Transcript Time-Dependent Studies of Atomic Ionization

Atomic data for heavy elements relevant to
magnetic fusion and astrophysics using
the Los Alamos atomic physics codes
James Colgan, Honglin Zhang,
and Christopher Fontes,
Los Alamos National Laboratory, NM, USA
Overview
• In the last 5 years we have used the LANL codes to
compute various sets of atomic data.
• In this talk I give some highlights of these
calculations and comparisons with other methods
and experiment, if available.
• I first discuss, very briefly, the methods used to
generate the atomic data.
• Then some comparisons of our results with
experiment are made for a selection of transitions.
• I also discuss a recent study of radiative losses from
the solar corona, in which significant amounts of
atomic data were generated and used.
• End with some conclusions and future outlooks.
The LANL Suite of Atomic Modeling Codes
20+ years of development, created by Joe Abdallah & Bob Clark, updated
for fully relativistic calculations by Chris Fontes & Honglin Zhang
Atomic Physics Codes
Atomic Models
ATOMIC
CATS: Cowan Code
fine-structure
config-average
LTE or NLTE
low or high-Z
populations
energy levels
gf-values
e- excitation
e- ionization
photoionization
autoionization
spectral modeling
emission
absorption
transmission
power loss
RATS: relativistic
ACE: e- excitation
GIPPER: ionization
http://aphysics2.lanl.gov/tempweb
Method
• Atomic collisional data may be computed in a variety of
approximations in the LANL codes:
• Excitation cross sections:
– Plane-wave Born (PWB) approach.
– Distorted-wave/First-order many-body perturbation theory
(DW/FOMBT).
• Ionization cross sections:
– Scaled hydrogenic method.
– Distorted-wave (DW) approach.
• We use FOMBT for excitation and the DW approach for
ionization in the data discussed here.
• Once the atomic data for all relevant ion stages have been
calculated, the Los Alamos plasma kinetics modeling code,
ATOMIC, can be used to compute the radiative losses.
Los Alamos Atomic Physics Codes:
Strengths/Weaknesses
• Consistent treatment of all
states and ion stages; accurate
and fast calculations for highly
ionized species.
• Storage of atomic data in a
compact binary format
(IPCRESS files) which allows
very large amounts of data to be
stored in a manageable form.
• Codes are now in a mature
state, are portable, and well
tested on a variety of platforms.
• Accuracy of PWB and/or DW
approach may be poorer in
collisional calculations
including neutral or near-neutral
systems (less of a problem for
hot plasmas where ions are
likely to be more stripped).
• No current ability to insert
(more accurate) other
calculations instead of
PWB/DW if required.
• This is due to problems of
consistently treating the
resonance contribution of
autoionizing states when
combining different types of
calculations.
Data generated for the CRP
•
•
Electron-impact excitation cross sections
for all ion stages of Si, Cl, and Ar.
n=0 and n=1 (and selected n=2)
excitation cross sections calculated from
the fine-structure levels of the ground
complex.
•
FOMBT method used to generate all
excitation cross sections.
• Photoionization cross sections from
all fine-structure levels.
• Autoionization rates from all finestructure levels.
• Electron-impact ionization cross
sections for all ion stages of Si,
Cl, and Ar.
• Scaled-hydrogenic cross sections
from all fine-structure levels.
• Distorted-wave ionization cross
sections from all levels associated
with the ground configuration.
• Selected close-coupling (TDCC)
ionization cross sections from the
ground and first excited
configurations of a few Si ions.
• DW inner-shell ionization cross
sections for selected neutral heavy
atoms.
Excitation: Comparison with RDW results
• Comparison with the published RDW
data for Be- through Na-like Si, Cl,
and Ar is generally good.
• One example is shown in the figure
for B-like Si, Cl, and Ar ; the RDW
results are from Zhang & Sampson,
ADNDT, 56, 41 (1994).
• The discrepancy for higher energies is
not due to the relativistic effects, but
to configuration-interaction.
• The red curves represent another
calculation from CATS/ACE with
only three n=2 configurations, as in
the RDW calculation, which almost
overlap with the RDW curves.
• The present collision strength data
thus appear to be quite accurate.
Colgan et al, Phys. Rev. A 77, 062704 (2008).
Electron-impact ionization calculations
• Electron-impact ionization of Si2+ (3s2).
DW calculations compared with
2+
measurements of Djuric et al [PRA 47,
4786 (1993)].
• Non-perturbative time-dependent closecoupling (TDCC) method used to check
the accuracy of the DW calculations.
• TDCC calculations somewhat lower than
DW, and in better agreement with
experiment.
• At around 125 eV, excitationautoionization makes a significant
contribution.
• We choose to only present direct
ionization cross sections, since the data
we submit will include DW ionization
from all configurations, including
autoionizing configurations.
• Since we have calculated excitation and
autoionization data, the excitationautoionization contribution can be
Colgan et al, Phys. Rev. A 77, 062704 (2008).
included.
Si
Electron-impact ionization calculations
• Electron-impact ionization of Si3+
(3s). DW calculations compared
with measurements of Crandall et al
[PRA 25, 143 (1982)].
Si3+
• TDCC calculations are only slightly
lower than DW; both are in good
agreement with experiment for the
direct ionization component.
• For the more highly charged Si ions,
this good agreement allows us to use
only the DW method to compute
ionization cross sections.
• At above 100 eV, excitationautoionization again dominates the
cross section.
Colgan et al, Phys. Rev. A 77, 062704 (2008).
Electron-impact ionization calculations
• Electron-impact ionization of Si7+ (2s2
2p3). DW calculations compared with
measurements of Zeijlmans et al [PRA
47, 2888 (1993)].
• Good agreement with experiment is
found.
• For the more highly charged ions, the
DW results appear to be in good
agreement with available experiment,
and should be of sufficient accuracy for
modeling purposes.
Colgan et al, Phys. Rev. A 77, 062704 (2008).
Si7+
Electron-impact ionization calculations
• Electron-impact ionization of Si-like
Cl3+ and Ar4+ (3s2 3p2).
•
For Ar4+
DW calculations compared
with measurements of Müller et al [JPB
13, 1877 (1980)].
Cl3+
• Again, good agreement with experiment
is found.
• For the more highly charged ions, the
DW results appear to be in good
agreement with available experiment,
and should be of sufficient accuracy for
modeling purposes.
Colgan et al, Phys. Rev. A 77, 062704 (2008).
Ar4+
Electron-impact ionization – K-shell
Mn: K-shell
• We also have been able to use a fully
relativistic DW approach (RDW) to
calculate K-shell ionization of heavy
neutral targets, to compare with the
experimental measurements of
Professor Luo’s group.
• Even though the ionization
measurements are from a solid target,
DW (isolated atom) calculations appear
to work well.
• Agreement with experiment is
excellent.
• Further semi-relativistic DW (SRDW)
calculations show that a fully relativistic
approach is necessary for these tightly
bound electrons to obtain good
Colgan et al, Phys. Rev. A 73, 062711 (2006).
agreement with experiment.
Electron-impact ionization – K-shell
• Similar conclusions can be drawn
from the other targets in our
study, for example Fe.
Fe: K-shell
• K-shell ionization cross sections
were calculated for Mn, Fe, Ni,
and Cu.
• We can investigate the K-shell
ionization of other heavy targets
of interest to this working group,
if necessary.
Colgan et al, Phys. Rev. A 73, 062711 (2006).
Electron-impact ionization – L-shell
• The Luo group has also been able to
study ionization from the L-shell of
W.
• We again use our RDW method to
compute ionization from the 2s1/2
and 2p1/2 & 2p3/2 sub-shells. The
inset shows the individual shell
contributions.
• The agreement with experiment is
still quite good, although not as
spectacular as for the K-shell
studies.
• Differences here may be due to
interactions of the ejected electron
with bound electrons, which are
only approximately taken into
account in the RDW calculations. Colgan et al, Phys. Rev. A 73, 062711 (2006).
W: L-shell
Solar coronal plasmas
• The radiative cooling, or radiative loss, of the solar
corona is an important quantity in solar physics.
– Crucial in evaluating the plasma’s energy balance
– Provides an understanding of the corresponding energy
source of the Sun
• But…
– Observational data are scarce
– Calculations are difficult, since the contributions from all
elements present in the solar corona must be taken into
account.
– One must consider atomic processes in all ion stages of
all the (15) elements which contribute
– Implies a model spanning almost 200 ion stages!
Method
• A recent addition to ATOMIC also allows us to consider all
elements under the influence of a single electron density.
This density may be determined using an iterative procedure,
under the constraints of a user-input electron temperature
and user-input set of elemental abundances.
• We also must satisfy global charge conservation.
• Radiative losses (bound-bound) are computed using:
Electron temperature &
density
R (T e , N e ) 
Ion level population
1
N tot N e


i



Ai ; j  k  E i ; j  k N ij
k j
Transition energy
Total ion number density
Spontaneous emission rate
• We also include bound-free (from radiative recombination)
and free-free (from bremsstrahlung) contributions to the
radiative losses.
Sherrill et al, Phys. Rev. E 76, 056401 (2007).
The Calculation
• Atomic structure and data were computed for all ion stages of
the 15 most abundant solar coronal elements:
• Collisional data was computed using PWB and FOMBT/DW
approaches, to test the sensitivity of the radiative losses to the
quality of the excitation data.
Results – comparisons to previous work
We compare the ATOMIC
calculations to previous work
of Landi & Landini (1999)
and from the CHIANTI
database.
Substantial differences are
found with Landi & Landini
(1999) over most of the
temperature range.
Significant differences with
the latest CHIANTI
calculations persist at low
temperatures below 106 K
(~ 100 eV).
Landi & Landini, A & A, 347, 401 (1999).
Radiative losses from the individual elements
We present the contribution of
the radiative losses from the
individual 15 elements included
in our calculation.
At low temperatures H, He, C,
and O are the dominant
contributors.
At high temperatures, Si and Fe
contribute most to the radiative
loss, although at the highest
temperatures, continuum
radiation from H also
contributes strongly.
Colgan et al, Astrophysical Journal 689, 585 (2008).
Results – comparisons to previous work
• Currently generating a finestructure model for all 15
coronal elements with which
to compute the radiative
losses.
• Differences between LANL
calculations and CHIANTI
results may be partly due to
FS vs CA differences, and
partly due to the LANL
inclusion of many more
excited and autoionizing
states.
Assessing the sensitivity to the atomic data
If we replace the DW excitation
cross sections with PWB, we
find that the DW data makes a
significant difference only at
low temperatures, less than ~ 2
X 105 K.
Nonperturbative cross sections
for neutral H are known to be ~
factor of two lower than DW
cross sections.
We can also then scale the H
cross sections by a factor of
two to gauge the sensitivity.
This only makes a difference at
the very lowest temperatures of
2 X 104 K (~ 1-2 eV).
Ion fractions of C, O, Si, and Fe
It is also instructive to know
which ion stages contribute
most to the radiative losses.
For C, at the temperatures at
which it is the strongest
contributor, we find that most
of the emission arises from
C IV and C V ions.
For O, O V, O VI, and O VII
contribute most at the
temperatures for which O
dominates.
We also see that many ion
stages of Si and Fe are present
for a wide temperature range
and so must be included.
Sensitivity to elemental abundances
The previous radiative losses
were computed using “quiet
region” elemental abundances.
Some regions of the corona
have different abundances of
elements with low (< 10 eV)
first-ionization-potentials
(FIPs).
These include the elements Na,
Mg, Al. Si, Ca, Fe, and Ni.
Thus we present the radiative
losses for a variety of FIP bias
values (relative to the
photospheric abundances).
Colgan et al, Astrophysical Journal 689, 585 (2008).
Conclusions/Future Work
• In the last 5 years we have generated significant amounts of
atomic data for use in the plasma modeling communities.
• We are in the process of converting these data to the
ALADDIN format for inclusion in the IAEA database.
• We note that the LANL atomic physics codes can be run
through our webpage, http://aphysics2.lanl.gov/tempweb,
where limited datasets may also be created.
• We hope to add to the data generated to the fusion modeling
community, particularly for tungsten (W). Preliminary
radiative power loss calculations for W have been generated
and appear to compare well with other calculations.
Conclusions/Future Work
• Our published data are already starting to be used by the
atomic physics and plasma modeling communities:
– We have had requests from M. Reinke (MIT) for Ar ionization data so
that ionization balance for Ar may be generated for use in plasma
transport codes.
– We have had requests from M. Ali (NIST) for comparison of our
ionization data for selected near-neutral ions as tests of new BEB
calculations.
• We are willing to generate data to meet future data needs – if
the time & funding are there!