Innovative XUV- und X-ray-Spectroscopy to explore Warm Dense Matter

Transcription

1 3rd EMMI Workshop on Plasma Physics with intense Lasers and Heavy Ion Beams Innovative XUV- und X-ray-Spectroscopy to explore Warm Dense Matter Eckhart Förster X-ray Optics Group - IOQ - Friedrich-Schiller-University Jena 0

2 Contents Physics of Warm Dense Matter (WDM) WDM generated by relativistic electrons using High-Intensity Lasers WDM generated by XUV-pulses at the Free-Electron Laser (FEL) FLASH Summary 1

3 Warm Dense Matter Condensed Matter <> Warm Dense Matter E therm ~ E Fermi ev ρ WDM ρ solid strong coupling Γ 1 E coulomb ~ E therm <> Ideal Plasma High Electron Density: Access of plasma parameters only possible by short wavelength radiation (ω > ω P ) penetration up to critical density n c = ω²ε 0 m/e² after R.W. Lee 2

4 Spectroscopy of Solid Density Plasmas by Using XUV and X-ray Photons Absorption length of λ=0.27 nm in Titanium (Z=22) : ~ 20 µm Absorption length of λ=13.5 nm in Aluminum (Z=13) : ~ 40 nm in laboratory always transient micro-plasmas with strong gradients spectroscopy with high spatial and temporal resolutions 3

5 Contents Physics of Warm Dense Matter (WDM) WDM generated by relativistic electrons using High-Intensity Lasers WDM generated by XUV-pulses at the Free-Electron Laser (FEL) FLASH Summary 4

6 Relevance of Laser-produced Plasmas Fundamental Parameter: Brightness number of X-ray photons time [s] emitting size [mm²] divergence [mrad²] spectral bandwidth [%] time-resolved X-ray diffraction point-source for radiography backlighter for Thomson scattering electron and ion acceleration (TNSA) laser-fusion and the Fast Ignitor -scheme Wilks 5

7 Physics of IR-Laser-Target Interaction few µm thin Titanium foil Ponderomototive Potential T hot ~ φ pond ~ Iλ² focused laser puls W/cm² IR-laser pulse creates fast electrons Plasma at the target surface Ti Kα n e fast electrons and X-ray emission electrons with energies up to MeV heat the cold target by collisions electrons with E > 5keV in Titanium are capable of K-shell ionization High Density & Fields, refluxing, filamentation, Hybrid PIC-fluid model: Evans et al., HEDP 2 (2006) we observe Kα emission from the heated target 6

8 LULI 100TW Laser Ti:Sa + Nd:Glas 1057 nm central wavelength 330 fs pulse duration max. 13 J energy in focus 8 µm focal diameter Experiment at 100TW Laser, LULI standard operation (ω) and frequency doubling (2ω) to obtain higher prepulse contrast X-ray film LASER 11 X-ray Spectrometer Intensity ~ W/cm² x α=50 Ti-Kα 2p-1s y z y r x Titanium Foil different titanium samples: massive (bulk) and foils of 25, 10 und 5µm d toroidal bent GaAs crystal U. Zastrau et al., PRE 81 (2010),

9 2D inverse Abel transformation W/cm² - single pulse spectra y Absolute Photons (in ΔλΔx) lateral Spectrum (y) 10µm Ti foil, 45 obvervation Wavelength [Å] y = 135µm 108µm 81µm 54µm 27µm 0µm spatial resolution: 13.5µm assuming cylindrical symmetry Peak-Normalized Intensity [a.u.] r radial Spectrum (r) µm 135µm 108µm µm r = 54µm 27µm 0.6 0µm Wavelength [Å] 8

10 Radial Temperature Distribution 10 µm foil transition from cold to warm Titanium plasma: blue-shift due to thermal M-shell ionization Theoretical line shape models: Stambulchik,.., Zastrau, et al., J. Phys. A 42 (2009), Sengebusch,.., Zastrau, et al., J. Phys. A 42 (2009), Laser Model: Spectrum of a simple, Laser spatially integrating spectrograph yields µm-folie 3 x lower temperature bulk! U. Zastrau et al., PRE 81 (2010),

11 Contents Physics of Warm Dense Matter (WDM) WDM generated by relativistic electrons using High-Intensity Lasers WDM generated by XUV-pulses at the Free-Electron Laser (FEL) FLASH Summary 10

12 Goal: Generating Warm Dense Matter without hot plasma NEEDS To generate WDM one needs femtosecond radiation sources of high brilliance (no hydro-dynamics). optical short pulse laser radiation (λ ~ 1 µm) : high reflectivity at plasma (n c = ω²ε 0 m/e² ~ e - /cm³) very hot electron distribution ~ kev, MeV steep gradients in n e und T - highly ionized - strong E- und B-fields POSSIBLE SOLUTIONS (laser-generated) electron or ion pulses heat matter indirectly and isochorically or: ultrashort XUV pulses from Free-Electron-Lasers (FELs) fs-pulses of high intensity penetration into the volume, no reflection (n c > e - /cm³) linear XUV photo-absorption of bound electrons defined warm photo-electrons and Auger electrons with E < 100 ev due to short wavelength very low ponderomotive force (F ~ Iλ²) at high intensity heating is isochorical and homogeneous 11

13 FLASH: the Free Electron Laser in Hamburg Fundamental Wavelength nm Average Pulse Energy < 100 µj 4th generation of synchrotron light sources Pulse Duration fs Repetition Rate 5 Hz Experimental Hall since 2005 W. Ackermann et al., Nature Photonics 1, 336 (2007) Source:DESY 12

14 XUV photo absorption Aluminum 93 ev conduction band Z*=3 Energy M [Ne] 3s²2p 1 L [He] 2s²2p 6 K 1s² 13+ Absorption coefficient above L-edge (72 ev): µ (L) / µ (M) ~ 10 13

15 Al plasma excitation processes t = fs XUV Photo-ionization t = fs L-hole recombination t ~ ps Lattice thermalization DOS Ε auger ~73eV DOS DOS ω fel =92eV E f E f E f 73eV 2p eV 2p 5 73eV 2p 6 118eV 2s 2 118eV 2s 2 118eV 2s 2 Free excited electrons have ~ 20 ev above E f Conduction band is slightly heated ~ 1 ev Auger & radiative decay fast Auger electrons & bremsstrahlung ( j ~ n e ²) e - -gas in equilibrium: thermal ionization, hydrodynamic expansion, ion-line emission 14

16 Experimental Setup at FLASH 2m FEL- Undulators Ellipsoidal Mirror Aluminum Sample Jena XUV Grating Spectrometer FLASH : 13.5 nm / 91.8 ev 33 µj average, 15 fs duration bulk aluminum 30-µm Focus I=10 14 W/cm² U. Zastrau, C. Fortmann, et al., Physical Review E 78 (2008),

17 Analysis of XUV emission spectrum Bremsstrahlung Kramers Law & Gaunt-factor: (includes quantum corrections) Z : Ion Charge, n e : fr. electron density Bremsstrahlung T e = (40±5) ev ratio of ion lines calculated via Boltzmann: Ratio of Al IV-Dublett T e = (34±6) ev U. Zastrau et al., Physical Review E 78 (2008),

18 Recombination Time and Conduction-Band Temperature E f 0.6 J/cm² 0.3 J/cm² 0.04 J/cm² 0.1eV Recombination time ~ fs Dominated by Auger time-scale. Should see fluorescence before lattice moves. Fluorescence proportional to ω 3 g(e)f(e,t) (better DFT model: Vinko, Zastrau, et al., PRL 2010 (in press)) Nagler, Zastrau, Fäustlin, Vinko, et al., Nature Physics 5, (2009) 17

19 Peak Intensities of W/cm² (50 nm) Is the absorption coefficient a function of intensity? 18

20 Transmission through a thin Al-Foil Colors represent different focal spot conditions A photon energy of E ph >93.5 ev is necessary to further ionize a 2p 5 electron every atom absorbs only one XUV photon saturable absorption significant larger absorption length The energy is in the electron sub-system, not yet in the lattice. This exotic state of matter ist still crystalline. Nagler, Zastrau, Fäustlin, Vinko, et al., Nature Physics 5, (2009) 19

21 Summary - LP Titan-Plasmas: radial Distribution of the Plasma Temperature with Δr = 13.5 µm - Toroidally bent crystal spectrometer single-puls spectra - 2D Abel-inversion - Homogeneously heated central region at k B T = 30 ev - up to 10x the laser focal diameter in size - spatially integrated spectra show a 3x lower plasma temperature - First-time creation and characterization of WDM at FLASH - Fundamental plasma parameters are accessible via XUV spectroscopy - heated conduction band of aluminum during the first 60 fs - fist demonstration of saturable absorption in the soft x-ray regime 20

22 Thanks to international Collaborations High Energy Density Physics Peak-Brightness Collaboration AG Röntgenoptik, IOQ, Universität Jena U. Zastrau, S. Höfer, T. Kämpfer, R. Loetzsch, I. Uschmann, O. Wehrhan, colleagues, workshop DESY, FLASH & XFEL Hamburg T. Tschentscher, S. Toleikis, R. Fäustlin, H. Chapman Universität Rostock G. Röpke, A. Sengebusch, C. Fortmann, R. Redmer Technische Universität Kaiserslautern B. Rethfeld, N. Medvedev Thanks to DFG Clarendon Laboratory, University of Oxford, U.K. G. Gregori, J. Wark, B. Nagler, S. Vinko NIF, LLNL, California, USA S.H. Glenzer, R.W. Lee Weizmann Institute of Science, Israel I. Maron, E. Kroupp, E. Stambulchik LULI, Ecole Polytechnique, Palaiseau, France P. Audebert, E. Brambrink 21

23 Thank you for your attention. 22