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


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


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


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


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+


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


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)


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


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


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).


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).


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


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.


Anhang


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


Existing Jena TG spectrometer

Name/Type Size(source-CCD)

SolidAngle(E-4sr)

[email protected]

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


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

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