Laser Inertial Fusion Energy

Transcription

1 Laser Inertial Fusion Energy presentation before the HEDLP committee at the WORKSHOP ON SCIENTIFIC OPPORTUNITIES IN HIGH ENERGY DENSITY PLASMA PHYSICS 25 August 2006 Washington DC presented by A. J. Schmitt, Naval Research Lab with contributions from: Jason Bates, Riccardo Betti, Denis Colombant, Valeri Goncharov, Max Karasik, Pat McKenty, Masa Murakami, Steve Obenschain, John Sethian, Sasha Velikovich, Jim Weaver, and many others work supported by US DoE/NNSA

2 Laser Inertial Fusion Energy: Overview Direct drive laser fusion is the simplest way to fusion energy ablator DT ice (fuel) Hot fuel Cold fuel burn lasers heat outer pellet shell, imploding fuel to v ~ 300 km/s Only central portion of DT fuel heats to ignition The basic idea is simple: simple spherical targets apply laser directly inertia provides confinement known physics high gains possible Thermonuclear burn propagates outward from spark into compressed DT fuel

3 Critical Issues and Progress Symmetry: Illumination by multiple beams around pellet, many configurations exist Hydro instability: ablative Richtmyer-Meshkov and Rayleigh-Taylor instabilities Coupling: Inverse bremsstrahlung absorption LPI: 2ω pe, SBS, SRS, PDI Preheat: nonlocal thermal conduction Significant advances since inception: shorter laser wavelengths: more absorption, less laser-plasma instability (LPI) optical smoothing (ISI/SSD): controls uniformity and shape of spot, reduces LPI target design advances: adiabat shaping techniques shock ignition

4 Advances: short wavelength lasers 10µm 1 µm 0.5 µm 0.35 µm 0.25 µm CO 2 Nd:glass KrF Short wavelength advantages: mitigate LPI: ponderomotive driving force is lower: F pond ~ v osc 2 ~ Iλ 2 (this allows higher intensity and higher pressure drive) enhances collisional absorption: absorption rate κ ib ~ n 2 T e -3/2 ω -2 : n ~ n crit ~ λ -2 ; Te ~ (Iλ 2 ) 2/3, ω 2 ~ λ -2 so κ ib ~ I -1 λ -4 higher mass ablation rates: m ~ I 1/3 λ -4/3, and higher pressures P~(I/λ) 2/3 give higher hydrodynamic efficiency Short wavelength disadvantages: smaller absorption-ablation distance less thermal smoothing (minor or no issue if optical smoothing is allowed) optics become more challenging

5 Optical smoothing and zooming 10% the laser nonuniformity (rms) decreases with time Provides spatial and temporal incoherence spatial incoherence limits LPI spatial extent temporal incoherence limits LPI growth times ( ν -1 ~ 1THz ~ 1 psec ~ λ 0 /c s [ion-acoustic scale]) together: synergism! supresses filamentation Beam envelope and characteristics are controllable over many averaging times Spot profile is given by phase mask (SSD/RPP) or by front-end aperture (ISI) laser intensity distribution is well defined. imperfect coherence spatial incoherence 1% 0.1% 1 beam 40 beams averaging time (ps) spatial & temporal incoherence (Ordinary, < ) (Random Phase Plate) (Induced Spatial Incoherence)

6 Illumination Symmetry: Controlled beams that give cos 2 θ absorption on a sphere can be arranged to produce perfect uniformity If each beam deposits an energy distribution on the pellet of the form cos 2 θ, then the pellet can be perfectly illuminated by systems with beams incident from angles {φ jk,ϑ k } Aiming angles must satisfy: K rings of ϑ beams; ring k has J φ k beams { -1/3} = 0 Examples: cube, octahedron, dodecahedron, truncated icosahedron (Omega), +... Control of spot profile is accomplished via optical smoothing Alignment of beams, energy and power balance (timing): engineering? This addresses only long-wavelength (low mode) asymmetry. Beam-beam interference and spatial incoherence from optical smoothing produce small scale (order λ) energy deposition nonuniformity. - Much of this will be too small in size to affect stability (ablative stabilization) but remaining portion (laser imprint) needs to be considered. - LPI can/will be affected by small scale nonuniformity

7 Zooming -- shrinking the focal spot during the implosion -- improves the laser-target coupling and symmetry Zooming: reduces refractive losses decreases drive energy required can increase drive symmetry For image-relayed lasers (e.g., KrF), zooming is done by switching between differently sized apertures during the pulse. All implementation is at the laser s lowpower front end. simplified KrF laser front end radius (cm) % r density flow diagram 74% r time (nsec) 54% r 0 Osc. Amps Glass: zoom by switching to different beams during implosion (Canaud et al., Nucl.Fus (2007)) Yield Laser Energy Gain zooming can decrease laser energy needed Example: With Zoom Without Zoom 165 MJ 151 MJ 1.3 MJ 2.1 MJ

8 Hydrodynamic stability is important issue in direct drive DT vapor DT fuel Density Flow Diagram compression phase acceleration phase 0.15 radius 0.10 (cm) During compression, there is Richtmyer-Meshkov (RM) growth. Growth ( < ~ 10X) is geometric (not exponential), but seeds RT time (nsecs) 1000 Power (TW) compression phase laser pulse acceleration phase Rayleigh-Taylor (RT) exponential growth begins with acceleration of shell. Growth can be ~ or more time (nsecs)

9 Adiabat Shaping: additional suppression of RM & RT Stability is optimized when the ablation density is low. The exponential growth rate is: growth term: g is the target acceleration Ablative stabilization term: Conversely, the gain is optimized when fuel density is high: burnup fraction ~ ρ fuel R / (ρ fuel R+7) Adiabat shaping tries to have it both ways: Lowered density in the ablator (where perturbations grow) to maximize stabilization ( high adiabat ) Higher density in the fuel) to maximize gain ( low adiabat ) Recent work has focused on ways of producing shaped adiabats: RT mitigation: Decaying shock pulses: Goncharov et al., Phys. Plasmas 10, 1906 (2003) Relaxation pulses: Anderson and Betti, Phys. Plasmas 10, 4448 (2003) Foam ablators: Bodner et al., Phys. Plasmas 5, 1901 (1998). RM mitigation: Spike pulses: Metzler et al., Phys. Plasmas 6, 3283 (1999) Radiative layers: Obenschain et al., Phys, Plasmas 9, 2234 (2002).

10 Laser Plasma Instabilities: the scorecard Filamentation: laser hot spot forms its own lens, focuses and becomes more intense worry: increase all other LPIs. mitgation: optical smoothing prevents this SBS (stimulated brillouin scatter): light decays into light and ion-acoustic wave (IAW) worry: reduced absorption, nonuniform absorption mitgation: optical smoothing and shorter plasma scale-lengths (than in hohlraums) 2ω pe (two plasmon decay): near n crit /4 density, light decays into two plasmons worry: resulting plasmons accelerate electrons to high velocity, hot electrons can preheat fuel and reduce compression/gain mitgation: optical smoothing? SRS (stimulated raman scatter): light decays into light plus plasmon worry: reflects light (reduce/nonuniform absorption), plasmons accelerate electrons and cause fuel preheat mitgation: short plasma scale-lengths, optical smoothing? PDI (parametric decay instability), resonance absorption: near critical density, light decays into plasmons and IAWs worry: plasmons accelerate electrons, may cause preheat mitgation: keep light away from critical surface (high absorption) Based on current experimental and theoretical results (predicted thresholds), we expect 2ω pe to have the greatest impact on direct drive. It may ultimately limit the intensity, and thus pressure, that can be used to drive the targets.

11 Direct drive approaches: 1. Conventional: (Nuckolls, Kidder, 1970's) Laser compresses and accelerates target/shell. Ignition occurs as low-density void and blown-off plasma stagnates and is compresses at center. Ignition is controlled by velocity of incoming shell. uncertainties: how high can drive pressure go? (more stable). How much preheat (LPI) can be tolerated? why is yield in low-adiabat experiments difficult to model? cm 5 µm 256 µm 334 µm cm possible Pd layer ~1000 Å CH(DT) 32 CH; 1.07 g/cm 3 ρ DT ice CH =70 mg/cm g/cm 3 DT vapor 0.3 mg/cm 3 High Gain KrF target E laser 2.6 MJ 2D Gain Shock ignition: (R. Betti, 2005) Laser compresses target to subignition core surrounded by high-density fuel shell. Final spike on laser pulse produces converging shock that heats and compresses the core causing ignition. uncertainties: how high can drive pressure go? How high can spike pressure be -- allowing for possible LPI? Will LPI in ignition spike help? (target is better shielded --high ρr -- at late times) 3. Fast ignition: (M. Tabak, 1994) (see Mike Key s earlier talk) 4. Impact ignition: (M. Murakami, 2005) Like fast ignition, but uses incoming slug of material to ignite. uncertainties: can cone be used to confine incoming slug? Can we accelerate dense material to ~1000 km/sec? How small can ignition point be made? 854 µm 105 µm 237 µm 512 µm CH[100mg/cc foam] + DT 0.33 g/cm 3 DT ice 0.25 g/cm 3 DT vapor 0.2 mg/cm kj KrF Shock-Ignition target 2D Gain Conceptual Impact Ignition target Murakami et al., Nucl Fuson 46, 99 (2006)

12 High gain target design uses KrF laser with zooming and "spike" prepulse and gets gain >150 in 2D cm 5 µm 256 µm 334 µm cm possible Pd layer ~1000 Å CH(DT) 32 DT ice DT vapor CH; 1.07 g/cm 3 ρ CH =70 mg/cm g/cm mg/cm 3 High Gain KrF target E laser 2.6 MJ Gain Power (TW) 10 1 spike Laser Pulse Shape zooms time (nsec) High Gain KrF pellet with stabilizing spike : 0.125µm rms outside, 1µm rms inside surface & 1 THz optical smoothing: gives Gain >150 (simulation: 408 x 2048 grid pts, resolves l=2-256) 0.3 density flow diagram radius (cm) time (nsec) images of density at six different times during the implosion Schmitt et al., Phys Plasmas 11, 2716 (2004).

13 Shock Ignition: Like fast ignition, compression and ignition are independently controlled Advantages: hydrodynamics are dominant; LPI may help (during ignition spike) but is not needed Shock ignition target * is thicker (aspect ratio ~2) and driven to low velocity (2.5x10 7 cm/s) before ignition spike. Laser energy ~300kJ, (160kJ w/o spike) 854 µm 105 µm 237 µm 512 µm DT vapor 0.2 mg/cm 3 CH[100mg/cc foam] + DT 0.33 g/cm 3 DT ice 0.25 g/cm Power 10 1 typical laser pulse *R.Betti, et al., PRL 98, ) 250 kj Shock-Ignition target time (nsec) 1.2 Total Surface1.0 Perturbation (µm) 0.8 2D simulations with surface perturbations show gain at 250 kj Gain, 250 kj target SPIKE TIME (NS) Schmitt et al., Anomalous Absorption (2008).

14 Laser IFE gain curve circa Target Gain 150 NRL, 1/4 µm v max ~3x10 7 cm/s 100 NRL, 1/4 µm (foam ablator) NRL, 1/3 µm 50 0 UR, 1/3 µm LLNL, NIF hohlraum Laser Energy (MJ) Bodner et al., IAEA Madrid meeting, June 2000.

15 Laser IFE gain curve extension to lower ignition energy (Fusion Test Facility) 200 Target Gain 150 NRL, 1/4 µm v max ~3x10 7 cm/s 100 NRL, 1/4 µm (foam ablator) 50 0 NRL 2006 v max =4-5x10 7, higher intensity NRL, 1/3 µm UR, 1/3 µm LLNL, NIF hohlraum Laser Energy (MJ) Bodner et al., IAEA Madrid meeting, June Colombant et al., Phys. Plasmas (2007).

16 Laser IFE gain curve, fast ignition comparison 200 Target Gain 150 Fast Ignition (Betti, 2006) NRL, 1/4 µm v max ~3x10 7 cm/s 100 NRL, 1/4 µm (foam ablator) 50 0 NRL 2006 v max =4-5x10 7, higher intensity NRL, 1/3 µm UR, 1/3 µm LLNL, NIF hohlraum Laser Energy (MJ) Bodner et al., IAEA Madrid meeting, June Colombant et al., Phys. Plasmas (2007). Betti et al., Phys. Plasmas 13, (2006).

17 Laser IFE gain curve shock ignition designs 200 Shock Ignition (NRL, Betti, 2008) Target Gain 150 Fast Ignition (Betti, 2006) NRL, 1/4 µm v max ~3x10 7 cm/s 100 NRL, 1/4 µm (foam ablator) 50 0 NRL 2006 v max =4-5x10 7, higher intensity NRL, 1/3 µm UR, 1/3 µm LLNL, NIF hohlraum Laser Energy (MJ) Bodner et al., IAEA Madrid meeting, June Colombant et al., Phys. Plasmas (2007). Betti et al., Phys. Plasmas 13, (2006). Schmitt et al., Anomalous Absorption Conference (2008).

18 Current status of laser IFE Theory: Targets have been designed in 1D and 2D consistent with high gain. Hydrodynamics theory is on good footing Better models are needed for: LPI - especially 2ω pe decay and hot electron generation Non-local electron thermal transport Experimental: Current lasers are too small to create ignition conditions Currently, many low-adiabat cryogenic implosions on Omega are not explained by simulation and theory The NIF and LMJ lasers may create ignition targets, and possibly ignition conditions for direct-drive. (LMJ ports allow symmetric illumination) The HAPL program is developing many of the other sciences and technologies needed for Laser Inertial Fusion Energy

19 The HAPL Program is developing two types of lasers Diode Pumped Solid State Lasers (DPPSL) --- "Mercury" at LLNL E-beam Pumped Krypton Fluoride Laser (KrF) ---- "Electra" at NRL 55 Joules up to 10 Hz 100 k shots continuous, 350 k total = 1053 nm (527 nm demonstrated) Experiments demo overall efficiency 6.5% (@ 1053 nm). Goal is 10% Joules 2.5 to 5 Hz 35 k shots continuous, 300 k total = 248 nm (shorter helps target) Experiments predict overall efficiency > 7%

20 The HAPL Program: Advances in other areas needed for IFE Targets: Mass produced foam shells that meet specs Target Engagement: Bench demonstration that mirrors can steer laser on to surrogate injected target with accuracy 50 µm Final Optics: Grazing Incidence Metal Mirror (GIMM) has withstood laser fluence of 4 J/cm 2 for > 6 M shots Chamber Concepts under development "Magnetic Intervention" "Nano-structured armor" first wall

21 Future directions in HEDLP: Areas where progress is needed Nonlocal thermal transport: Need kinetic-based description that limits the thermal flux and describes the preheat tail/distribution. LPI: LPI theory and modelling needs much improvement to be predictive LPI limits the usable laser power, which limits design flexibility - account for real laser field spatial and temporal incoherence of the light light propagation with turning/scattering in the underdense plasma - account for importance of kinetic effects: electron distribution modification and evolution. Hydrodynamics: stabilization methods are always welcome. Basic physics is well known, but modeling the interplay between growth, convergence and dynamics (shocks) can be improved NonLTE radiation modeling radiation (thin radiation layers?) may be used for hydrodynamic stabilization EOS of compressed material well known? Are magnetic fields important? Experimental: Improved diagnostics desired (e.g., in-flight density)

22 Laser plasma instability theory and modeling needs to be predictive 1/3 µm laser µ 1/4 µm laser ω Intensity (10 14 W/cm 2 ) 5 kev 3 kev 1 kev Nike ω pe thresholds 3ω 0 /2, Hard x-rays ω 0 /2 SRS Threshold Nike 2006 FTF (KrF 0.5 MJ) Stoeckl et al, LLE Rev. 94, 76 (2003) Weaver et al, 2008 LPI theory can provide uniform plane-wave thresholds, which is suspect when optical smoothing is used. LPI modeling must include kinetics. Available Vlasov or PIC simulations lack the time and space scales needed to describe the experiments. Currently, the only way to determine if LPI is a problem in direct-drive scenario is to actually do the experiment.

23 Nonlocal thermal transport of electrons modeling needs improvement Models: Diffusion w/spitzer conductivity Diffusion w/spitzer & flux limit Kernel methods Kinetic simulations (Fokker-Planck) UR 1.5 MJ all-dt design NRL 0.5MJ FTF (KrF) designs { T(r) Spitzer Spitzer+Flux-limit r Kernel-method, Fokker-Planck The preheat tail is missing in most simulations, (Kernel and kinetic methods slow calculations) Simulations that include models allowing preheat often don t match experiment Nonlocal transport is particularly important for: - small targets - low adiabat targets - higher laser intensities Relative Gain TC Fractional energy in DT-ice layer (%) Fractional energy into fast electrons (%) The effect of varying the fraction of energy dumped into an 80-keV hotelectron tail on the target gain. McKenty et al., LLE Review, Volume 79 (1999)

24 A thin high-z layer can be used to reduce initial perturbation growth Thin high Z layer Flat CH: strong imprint growth Flat CH + 450Å Au: imprint is suppressed DT ice X-rays from high-z layer enhance energy transport into the target, and produce a large plasma that can buffer and reduce the small scale laser nonuniformity low opacity ablator (DT-loaded CH foam) Time (ns) Space ( m) Space ( m) streak camera results of face-on x-ray backlight targets shot on Nike KrF laser M. Karasik et al., APS/Div. Plasma Phys. Conf S. Obenschain et al., Physics of Plasmas 9, 2234 (2002).

25 Issue: why is simulation neutron yield larger than experiment? 2D DRACO simulations of low-adiabat Omega D 2 -cryo experiments are best for lowmiddle laser Intensities, and where experimental <ρr> values are similar to 1D simulations. - When intensities are higher, nonlocal thermal transport, LPI proves harder to model (preheat, shock timing are important.). I ~ 2-6x10 14 W/cm 2 <ρr> exp ~ g/cm 2, where <ρr> exp /<rr> 1D-sim > 60% S.X. Hu et al., to be published courtesy V. Goncharov

26 Summary Laser IFE has made significant progress since early days, but some areas still need more development and progress LPI: Theory and modeling needs much improvement to be predictive. Experiments in relevant regimes need to be performed. Nonlocal thermal transport: Affects laser absorption and shock timing, as well as preheat due to LPI-generated hot electrons Hydrodynamics: you can t have too much stabilization (allowing for high gain) NonLTE radiation modelling: needed for predicting radiating layers, high-z dopants EOS, Magnetic fields: important or not? simulations don t include B fields. Experimental diagnostics