100 research outputs found
Investigation of cryogenic mixed-refrigerant cooled current leads in combination with peltier elements
Current leads supply electrical energy from a room-temperature power supply to a superconducting application, representing thus a major thermal load. State-of-the-art cooling solutions use either open (vapor cooled) or multi-stage closed cycle systems. The multi-stage concept can be integrated in one cryogenic mixed refrigerant cycle (CMRC), where a wide-boiling fluid mixture absorbs the heat load continuously along the current lead. In this paper, we study the combination of CMRC cooling with Peltier elements at the warm end of DC current leads. The Peltier cooling may cause a temperature drop on the order of 80 K. This allows an optimization of the CMRC mixture composition towards lower temperatures, avoiding the use of high-boilers that risk to freeze out at low temperatures. Our studies suggest that Peltier and CMRC cooling can reduce the thermal load at the cold end by 30 to 45% compared to conventional conduction-cooled current leads
Analytic Perturbation Theory: A New Approach to the Analytic Continuation of the Strong Coupling Constant into the Timelike Region
The renormalization group applied to perturbation theory is ordinarily used
to define the running coupling constant in the spacelike region. However, to
describe processes with timelike momenta transfers, it is important to have a
self-consistent determination of the running coupling constant in the timelike
region. The technique called analytic perturbation theory (APT) allows a
consistent determination of this running coupling constant. The results are
found to disagree significantly with those obtained in the standard
perturbative approach. Comparison between the standard approach and APT is
carried out to two loops, and threshold matching in APT is applied in the
timelike region.Comment: 16 pages, REVTeX, 7 postscript figure
Quantum Interaction : the Construction of Quantum Field defined as a Bilinear Form
We construct the solution of the quantum wave equation
as a bilinear form which can
be expanded over Wick polynomials of the free -field, and where
is defined as the normal ordered product with
respect to the free -field. The constructed solution is correctly defined
as a bilinear form on , where is a
dense linear subspace in the Fock space of the free -field. On
the diagonal Wick symbol of this bilinear form
satisfies the nonlinear classical wave equation.Comment: 32 pages, LaTe
Tall tales from de Sitter space II: Field theory dualities
We consider the evolution of massive scalar fields in (asymptotically) de
Sitter spacetimes of arbitrary dimension. Through the proposed dS/CFT
correspondence, our analysis points to the existence of new nonlocal dualities
for the Euclidean conformal field theory. A massless conformally coupled scalar
field provides an example where the analysis is easily explicitly extended to
'tall' background spacetimes.Comment: 31 pages, 2 figure
On the role of power expansions in quantum field theory
Methods of summation of power series relevant to applications in quantum
theory are reviewed, with particular attention to expansions in powers of the
coupling constant and in inverse powers of an energy variable. Alternatives to
the Borel summation method are considered and their relevance to different
physical situations is discussed. Emphasis is placed on quantum chromodynamics.
Applications of the renormalon language to perturbation expansions (resummation
of bubble chains) in various QCD processes are reported and the importance of
observing the full renormalization-group invariance in predicting observables
is emphasized. News in applications of the Borel-plane formalism to
phenomenology are conveyed. The properties of the operator-product expansion
along different rays in the complex plane are examined and the problem is
studied how the remainder after subtraction of the first terms depends on
the distance from euclidean region. Estimates of the remainder are obtained and
their strong dependence on the nature of the discontinuity along the cut is
shown. Relevance of this subject to calculations of various QCD effects is
discussed.Comment: 50 pages, Latex, 1 Postscript figur
Experimental Constraints on the Neutrino Oscillations and a Simple Model of Three Flavour Mixing
A simple model of the neutrino mixing is considered, which contains only one
right-handed neutrino field, coupled via the mass term to the three usual
left-handed fields. This is a simplest model that allows for three-flavour
neutrino oscillations. The existing experimental limits on the neutrino
oscillations are used to obtain constraints on the two free mixing parameters
of the model. A specific sum rule relating the oscillation probabilities of
different flavours is derived.Comment: 10 pages, 3 figures in post script, Latex, IFT 2/9
Study of shock waves generation, hot electron production and role of parametric instabilities in an intensity regime relevant for the shock ignition
We present experimental results at intensities relevant to Shock Ignition
obtained at the sub-ns Prague Asterix Laser System in 2012 . We studied shock waves
produced by laser-matter interaction in presence of a pre-plasma. We used a first beam at
1ω (1315 nm) at 7 × 10 13 W/cm 2 to create a pre-plasma on the front side of the target and
a second at 3ω (438 nm) at ∼ 10 16 W/cm 2 to create the shock wave. Multilayer targets
composed of 25 (or 40 μm) of plastic (doped with Cl), 5 μm of Cu (for Kα diagnostics)
and 20 μm of Al for shock measurement were used. We used X-ray spectroscopy of Cl
to evaluate the plasma temperature, Kα imaging and spectroscopy to evaluate spatial and
spectral properties of the fast electrons and a streak camera for shock breakout measurements.
Parametric instabilities (Stimulated Raman Scattering, Stimulated Brillouin Scattering and
Two Plasmon Decay) were studied by collecting the back scattered light and analysing its
spectrum. Back scattered energy was measured with calorimeters. To evaluate the maximum
pressure reached in our experiment we performed hydro simulations with CHIC and DUED
codes. The maximum shock pressure generated in our experiment at the front side of the
target during laser-interaction is 90 Mbar. The conversion efficiency into hot electrons was
estimated to be of the order of ∼ 0.1% and their mean energy in the order ∼50 keV.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributio
Asymptotic Improvement of Resummation and Perturbative Predictions in Quantum Field Theory
The improvement of resummation algorithms for divergent perturbative
expansions in quantum field theory by asymptotic information about perturbative
coefficients is investigated. Various asymptotically optimized resummation
prescriptions are considered. The improvement of perturbative predictions
beyond the reexpansion of rational approximants is discussed.Comment: 21 pages, LaTeX, 3 tables; title shortened; typographical errors
corrected; minor changes of style; 2 references adde
The Determination of alpha_s from Tau Decays Revisited
We revisit the determination of alpha_s(m_tau) using a fit to inclusive tau
hadronic spectral moments in light of (1) the recent calculation of the
fourth-order perturbative coefficient K_4 in the expansion of the Adler
function, (2) new precision measurements from BABAR of e+e- annihilation cross
sections, which decrease the uncertainty in the separation of vector and
axial-vector spectral functions, and (3) improved results from BABAR and Belle
on tau branching fractions involving kaons. We estimate that the fourth-order
perturbative prediction reduces the theoretical uncertainty, introduced by the
truncation of the series, by 20% with respect to earlier determinations. We
discuss to some detail the perturbative prediction and show that the effect of
the incomplete knowledge of the series is reduced by using the so-called
contour-improved calculation, as opposed to fixed-order perturbation theory
which manifests convergence problems. The corresponding theoretical
uncertainties are studied at the tau and Z mass scales. Nonperturbative
contributions extracted from the most inclusive fit are small, in agreement
with earlier determinations. Systematic effects from quark-hadron duality
violation are estimated with simple models and found to be within the quoted
systematic errors. The fit gives alpha_s(m_tau) = 0.344 +- 0.005 +- 0.007,
where the first error is experimental and the second theoretical. After
evolution to M_Z we obtain alpha_s(M_Z) = 0.1212 +- 0.0005 +- 0.0008 +- 0.0005,
where the errors are respectively experimental, theoretical and due to the
evolution. The result is in agreement with the corresponding NNNLO value
derived from essentially the Z width in the global electroweak fit. The
alpha_s(M_Z) determination from tau decays is the most precise one to date.Comment: 22 pages, 7 figure
Evaluating the Community Land Model (CLM4.5) at a coniferous forest site in northwestern United States using flux and carbon-isotope measurements
Droughts in the western United States are expected to intensify with climate
change. Thus, an adequate representation of ecosystem response to water
stress in land models is critical for predicting carbon dynamics. The goal of
this study was to evaluate the performance of the Community Land Model (CLM)
version 4.5 against observations at an old-growth coniferous forest site in
the Pacific Northwest region of the United States (Wind River AmeriFlux
site), characterized by a Mediterranean climate that subjects trees to water
stress each summer. CLM was driven by site-observed meteorology and
calibrated primarily using parameter values observed at the site or at
similar stands in the region. Key model adjustments included parameters
controlling specific leaf area and stomatal conductance. Default values of
these parameters led to significant underestimation of gross primary
production, overestimation of evapotranspiration, and consequently
overestimation of photosynthetic 13C discrimination, reflected in
reduced 13C : 12C ratios of carbon fluxes and pools. Adjustments
in soil hydraulic parameters within CLM were also critical, preventing
significant underestimation of soil water content and unrealistic soil
moisture stress during summer. After calibration, CLM was able to simulate
energy and carbon fluxes, leaf area index, biomass stocks, and carbon isotope
ratios of carbon fluxes and pools in reasonable agreement with site
observations. Overall, the calibrated CLM was able to simulate the observed
response of canopy conductance to atmospheric vapor pressure deficit (VPD)
and soil water content, reasonably capturing the impact of water stress on
ecosystem functioning. Both simulations and observations indicate that
stomatal response from water stress at Wind River was primarily driven by VPD
and not soil moisture. The calibration of the Ball–Berry stomatal
conductance slope (mbb) at Wind River aligned with findings from recent CLM experiments at sites characterized by the same plant functional
type (needleleaf evergreen temperate forest), despite significant differences
in stand composition and age and climatology, suggesting that CLM could
benefit from a revised mbb value of 6, rather than the default
value of 9, for this plant functional type. Conversely, Wind River required a
unique calibration of the hydrology submodel to simulate soil moisture,
suggesting that the default hydrology has a more limited applicability. This
study demonstrates that carbon isotope data can be used to constrain stomatal
conductance and intrinsic water use efficiency in CLM, as an alternative to
eddy covariance flux measurements. It also demonstrates that carbon isotopes
can expose structural weaknesses in the model and provide a key constraint
that may guide future model development
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