974 research outputs found
Theory of attosecond delays in laser-assisted photoionization
We study the temporal aspects of laser-assisted extreme ultraviolet (XUV)
photoionization using attosecond pulses of harmonic radiation. The aim of this
paper is to establish the general form of the phase of the relevant transition
amplitudes and to make the connection with the time-delays that have been
recently measured in experiments. We find that the overall phase contains two
distinct types of contributions: one is expressed in terms of the phase-shifts
of the photoelectron continuum wavefunction while the other is linked to
continuum--continuum transitions induced by the infrared (IR) laser probe. Our
formalism applies to both kinds of measurements reported so far, namely the
ones using attosecond pulse trains of XUV harmonics and the others based on the
use of isolated attosecond pulses (streaking). The connection between the
phases and the time-delays is established with the help of finite difference
approximations to the energy derivatives of the phases. This makes clear that
the observed time-delays is a sum of two components: a one-photon Wigner-like
delay and an universal delay that originates from the probing process itself.Comment: 15 pages, 10 figures, special issue 'Attosecond spectroscopy' Chem.
Phy
ANSYS HFSS as a new numerical tool to study wave propagation inside anisotropic magnetized plasmas in the Ion Cylotron Range of Frequencies
The paper demonstrates the possibility to use ANSYS HFSS as a versatile
simulating tool for antennas facing inhomogeneous anisotropic magnetized
plasmas in the Ion Cyclotron Range of Frequencies (ICRF). The methodology used
throughout the paper is first illustrated with a uniform plasma case. We then
extend this method to 1D plasma density profiles where we perform a first
benchmark against the ANTITER II code. The possibility to include more complex
phenomena relevant to the ICRF field in future works like the lower hybrid
resonance, the edge propagation of slow waves, sheaths and ponderomotive forces
is also discussed. We finally present a 3D case for WEST and compare the
radiation resistance calculated by the code to the experimental data. The main
result of this paper - the implementation of a cold plasma medium in HFSS - is
general and we hope it will also benefit to research fields besides controlled
fusion.Comment: 15 pages, 14 figure
Probing single-photon ionization on the attosecond time scale
We study photoionization of argon atoms excited by attosecond pulses using an
interferometric measurement technique. We measure the difference in time delays
between electrons emitted from the and from the shell, at
different excitation energies ranging from 32 to 42 eV. The determination of
single photoemission time delays requires to take into account the measurement
process, involving the interaction with a probing infrared field. This
contribution can be estimated using an universal formula and is found to
account for a substantial fraction of the measured delay.Comment: 4 pages, 4 figures, under consideratio
Relativistic central--field Green's functions for the RATIP package
From perturbation theory, Green's functions are known for providing a simple
and convenient access to the (complete) spectrum of atoms and ions. Having
these functions available, they may help carry out perturbation expansions to
any order beyond the first one. For most realistic potentials, however, the
Green's functions need to be calculated numerically since an analytic form is
known only for free electrons or for their motion in a pure Coulomb field.
Therefore, in order to facilitate the use of Green's functions also for atoms
and ions other than the hydrogen--like ions, here we provide an extension to
the Ratip program which supports the computation of relativistic
(one--electron) Green's functions in an -- arbitrarily given -- central--field
potential \rV(r). Different computational modes have been implemented to
define these effective potentials and to generate the radial Green's functions
for all bound--state energies . In addition, care has been taken to
provide a user--friendly component of the Ratip package by utilizing features
of the Fortran 90/95 standard such as data structures, allocatable arrays, or a
module--oriented design.Comment: 20 pages, 1 figur
Recent modeling for the ITER ion cyclotron range of frequency antennas with the TOPICA code
This paper documents the analysis of the ITER ion cyclotron resonance heating (ICRF) launcher using the TOPICA code, throughout recent years' design activities. The ability to simulate the detailed geometry of an ICRF antenna in front of a realistic plasma and to obtain the antenna input parameters, the electric currents on conductors and the radiated field distribution next to the antenna is of significant importance to evaluate and predict the overall system performances. Starting from a reference geometry, we first investigated the impact of some geometrical and numerical factors, such as the Faraday Screen geometry or the mesh quality. Then a final geometry was the object of a comprehensive analysis, varying the working frequency, the plasma conditions and the poloidal and toroidal phasings between the feeding lines. The performance of the antenna has been documented in terms of input parameters, power coupled to plasma and electric fields. Eventually, the four-port junction has also been included in TOPICA models
DEMO ion cyclotron heating: Status of ITER-type antenna design
The ITER ICRF system will gain in complexity relative to the existing systems
on modern devices, and the same will hold true for DEMO. The accumulated
experience can help greatly in designing an ICRF system for DEMO. In this paper
the current status of the pre-conceptual design of the DEMO ICRF antenna and
some related components is presented. While many aspects strongly resemble the
ITER system, in some design solutions we had to take an alternative route to be
able to adapt to DEMO specific. One of the key points is the toroidal antenna
extent needed for the requested ICRF heating performance, achieved by splitting
the antenna in halves, with appropriate installation. Modelling of the so far
largest ICRF antenna in RAPLICASOL and associated challenges are presented.
Calculation are benchmarked with TOPICA. Results of the analysis of the latest
model and an outlook for future steps are given.Comment: Published in Fusion Engineering and Design 165 (2021) 11226
Photoemission time-delay measurements and calculations close to the 3s ionization minimum in Ar
We present experimental measurements and theoretical calculations of
photoionization time delays from the and shells in Ar in the photon
energy range of 32-42 eV. The experimental measurements are performed by
interferometry using attosecond pulse trains and the infrared laser used for
their generation. The theoretical approach includes intershell correlation
effects between the 3s and 3p shells within the framework of the random phase
approximation with exchange (RPAE). The connection between single-photon
ionization and the two-color two-photon ionization process used in the
measurement is established using the recently developed asymptotic
approximation for the complex transition amplitudes of laser-assisted
photoionization. We compare and discuss the theoretical and experimental
results especially in the region where strong intershell correlations in the 3s
to kp channel lead to an induced "Cooper" minimum in the 3s ionization
cross-section.Comment: submitted to PR
- …