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NRL Target Physics Experiments

NRL Target Physics Experiments . J. Weaver a , M. Karasik a , V. Serlin a , J. Oh b , Y. Aglitskiy c ,S. Obenschain a , J. Sethian a , L-Y. Chan a , D. Kehne a , A. N. Mostovych d , J. Seely e , U. Feldman f ,

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NRL Target Physics Experiments

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  1. NRL Target Physics Experiments J. Weavera, M. Karasika, V. Serlina,J. Ohb, Y. Aglitskiyc ,S. Obenschaina, J. Sethiana,L-Y. Chana,D. Kehnea ,A. N. Mostovychd ,J. Seelye, U. Feldmanf, C. Browne, G. Hollandg, A. Fieldingh, C. Mankab,B. Afeyani, R. H. Lehmberga, J. Batesa, A. J. Schmitta, L. Phillipsj, A. Velikovicha, N. Metzlerf • Plasma Physics Division, Naval Research Laboratory, b. Research Support Instruments, • c. Science Applications International Corporation, d. Enterprise Sciences Inc., • e. Space Sciences Division, Naval Research Laboratory, f. ARTEP Inc., g. SFA Inc., • h. Commonwealth Technologies Inc., i. Polymath Research,j. Lab. for Comp. Physics & Fluid Dynamics, Naval Research Laboratory Presented at 15th High Average Power Lasers Workshop, San Diego, CA August 8, 2006

  2. Goal: Reduce the total laser energy required to achieve significant gain for direct-drive ICF implosions NRL Laser Fusion ablator Hot fuel Cold fuel Burn DT ice (fuel) D Pellet shell imploded by laser ablation to v  300 km/sec for >MJ designs • Reduce pellet mass while increasing implosion velocity (v 400 km/sec) • Increase peak drive irradiance and concomitant ablation pressure (~2x) • Use advanced pellet designs that are resistant to hydro-instability • Use the KrF laser’s deep UV light and large 

  3. NRL target physics experiments provide data relevant to pellet designs NRL Laser Fusion • Hydrodynamic Instabilities • Core experiments explore strategies for mitigation of pertubation growth: • High-Z coatings • Spike prepulses • Exploratory hydrodynamics experiments: • Richtmyer-Meshkov instability in colliding foils • Convergence effects in hemispherical targets • Laser Plasma Instabilities • Establish high intensity laser pulse operation in the range of actual implosions • Study instability thresholds with enhanced diagnostic capabilities • Study hot electron generation and possible threat to target conditions

  4. Intensity 3 Single-beam measurements Single-beam ISI theory 1 RMS Nonuniformity (%) x y 40 overlapping Nike beams 0.3 5×104 103 104 (Averaging time)/(Coherence time) Nike laser optimized for laser-driven hydrodynamics NRL Laser Fusion Overlapping Nike beams produce the smoothest laser irradiation in ICF, <0.3% variation in a 2-3 kJ, 4 ns long pulse at 248 nm Induced Spatial Incoherence beam smoothing technique: time-averaged focal distribution with residual speckle non-uniformities of 1% rms in a single beam and <0.3% in a 37 beam overlap at Dn = 1 THz. Nike operates at bandwidths up to 3 THz.

  5. BACKLIGHTER LASER BEAMS 2D IMAGE RIPPLED TARGET Sample RT Data Time X-ray radiography is major tool to study hydrodynamic evolution of laser-accelerated planar targets NRL Laser Fusion 1.86 keV imaging QUARTZ CRYSTAL MAIN LASER BEAMS BACKLIGHTER TARGET Si 0 to 100 km/sec in <4 ns STREAK CAMERA Y. Aglitskiy, et al. , Phys. Rev. Letters, 86, 265001 (2001)

  6. High Intensity Acceleration phase Low Intensity compression phase Time Plastic 0.4 mm Plastic + Au layer Laser Laser imprint is effectively smoothed by early time “indirect-drive” NRL Laser Fusion Thin high-Z layer DT-loaded CH foam Au layer X-rays High-Z layers may also help mitigate RM and RT due to increased mass ablation rates & softer ablation profiles Side views of X-ray emission

  7. Laser pulse Time (ns) Space (µm) Space (µm) Laser imprint suppression with high-Z layers is working at higher foot intensities (8 TW/cm2 - within a factor of 2 of the pellet designs) NRL Laser Fusion Flat CH: strong imprint growth Flat CH + 450Å Au: imprint issuppressed We need to verify that fuel preheat remains small.

  8. Shock front no spike a spike r g Target with pre-formed density gradient Laser beam spike main Ablation front r main spike foot Spike prepulse can help mitigate perturbation growth NRL Laser Fusion Strong reduction of growth rates due to increased ablation velocity, particularly for high modes. Goncharov et al. PoP 10, 1906 (2003). Relaxation spike used for present Nike experiments Decaying shock (DS) Strong spike, target adiabat is shaped by the decaying shock from the spike Relaxation (RX) Weak spike shapes a graded density profile, target adiabat is shaped by the decelerating shock from the foot J. P. Knauer et al., PoP 12, 056306 (2005). Theory: K. Anderson and R. Betti, PoP 10, 4448 (2003); R. Betti et al., PoP 12, 042703 (2005). N. Metzler et al., PoP 6, 3283 (1999).

  9. 0.5 12 2 I = 8.3 × 10 W/cm spike 12 2 I = 5.1 × 10 W/cm spike 12 2 I = 3.5 × 10 W/cm spike 0.5 0 y (mm) Velocity (km/s) - - 2 0 2 4 Time ( nsec ) Time (ns) Well characterized spike prepulse capability installed on Nike Spike pulse in Nike front end Pulse shape after final amplifier NRL Laser Fusion Normalized Signal Signal (arb. units) Time (ns) Time (ns) VISAR Streak Image Theory matches Observation Jaechul Oh, Andrew Mostovych, et al.

  10. Low-amplitude spike prepulse suppresses ablative RM growth triggered by target surface roughness NRL Laser Fusion Early Late

  11. Plastic 30 μm p-to-v 5 μm Double–foil experiment, first results NRL Laser Fusion • New capability: orthogonal simultaneous imaging • Promising technique to study perturbation growth in • decelerating systems • Applications to studies related to impact ignition 70 μm 30 μm Plastic

  12. t1 t2 t3 t4 Convergent geometry, planned experiment NRL Laser Fusion Target thickness 2.47 mg/cc - max shim thickness 1.81 mg/cc

  13. Space (µm) Side-on streak images show variation depending on laser spot size NRL Laser Fusion Hemispherical targets made by GA mounted at ILE Shell specs: • Inner diameter ≈ 940 µm • Thickness ≈ 20 µm • Composition: CH1.3O5 CH shell Be plate Spot size ~ hemisphere radius Spot size < hemisphere radius Time (ns) Laser Space (µm) 500 µm spot (with KPP) 300 µm spot (no KPP)

  14. Laser Plasma Instabilities NRL Laser Fusion Plasma Mode Or EM Wave Laser plasma instabilities: Three wave parametric processes in which laser light couples to natural modes in the coronal plasma thereby generating new radiation and altering target conditions Laser Long history of research, still many unanswered questions – KrF lasers relatively unexplored territory Plasma Mode • Two primary plasma modes: • Electron plasma waves – Stimulated Raman scattering, Two-plasmon decay • Ion acoustic waves – Stimulated Brillouin scattering, filamentation Primarily interested in generation of hot electrons that could lead to target preheat but will look for all evidence of LPI in initial stages

  15. Thresholds for the 3 wave parametric instabilities in inhomogeneous plasmas for 0.248 m light NRL Laser Fusion EMW --> EPW + EPW EMW --> EMW + IAW EMW --> EMW + EPW

  16. LPI threat to sub-MJ targets: 2p could be problematicSRS & SBS do not appear dangerous NRL Laser Fusion Estimates of LPI risk near peak intensities for FTF implosions show 2wp is most highly over threshold There is a lack of experimental data for LPI physics for ~0.25 mm lasers with broad bandwidth, and ISI smoothing

  17. 135o Initial geometry for LPI experiments NRL Laser Fusion 2 Redirected Main Beams Nike Target Facility 10-12 Backlighter Beams Target Vacuum Vessel X-ray Pinhole Camera F/20 Lens array 44 Main Beams Target Crystal Imager F/40 Lens array Main Target • Use of backlighter array allows • smaller focal distribution X-ray Streak Camera • Main beams with independently controlled spot size, • energy, and pulse shape can be introduced into backlighter beam path • Can vary plasma conditions with main beams and vary LPI interaction by controlling • backlighter beams • Focal spots data at full power used face-on imaging with streak and pinhole camera

  18. Amplification of short pulse through final amplifiers increases intensity NRL Laser Fusion Standard Backlighter Pulse Spike-only Backlighter Pulse Energy: 36 J Energy: 18 J Diode Signal (V) Diode Signal (V) 0.4 ns 5 ns Time (ns) Time (ns) Spike-only pulse through time-multiplexed KrF amplifier generates higher intensity pulses Power increase 4-5x Pulse length decreased by factor 10-12, energy only down by ½ Studies of spike propagation incomplete, but above result appears robust over many shots

  19. Beam 32 Beam 1 Beam 4 100 mm Low-energy, time integrated focal distributions NRL Laser Fusion • Spot size at target chamber center measured with thin UV fluorescent glass, microscope, • and CCD; only oscillator and first stage of amplification used (low energy laser pulses) • Spot size controlled by selection of initial apertures for ISI beam optics • Measurements show FWHM of 70 – 110 mm • Relative shot to shot overlap error is estimated to be less than spot diameter (s<50 mm)

  20. Position (mm) Time (ns) Time-resolved, single beam focal distributions at high intensity NRL Laser Fusion Single beam spot on Si target Counts 115 mm Position (mm) 375 ps Counts Spot diameter ~ 115 mm, pulse width ~ 375 ps Working on time-resolved multibeam overlap image for small spot, spike pulse Time (ns)

  21. Estimated range of focal intensities for LPI experiments NRL Laser Fusion Spot Size (mm) Total Energy (J) Intensity (1014 W/cm2) 120 68 75 160 91 113 200 38 120 160 100 Range most consistent with current observations 51 200 63 24 120 125 160 33 200 41 17 120 150 160 23 200 28 Assumes 400 ps pulse duration

  22. LPI diagnostics are being fielded at Nike laser for next stage NRL Laser Fusion Detector plane of 165 nm spectrometer 165 nm Tandem Wadsworth Spectrometer Diode array Dual grating mount Telescope Time resolution ~ 300 ps Spectral resolution ~2.5 Ang/mm Spectrometer developed in collaboration with Space Science Division at NRL Bandpass hard x-ray photodetectors Visible time-resolved spectrometers X-ray pinhole cameras X-ray spectrometers Absolute calibrations for 165 nm spectrometer have been performed at Brookhaven National Laboratory

  23. LPI experimental program is still in preliminary stages NRL Laser Fusion • Preliminary experiments will determine instability thresholds as a functions of • Total intensity (energy per beam, spot size) • Pulse shapes • Target type (CH, BN, Si, Au, foam – either CH or Si aerogel, cryo D2) • Geometry (target tilt, angle of beam overlap, instrumental line of sight) • Laser bandwidth • First physics experiments will focus on hot electron generation and target heating • Hard x-ray monitors (1-100 keV) will serve as first diagnostics • X-ray spectrometers • Specialized target designs • Second stage physics experiments will take more detailed exploration of LPI physics • to enhance predictive capabilities • Saturation mechanisms • Hot spot effects (size of hot spot, beam overlap, bandwidth) • Advanced diagnostics – Thomson scattering

  24. Summary: Near term goals for NRL target physics experiments NRL Laser Fusion Target physics program will evaluate hydrodynamic instabilities Relevant to pellet designs and restrictions on laser intensity due to laser-plasma instabilities • Essential data to support physics for pellet designs: • Continued examination of high-Z layers and spike prepulses as • mitigation techniques for early time perturbations • Develop techniques with double foils for RM physics and target diagnostics • Study convergence effects in hemispherical targets • Characterization of relevant thresholds for parametric instabilities • Generation of hot electron and target heating by hot electrons

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