430 research outputs found
The Unexpected Role of Evolving Longitudinal Electric Fields in Generating Energetic Electrons in Relativistically Transparent Plasmas
Superponderomotive-energy electrons are observed experimentally from the
interaction of an intense laser pulse with a relativistically transparent
target. For a relativistically transparent target, kinetic modeling shows that
the generation of energetic electrons is dominated by energy transfer within
the main, classically overdense, plasma volume. The laser pulse produces a
narrowing, funnel-like channel inside the plasma volume that generates a field
structure responsible for the electron heating. The field structure combines a
slowly evolving azimuthal magnetic field, generated by a strong laser-driven
longitudinal electron current, and, unexpectedly, a strong propagating
longitudinal electric field, generated by reflections off the walls of the
funnel-like channel. The magnetic field assists electron heating by the
transverse electric field of the laser pulse through deflections, whereas the
longitudinal electric field directly accelerates the electrons in the forward
direction. The longitudinal electric field produced by reflections is 30 times
stronger than that in the incoming laser beam and the resulting direct laser
acceleration contributes roughly one third of the energy transferred by the
transverse electric field of the laser pulse to electrons of the
super-ponderomotive tail
Exploration of Resonant Continuum and Giant Resonance in the Relativistic Approach
Single-particle resonant-states in the continuum are determined by solving
scattering states of the Dirac equation with proper asymptotic conditions in
the relativistic mean field theory (RMF). The regular and irregular solutions
of the Dirac equation at a large radius where the nuclear potentials vanish are
relativistic Coulomb wave functions, which are calculated numerically.
Energies, widths and wave functions of single-particle resonance states in the
continuum for ^{120}Sn are studied in the RMF with the parameter set of NL3.
The isoscalar giant octupole resonance of ^{120}Sn is investigated in a fully
consistent relativistic random phase approximation. Comparing the results with
including full continuum states and only those single-particle resonances we
find that the contributions from those resonant-states dominate in the nuclear
giant resonant processes.Comment: 16 pages, 2 figure
Identification of the Beutler-Fano formula in eigenphase shifts and eigentime delays near a resonance
Eigenphase shifts and eigentime delays near a resonance for a system of one
discrete state and two continua are shown to be functionals of the Beutler-
Fano formulas using appropriate dimensionless energy units and line profile
indices. Parameters responsible for the avoided crossing of eigenphase shifts
and eigentime delays are identified. Similarly, parameters responsible for the
eigentime delays due to a frame change are identified. With the help of new
parameters, an analogy with the spin model is pursued for the S matrix and time
delay matrix. The time delay matrix is shown to comprise three terms, one due
to resonance, one due to a avoided crossing interaction, and one due to a frame
change. It is found that the squared sum of time delays due to the avoided
crossing interaction and frame change is unity.Comment: 17 pages, 3 figures, RevTe
Review of Available Data for Validation of Nuresim Two-Phase CFD Software Applied to CHF Investigations
The NURESIM Project of the 6th European Framework Program initiated the development of a new-generation common European Standard Software Platform for nuclear reactor simulation. The thermal-hydraulic subproject aims at improving the understanding and the predictive capabilities of the simulation tools for key two-phase flow thermal-hydraulic processes such as the critical heat flux (CHF). As part of a multi-scale analysis of reactor thermal-hydraulics, a two-phase CFD tool is developed to allow zooming on local processes. Current industrial methods for CHF mainly use the sub-channel analysis and empirical CHF correlations based on large scale experiments having the real geometry of a reactor assembly. Two-phase CFD is used here for understanding some boiling flow processes, for helping new fuel assembly design, and for developing better CHF predictions in both PWR and BWR. This paper presents a review of experimental data which can be used for validation of the two-phase CFD application to CHF investigations. The phenomenology of DNB and Dry-Out are detailed identifying all basic flow processes which require a specific modeling in CFD tool. The resulting modeling program of work is given and the current state-of-the-art of the modeling within the NURESIM project is presented
Photodetachment study of the 1s3s4s ^4S resonance in He^-
A Feshbach resonance associated with the 1s3s4s ^{4}S state of He^{-} has
been observed in the He(1s2s ^{3}S) + e^- (\epsilon s) partial photodetachment
cross section. The residual He(1s2s ^{3}S) atoms were resonantly ionized and
the resulting He^+ ions were detected in the presence of a small background. A
collinear laser-ion beam apparatus was used to attain both high resolution and
sensitivity. We measured a resonance energy E_r = 2.959 255(7) eV and a width
\Gamma = 0.19(3) meV, in agreement with a recent calculation.Comment: LaTeX article, 4 pages, 3 figures, 21 reference
Structure of the mirror nuclei Be and B in a microscopic cluster model
The structure of the mirror nuclei Be and B is studied in a
microscopic and three-cluster model
using a fully antisymmetrized 9-nucleon wave function. The two-nucleon
interaction includes central and spin-orbit components and the Coulomb
potential. The ground state of Be is obtained accurately with the
stochastic variational method, while several particle-unbound states of both
Be and B are investigated with the complex scaling method.The
calculation for Be supports the recent identification for the existence of
two broad states around 6.5 MeV, and predicts the and
states at about 4.5 MeV and 8 MeV, respectively. The
similarity of the calculated spectra of Be and B enables one to
identify unknown spins and parities of the B states. Available data on
electromagnetic moments and elastic electron scatterings are reproduced very
well. The enhancement of the 1 transition of the first excited state in
Be is well accounted for. The calculated density of Be is found to
reproduce the reaction cross section on a Carbon target. The analysis of the
beta decay of Li to Be clearly shows that the wave function of Be
must contain a small component that cannot be described by the simple model. This small component can be well accounted for by extending a
configuration space to include the distortion of the -particle to
and partitions.Comment: 24 page
High-intensity laser-accelerated ion beam produced from cryogenic micro-jet target
We report on the successful operation of a newly developed cryogenic jet target at high intensity
laser-irradiation. Using the frequency-doubled Titan short pulse laser system at Jupiter Laser Fa-
cility, Lawrence Livermore National Laboratory, we demonstrate the generation of a pure proton
beam a with maximum energy of 2 MeV. Furthermore, we record a quasi-monoenergetic peak at
1.1 MeV in the proton spectrum emitted in the laser forward direction suggesting an alternative
acceleration mechanism. Using a solid-density mixed hydrogen-deuterium target, we are also able
to produce pure proton-deuteron ion beams. With its high purity, limited size, near-critical density,
and high-repetition rate capability, this target is promising for future applications
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Multiple programs: essential to the scientific vitality of the DOE Defense Program Laboratories
The future of the Department of Energy� s Defense Program (DP) laboratories-Los Alamos, Livermore, and Sandia-has been extensively debated and examined over the past several years. To assist in this process, I have asked that a set of documents be prepared, which, when taken together, present a comprehensive picture of the three laboratories. This document describes the multiprogram nature of the DP laboratories and the value of their involvement in non-DP work as it relates to the nuclear weapons program. The other two documents, Integration and Collaboration.. Solving Science and Technology Problems for the Nation (DOE/DP-96009797) and Roles and Responsibilities of the Department of Energy Nuclear Weapons Laboratories in the Stockpile Stewardship and Management Program (DOE/DP-97000280), describe respectively the integrated nature of the DP laboratories and the roles of the laboratories as they meet their individual and collective responsibilities of ensuring the safety and reliabilities of the U.S. nuclear weapons stockpile. The scientific and technical challenges inherent in the DP laboratories� national security responsibilities today are as complex as those during the Manhattan Project and the Cold War years. Science-based stockpile stewardship and management require in-depth understanding of the full spectrum of nuclear weapons science and technology- physics, chemistry, materials, manufacturing, computational modeling, engineering, and electronics, to name a few-as well as a combination of capabilities and facilities unavailable anywhere else in the country. In addition to stockpile stewardship and management, many other nationally important issues involve science and technology-for example, nuclear nonproliferation, energy security, and environmental protection and remediation. Over the years, the DP laboratories have applied expertise and technologies developed in their nuclear weapons work to these other issues, focusing on those areas where they can make unique and valuable contributions. The nation has invested substantially in the three DP laboratories, creating an unmatched resource of scientific and engineering expertise, facilities, and capabilities. In this era of tight budgets, it is important that the laboratories extract maximum leverage from this investment and fulfill their nuclear weapons responsibilities as cost-effectively as possible. The multiprogram nature of the DP laboratories has been key to their success in achieving the outstanding level of scientific and technical excellence that has become their hallmark and in carrying out their national security mission. The multiprogram work of the laboratories also provides an extremely effective way of leveraging the nation� s investment in science and technology. It makes sense for the DP laboratories to apply their expertise to non-nuclear-weapons programs of national importance. It also makes sense for the DP laboratories to collaborate with other government laboratories, universities, and industry to apply the unique expertise, facilities, and capabilities of these institutions to national security challenges. This report briefly reviews the challenges faced by the DP laboratories in fulfilling their stockpile stewardship and management responsibilities. It then discusses the benefits of the synergy and the accelerated pace of scientific achievement that arise from the laboratories� multiple programs. A representative selection of accomplishments is presented that illustrates the importance of the contributions made to the laboratories� national security mission by their non-nuclear-weapons projects and their connections with the wider scientific community
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