Progress in Direct-Drive Inertial Confinement Fusion Research

Transcription

1 Progress in Direct-Drive Inertial Confinement Fusion Research Ignition and Gain Total GtRH n (g/cm 2 ) IAEA 21 DT, 22 kj IAEA 28 DT, 16 kj NIF.5 MJ NIF point design 1.5 MJ 1-D marginal ignition OMEGA DT equivalent of the NIF point design Hydro equivalent curve V i = const., a = const. (4 1 7 cm/s) (2.5) David D. Meyerhofer Deputy Director Laboratory for Laser Energetics University of Rochester GT i H n (kev) (no alpha deposition) 23rd IAEA Fusion Energy Conference Daejeon, South Korea October 21

2 Summary The OMEGA/OMEGA EP Laser Facility is being used to investigate several approaches to inertial confinement fusion High compression (tr ~ 3 mg/cm 2 ) has been demonstrated in the symmetric hot-spot approach to ignition 1 Two advanced (two-step) concepts are being explored at LLE shock ignition 2 fast ignition 3 Cone-in-shell fast-ignition experiments have quadrupled the neutron yield with a 1-kJ, 1-ps laser pulse Experiments with shock ignition show a 4 improvement in neutron yield The Omega Laser Facility is ideal for studying inertial fusion energy concepts. TC V. N. Goncharov et al., Phys. Rev. Lett. 14, 1651 (21). 2 R. Betti et al., Phys. Rev. Lett. 98, 1551 (27). 3 M. Tabak et al., Phys. Plasmas 1, 1626 (1994).

3 Collaborators V. N. Goncharov, R. Betti, T. R. Boehly, T. J. B. Collins, R. S. Craxton, J. A. Delettrez, D. H. Edgell, R. Epstein, V. Yu. Glebov, D. R. Harding, S. X. Hu, I. V. Igumenschev, J. P. Knauer, S. J. Loucks, J. A. Marozas, F. J. Marshall, R. L. McCrory, P. W. McKenty, P. M. Nilson, P. B. Radha, S. P. Regan, T. C.Sangster, W. Seka, R. W. Short, D. Shvarts,* S. Skupsky, V. A. Smalyuk, J. M. Soures, C. Stoeckl, W. Theobald, and B. Yaakobi Laboratory for Laser Energetics University of Rochester *also at Nuclear Research Center, Negev, Israel J. A. Frenje, D. T. Casey, C.K. Li, R. D. Petrasso, and F. H. Séguin Massachusetts Institute of Technology Plasma Science and Fusion Center S. P. Padalino and K. Fletcher SUNY Geneseo

4 Hot-spot and two-step-ignition research is making good progress Hot-Spot Self Ignition External Spark External Spark Conventional ICF Fast Ignition Shock Ignition T Hot spot t Fast injection of heat T t Shock pulse T t r Low-density central spot ignites Fast-heated side spot ignites Spherical shock wave ignites a high-density cold shell a high-density fuel ball a high-density fuel ball tt hot. tt cold (isobaric) t hot. t cold (isochoric) tt hot & tt cold r r In conventional ignition the hot spot contains 1%~2% of the mass but 5% of the compressed energy. Two step designs require lower implosion velocity, allowing more massive targets for the same driver energy higher gain. E12526k

5 A new hot-spot ignition design uses a multi-picket, multishock drive instead of the continuous low-intensity foot Gain 1-D = 48 3 Three-picket NIF design 37 nm CH Power/beam (TW) nm DT DT gas 17 nm E184f Time (ns) 1 12 OMEGA cryogenic targets are ~1/4 scale of the NIF target The multiple picket design is more stable and easier to tune for shock coalescence.

6 The adiabat must be kept close to one to minimize the ignition energy Multiple relatively weak shock waves are launched in the target to approximate adiabatic compression Pressure (Mb) Principal Hugoniot Single shock Higher Isentrope Mistimed shock a = 1 First shock Triple shock Double shock Second shock Third shock Time Fourth rise a = P P Fermi Minimum energy for ignition E MIN ~ a Density (g/cc) 1. Shock timing is measured in cone-in-shell targets* - good agreement with theoretical predictions E18674b *T. R. Boehly et al., Phys. Plasmas 16, 5632 (29).

7 Multiple-picket pulse shapes are being used to drive cryogenic-dt implosions on OMEGA I192e Power/beam (TW) Current drive pulse used to implode cryogenic-dt targets.4 Peak drive intensity ~ W/cm 2.3 Design a ~ 2 3 V imp = cm/s.1. 2 Time (ps) 4 Picket energies and relative timing are adjusted to optimize the shock coalescence Pixels Shadowgraph of a stalkmounted cryogenic-dt capsule nm thick DT ice <2-nm rms Pixels Target vibration at T is significantly reduced with stalk-mounted targets

8 Areal densities of up to 3 mg/cm 2 have been measured in cryogenic target implosions on OMEGA* Power/beam (TW) Power/beam (TW) E1846e V imp ~ cm/ns DT fuel V imp ~ cm/ns D 2 fuel 24 kj Time (ns) 18 kj tr exp (mg/cm 2 ) D areal densities have been achieved for drive intensities from < up to W/cm nm CD 65 DT or 95 D 2 D 2 /DT gas tr 1-D (mg/cm 2 ) V imp ~ cm/s, I ~ W/cm 2 65-nm-thick DT, a ~ 2. V imp ~ cm/s, I ~ W/cm 2 65-nm-thick DT, a ~ 2.5 V imp ~ cm/s, I ~ W/cm 2 95-nm-thick D 2, a ~ 2.5 V imp ~ cm/s, I ~ W/cm 2 95-nm-thick D 2, a ~ 2.5 V. N. Goncharov et al., Phys. Rev. Lett. 14, 1651 (21).

9 Direct-drive research is making continued progress Ignition-relevant areal densities have been achieved The next step is to increase T i (implosion velocity) Ignition and Gain Total GtRH n (g/cm 2 ) IAEA 21 DT, 22 kj IAEA 28 DT, 16 kj NIF.5 MJ NIF point design 1.5 MJ 1-D marginal ignition OMEGA DT equivalent of the NIF point design Hydro equivalent curve V i = const., a = const. (4 1 7 cm/s) (2.5) GT i H n (kev) (no alpha deposition) I1772g

10 Betti et al.* derived an ICF Lawson criterion based on quantities that can be measured in ICF implosions In 1-D and = 8tR_ g/ cm ib 6T^keVh / 4. > 1 3-D effects can be included through the measured neutron yield divided by that predicted by 1-D simulations (YOC) Px^ atm-s h~ 89tR_ g/ cm it^kevhc Px^ atm-s h~ 89tR_ g/ cm it^kevhc YOC and = 8tR_ g/ cm ib 6T^keVh / 4. YOC > 1 E18673a * C. D. Zhou and R. Betti, Phys. Plasmas 15, 1277 (28). * R. Betti et al., Phys. Plasmas 17, 5812 (21).

11 OMEGA cryogenic implosions have achieved Px ~ 1.7 atm-s On OMEGA, ignition-equivalent performance requires ~.4, GTH ~ 3.4 kev Px ~ 2.6 atm-s Cryogenic implosions to date tr =.3 g/cm 2, GTH = 2 kev YOC = 1% give ~.3, Px ~ 1.7 atm-s For comparison, the Joint European Tokamak has produced ~.14, Px ~ 1 atm-s GPHx E (atm s) OMEGA hydro-equivalent ignition OMEGA (29) 1 1 LHD LSX SSPX C-mod MST DIIID NSTX Jet GTH (kev) ITER TFTR Q = 2 Ignition CTF FRC Spheromak RFP ST Stellarator Tokamak Tokamak Tokamak Tokamak ITER E18675c

12 LLE proposed Polar Drive in 23 to allow direct-drive implosions on the NIF without moving the beams Pointing for x-ray drive Repointing for polar drive* Polar drive provides Ignition alternative Diagnostic qualification on the NIF HED platform for the NIF TC63o *S. Skupsky et al., Phys. Plasmas 11, 2763 (24).

13 A NIF hot spot, triple picket, polar drive design achieves a gain of 18 with a 1.7 MJ laser pulse This design uses a 9 nm,, = 2 shim in the ice layer, lowering the equatorial drive required The peak power of the equatorial beams is 2.2 TW (42 TW equivalent) To date, only 2-D drive non-uniformities have been included z (nm) 1.6 ns t (g/cc) r (nm) E19349 This design will be further optimized and all sources of non-uniformity will be included.

14 Integrated fast-ignition cone-in-shell experiments* are being performed on OMEGA/OMEGA EP FSC CD Void 4 nm 39 nm Implosion Energy ~18 kj (54 beams) Wavelength 351 nm Pulse duration ~3 ns Implosion velocity ~2 1 7 cm/s Drive-pulse shape Hard x-ray pulse by OMEGA EP laser P51 power (TW) Time (ps) CCD (counts/pixel) Heating beam Energy ~1. kj Wavelength 153 nm Pulse duration ~1 ps Intensity ~ W/cm 2 Relative timing varied TC8875a *W. Theobald et al., Plasma Phys. Control. Fusion 51, (29).

15 The neutron yield increased by a factor of 4 with an appropriately timed OMEGA EP beam FSC 25 Neutron yield ( 1 6 ) Without OMEGA EP OMEGA EP arrival time (ns) 1.4± additional neutrons were produced with the short-pulse laser. E1932

16 Shock-breakout measurements confirm an intact cone tip at peak neutron production FSC CD shell Au cone Streaked optical pyrometer Shock wave Shock breakout (ns) Optical emission Imaging VISAR 1 & 2 streak cameras Peak neutron production with EP laser E Tip thickness (nm)

17 CH shells have been imploded on OMEGA to test the performance of shock-ignition pulse shapes FSC The neutron yield increases considerably when a shock is launched at the end of the pulse. E16128e E L = 19 kj, a = 1.3, V i = cm/s CH 7 to 25 atm D 2 gas 4 nm 39 nm Power (TW) Y n = 2± Y n = 8± Time (ns) Intensity (W/cm 2 ) ( 1 14 ) W. Theobald et al., Bull. Am Phys. Soc. 52, 97 (27).

18 Significant improvement in target performance* is observed with shock ignition FSC Neutron YOC (%) With spike Without spike Hot-spot convergence ratio Measured GtRH n (g/cm 2 ) With spike (optimized) Without spike 1 D calculation Simulated hot-spot convergence The shock-ignition implosions show improved performance with respect to areal density and neutron yields. E17294b *W. Theobald et al., Phys. Plasmas 15, 5636 (28).

19 Summary/Conclusions The OMEGA/OMEGA EP Laser Facility is being used to investigate several approaches to inertial confinement fusion High compression (tr ~ 3 mg/cm 2 ) has been demonstrated in the symmetric hot-spot approach to ignition 1 Two advanced (two-step) concepts are being explored at LLE shock ignition 2 fast ignition 3 Cone-in-shell fast-ignition experiments have quadrupled the neutron yield with a 1-kJ, 1-ps laser pulse Experiments with shock ignition show a 4 improvement in neutron yield The Omega Laser Facility is ideal for studying inertial fusion energy concepts. TC8866a 1 V. N. Goncharov, Phys. Rev. Lett. 14, 1651 (21). 2 R. Betti et al., Phys. Rev. Lett. 98, 1551 (27). 3 M. Tabak et al., Phys. Plasmas 1, 1626 (1994).

20 Backup

21 Two-step ignition concepts ease constraints on the compression laser Fast-ignitor concept 1 Shock-ignition concept 2 Compression laser power Compression laser power Power spike Time Time Fast injection of heat using a PW ignitor beam Return shock Two-step ignition offers lower drive energies with the possibilty of higher gain. TC8868a T Burn wave t r Spike shock wave 1 M. Tabak et al., Phys. Plasmas 1, 1626 (1994). 2 R. Betti et al., Phys. Rev. Lett. 98, 1551 (27).

22 Simulations indicate improvements in yield for a range of fast-electron-conversion efficiences FSC The 2-D hydrodynamic code DRACO 2 was coupled to the hybrid PIC code LSP 3 Simulations for 1-kJ, 1-ps, 4-nm focus r (nm) 4 2 Plasma density (g/cm 3 ) z (nm) Fast-electronconversion efficiency Increase in neutron yield 1% % D yield: DRACO fuel assembly Fast-electron injection E17743d 1 A. A. Solodov et al., Phys. Plasmas 15, (28); ibid. 16, 5639 (29). 2 P. B. Radha et al., Phys. Plasmas 12, 5637 (25). 3 D. R. Welch et al., Phys. Plasmas 13, 6315 (26).

23 6 OMEGA beams were split into 4 low-intensity drive beams and 2 tightly focused delayed beams to study LPI at relevant spike intensities FSC Critical density surface Fast electrons Shock beams ~1 16 W/cm 2 Density scale length ~2 nm Laser intensity (W/cm 2 ) Shock beam 2 Drive Time (ns) The delay and intensity of the tightly focused beams are varied Laser backscattering and hot-electron generation were studied Areal density (mg/cm 2 ) E17863e

24 Up to 35% of the shock-beam laser energy is lost due to backscatter FSC Reflectivity (%) (SRS + SBS) E18435a Laser intensity ( 1 15 W/cm 2 ) No measurable signal of the 3/2 harmonic SRS dominates back reflection at highest intensity SBS reflection is relatively stable at ~1% Reflectivity (SRS)(%) Reflectivity (SBS)(%) Laser intensity ( 1 15 W/cm 2 )