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Visualizing fast electron energy transport into laser-compressed high-density fast-ignition targets

Abstract

Recent progress in kilojoule-scale high-intensity lasers has opened up new areas of research in radiography, laboratory astrophysics, high-energy-density physics, and fast-ignition (FI) laser fusion. FI requires efficient heating of pre-compressed high-density fuel by an intense relativistic electron beam produced from laser–matter interaction. Understanding the details of electron beam generation and transport is crucial for FI. Here we report on the first visualization of fast electron spatial energy deposition in a laser-compressed cone-in-shell FI target, facilitated by doping the shell with copper and imaging the K-shell radiation. Multi-scale simulations accompanying the experiments clearly show the location of fast electrons and reveal key parameters affecting energy coupling. The approach provides a more direct way to infer energy coupling and guide experimental designs that significantly improve the laser-to-core coupling to 7%. Our findings lay the groundwork for further improving efficiency, with 15% energy coupling predicted in FI experiments using an existing megajoule-scale laser driver.

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Figure 1: Configuration of the target, experimental layout, and laser parameters.
Figure 2: Experimentally measured and simulated Cu Kα images.
Figure 3: Spectrally measured Cu Kα yield as a function of OMEGA-EP short-pulse energy.
Figure 4: Dependence of fast electron energy-coupling efficiency on core areal density and electron beam temperature.

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Acknowledgements

This material is based on work supported by the US Department of Energy National Nuclear Security Administration under the National Laser User Facility programme with Award Number DE-NA0000854, DE-NA0002033, the OFES Fusion Science Center (FSC) grant No DE-FC02-04ER54789, the OFES ACE Fast Ignition grant No. DE-FG02-95ER54839, and NNSA cooperative agreement DE-NA0001944. The support of the DOE does not constitute an endorsement by the DOE of the views expressed in this article. J.J.S. participated in this work thanks to funding from the French National Agency for Research (ANR) and the competitiveness cluster Alpha—Route des Lasers through project TERRE ANR-2011-BS04-014. The authors would like to acknowledge excellent support provided by the Omega Laser Facility staff and the GA target fabrication group. The authors are thankful to S. Chawla for HYDRA simulations and C. Dorrer for the measured OMEGA-EP pre-pulse information.

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Contributions

F.N.B. and M.S.W. designed and executed the experiment as principal investigators with help from L.C.J., C.McGuffey, W.T., A.A.S., R.B., J.D., M.H.K., F.J.M., H.S.M., P.K.P., J.J.S., H.S., T.Y. and R.B.S.; C.S., H.C., T.D., V.Y.G., H.H., T.I. and C.Mileham developed and operated diagnostics; data analysis was performed by L.C.J., C.McGuffey and W.T.; simulations were performed by A.A.S. for the DRACO/LSP modelling, B.Q. for the LSP PIC modelling, L.C.J. for the ZUMA modelling; targets were manufactured by E.M.G., R.W.L. and R.B.S.; M.S.W., C.McGuffey and F.N.B. led the writing of the manuscript with significant contributions from A.A.S., L.C.J., W.T., H.S.M. and R.B.S.; figures were prepared by L.C.J., A.A.S. and M.S.W.

Corresponding authors

Correspondence to M. S. Wei or F. N. Beg.

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The authors declare no competing financial interests.

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Jarrott, L., Wei, M., McGuffey, C. et al. Visualizing fast electron energy transport into laser-compressed high-density fast-ignition targets. Nature Phys 12, 499–504 (2016). https://doi.org/10.1038/nphys3614

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