Transcript Tuxedo - University of California, San Diego

Radiation-Hydrodynamic Analysis of Doped
Underdense Targets for HED Studies
M. S. Tillack
8 February 2005
Mechanical and Aerospace Engineering Department
and the Center for Energy Research,
Jacobs School of Engineering
We are collaborating with LLNL on a
3-year study of non-LTE laser plasmas
Overall goal is to develop absolutely calibrated spectroscopic diagnostics
and benchmark data from well-characterized “non-LTE” plasmas
• Address a class of problems in which temperature
can not be uniquely related to energy
TGS
• Establish credibility in non-LTE calculations
• Resolve long-standing problems in the literature
concerning emission from low density plasmas
“Non-LTE”: energy content and radiation emission depend on
the full time-dependent set of rate equations for atomic processes
• Collisional ionization, recombination, excitation, and deexcitation
• Photoionization and stimulated recombination
• Photoexcitation and stimulated emission
• Spontaneous decay
• Radiative recombination
• Dielectronic recombination, autoionization, and electron capture
A generic problem with laser plasmas is the
large gradients and transient nature, which
complicate analysis and data interpretation
Te and ne in 100 mm DT film at 1 ns, 5x1014 W/cm2
Low density targets (gas bags or foams) can provide more uniform
density and temperature:
1. mass limited so that all of the target mass heats
2. optical thickness comparable to target thickness at desired Te
3. larger than the hydrodynamic expansion length during the pulse
>1 mm thick, 1-10 mg/cm3 for 4 ns laser pulse
40 shots (7 days) were obtained at Nike in 2004;
Future experiments are planned at Janus
Obtained absolutely calibrated Ti L-shell emission from aerogel targets
– Measured time-resolved spectra in 470-3000 keV region covering Ti L-shell
Determined accessible plasma conditions by variation of laser parameters
– Able to heat plasma to threshold of K-shell emission (He-like Ti emission)
– Determined experimental conditions for creating L-shell emission
(135 J, 4 ns, 940 µm spot)
– Time-integrated measurements of plasma Te via Si and Ti K-shell line ratios
Examined plasma uniformity with x-ray imaging diagnostics
Nike chamber (5 kJ)
QuickTime™
and
QuickTime™
and a a
TIFF (Uncompressed) decompressor
TIFF (LZW)
decompressor
are needed
to see this picture.
are needed to see this picture.
Model predictions often disagree with data,
and with each other
Non-LTE plasma simulations are computationally
expensive and often not self-consistent
To model realistic plasmas, simulations must
implement approximations in the atomic physics
data and radiation algorithm
– A relatively complete model of a Ti ion can have
up to 20,000 levels
– An approximate model of the same ion may be
reduced to ~100 levels in order to run simulations
This project is an element of the growing field
of High Energy Density (HED) physics
The topic of HED on NNSA facilities is divided into 4 thrust areas: *
*
1. Material Properties
3. Radiative Hydrodynamics
2. Compressible Dynamics
4. Inertial Confinement Fusion
HED in NNSA Facilities, “Bruce Remington (LLNL), Chris Deeney (SNLA), David Hammer (Cornell),
Dick Lee (LLNL), David Meyerhofer (LLE), Dieter Schneider (LLNL), Isaac Silvera (Harvard), Bernie
Wilde (LANL),” A presentation to the High Energy Density Physics Workshop, May 24-26, 2004,
Gaithersburg, Maryland. http://www.ofes.fusion.doe.gov/More_HTML/HEDPWorkshop5-04.html
Our work addresses two of these
“Compelling question” for material properties thrust area:
Can matter in the difficult warm dense matter (WDM) regime be isolated,
defining its state while measuring the material properties of interest?
“Compelling question” for radiation hydrodynamics area:
Can HED experimental facilities become a routine tool for testing rad-hydro models
and simulations of powerful astronomical phenomena in a scaled laboratory setting?
The goal of the material properties thrust is to map
material properties across the WDM regime
Hot Dense Matter occurs in:
• stellar interiors, accretion disks
• laser plasmas, z-pinches
• radiatively heated foams
• ICF capsule impoded cores
Warm Dense Matter occurs in:
• cores of giant planets
• strongly shocked solids
• radiatively heated solid foils
The methodology involves two steps:
1. Prepare the state (at desired density, temperature, etc.)
– verify that gradients are small, time-dependent effects are unimportant
2. Measure the material property of interest
– opacity, ionization state, EOS, conductivity, etc.
The Radiative Hydrodynamics thrust focuses on
“hot flowing matter”, where the radiation and
material flows are coupled
• Quantitative modeling of such flows is difficult;
benchmark data is needed
• Examples: radiative shocks & jets, supersonic radiation flow,
photoionized plasmas, radiation-dominated dynamics
• Radiative hydrodynamics abounds in astrophysical plasmas
Radiative shock in Janus
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Radiative shocks in the Cygnus loop SNR
At UCSD we are contributing to both
experiments and modeling activities
• 1D LTE rad-hydro simulations using Hyades and Helios
– gray and spectrally resolved simulations
– purpose is to explore and verify ne and Te behavior,
and determine whether spectral detail affects hydro
• Non-LTE simulations using Cretin and Helios-CR
• Experimental support of Te diagnostics
– Single-channel calibrated PCD detector
– Multi-channel filtered diode detector
During 2004 we acquired several
new modeling tools
Standard 1D rad-hydro codes
•
Lagrangian grid, flux limited, diffusion approximation
1. Hyades
•
•
Cascade Applied Sciences (Jon Larsen), history of use at LLNL
Limited capabilities for spectrally resolved opacities, LTE only
2. Helios
•
•
Prism Computational Sciences (MacFarlane/Golovkin), used a lot at SNLA
Propaceos code provides spectral data, Helios-CR for non-LTE
Cretin
•
3D non-LTE collisional-radiative rate equation solver (Howard Scott)
HULLAC
•
Parametric potential method to generate atomic data (Klapisch/Busquet)
Cases were examined using Hyades at high and low
density, high and low intensity, with and without doping
C ase #
0
1
2
3
4
5
6
I
Descri ption
Ti wt%
modeling base case
experimental base case
high T i dopant case
high SiO2 density case
thin target case
high laser intensit y case
high intensity, high doping
All cases used a 4-ns flat-top
intensity profile at 248 nm
3.6 ns
t
0
2
6
2
2
2
6
SiO2 density
(mg/cm3)
2
2
2
8
2
2
2
4 ns
An unfeathered grid with 50
zones was used to simplify
graphical interpretation
(constant mass per zone)
laser intensi ty
(TW/cm2)
4.9
4.9
4.9
4.9
4.9
370
370
Base case temperature evolution
(5x1012 W/cm2, SiO2, 2 mg/cm3 )
Spatial profile at 2.5 ns
~50 eV
~1/2 ns
Base case density evolution
(note: ncr=16x1021/cm3)
Spatial profile at 2.5 ns
Charge state, target expansion, laser absorption
(100 nodes)
(nodes 25-50)
(Denavit, PoP 1994)
Two key physical processes are involved in
underdense laser plasma energy transfer
1. Laser absorption in underdense
plasma (inverse bremsstrahlung)
 = 10–16 Te–3/2 Z ln (ne2/ncr)
L = 1 mm
2. Emission and absorption
of thermal radiation
The radiation absorption wave propagates more
slowly at higher density (8 mg/cm3)
~50 eV
Scaling depends on opacity rather
than inverse bremsstrahlung
... but the final temperature, density and
charge state are remarkably similar
At 4x1014 W/cm2, the plasma is fully stripped
and expands more rapidly
This plasma becomes transparent; the density is
initially uniform, but is quickly lost due to expansion
8.E+20
increasing time
6.E+20
5.E+20
4.E+20
3.E+20
2.E+20
1.E+20
0.E+00
0
10
20
30
40
50
40
50
Zone index
1.8
increasing time
1.6
1.4
Temperature, keV
Electron density, cm -3
7.E+20
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
Zone index
30
Properly modeling opacities is challenging
Planck averaging:
Rosseland averaging:
SiO2
We tried several variations to explore the
influence of opacity models on the results
•
•
•
•
•
•
Sesame data in Hyades
Built-in multi-group model in Hyades
Propaceos (spectral) data averaged and imported into Hyades
Propaceos (spectral) data used in Helios
Sesame data used in Helios
Reduced frequency group (averaged) modeling with Helios
Due to problems implementing multi-group radiation transport
in Hyades, we relied upon Helios to study the effect of doping
Unfortunately, temperatures from Hyades
and Helios do not agree very well
Helios
Hyades
spatial profiles at 2.5 ns
Opacity data is surely part of the reason
Comparison of spectrally averaged opacities
range of interest
• Helios plasmas are much more opaque
• Which is correct?
The energy balance looks completely different
16 kJ/cm2
7 kJ/cm2
2 kJ/cm2
8 kJ/cm2
Helios stores ~50% of the energy,
whereas Hyades promptly
radiates 90%. Helios plasma is
far more opaque.
The effect of 6% Ti dopant on Te (using Helios)
base case
The doped case cools
faster and is less uniform
6% Ti
The effect of 6% Ti dopant on ne (using Helios)
base case
The lower temperature leads
to slightly lower density
6% Ti
Summary
•
Plasmas with uniform ne and “relatively” uniform
Te were obtained and parametrically studied in the
range 5x1012–5x1014 W/cm2; not quite good enough yet
•
The best results seem to occur when the target is
optically thick to the laser
•
Codes disagree, even with single-group opacities.
Hyades needs more work to produce accurate
spectrally resolved results
•
Doping significantly affects temperatures
(based on Helios simulations); makes them worse!
Our plans in 2005-06 include more modeling
and experimental collaborations
•
Further optimization of rad hydro
•
Increased use of Cretin to study non-LTE emissions
•
Explore atomic data for non-LTE work
Hullac, Propaceos, new averaging schemes, ...
•
Development of PCD detectors for Te measurements
– single, calibrated diamond diode
– filtered diode array
•
Experiments at Janus