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. Förster IOQ - X-Ray Optics Group - Friedrich-Schiller-University Jena C. Fortmann, G. Röpke – University Rostock R. Fäustlin – DESY Hamburg

Hirschegg 2008Ulf Zastrau1 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, 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, Rostock,Germany R. Fäustlin, T. Laarmann, S. Toleikis, S. Düsterer, P. Radcliffe, T. Tschentscher c.Deutsches Elektron-Synchrotron DESY, Notkestr. 85, Hamburg, Germany S.H. Glenzer, T. Döppner d.L-399, Lawrence Livermore National Laboratory, University of California, P.O. Box 808, Livermore, CA 94551, USA

Hirschegg 2008Ulf Zastrau2 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

Hirschegg 2008Ulf Zastrau3 Warm Dense Matter Condensed Matter <> Warm Dense Matter <> Ideal Plasma E thermal ~ E Fermi eV Plasma Coupling (E coulomb / E thermal )  >1  = x  solid Condensed Matter theory fails: Electrons are too hot Classical Plasma theory fails: collective behavior of electrons and atoms

Hirschegg 2008Ulf Zastrau4 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 (~I ²) 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 T e of a few tens eV high density (  ~ solid density) JETI, IOQ Jena

Hirschegg 2008Ulf Zastrau5 K 1s² L [He] 2s²2p 6 M [Ne] 3s²2p 1 The target - Aluminum 93 eV Z*=2.6 Energy Conduction band Absorption coefficient above L-egde (72 eV): µ (L) / µ (M) ~

Hirschegg 2008Ulf Zastrau6 Experimental Setup Al bulk 45° FEL pulses 30 fs 13.5 nm / 93 eV 30µm focal spot 50µJ/pulse  VUV Spectrometer covers 7..19nm with /  ~ ×10 14 Wcm -2 n crit = 6  cm -3 ~ 60 n solid  direct energy transfer into the bulk Absorption length ~ 40nm [Henke] Accumulation of exposures ≥10 -6 efficiency

Hirschegg 2008Ulf Zastrau7 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  )

Hirschegg 2008Ulf Zastrau8 Analysis I - bremsstrahlung provided by C. Fortmann et al. Simulation of continuum radiation: Kramer’s law, with Z : mean ion charge Assumption of reabsorption by Gaunt factor g T ( ) in Sommerfeld approximation best fit of bremsstrahlung  T e = 22.5 eV

Hirschegg 2008Ulf Zastrau9 Analysis II – emission lines Ratio of lines given by Boltzmann ratio of dubletts of Al IV  T e = 22.6 eV provided by C. Fortmann et al. COMPTRA04

Hirschegg 2008Ulf Zastrau10 Comparison XUV – UV Laser FLASH Experiment 13.5nm, 30fs 1.5  W/cm² 25µm focus Recorded with a focusing spectrometer and CCD  no high ionization (>4+) detectable Al 6+ Lines FEL UV Laser Experiment 248nm, 500fs (with prepulse) 5  W/cm² 22µm focus Recorded with x-ray film on Rowland circle U. Teubner et al., Appl. Phys. Lett. 59, 2672 (1991).

Hirschegg 2008Ulf Zastrau11 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 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).

Hirschegg 2008Ulf Zastrau12 Hydrodynamic predictions (preliminary) provided by R. Fäustlin Electron Temperature Mean Ion Charge Depth [µm] Same FEL parameters as in the experiment Includes 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

Hirschegg 2008Ulf Zastrau13 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 T e = (23±7) eV was deduced from bremsstrahlung fit and ratio of Al IV emission lines Al III and Al IV emission lines are detected, but no Al VII and higher although 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 T e and ion charge, expected emission time  ~ 200 fs.

Hirschegg 2008Ulf Zastrau14 Anhang

Hirschegg 2008Ulf Zastrau15 Reflection optics 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

Hirschegg 2008Ulf Zastrau16 Existing Jena TG spectrometer Name/TypeSize (source- CCD) Solid Angle (E-4sr) 13.5nm AlignmentFragility(photons Out/In) costcomment Jena72 cm4.1~0.1nmMechanically, Timeconsuming Sturdy housing, Fragile grating Readily available Source Photons to CCD: 2.6  Thanks to R. Fäustlin

Hirschegg 2008Ulf Zastrau17 Reabsorption of Bremsstrahlung in solid aluminium target – effect of „hole boring“ Fresh target surface after many FEL pulses target surface target surface FEL pulse constant angle of incidence FEL pulse EUV spectrometer constant observation angle EUV spectrometer deep penetration weak penetration, low intensity Weak emission and reabsorption Strong reabsorption L edge contrast strong plasma near surface only Penetration depth 40nm, focus diameter 20µm  factor of 500