Hirschegg 2008 January 31th, 2007 Emission Spectra from the Interaction of VUV FEL Radiation with...
-
Upload
helen-taylor -
Category
Documents
-
view
215 -
download
0
Transcript of Hirschegg 2008 January 31th, 2007 Emission Spectra from the Interaction of VUV FEL Radiation with...
Hirschegg 2008 January 31th, 2007
Emission Spectra from the Interaction
of VUV FEL Radiation with solid Aluminium
at FLASH
U. Zastrau, L. Cao, I. Uschmann & E. FörsterIOQ - X-Ray Optics Group - Friedrich-Schiller-University Jena
C. Fortmann, G. Röpke – University Rostock
R. Fäustlin – DESY Hamburg
31.1.2008 Hirschegg 2008 Ulf Zastrau 2
VI people in theory & experiment
• L. Cao, U. Zastrau, I. Uschmann, E. Förster a. Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität Jena,
Max-Wien-Platz 1, 07743 Jena, Germany
• C. Fortmann, G. Röpke, J. Tiggesbäumker, A. Przystawik, K.-H. Meiwes-Broer , H. Reinholz, R. Thiele, Th. Bornath, N.X. Truong, A. Höll, R. Redmer
b. Institut für Physik, Universität Rostock, Universitätsplatz 3, 18051 Rostock,Germany
• R. Fäustlin, T. Laarmann, S. Toleikis, S. Düsterer, P. Radcliffe, T. Tschentscherc. Deutsches Elektron-Synchrotron DESY,
Notkestr. 85, 22607 Hamburg, Germany
• S.H. Glenzer, T. Döppnerd. L-399, Lawrence Livermore National Laboratory,
University of California, P.O. Box 808, Livermore, CA 94551, USA
31.1.2008 Hirschegg 2008 Ulf Zastrau 3
Contents
• Warm Dense Matter – Creation in isolation state?
• Experimental Setup – FEL irradiates Aluminum target
• Experimental Results – VUV emission spectra
• Analysis – Bremsstrahlung, Line Intensities and L-edge
• Simulation – Hydrodynamic Predictions from HELIOS
• Summary of the Talk
31.1.2008 Hirschegg 2008 Ulf Zastrau 4
Warm Dense Matterwww.arcadiastreet.com
Condensed Matter <> Warm Dense Matter <> Ideal Plasma
Ethermal ~ EFermi
1..100 eV
Plasma Coupling(Ecoulomb / Ethermal)
>1
= 0.1..10 x solid
Condensed Mattertheory fails:
Electrons aretoo hot
-----
Classical Plasmatheory fails:
collective behaviorof electrons and atoms
31.1.2008 Hirschegg 2008 Ulf Zastrau 5
To create WDM, one needs…
PROBLEMS with “long” laser pulses, hydrodynamic motion takes place while laser is heating
need of fs laser pulses, most common VIS or IR lasers
optical density is high at VIS:multi-photon absorption - high reflectivity of surface
hot electrons (~I²)steep gradient at surface - high ion charge (~10+)
U. Teubner et al., Appl. Phys. Lett. 59, 2672 (1991).
SOLUTIONS layer target: laser generated electrons heat WDM indirectly
Benattar, Geindre et al, Opt. Comm. (1992)
XUV FEL : linear absorption - short pulses with high intensity ‘warm’ electrons
Homogeneous temperature distribution - low ion charge (~3+)
electron temperature Te of a few tens eV high density ( ~ solid density)
JETI, IOQ Jena
31.1.2008 Hirschegg 2008 Ulf Zastrau 6
K 1s²
L [He] 2s²2p6
M [Ne] 3s²2p1
The target - Aluminum
93 eV
Z*=2.6
Ene
rgy
Conduction band
Absorption coefficient above L-egde (72 eV):
µ (L) / µ (M) ~ 10
13+
31.1.2008 Hirschegg 2008 Ulf Zastrau 7
Experimental Setup
Al bulk45°
FEL pulses
30 fs 13.5 nm / 93 eV30µm focal spot
50µJ/pulse
VUV Spectrometercovers 7..19nm with /~100
1.7×1014 Wcm-2
ncrit = 61024cm-3 ~ 60 nsolid
direct energy transfer into the bulkAbsorption length ~ 40nm [Henke]
Accumulation of 81000 exposures
≥10-6 efficiency
31.1.2008 Hirschegg 2008 Ulf Zastrau 8
Results – EUV emission spectrum
Characteristics:
-FEL Scatter at 13.5nm (bandwidth ~ 1.4nm)
-continuum emission (bremsstrahlung)
-spectral line emission (ratio -> temperature)
-Al L absorption edge (penetration depth)
(in 4)
31.1.2008 Hirschegg 2008 Ulf Zastrau 9
Analysis I - bremsstrahlungprovided by C. Fortmann et al.
Simulation of continuum radiation:Kramer’s law, with Z : mean ion charge
Assumption of reabsorption by
Gaunt factor gT()in Sommerfeld approximation
best fit of bremsstrahlung
Te = 22.5 eV
31.1.2008 Hirschegg 2008 Ulf Zastrau 10
Analysis II – emission lines
Ratio of lines given by Boltzmann
ratio of dubletts of Al IV
Te = 22.6 eV
provided by C. Fortmann et al.
COMPTRA04
31.1.2008 Hirschegg 2008 Ulf Zastrau 11
Comparison XUV – UV Laser
4 6 8 10 12 14 16
FLASHExperiment
13.5nm, 30fs
1.51014 W/cm²
25µm focus
Recorded witha focusing
spectrometerand CCD
no highionization (>4+)
detectable
Al 6+ Lines
FELUV LaserExperiment
248nm, 500fs(with prepulse)51015 W/cm²
22µm focus
Recorded withx-ray film
on Rowland circle
U. Teubner et al., Appl. Phys. Lett. 59,
2672 (1991).
31.1.2008 Hirschegg 2008 Ulf Zastrau 12
Reabsorption of Bremsstrahlung
The effect of reabsorption can be determined by the transmission
close to the L – edge,
by assuming tabulated attenuation coefficients
Notice: mean ion charge is Z=3.5
5 10 15 20 25 30
0.0
0.2
0.4
0.6
0.8
1.0
Thickness=0,1µm Thickness=0,04µm correspond to XUV-FEL Experiment
at 45 deg observationTransmission data from LBL-tables
I / I
0
wavelength / nm
Al L-edge
If hot plasma is present one would observe a change of the edge.Absorption through WDM Possilbe EXAFS WDM diagnostics ?
Ultrafast melting Si L-edge EXAFS: Oguri, Okano et al., Appl. Phys. Lett. 87 (2005).
31.1.2008 Hirschegg 2008 Ulf Zastrau 13
Hydrodynamic predictions (preliminary)provided by R. Fäustlin
Electron Temperature Mean Ion Charge
Depth [µm] Depth [µm]
Same FEL parameters as in the experimentIncludes absorption via bound-free and inverse bremsstrahlung
~200 fs emission duration(Murnane, Kaypeyn et al. Science ‘91)
0 fs
500 fs
1 ps
0 fs
500 fs
1 ps
31.1.2008 Hirschegg 2008 Ulf Zastrau 14
Summary of the talk
Thank you for your attention.
• Warm Dense Matter can be created in isolation (without hot plasma) using FLASH XUV pulses and solid target
• An electron temperature of Te = (23±7) eV was deduced frombremsstrahlung fit and ratio of Al IV emission lines
• Al III and Al IV emission lines are detected, but no Al VII and higheralthough there are present in optical laser plasmas
• The penetration depth is l =(40±10) nm deduced from L-egde,with agrees with the absorption length of FEL radiation in Al
• Hydrodynamic simulations are in agreement with Te and ion charge, expected emission time ~ 200 fs.
31.1.2008 Hirschegg 2008 Ulf Zastrau 15
Anhang
31.1.2008 Hirschegg 2008 Ulf Zastrau 16
Reflection optics
20 40 60 80 100 120 140 1600.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
ideal surface
20 Angstroem roughness
FEL
refl
ectiv
ity
photon energy / eV
Advantages: - better performance - higher aperture (use of the whole blazed surface)
Possible problem: - loss of reflectivity by surface contamination - loss of reflectivity by surface roughness induced by debris
Reflectivity of Nickel with a 2 nm carbon surface
20 40 60 80 100 120 140 1600.000.050.100.150.200.250.300.350.400.450.500.550.600.650.700.750.800.850.900.95
clean surface
2 nm carbon contamination
refl
ectiv
ity
photon energy / eV
31.1.2008 Hirschegg 2008 Ulf Zastrau 17
Existing Jena TG spectrometer
Name/Type Size(source-CCD)
SolidAngle(E-4sr)
Alignment Fragility (photons Out/In)
cost comment
Jena 72 cm 4.1 ~0.1nm Mechanically,Timeconsuming
Sturdy housing,Fragile grating
0.082 -- Readilyavailable
Source Photons to CCD:2.6 10-6
Thanks to R. Fäustlin
31.1.2008 Hirschegg 2008 Ulf Zastrau 18
Reabsorption of Bremsstrahlung in solid aluminium target – effect of „hole boring“
Fresh target surface after many FEL pulses
target surface
target surface
FEL pulseconstant angle of incidence
FEL pulse
EUV spectrometerconstant observation angle
EUV spectrometer
deep penetration
weak penetration,low intensity
Weak emission and reabsorption
Strong reabsorptionL edge contrast strongplasma near surface only
Penetration depth 40nm, focus diameter 20µm factor of 500