thermal annealing of SqP and BC BHJ films depend on polymer regioregularity. For SqP devices, low regioregularity P3HT films that possess more amorphous regions allow for more PCBM crystallite growth and thus show better photovoltaic device efficiency. On the other hand, highly regioregular P3HT leads to a more favorable morphology and better device efficiency for BC BHJ films. Comparing the photovoltaic performance and structural characterization indicates that the mechanisms controlling morphology in the active layers are fundamentally different for BHJs formed via SqP and BC. Most importantly, we find that nanoscale morphology in both SqP and BC BHJs can be systematically controlled by tuning the amorphous fraction of polymer in the active layer. © 2014 American Chemical Society
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