Transport using Second Harmonic. University of Alberta

Transcription

1 MeV Electron Generation and Transport using Second Harmonic Laser Pulses for Fast Ignition R. Fedosejevs University of Alberta Frontiers of Plasma Physics and Technology Frontiers of Plasma Physics and Technology Singapore, April 18-22, 2011

2 H Friesen, A Beaudry, J Tait, J Westwood, HF Tiedje, S Singh, MZ Mo, YY Tsui University of Alberta DP Higginson, A Sorokovikova, CC Jarrott, B Westover, FN Beg University it of California i San Diego A Link, GE Kemp, KU Akli, RR Freeman, LD Van Woerkom Ohio State University D Hey, Y Ping, C Chen, MH Key, HS McLean, P Patel, T Doeppner Lawrence Livermore National Laboratory RB Stephens General Atomics I Bush, J Pasley University of York

3 Laser Fusion Direct Drive Ignition Conditions 500 g cm -3 Indirect Drive T ~ 10 kev

4 Fast Ignition Ignition Requirements 300 g cm -3 dep ~ 20 ps D dep ~ 40 m E dep ~ 20 kj p E laser ~ 100 kj Requires electrons or ions to carry the energy from the laser absorption region to the compressed core laser ~ m l I laser ~ x W cm -2

5 Electron Energy Scaling Required electron energies ~ 1-3 MeV Scaling Laws: Ideal 1-3 MeV range Wilks Haines 1 m Wilks (Ponderomotive) PRL 69, 1383 (1992) Beg (Exp Bremsstrahlung) 05 m 0.5 Phys.PlasmasPlasmas 4, (1997) Beg Haines (Energy/Momentum) PRL 102, (2009)

6 2 Experiment Objectives Determine scaling at 2 for hot electron generation Measure T hot Electron generation efficiency Divergence Specular Beam reflection and chirp Geometries Flat Foils with Cu tracer layer Buried Cones with Cu tracer layer Cone wire Diagnostics HOPG CU K x-ray spectrometers t Electron spectrometers X-ray Bremsstrahlung versus angle Cu K imaging g crystals s KB x-ray microscope Specular Imaging and FROG

7 TITAN Laser Facility LLNL Experimental Diagnostic Layout Diagnostic Setup Electron Spectrometers at 15 o and 25 o off axis Von Hamos HOPG Bremsstrahlung Cannons over range of angles KB Microscope Locations Pellicle lens DC HOPG Kα Crystals F/3 OAP focussing on target Spectralon plate

8 2 Titan Run Parameters 50 J 700 fs 053 m x W cm -2 Planar Foil Targets Buried Cone targets Cone foil Targets Cone Wire Targets

9 Laser Diagnostic Setup Layout - 2 Use leakage light through final turning mirror to target chamber Prepulse energy less than 10 J Near Field Monitor and input spectrometer t Autocorrelator Input Frog Prepulse Monitor Laser 6 m Focal Length Lens Equivalent Focal Plane Monitor

10 Input FROG Signals Duration and Phase 710 fs FWHM Spectrum and Phase 710 fs pulse duration with slight chirp

11 Conversion Efficiency Peak Conversion efficiencies of over 60% obtained - 2mm KDP crystal

12 Typical Low Energy Focal Spot on Target Low Intensity Focal Spot at TCC Estimated Radially Symmetric Target Intensity Distribution for 50J 700 fs Equivalent FWHM Spot Diameter = 8 m

13 Targets used Planar with buried Cu tracer layer Solid Al cone with buried Cu tracer layer Thin Al cone Cu wire Shots taken with no prepulse ( <10 J) Shots taken with no prepulse ( <10 J) or with injected 3mJ 3ns 2 prepulse

14 Spectralon Reflectivity Mesurement Setup Parabola TCC Backscatter Imaging System Spectralon Imaging System On Door H Looking through Port H1 Spectralon 10 x 10 spectralon In front of Door E Holes aligned to allow FROG through port E2 F/1.5 beam collection

15 Specular Reflectivity Images Titan 2w run No prepulse 3mJ prepulse Speckle pattern seen from no prepulse p shots perhaps speckle from surface roughness Smooth pattern seen for shots with prepulse smoothing from preplasma

16 Reflectivity 1 vs Intensities either Good EPM or Extrapolated from Good EPM 3 mj, 3ns Prepulse < 10 J of Prepulse 6 Al 17 S Feb 2011 Al 28 S Aug 2007 Reflectiv vity (%) Have EPM ity (%) Reflectivi E E E E E E+019 Peak Intensity (Wcm -2 ) 2 Data 2 Reflectivty R npp ~ 27% npp R wpp ~ 14% 1J Prepulse Peak Power (TW) 1 Data 2 vs 1 with pp R 1 ~ 4% R 2 ~ 14%

17 FROG data shows prepulse effect No prepulse 3mJ prepulse Time Wavelength Large red shift at the beginning: due to pushing in of preplasma? Very reproducible

18 Electron Spectrometer Setup Especs Located Above Cannon 1 ESpecs Horizontal Angles 0 wrt Target Normal 16 wrt to Laser Axis Vertical Angles 15 out of the plane 25 o out of the plane plane, looking down to TCC Distance center of slit 80 cm from stalk at TCC TCC Height

19 Typical 2 Electron Spectra 15 o No Prepulse With Prepulse SR) dn / de / dw (Electrons / MeV / 1E9 1E8 1E Aug Shot 1 E = 28.2J = 0.7 ps I ~ Wcm -2 peak A m Al/20 mcu/ 22 m B-doped Al/ 1mm CH Laser axis 16 (H) 15.1 (V) Target Normal 0 (H) 15.1 (V) kt = 0.93 MeV Energy (MeV) 2 T hnpp ~ 1.5 MeV T hwpp ~ 1.9 MeV T hnpp ~ 0.8 T hwpp dw (Electrons / MeV / SR) dn / de / 1E10 1E9 kt =1.83 MeV Aug Shot 1 E = 41.9J = 0.7 ps 3.3 mj PREPULSE 1E8 I ~ Wcm -2 peak A m Al/20 mcu/ 22 mb-doped Al/1mm CH Laser axis 16 (H) 15.1 (V) Target Normal 0 (H) 15.1 (V) 1E Energy (MeV) dn / de / dw (Electron ns / MeV / SR) 1E9 1E8 1E7 kt = 1.55 MeV Aug Shot 4 E = 43.0J = 0.7 ps I peak ~ Wcm -2 A m Al/20 mcu/ 22 m B-doped Al/ 1mm CH Laser axis 16 (H) 15.1 (V) Target Normal 0 (H) 15.1 (V) Energy (MeV) / MeV / SR) dn / de / dw (Electrons / 1E10 1E9 1E8 1E7 kt =2.4 MeV Aug Shot 7 E = 37.2J = 0.7 ps 3.2 mj PREPULSE I peak ~ Wcm -2 A m Al/20 mcu/ 22 mb-doped Al/1mm CH Laser axis 16 (H) 15.1 (V) Target Normal 0 (H) 15.1 (V) Energy (MeV)

20 Typical 1 Electron Spectra 15 o Measured with the same spectrometer and similar targets dn / de / d (Electro ons / MeV /SR) 1E10 1E9 1E8 1E7 kt Jul Shot 1 E = J = 0.7 ps I peak ~ Wcm -2? PrePulse A m Al/21 m Cu/ 26 m Al/1mm CH Laser axis 16 (H) 15.1 (V) Target Normal 0 (H) 15.1 (V) 6.2 MeV Kinetic Energy (MeV) dn / de / d (Electr rons / MeV /SR R) 1E10 1E9 1E8 1E7 kt 6.64 MeV Jul Shot 2 E = J = 0.7 ps I peak ~ Wcm -2 (Estimated) 10 mj Prepulse A m Al/21 m Cu/26 m Al/1mm CH Laser axis 16 (H) 15.1 (V) Target Normal 0 (H) 15.1 (V) Kinetic Energy (MeV) T hot ~ 6 MeV at 1

21 Electron Spectrometer 25 o off axis T h at 25 o ~ 0.6 x T h at 15 o dn/de (a arb units) 10, , Slope Temperature 11M 1.1 MeV Electron Energy (MeV)

22 Electron Spectrometer Summary 1 vs Slabs 2 Slabs kt = mc2 * (sqrt(1+(3*a 2 0 ))-1) Cones 2 Cones kt = mc2 * (sqrt(1+(3*a 2 0 ))-1) kt (MeV) kt (MeV) I (Wcm -2 m 2 ) I (Wcm -2 m 2 ) T fits modified Pondermotive scaling law T h fits modified Pondermotive scaling law ( T h ~ 1.7 T PM )

23 Bremsstralung Cannon Data Filtered image plate stack with Pb collimator sensitive up to 500keV

24 Bremsstralung Cannon Data Fits vs Electron source Divergence

25 Bremsstralung Cannon Preliminary Summary Compare to 1 data for Ag target (Westover APS 2010): Electron Divergence ~ 60 o (HW) Conversion Efficiency ~ 32%-38% Compare to 1 data for Al target (Chen PoP 16, ): T hot ~13MeV 1.3 Conversion Efficiency ~ 20-40%

26 Cu K HOPG Data: 1ω vs 2ω -Planar 1E+11 1w Planar multilayers 2w Planar multilayers # photons per joule 1E+10 1E Copper Depth (micron)

27 Cu K HOPG Data: Buried Cones vs Planar (2ω) 1.00E+11 2w Planar multilayers 2w Buried cones photons per joule 1.00E+10 # 1.00E Copper Depth (micron)

28 Electron Imaging Divergence Measurements SLAB Targets Buried Cone Targets Cu 30 m tip diameter Cu Al Al Al Al CH 17 half angle CH Variable Cu fluor depths for divergence calculation Surrogate for a cone surrounded by a conducting plasma Thick 1mm CH layer to avoid electron refluxing

29 Electron Beam Divergence from K images Electron Source

30 Preliminary Electron Beam Divergence - Planar HWHM [μm] SHALLOW 2 with pp 54 o FW 2 no pp 36 o FW DEEP y = x R² = Full Angle y = x R² = Full Angle 2ω Slab without Prepulse 2ω Slab with Prepulse 1ω Slab with Prepulse with pp 33 o FW y = x R² = Full Angle Fluor Depth [μm]

31 Preliminary Electron Beam Divergence Buried Cones 200 HWHM [μm] SHALLOW DEEP y = 0.599x R² = Full Angle y = x R² = Full Angle 2 with pp 62 o FW y = x R² = Full Angle 2ω Buried Cone without Prepulse 2ω Buried Cone with Prepulse 1ω Buried Cone with Prepulse 1 with pp 50 o FW 40 2 no pp 50 o FW Fluor Depth [μm]

32 Initial LSP simulations with Laser Plasma Interaction Physics Comparison to Data TIME INTEGRATED CU Kα DISTRIBUTIONS NO PREPULSE CU FLUOR DEPTH 20.5 μm DEEP _s01_CuKaSouth

33 Initial LSP Code Comparison to data TIME INTEGRATED CU Kα DISTRIBUTIONS NO PREPULSE CU FLUOR DEPTH 111 μm DEEP _s06_CuKaSouth

34 Cone Wire Target KB Image with no Prepulse Viewed at approximately 45 o angle Laser

35 Summary - Electron Energy Scaling Experimental Results E-spectrometer 2 1 Link et al. (Titan 2010 ) Scaling Laws: 1 m Wilks (Ponderomotive) PRL 69, 1383 (1992) Ideal 1-3 MeV range 05 m 0.5 Beg (Exp Bremsstrahlung) Phys.Plasmas 4,447 (1997) Bremsstrahlung Chen et al. (PoP 16, (09)) Haines (Energy/Momentum) PRL 102, (2009)

36 Summary Successful implementation of 2 target experiments at 50J 700fs level at the TITAN facility Conversion efficiencies over 60% obtained Intrinsic prepulse less than 10 J Controlled injection of prepulse of 3ns 3mJ Preliminary i analysis of the experimental results carried out thus far Initial electron temperatures colder than 1 as expected from I 2 scaling T hot ~ MeV (Bremsstrahlung) T hot MeV (Bremsstrahlung) ~ MeV (escaping hot electrons)

37 Summary Initial divergence angles similar to 1 Difference seen between angularly l resolved Bremsstrahlung and K imager angles FW ~ 120 o 142 o Bremsstrahlung FW ~ 36 o 62 o K imaging Buried Cones show slightly larger divergence angles Absorption and electron yield lower than 1 as Absorption and electron yield lower than 1 as expected for lower I e- ~ % R ~ 27 14% Red shift and smoothing seen in specular reflection with prepulse indicating preplasma effects Extensive modeling under way to better understand the results

38 End