Advanced Ignition Experiments on OMEGA

Transcription

1 Advanced Ignition Experiments on OMEGA C. Stoeckl University of Rochester Laboratory for Laser Energetics 5th Annual Meeting of the American Physical Society Division of Plasma Physics Dallas, TX November 28

2 Summary A comprehensive scientific program to study advanced ignition concepts is being pursued at LLE Two advanced concepts beyond conventional hot-spot ignition are explored at LLE: Shock ignition 1 Fast ignition 2 Experiments with shock-ignition pulses show a 4# improvement in yield and 3% more areal density compared to conventional pulses. Fast-ignition-relevant experiments show favorable hydro performance of cone-in-shell targets and a ~2% conversion efficiency from shortpulse laser energy into electrons. OMEGA EP, a new high-energy, ultrashort-pulse laser system was completed in April 28 and is used for advanced ignition experiments. E R. Betti et al., Phys. Rev. Lett. 98, 1551 (27). 2 M. Tabak et al., Phys. Plasmas 1, 1626 (1994).

3 Collaborators K. S. Anderson, T. R. Boehly, R. Betti,* J. A. Delettrez, V. N. Goncharov, V. Yu. Glebov, F. J. Marshall, R. L. McCrory*, D. D. Meyerhofer,* J. F. Myatt, P. M. Nilson, T. C. Sangster, A. A. Solodov, M. Storm, W. Theobald, B. Yaakobi, and C. D. Zhou University of Rochester, Laboratory for Laser Energetics, Fusion Science Center *Depts. of Mechanical Engineering and Physics, University of Rochester J. A. Frenje and R. D. Petrasso Massachusetts Institute of Technology A. J. MacKinnon, P. Patel Lawrence Livermore National Laboratory P. A. Norreys Rutherford Appleton Laboratory R. B. Stephens General Atomics

4 Outline Motivation Shock ignition uses a shock generated by the compression driver later in the implosion to reduce the energy required for ignition Fast ignition separates the fuel assembly and heating using an ultrafast laser to heat the fuel in addition to a compression driver Shock-ignition experiments Fast-ignitor-relevant experiments OMEGA EP Laser System E17293

5 Fast ignition and shock ignition can trigger ignition in massive (slow) targets, leading to high gains* Gain G > 1 1-D maximum gain if ignition occurs ~ 1 V 1.3 i Potentially high gains, stable implosions G ~ 5 Conventional hot-spot ignition Accessible by fast and shock ignition V min ~ Hot-spot ignition fails V i (cm/s) V max ~ Quenching by hydro-instabilities TC7815f *R. L. McCrory et al., Phys. Plasmas 15, 5553 (28).

6 A shaped laser pulse with a high-intensity spike launches a strong shock wave for ignition* FSC Laser power Power spike Return shock Time Spike shock wave The ignitor shock wave significantly increases its strength as it propagates through the converging shell. E16323 *R. Betti et al., Phys. Rev. Lett. 98, 1551 (27).

7 A conventional hot-spot target requires ~3# more energy than a shock-ignition target Low-velocity hot-spot ignition (Gain ~1) Shock ignition (Gain ~1) CH(DT) 6 DT ice DT gas 167 nm 378 nm CH(DT) 6 DT ice DT gas 16 nm 24 nm 56 nm 796 nm Hydro-equivalent shock ignition E16312a Power (TW) Time (ns) E L = 1.2 MJ Power (TW) Time (ns) A high-velocity hot-spot design achieves a gain of 49 at 1 MJ.* E L = 35 kj *T. J. B. Collins, Phys. Plasmas 14, 5638 (27).

8 The two viable fast-ignition concepts share fundamental issues: hot-electron production and transport to the core Channeling Concept 1 Cone-Focused Concept 2 Hole boring Ignition Light pressure bores hole in coronal plasma 1 ps ~1-MeV electrons heat DT fuel to ~1 kev, ~3 g/cc Au cone e Single ignitor beam: 1 ps E1171d 1 M. Tabak et al., Phys. Plasmas 1, 1626 (1999). 2 P. A. Norreys, Phys. Plasmas 7, 3721 (2).

9 In integrated simulations an optimized target 1,2 ignites with a gain = 1 when heated with 43 kj of electrons FSC 3-kJ fuel assembly CH(DT) 6 DT ice DT gas CH 2 nm 146 nm 34 nm 56 nm Gaussian, relativistic-maxwellian e-beam r (nm) FWHM Duration x T e 3 nm 1 ps 2 MeV Divergence half-angle 2º Distance to the target x (nm) nm e-beam n e 3 nm r TC7792d 1 R. Betti and C. Zhou, Phys. Plasmas 12, 1172 (25). 2 A. A. Solodov (YI1.2).

10 The self-generated magnetic field collimates the electron beam 1,2 FSC Snapshots at t = 8 ps after the beginning of the e-beam r (nm) r (nm) z (nm) Plasma electron temperature (kev) Plasma density ( 1 26 cm 3 ) Hot-electron density ( 1 23 cm 3 ) 5 1 z (nm) 5 r (nm) r (nm) z (nm) Azimuthal magnetic field (MG) 5 1 z (nm) Total e-beam energy = 43 kj, angular divergence = TC7793e 1 L. Gremillet et al., Phys. Plasmas 9, 941 (22). 2 J. J. Honrubia and J. Meyer-ter-Vehn, Nucl. Fusion 46, L25 (26).

11 Outline Motivation Shock-ignition experiments CH shells have been used on OMEGA to test the performance of shock-ignition pulse shapes timing of the shocks is critical to optimize the performance of the implosion with optimized shock timing the neutron yields improve by up to a factor of 4, the areal density by up to 3% Fast-ignitor-relevant experiments OMEGA EP Laser System E17293a

12 CH shells have been imploded on OMEGA to test the performance of shock-ignition pulse shapes FSC E L = 19 kj, a = 1.3, V i = cm/s, SSD off CH 7 to 25 atm D 2 gas 4 nm 39 nm Power (TW) 2 Y n = 2± Y n = 8± Intensity (W/cm 2 ) ( 1 14 ) Time (ns) E16128b The neutron yield increases considerably when a shock is launched at the end of the pulse.

13 The correct timing of the shock waves is crucial for optimized implosion performance* FSC (1) Compression (2) wave Spike shock wave Return shock Picket shock wave Power (TW) 1 Picket delay Time (ns) 4 2 Intensity (W/cm 2 ) ( 1 14 ) Power (TW) Spike delay Time (ns) Intensity (W/cm 2 ) ( 1 14 ) E1613a *W. Theobald, Phys. Plasmas 15, 5636 (28).

14 The picket and spike timing has a significant effect on the measured neutron yield and areal density GtRH FSC Measured neutron yield ( 1 9 ) Picket delay 1 ps Picket delay 3 ps kj No spike Measured GtRH n (g/cm 2 ) With spike (optimized) Without spike 1 D calculation 2 25 Spike delay (ns) Simulated hot-spot convergence The shock-ignition implosions show an improved performance with respect to areal density and neutron yields. E17294

15 Outline Motivation Shock-ignition experiments Fast-ignitor-relevant experiments fuel-assembly experiments with cone in shell targets show good performance and no early filling of the cone with plasma a conversion efficiency from laser energy into hot electrons of ~2% was inferred using two independent experimental methods OMEGA EP Laser System E17293b

16 Cone-in-shell fuel-assembly experiments were performed on OMEGA* FSC 24 nm Gold CH Radiograph of target 43 nm Step r (nm) 1 1 DRACO Backlit image Density (g/cc) z (nm) The measured areal density was >6% of the 1-D prediction for a full sphere. TC7487b *C. Stoeckl, Phys. Plasmas 14, (27).

17 Shock breakout is close to peak compression in the low-adiabat experiments with a 15-nm-thick cone tip Streaked Optical Pyrometer (SOP)* Cone target Imaging optics Streak camera Capsule center Hyperbolic tip 4 nm 3 nm 15 nm Flat tip 1 nm Signal (arbitrary units) E17295 *J. A. Oertel, Rev. Sci. Inst. 7, 83 (1999) Laser 1 2 Time (ns) 3 4 SOP signal 2-D areal density 4982

18 A conversion efficiency of h L"e = 2% into energetic electrons can be inferred from K a yields FSC K a production is insensitive to fastelectron energy spectrum and range for I > 1 18 W/cm 2 Cu targets: nm 3 K a signal/ laser energy MTW 1 J, 1 ps VULCAN PW 5 J, 1 ps* h L"e = 3% h L"e = 2% h L"e = 1% Intensity (W/cm 2 ) E1615b *W. Theobald et al., Phys. Plasmas 13, 4312 (26).

19 Target bulk heating affects L"K and M"K electron transitions 1,2 FSC Inelastic electron electron collisions heat the target. Collisional ionization with thermal background plasma occurs. T e > 1 ev causes significant M-shell depletion. N M L K a (L8.5 kev) Ionization Depleted population K b (L8.91 kev) Target heating is inferred from K b /K a. K Copper energy levels E16143b 1 J. Myatt et al., Phys. Plasmas 14, 5631 (27). 2 G. Gregori et al., Contrib. Plasma Phys. 45, 284 (25).

20 A comparison of K b /K a to LSP calculations gives h L"e c 2% consistent with the K a -yield measurements* FSC 1.2 Provides a self-consistency check on h L"e Confirms that the dominant physics in the simple refluxing K a -production model are correctly accounted for. K b /K a h L"e = 3% h L"e = 1% Temperature (ev) Laser energy (J)/target volume (mm 3 ) Solid density targets are heated to temperatures greater than 2 ev with 5 J of laser energy E16314b *P. M. Nilson et al., Phys. Plasmas 15, 5638 (28).

21 Outline Motivation Shock-ignition experiments Fast-ignitor-relevant experiments OMEGA EP Laser System OMEGA EP can provide up to 2.6 kj in 1 ps and 2.6 kj in 1 ps into the OMEGA target chamber for integrated FI experiments Simulations of integrated FI experiments using OMEGA EP have shown core heating of up to 1 kev LLE has the infrastructure in place to field cryogenic cone-in-shell targets E17293c

22 Short-pulse OMEGA EP beams can be directed either to OMEGA or to the new OMEGA EP target chamber Short pulse combined Beam 1 Beam 2 IR energy (kj) Pulse duration at full energy (ps) 1 to 1 8 to 1 >8% in >8% in Focusing (diam) 2 nm 4 nm Intensity (W/cm 2 ) In October 28 OMEGA EP delivered 1.3 kj in 1 ps to a target. G6957u

23 Integrated FI experiments with cone-in-shell targets have started on OMEGA CD shell Driver energy Short pulse Pulse duration Focus ~87-nm diam ~18 kj ~1.3 kj ~1 ps ~4-nm diam No short pulse With short pulse E1748 The hard x-rays produced by the short-pulse interaction saturate the current neutron detectors.

24 With 2.6 kj of short-pulse energy the hot electrons are predicted to heat up the core by up to 1 kev* FSC r (nm) r (nm) 4 2 Snapshots at t = 6 ps after the beginning of the e-beam Plasma density (g/cm 3 ) z (nm) Plasma temperature increase (kev) z (nm) r (nm) r (nm) Electron-beam density ( 1 21 cm 3 ) z (nm) Azimuthal magnetic field (MG) z (nm) With hot electrons the neutron yield increases from to E16811b *A. A. Solodov (YI1.2).

25 LLE has the infrastructure to field cryogenic DT-filled fast-ignitor targets FSC Vibration isolating stand Fill-tube-based FI cone-in-shell target Target chamber Shroud retractor Cryogenic refrigerator Visible camera X-ray camera X-ray source + shield Vacuum chamber Moving Cryostat Transfer Cart Visible or near-ir illuminator The cryogenic layering test stand E17296

26 Summary/Conclusions A comprehensive scientific program to study advanced ignition concepts is being pursued at LLE Two advanced concepts beyond conventional hot-spot ignition are explored at LLE: Shock ignition 1 Fast ignition 2 Experiments with shock-ignition pulses show a 4# improvement in yield and 3% more areal density compared to conventional pulses. Fast-ignition-relevant experiments show favorable hydro performance of cone-in-shell targets and a ~2% conversion efficiency from shortpulse laser energy into electrons. OMEGA EP, a new high-energy, ultrashort-pulse laser system was completed in April 28 and is used for advanced ignition experiments. E R. Betti et al., Phys. Rev. Lett. 98, 1551 (27). 2 M. Tabak et al., Phys. Plasmas 1, 1626 (1994).