172 research outputs found
Lower entropy bounds and particle number fluctuations in a Fermi sea
We demonstrate, in an elementary manner, that given a partition of the single
particle Hilbert space into orthogonal subspaces, a Fermi sea may be factored
into pairs of entangled modes, similar to a BCS state. We derive expressions
for the entropy and for the particle number fluctuations of a subspace of a
fermi sea, at zero and finite temperatures, and relate these by a lower bound
on the entropy. As an application we investigate analytically and numerically
these quantities for electrons in the lowest Landau level of a quantum Hall
sample.Comment: shorter version, typos fixe
Ionization state, excited populations and emission of impurities in dynamic finite density plasmas: I. The generalized collisional-radiative model for light elements
The paper presents an integrated view of the population structure and its role in establishing the ionization state of light elements in dynamic, finite density, laboratory and astrophysical plasmas. There are four main issues, the generalized collisional-radiative picture for metastables in dynamic plasmas with Maxwellian free electrons and its particularizing to light elements, the methods of bundling and projection for manipulating the population equations, the systematic production/use of state selective fundamental collision data in the metastable resolved picture to all levels for collisonal-radiative modelling and the delivery of appropriate derived coefficients for experiment analysis. The ions of carbon, oxygen and neon are used in illustration. The practical implementation of the methods described here is part of the ADAS Project
Threshold detachment of negative ions by electron impact
The description of threshold fragmentation under long range repulsive forces
is presented. The dominant energy dependence near threshold is isolated by
decomposing the cross section into a product of a back ground part and a
barrier penetration probability resulting from the repulsive Coulomb
interaction. This tunneling probability contains the dominant energy variation
and it can be calculated analytically based on the same principles as Wannier's
description for threshold ionization under attractive forces. Good agreement is
found with the available experimental cross sections on detachment by electron
impact from , and .Comment: 4 pages, 4 figures (EPS), to appear in Phys.Rev.Lett, Feb. 22nd, 199
Dielectronic recombination data for dynamic finite-density plasmas I. Goals and methodology
A programme is outlined for the assembly of a comprehensive dielectronic
recombination database within the generalized collisional--radiative (GCR)
framework. It is valid for modelling ions of elements in dynamic finite-density
plasmas such as occur in transient astrophysical plasmas such as solar flares
and in the divertors and high transport regions of magnetic fusion devices. The
resolution and precision of the data are tuned to spectral analysis and so are
sufficient for prediction of the dielectronic recombination contributions to
individual spectral line emissivities. The fundamental data are structured
according to the format prescriptions of the Atomic Data and Analysis Structure
(ADAS) and the production of relevant GCR derived data for application is
described and implemented following ADAS. The requirements on the dielectronic
recombination database are reviewed and the new data are placed in context and
evaluated with respect to older and more approximate treatments. Illustrative
results validate the new high-resolution zero-density dielectronic
recombination data in comparison with measurements made in heavy-ion storage
rings utilizing an electron cooler. We also exemplify the role of the
dielectronic data on GCR coefficient behaviour for some representative light
and medium weight elements.Comment: 14 Pages, 9 Figures. Submitted to Astronomy & Astrophysics April 12,
200
A large-scale R-matrix calculation for electron-impact excitation of the Ne O-like ion
The five J levels within a or ground state complex provide
an excellent testing ground for the comparison of theoretical line ratios with
astrophysically observed values, in addition to providing valuable electron
temperature and density diagnostics. The low temperature nature of the line
ratios ensure that the theoretically derived values are sensitive to the
underlying atomic structure and electron-impact excitation rates. Previous
R-matrix calculations for the Ne O-like ion exhibit large spurious
structure in the cross sections at higher electron energies, which may affect
Maxwellian averaged rates even at low temperatures. Furthermore, there is an
absence of comprehensive excitation data between the excited states that may
provide newer diagnostics to compliment the more established lines discussed in
this paper. To resolve these issues, we present both a small scale 56-level
Breit-Pauli (BP) calculation and a large-scale 554 levels R-matrix Intermediate
Coupling Frame Transformation (ICFT) calculation that extends the scope and
validity of earlier JAJOM calculations both in terms of the atomic structure
and scattering cross sections. Our results provide a comprehensive
electron-impact excitation data set for all transitions to higher shells.
The fundamental atomic data for this O-like ion is subsequently used within a
collisional radiative framework to provide the line ratios across a range of
electron temperatures and densities of interest in astrophysical observations.Comment: 17 pages, 8 figure
Photoelectron Angular Distributions for Two-photon Ionization of Helium by Ultrashort Extreme Ultraviolet Free Electron Laser Pulses
Phase-shift differences and amplitude ratios of the outgoing and
continuum wave packets generated by two-photon ionization of helium atoms are
determined from the photoelectron angular distributions obtained using velocity
map imaging. Helium atoms are ionized with ultrashort extreme-ultraviolet
free-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3
eV, produced by the SPring-8 Compact SASE Source test accelerator. The measured
values of the phase-shift differences are distinct from scattering phase-shift
differences when the photon energy is tuned to an excited level or Rydberg
manifold. The difference stems from the competition between resonant and
non-resonant paths in two-photon ionization by ultrashort pulses. Since the
competition can be controlled in principle by the pulse shape, the present
results illustrate a new way to tailor the continuum wave packet.Comment: 5 pages, 1 table, 3 figure
Toward a first-principles integrated simulation of tokamak edge plasmas
Performance of the ITER is anticipated to be highly sensitive to the edge plasma condition. The edge pedestal in ITER needs to be predicted from an integrated simulation of the necessary first-principles, multi-scale physics codes. The mission of the SciDAC Fusion Simulation Project (FSP) Prototype Center for Plasma Edge Simulation (CPES) is to deliver such a code integration framework by (1) building new kinetic codes XGC0 and XGC1, which can simulate the edge pedestal buildup; (2) using and improving the existing MHD codes ELITE, M3D-OMP, M3D-MPP and NIMROD, for study of large-scale edge instabilities called Edge Localized Modes (ELMs); and (3) integrating the codes into a framework using cutting-edge computer science technology. Collaborative effort among physics, computer science, and applied mathematics within CPES has created the first working version of the End-to-end Framework for Fusion Integrated Simulation (EFFIS), which can be used to study the pedestal-ELM cycles
Ejection of quasi-free electron pairs from the helium atom ground state by single photon absorption
We investigate single photon double ionization (PDI) of helium at photon
energies of 440 and 800 eV. We observe doubly charged ions with close to zero
momentum corresponding to electrons emitted back-to-back with equal energy.
These slow ions are the unique fingerprint of an elusive quasi-free PDI
mechanism predicted by Amusia et al. nearly four decades years ago [J. Phys. B
8, 1248, (1975)] . It results from the non-dipole part of the electromagnetic
interaction. Our experimental data are in excellent agreement with calculations
performed using the convergent close coupling and time dependent close coupling
methods
Single photon double ionization of Helium at 800 eV - observation of the Quasi Free Mechanism
In a kinematically complete experiment we have measured the photo double ionization of Helium at aphoton energy of 800 eV and observed He2+ ions with 0 momentum, corresponding to a back-to-back-emissionof the two electrons. The results are in good agreement with theoretical calculations
Evidence for significant projectileâtarget nucleus scattering in single ionization of helium
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