Speaker: Dr. Mingsheng Wei (General Atomics, San Diego, California, U.S.A.）

Chair Person: Prof. Bin Qiao

Date: March 31, 2016, 14:00

Place: Room 210 COE Building No.1

Report Introduction:

Fast ignition (FI) laser fusion scheme requires efficient energy coupling of a high intensity laser to a preassembled high-density core transferred by laser-produced MeV electrons. Knowledge of fast electron spatial energy deposition in integrated FI experiments is critical to assessing various FI approaches and forming credible predictions of performance. In this presentation, I will discuss results from a series of integrated FI experiments conducted at the Omega laser facility using novel copper-doped shell targets that enable the spatial energy deposition measurement. The cone-in-shell target was imploded by the 18 kJ OMEGA UV driver with a low-adiabat pulse shape resulting in a compressed plasma target that was well characterized1. The compressed target was then heated by injecting up to 1.5 kJ of short-pulse laser energy from the 10 ps OMEGA EP beam into the gold cone with varied timing delay, laser contrast and cone-tip diameter. Spatial energy deposition of fast electrons in the compressed target was visualized by imaging copper Ka radiation using a spherical crystal imager and the total Ka yield was measured by a calibrated x-ray spectrometer, from which key information of fast electron beam divergence, source location and temperature can be inferred by directly comparing the experimental results with the simulated fluorescence spatial distribution and yield. With the physics understanding gained from the data and supported by multi-scale modeling, we improved performance demonstrating up to ~7% short-pulse energy coupling to the compressed target by optimizing target and implosion parameters together with the use of the high contrast laser that minimized preplasma inside the cone and increased core areal density. This is the highest energy coupling efficiency ever reported from the OMEGA-scale FI experiments2.

The technique and the findings are highly relevant for the further study of FI energy coupling and assessing various approaches such as magnetic guiding of fast electrons with engineered targets, channeling approach as well as laser-driven proton FI on existing facilities. This work also presents the case for the predicted higher coupling efficiency (~15%) due to more effective stopping of fast electrons in the ignition-scale areal densities accessible on large-scale facilities such as the combined NIF and high intensity multi-kJ NIF-ARC laser. 

The work was performed in collaboration with F.N. Beg, L.C. Jarrott, C. McGuffey, B. Qiao at UCSD; A.A. Solodov, W. Theobald, C. Stoeckl, R. Betti, J. Delettrez, V.Y. Glebov, F.J. Marshall, C. Mileham at LLE, Univ. Rochester; H. Chen, T. Döppner, M.H. Key, H.S. McLean, P.K. Pate at LLNL; E.M. Giraldez, R.W. Luo, R.B. Stephens at GA; H. Sawada at UNR; H. Habara, T. Iwawaki, T. Yabuuchi at Osaka University; J.J. Santos at CELIA, University of Bordeaux.

This work was supported by U.S. Department of Energy (DOE) under contracts DE-FC02-04ER54789 (FSC), DE-FG02-05ER54834 (ACE), DE-NA0000854 (NLUF) and DE-NA0002033 (NLUF). The support of DOE does not constitute an endorsement by DOE of the views expressed here.

[1] W. Theobald, A.A. Solodov, C. Stoeckl et al., Nat. Commun. 5, 5785 (2014).

[2] L.C. Jarrott, M.S. Wei, C. McGuffey et al., Nature Physics (published Online, January 11, 2016), DOI:10.1038/NPHYS3614

About Speaker:
　　Dr. Mingsheng Wei received her PhD in Laser Plasma Physics from Imperial College, University of London in 2005. From 2005 to 2010, she was a Project Scientist at the Center for Energy Research, University of California at San Diego. In 2010, Dr. Wei joined the Inertial Fusion Technology Division of General Atomics, San Diego, where she currently serves as Senior Scientist and Principal Investigator (PI) working in the area of High Energy Density Physics (HEDP) as well as interfacing with PIs from national labs and academics on target development and fabrication to support their HEDP experiments. Dr. Wei’s research has focused on study of high power laser plasma interaction physics, the resultant fast electron and ion beam generation and transport in solid, warm and hot dense plasmas and their applications in alternative ignition schemes such as fast ignition (FI) and shock ignition (SI), and laser preheating in magnetized liner inertial fusion. She has led the designs and executions of many successful HEDP experiments using the world-leading high power lasers such as the Vulcan 100TW and PW lasers, the Titan laser, the Trident laser and the OMEGA and OMEGA EP lasers. Dr. Wei has authored and co-authored over 90 articles on these topics in high impact refereed journals including Nature, Nature Physics, Nature Communications, Physical Review Letters among others.