1,024 research outputs found
Design and fabrication of a long-life Stirling cycle cooler for space application. Phase 3: Prototype model
A second-generation, Stirling-cycle cryocooler (cryogenic refrigerator) for space applications, with a cooling capacity of 5 watts at 65 K, was recently completed. The refrigerator, called the Prototype Model, was designed with a goal of 5 year life with no degradation in cooling performance. The free displacer and free piston of the refrigerator are driven directly by moving-magnet linear motors with the moving elements supported by active magnetic bearings. The use of clearance seals and the absence of outgassing material in the working volume of the refrigerator enable long-life operation with no deterioration in performance. Fiber-optic sensors detect the radial position of the shafts and provide a control signal for the magnetic bearings. The frequency, phase, stroke, and offset of the compressor and expander are controlled by signals from precision linear position sensors (LVDTs). The vibration generated by the compressor and expander is cancelled by an active counter balance which also uses a moving-magnet linear motor and magnetic bearings. The driving signal for the counter balance is derived from the compressor and expander position sensors which have wide bandwidth for suppression of harmonic vibrations. The efficiency of the three active members, which operate in a resonant mode, is enhanced by a magnetic spring in the expander and by gas springs in the compressor and counterbalance. The cooling was achieved with a total motor input power of 139 watts. The magnetic-bearing stiffness was significantly increased from the first-generation cooler to accommodate shuttle launch vibrations
Moving towards 100% renewable electricity in Europe & North Africa by 2050
In spring 2010, European and international climate experts at PwC, the European Climate Forum, the Potsdam Institute for Climate Impact Research and the International Institute for Applied System Analysis published 100% Renewable Electricity - A roadmap to 2050 for Europe and North Africa. The report examined the potential for powering Europe and North Africa with renewable electricity exclusively by 2050. It set out a series of financial, market, infrastructure and government policy steps that would need to occur if such a "what if" vision was to be achieved.
Now, a year on, this latest report provides a complementary analysis to the original roadmap. PwC, the Potsdam Institute for Climate Impact Research and the International Institute for Applied System Analysis, look at whether the vision of 100% renewable electricity has moved closer or further away as a result of current and recent developments over the last 12 months. The report, intended to support the wider debate in this area, examines five areas that are most critical to achieving progress and, through the lens of these five areas, looks at the impact of recent and current events
On the terms violating the custodial symmetry in multi-Higgs-doublet models
We prove that a generic multi-Higgs-doublet model (NHDM) generally must
contain terms in the potential that violate the custodial symmetry. This is
done by showing that the O(4) violating terms of the NHDM potential cannot be
excluded by imposing a symmetry on the NHDM Lagrangian. Hence we expect
higher-order corrections to necessarily introduce such terms. We also note, in
the case of custodially symmetric Higgs-quark couplings, that vacuum alignment
will lead to up-down mass degeneration; this is not true if the vacua are not
aligned.Comment: 16 pages, 1 figure. Title and abstract are modified, conclusions
remain the same. Section on Yukawa couplings is extended. Published versio
Deep Emissions Reductions and Mainstreaming of Mitigation and Adaptation: Key Findings
Climate policy "mainstreaming", "proofing" and "integration" are concepts that are increasingly appearing in a range of EU policy discussions, including those concerning the 2014-2020 Multi-Annual Financial Framework (MFF). They reflect the view that all policy sectors need to play a part in both reducing emissions and increasing resilience to unavoidable climate impacts. Broadly defined, mainstreaming involves including climate considerations in policy processes, improving the consistency among policy objectives, and where necessary, giving priority to climate-related goals above others. Although often couched in technical language, profound political challenges, at multiple levels of governance, lie at the heart of the mainstreaming agenda. The RESPONSES project analysed how far adaptation and mitigation was being mainstreamed in EU policies, and assessed the potential opportunities and limits for the future
A Study of Quantum Error Correction by Geometric Algebra and Liquid-State NMR Spectroscopy
Quantum error correcting codes enable the information contained in a quantum
state to be protected from decoherence due to external perturbations. Applied
to NMR, quantum coding does not alter normal relaxation, but rather converts
the state of a ``data'' spin into multiple quantum coherences involving
additional ancilla spins. These multiple quantum coherences relax at differing
rates, thus permitting the original state of the data to be approximately
reconstructed by mixing them together in an appropriate fashion. This paper
describes the operation of a simple, three-bit quantum code in the product
operator formalism, and uses geometric algebra methods to obtain the
error-corrected decay curve in the presence of arbitrary correlations in the
external random fields. These predictions are confirmed in both the totally
correlated and uncorrelated cases by liquid-state NMR experiments on
13C-labeled alanine, using gradient-diffusion methods to implement these
idealized decoherence models. Quantum error correction in weakly polarized
systems requires that the ancilla spins be prepared in a pseudo-pure state
relative to the data spin, which entails a loss of signal that exceeds any
potential gain through error correction. Nevertheless, this study shows that
quantum coding can be used to validate theoretical decoherence mechanisms, and
to provide detailed information on correlations in the underlying NMR
relaxation dynamics.Comment: 33 pages plus 6 figures, LaTeX article class with amsmath & graphicx
package
Adaptation in integrated assessment modeling: where do we stand?
Adaptation is an important element on the climate change policy agenda. Integrated assessment models, which are key tools to assess climate change policies, have begun to address adaptation, either by including it implicitly in damage cost estimates, or by making it an explicit control variable. We analyze how modelers have chosen to describe adaptation within an integrated framework, and suggest many ways they could improve the treatment of adaptation by considering more of its bottom-up characteristics. Until this happens, we suggest, models may be too optimistic about the net benefits adaptation can provide, and therefore may underestimate the amount of mitigation they judge to be socially optimal. Under some conditions, better modeling of adaptation costs and benefits could have important implications for defining mitigation targets. © Springer Science+Business Media B.V. 2009
A Human Development Framework for CO2 Reductions
Although developing countries are called to participate in CO2 emission
reduction efforts to avoid dangerous climate change, the implications of
proposed reduction schemes in human development standards of developing
countries remain a matter of debate. We show the existence of a positive and
time-dependent correlation between the Human Development Index (HDI) and per
capita CO2 emissions from fossil fuel combustion. Employing this empirical
relation, extrapolating the HDI, and using three population scenarios, the
cumulative CO2 emissions necessary for developing countries to achieve
particular HDI thresholds are assessed following a Development As Usual
approach (DAU). If current demographic and development trends are maintained,
we estimate that by 2050 around 85% of the world's population will live in
countries with high HDI (above 0.8). In particular, 300Gt of cumulative CO2
emissions between 2000 and 2050 are estimated to be necessary for the
development of 104 developing countries in the year 2000. This value represents
between 20% to 30% of previously calculated CO2 budgets limiting global warming
to 2{\deg}C. These constraints and results are incorporated into a CO2
reduction framework involving four domains of climate action for individual
countries. The framework reserves a fair emission path for developing countries
to proceed with their development by indexing country-dependent reduction rates
proportional to the HDI in order to preserve the 2{\deg}C target after a
particular development threshold is reached. Under this approach, global
cumulative emissions by 2050 are estimated to range from 850 up to 1100Gt of
CO2. These values are within the uncertainty range of emissions to limit global
temperatures to 2{\deg}C.Comment: 14 pages, 7 figures, 1 tabl
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Using a game to engage stakeholders in extreme event attribution science
The impacts of weather and climate-related disasters are increasing, and climate change can exacerbate many disasters. Effectively communicating climate risk and integrating science into policy requires scientists and stakeholders to work together. But dialogue between scientists and policymakers can be challenging given the inherently multidimensional nature of the issues at stake when managing climate risks. Building on the growing use of serious games to create dialogue between stakeholders, we present a new game for policymakers called Climate Attribution Under Loss and Damage: Risking, Observing,co-Negotiating (CAULDRON). CAULDRON aims to communicate understanding of the science attributing extreme events to climate change in a memorable and compelling way, and create space for dialogue around policy decisions addressing changing risks and loss and damage from climate change. We describe the process of developing CAULDRON, and draw on observations of players and their feedback to demonstrate its potential to facilitate the interpretation of probabilistic climate information and the understanding of its relevance to informing policy. Scientists looking to engage with stakeholders can learn valuable lessons in adopting similar innovative approaches. The suitability of games depends on the policy context but, if used appropriately, experiential learning can drive co-produced understanding and meaningful dialogue
NMR Techniques for Quantum Control and Computation
Fifty years of developments in nuclear magnetic resonance (NMR) have resulted
in an unrivaled degree of control of the dynamics of coupled two-level quantum
systems. This coherent control of nuclear spin dynamics has recently been taken
to a new level, motivated by the interest in quantum information processing.
NMR has been the workhorse for the experimental implementation of quantum
protocols, allowing exquisite control of systems up to seven qubits in size.
Here, we survey and summarize a broad variety of pulse control and tomographic
techniques which have been developed for and used in NMR quantum computation.
Many of these will be useful in other quantum systems now being considered for
implementation of quantum information processing tasks.Comment: 33 pages, accepted for publication in Rev. Mod. Phys., added
subsection on T_{1,\rho} (V.A.6) and on time-optimal pulse sequences
(III.A.6), redid some figures, made many small changes, expanded reference
Positronium Portal into Hidden Sector: A new Experiment to Search for Mirror Dark Matter
The understanding of the origin of dark matter has great importance for
cosmology and particle physics. Several interesting extensions of the standard
model dealing with solution of this problem motivate the concept of hidden
sectors consisting of SU(3)xSU(2)_LxU(1)_Y singlet fields. Among these models,
the mirror matter model is certainly one of the most interesting. The model
explains the origin of parity violation in weak interactions, it could also
explain the baryon asymmetry of the Universe and provide a natural ground for
the explanation of dark matter. The mirror matter could have a portal to our
world through photon-mirror photon mixing (epsilon). This mixing would lead to
orthopositronium (o-Ps) to mirror orthopositronium oscillations, the
experimental signature of which is the apparently invisible decay of o-Ps. In
this paper, we describe an experiment to search for the decay o-Ps -> invisible
in vacuum by using a pulsed slow positron beam and a massive 4pi BGO crystal
calorimeter. The developed high efficiency positron tagging system, the low
calorimeter energy threshold and high hermiticity allow the expected
sensitivity in mixing strength to be epsilon about 10^-9, which is more than
one order of magnitude below the current Big Bang Nucleosynthesis limit and in
a region of parameter space of great theoretical and phenomenological interest.
The vacuum experiment with such sensitivity is particularly timely in light of
the recent DAMA/LIBRA observations of the annual modulation signal consistent
with a mirror type dark matter interpretation.Comment: 40 pages, 29 Figures 2 Tables v2: Ref. added, Fig. 29 and some text
added to explain idea for backscattering e+ background suppression, corrected
typos v3: minor corrections: Eq 2.1 corrected (6 lines-> 5 lines), Eq.2.17:
two extra "-" signs remove
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