Laser entrance window transmission and reflection measurements for preheating in magnetized liner inertial fusion

Journal Article · 03 June 2018 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.5030107 · OSTI ID:1460288

Davies, J. R. ^[1]; Bahr, R. E. ^[2]; Barnak, D. H.  ^[2]; Betti, R. ^[2]; Bonino, M. J. ^[2]; Campbell, E. M.  ^[2]; Hansen, E. C. ^[2]; Harding, D. R. ^[2]; Peebles, J. L.  ^[2]; Sefkow, A. B. ^[2]; Seka, W. ^[2]; Chang, P. -Y.  ^[3]; Geissel, M.  ^[4]; Harvey-Thompson, A. J. ^[4]

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1. Univ. of Rochester, NY (United States). Lab. for Laser Energetics; Laboratory for Laser Energetics, University of Rochester
2. Univ. of Rochester, NY (United States). Lab. for Laser Energetics
3. National Cheng Kung Univ., Tainan City (Taiwan). Inst. of Space and Plasma Sciences
4. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

Laser-driven magnetized liner inertial fusion (MagLIF) is being developed on the OMEGA Laser System to study scaling. MagLIF targets require a preheat laser entrance window that can hold the gas in the target yet allow sufficient laser energy to enter the gas. For OMEGA MagLIF targets, 1.8-μm-thick polyimide foils were found to be sufficient to hold a fuel pressure of up to 14 atm. Transmission and reflection of an OMEGA beam incident on such foils were measured with a calorimeter and time-resolved spectrometers for 2.5-ns square-shaped pulses, with energies from 60 to 200 J, focused to intensities from 0.65 to 2.2 × 10¹⁴ W/cm². The laser energy transmitted in every case exceeded that required to achieve the goal of preheating the gas to 100 eV. The time-resolved measurements showed an initial period with very low, decreasing transmission, the duration of which decreased with increasing intensity, followed by a rapid transition to full transmission, accompanied by brief sidescattering of the transmitted light with a significant red shift. Reflection was always negligible. Two-dimensional radiation–hydrodynamic simulations, using 3-D ray tracing with inverse bremsstrahlung energy deposition, did not capture the rapid transition to full transmission, showing instead a slow increase in transmission, without significant sidescatter or red shift. Finally, we propose that full transmission is achieved by relativistic self-focusing, enhanced by focusing related to the electron density profile that forms, followed by a ponderomotive blowout of the plasma.

• Research Organization: Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States); Univ. of Rochester, NY (United States). Lab. for Laser Energetics
• Sponsoring Organization: USDOE Advanced Research Projects Agency - Energy (ARPA-E); USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC04-94AL85000; AR0000568; NA0001944
• OSTI ID: 1460288
• Report Number(s): SAND--2018-9666J; 2018-25, 2378, 1420
• Journal Information: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 6 Vol. 25; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English