Mitigation of Cross-Beam Energy Transfer in Direct-Drive Implosions on OMEGA

Transcription

1 Mitigation of Cross-Beam Energy Transfer in Direct-Drive Implosions on OMEGA In-flight aspect ratio OMEGA cryogenic ignition hydro-equivalent design tr = 3 mg/cm 2, V imp = cm/s m = 48 ng a = R t (CH) Zooming (CH) m = 6 ng Hydro-equivalent a = 3.2 curve CBET No CBET Ablation pressure (Mbar) D. H. Froula Plasma and Ultrafast Physics Group Leader University of Rochester Laboratory for Laser Energetics Current stability threshold th Annual Meeting of the American Physical Society Division of Plasma Physics Denver, CO 11 November 13

2 Summary Reducing cross-beam energy transfer (CBET) on OMEGA will allow for more stable ignition-relevant implosions CBET can be mitigated by reducing the diameter of the laser beams Mitigating CBET increases the ablation pressure, allowing for thicker shelled targets and higher adiabats Two approaches are being investigated on OMEGA to reduce the laser beams smaller laser spots reduced beam-to-beam overlap two-state zooming increased single-beam imprint Experiments to validate these schemes are underway. E22428

3 Collaborators T. J. Kessler, I. V. Igumenshchev, V. N. Goncharov, H. Huang, S. X. Hu, E. Hill, J. H. Kelly, D. T. Michel, D. D. Meyerhofer, A. Shvydky, J. D. Zuegel, and R. Epstein University of Rochester Laboratory for Laser Energetics

4 CBET reduces the energy coupled to the fusion capsule CBET is spatially limited near M ~ 1 k 1 k 2 k a Energy is transferred between beams by ion-acoustic waves Target E19971d CBET reduces the most hydrodynamically efficient portion of the incident laser beams.

5 CBET modeling is required to match the experimental observables (scattered light, implosion velocity, and bang time)* 76 nm 122 ps 128 ps 133 ps 138 ps 182 ps 188 ps 193 ps 198 ps** 2 Simulations (Nonlocal + CBET models) Simulations (Nonlocal + CBET models) 2 4 Simulations (Nonlocal + CBET models) 2 P (TW) R (nm) 4 3 Data P (TW) V (km/s) 3 Data P (TW) t (ns) t (ns) t (ns) CBET reduces the ablation pressure by ~4%. E22473a * I. V. Igumenshchev et al., Phys. Plasmas 19, 6314 (12). ** D. T. Michel et al., Rev. Sci. Instrum. 83, E3 (12).

6 Experiments have demonstrated that CBET can be mitigated by reducing the radius of laser beams Scattered-light fraction.3 CBET model R beam /R target Absorption fraction OMEGA cryogenic hydro-equivalent design tr = 3 mg/cm 2, V imp = cm/s In-flight aspect ratio CBET m = 48 ng a = 1.7 Hydroequivalent curve.8 R t (CH) No CBET Ablation pressure (Mbar) Current stability threshold* m = 6 ng a = 3.2 Reducing the radius of the beams will allow the thickness of the shell and the adiabat to be increased in a hydro-equivalent design but the reduced overlap uniformity may increase the imprint. E14g *D. H. Froula et al., Phys. Rev. Lett. 8, 123 (12). *V. N. Goncharov, GI3.1, this conference (invited).

7 Simulations suggest that reducing the beam diameters by % (R b /R t =.8) will have minimal impact on the hotspot symmetry 2-D DRACO simulations (low-order nonuniformities only) 4 3 R b /R t = 1.7 YOC = R b /R t =.8 YOC = R b /R t =.7 YOC =.62 t (g/cm 3 ) t (g/cm 3 ) t (g/cm 3 ) z (nm) z (nm) z (nm) Reducing the beam diameters by more than % significantly degrades the target performance. TC9894 I. V. Igumenshchev et.al., Phys. Plasmas 19, 6314 (12).

8 Reducing the diameter of the laser beams beyond % after a sufficient conduction zone is generated ( zooming ) is predicted to maintain good low-mode uniformity rms deviation from round (v) R b /R t =.7 3 v - 17 nm 3 4 Power ( 12 W) Time (ns) R b /R t = 1. 3 v < 3 nm 3 4 t (g/cm 3 ) E21317l *I. V. Igumenshchev et al., Phys. Rev. Lett. 1, 141 (13).

9 Reducing the diameter of the laser beams beyond % after a sufficient conduction zone is generated ( zooming ) is predicted to maintain good low-mode uniformity rms deviation from round (v) R b /R t =.7 3 v - 17 nm 3 4 Power ( 12 W) R b /R t = 1. R b /R t = Time (ns) Zooming from R b /R t = 1. to.7 R b /R t = 1. 3 v < 3 nm 3 4 t (g/cm 3 ) v - 12 nm 1 E21317m 3 4 *I. V. Igumenshchev et al., Phys. Rev. Lett. 1, 141 (13).

10 Reducing the diameter of the laser beams beyond % after a sufficient conduction zone is generated ( zooming ) is predicted to maintain good low-mode uniformity rms deviation from round (v) R b /R t =.7 3 v - 17 nm 3 4 Power ( 12 W) R b /R t = 1. R b /R t = Time (ns) Zooming from R b /R t = 1. to.7 R b /R t = 1. 3 v < 3 nm 3 4 t (g/cm 3 ) v - 12 nm v - 3. nm E21317n *I. V. Igumenshchev et al., Phys. Rev. Lett. 1, 141 (13).

11 Reducing the diameter of the laser beams beyond % after a sufficient conduction zone is generated ( zooming ) is predicted to maintain good low-mode uniformity rms deviation from round (v) R b /R t =.7 3 v - 17 nm 3 4 Power ( 12 W) R b /R t = 1. R b /R t = Time (ns) Zooming from R b /R t = 1. to.7 R b /R t = 1. 3 v < 3 nm 3 4 t (g/cm 3 ) v - 12 nm v - 3. nm v < 1. nm E21317o *I. V. Igumenshchev et al., Phys. Rev. Lett. 1, 141 (13).

12 Zooming can be implemented on OMEGA using a radially varying phase plate and a dynamic near field Power ( 12 W) Time (ns) Dynamic near field Picket pulse Zooming phase plate (ZPP) Main pulse E239b D. H. Froula et al., Phys. Plasmas, 8274 (13).

13 Implementing zooming on OMEGA will provide a more-robust implosion to hydrodynamic instabilities In-flight aspect ratio OMEGA cryogenic ignition hydro-equivalent design tr = 3 mg/cm 2, V imp = cm/s Current stability threshold 14 m = 48 ng a = R t (CH) Zooming (CH) m = 6 ng Hydro-equivalent a = 3.2 curve CBET No CBET Ablation pressure (Mbar) In-flight aspect ratio Hydro-equivalence for current implosions Unstable Current threshold Adiabat tr/tr 1-D > <. Data Both CBET mitigation strategies on OMEGA will allow the mass of the shell and the adiabat to be increased while maintaining ignition-relevant conditions. E22229b T. C. Sangster et al., Phys. Plasmas, 6317 (13).

14 Summary/Conclusions Reducing cross-beam energy transfer (CBET) on OMEGA will allow for more stable ignition-relevant implosions CBET can be mitigated by reducing the diameter of the laser beams Mitigating CBET increases the ablation pressure, allowing for thicker shelled targets and higher adiabats Two approaches are being investigated on OMEGA to reduce the laser beams smaller laser spots reduced beam-to-beam overlap two-state zooming increased single-beam imprint Experiments to validate these schemes are underway. E22428