215 research outputs found
Electron Impact Ionization Close to the Threshold: Classical Calculations
In this paper we present Classical Trajectory Monte Carlo (CTMC) calculations
for single and multiple electron ionization of Argon atoms and ions in the
threshold region. We are able to recover the Wannier exponents a for the
power-law behavior of the cross section s versus excess energy: the exact value
of the exponent as well as the existence of its saturation for multiple
ionization appear to be related to how the total binding energy is shared
between target electrons.Comment: 9 pages. To be published in Journal of Physics
Laboratory simulation of cometary x rays using a high-resolution microcalorimeter
X-ray emission following charge exchange has been studied on the University
of California Lawrence Livermore National Laboratory electron beam ion traps
EBIT-I and EBIT-II using a high-resolution microcalorimeter. The measured
spectra include the K-shell emission from hydrogenlike and heliumlike C, N, O,
and Ne needed for simulations of cometary x-ray emission. A comparison of the
spectra produced in the interaction of O8+ with N2 and CH4 is presented that
illustrates the dependence of the observed spectrum on the interaction gas.Comment: 11 pages, 2 figure
Status of the heavy ion beam probe system in the Large Helical Device
A heavy ion beam probe (HIBP) system has been installed into the Large Helical Device (LHD) to measure the spatial profile of the plasma potential and density fluctuations. The optimization of the HIBP system, especially the beam injector, is described. The negative ion beam is required for the MeV beam production in a tandem accelerator. A sputter-type heavy negative ion source has been developed as an intense Au^? beam source to produce Au^+ beams with energy in the MeV range. The extraction electrodes and the Einzel lens system of the ion source have been designed taking into account the beam optics, and installed into the real machine. Throughout the plasma diagnostics on LHD experiments, the consumptions of vaporized caesium and gold target are being characterized for practical operations. In addition, the experimental charge fractions are compared with the theoretical fractions for understanding the charge-changing behavior of Au^? ions and optimizing the fraction of Au^+ ions at the exit of the tandem accelerator of the HIBP syste
Selective excitation of metastable atomic states by femto- and attosecond laser pulses
The possibility of achieving highly selective excitation of low metastable
states of hydrogen and helium atoms by using short laser pulses with reasonable
parameters is demonstrated theoretically. Interactions of atoms with the laser
field are studied by solving the close-coupling equations without
discretization. The parameters of laser pulses are calculated using different
kinds of optimization procedures. For the excitation durations of hundreds of
femtoseconds direct optimization of the parameters of one and two laser pulses
with Gaussian envelopes is used to introduce a number of simple schemes of
selective excitation. To treat the case of shorter excitation durations,
optimal control theory is used and the calculated optimal fields are
approximated by sequences of pulses with reasonable shapes. A new way to
achieve selective excitation of metastable atomic states by using sequences of
attosecond pulses is introduced.Comment: To be published in Phys. Rev. A, 10 pages, 3 figure
Electron Loss Cross Sections for a Heavy Ion Beam Probe
Electron loss cross sections of Au+ and Au2+ by electron and proton impact are calculated by using respectively the Lotz formula and the LOSS and CAPTURE computer codes. The corresponding rate coefficients have also been calculated. Using this information, the signal levels of heavy ion beam probe in the Large Helical Device are estimated. The calculated beam currents at the detector position are compared with the detected beamcurrents in the MeV energy range for a plasma electron density of 1 × 1019 m?3 and electron temperature of 1.5 keV. The obtained cross section / rate coefficient data can also be used for reconstruction of electron density and temperature profiles
Ionization of hydrogen and hydrogenic ions by antiprotons
Presented here is a description of the ionization of hydrogen and hydrogenic
ions by antiproton-impact, based on very large scale numerical solutions of the
time-dependent Schr\"odinger equation in three spatial dimensions and on
analysis of the topology of the electronic eigenenergy surfaces in the plane of
complex internuclear distance. Comparison is made with other theories and very
recent measurements.Comment: RevTex document, 11 pages, 4 Postscript figures are available from
the authors, in press Phys. Rev. Let
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