K-Shell Spectroscopy of Au Plasma Generated with a Short Pulse Laser
Calvin Zulick[1], Franklin Dollar[1], Hui Chen[2], Katerina Falk[3], Andy Hazi[2], Karl Krushelnick [1], Chris Murhpy[3], Jaebum Park[2], John
Seely[4], Ronnie Shepherd[2], Csilla I. Szabo[4], Riccardo Tommasini[2]
[1]
Abstract: The production of x-rays from electron transitions into K-shell
vacancies (K-α/β emission) is a well known process in atomic physics and has
been extensively studied as a plasma diagnostic in low and mid Z materials[1-2].
Such spectra from near neutral high-Z ions are very complex and therefore
difficult to describe with analytical models. In this experiment a high Z (gold)
plasma emission spectrum was measured with a transparent cylindrically bent
quartz crystal spectrometer with a hard x-ray energy window ranging from 17
to 102 keV.
Center for Ultrafast Optical Science, University of Michigan
[2] L-472, Lawrence Livermore National Laboratory
[3] Clarendon Laboratory, University of Oxford
[4] Space Science Division, Naval Research Laboratory
Image Plate showing Eu Spectra
Cylindrically Bent Crystal Spectrometer: A hard x-ray spectrometer
was used to time-integrate x-ray spectra and prove intensity and
energy data. The spectrometer utilized a cylindrically bent quartz
crystal to “focus” Bragg diffracted x-rays through a lead slit. The
dependence of the Bragg angle on the x-ray energy introduces
spatial separation in the x-ray energies which are then resolved on
image plate.
The spectrometer was designed with a
60cm stand off distance which
corresponds to a collection angle of
approximately 50 mrad.
Higher Energy
The axial symmetry of the spectrometer allows duplicate
information to be recorded on each side of the image plate,
providing additional signal and information about the
background noise. Second order Bragg peaks from the Kalpha signal are also evident in this image.
CAD drawing and schematic of the spectrometer
K-Shell Spectra: K-Alpha1,2 and K-Beta1,2 transitions were observed over a
series of 40 shots with varying target and laser conditions. The observed
spectra were consistent with tabulated energies[3] for both cold Au and Eu
targets.
Simulation: FLYCHK[4], an atomic NLTE code designed to provide ionization and
population distributions, was used to simulate the K-alpha and K-beta spectra for
Au targets with varying temperatures and ionization states. Temperature and
ionization estimates were calculated with HYADES (a 1-D hydro-code).
Tabulated Values (NIST)
Electron Temperature
Electron Density
22
10
3
Ions per cm
Z bar
3
Electrons per cm
KeV
0.1
30
21
4x10
21
2x10
10
21
10
20
10
19
10
18
10
0
-0.005
0
0.005
-0.02
Thickness [cm]
Kα1
100
Kβ1 Kβ2
80
-0.01
0
0.01
0.02
0.03
0
Thickness [cm]
0.01
-0.02
Thickness [cm]
-0.01
0
0.01
0.02
0.03
Thickness [cm]
Using these estimates, FLYCHK was used to establish a better estimate on the
plasma conditions by matching the simulated spectra to the observed intensity
ratios. Ultimately, this information will be used to establish a connection
between the plasma temperature and ionization states and the production of
positrons.
K-alpha / K-beta strengths for various Au conditions
Au Shot
120
60
40
20
1.00
0
0.90
60
65
70
75
80
85
90
0.80
Normalized Intensity
Energy (KeV)
Eu Shot
0.08
Tabulated Values (NIST)
Line
Kα2
Kα1
Kβ1
Kβ2
Kev
40.9
41.5
47.3
48.3
Intensity (Arb)
Electron Shell Diagram
Ion Density
23
Kev
66.9
68.8
78.5
80.3
Kα2
Intensity (Arb)
Experimental Setup and Background: The
Titan laser, part of the Jupiter Laser Facility
at Lawrence Livermore National Laboratory,
was used to deliver a 350 joule, 10 ps, 1054
nm laser pulse to a Au target.
The
absorption of laser energy by the resulting Au
plasma results in the production of
suprathermal
(“hot”)
electrons
which
propagate into the target. The high energy
electron beam knocks inner shell electrons
from their orbit leaving vacancies which can
be replaced by higher energy electrons. The
energy released as electrons relax into inner
shell vacancies is given off as x-rays
(commonly referred to as K-alpha and K-beta
radiation) which differ in energy depending
on the original shell position of the electron.
Ionization Level
10
0.2
Line
Kα2
Kα1
Kβ1
Kβ2
Spectrometer in the Titan target chamber
0.70
0.60
Neutral
0.50
100eV 10+
0.40
10eV 5+
0.30
50eV 10+
75eV 10+
0.20
0.06
0.10
0.04
0.00
60000.00
0.02
65000.00
70000.00
75000.00
80000.00
85000.00
90000.00
Energy (eV)
0
30
40
50
60
70
80
90
100
Energy (keV)
Summary:
We performed a series of shots in which the backside of the target was preheated and pre-ionized with a long pulse laser (3 nanosecond, 1-10 joule). We
observed an increase in the ratio of K-alpha to K-beta signal with increasing
short pulse laser energy.
Au Kα/Kβ
6
6
•The presence of a nanosecond pulse on the rear surface of the gold target
increased the K-alpha to K-beta ratio.
•The plasma conditions inferred by the K-shell x-rays may provide some insight into
the production of positrons.
Eu Kα/Kβ
6.5
•The cylindrically bent crystal spectrometer provides an effective way of measuring
K-alpha and K-beta x-rays from short pulse laser-matter interactions.
5.5
5
5
4.5
area
4
Peak
3.5
References:
4.5
Area
4
Peak
3.5
3
2.5
2
4
6
Joules
8
10
12
[1] Kneip, S. et al. HEDP 4 41-48 (2008)
[2] Jiang, Z. et al. Phys. Plasmas 2 5 1702-1711 (1995)
3
0
Titan Experimental Chamber
Ratio
Ratio
5.5
1
10
100
Joules
1000
[3] http://www.nist.gov/physlab/data/xraytrans/index.cfm
[4] Chung, H. et al. http://nlte.nist.gov/FLY/ (2008)
This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344 and was funded by LDRD #10-ERD-044