Session DI02: Inertial Confinement I

Thomas H. Stix Award for Outstanding Early Career Contributions to Plasma Physics Research: Developing Broadband Laser Drivers for a Step Change in ICF Performance through Laser-Plasma Instability Mitigation

Increasing ablation pressure to >200 Mbar is crucial for accessing the region of design space where robust ignition would be expected for a MJ-class laser driver. Currently, crossed-beam energy transfer (CBET) severely limits the ablation pressure in directly driven implosions. Operating intensities are further constrained by the need to avoid fuel preheat by suprathermal electrons generated by the two-plasmon decay (TPD) and stimulated Raman scattering instabilities. Simulations suggest that fractional bandwidths of ≈1.5% (Δω/ω)—about 50 larger than what is available currently—would completely eliminate all laser-plasma instabilities (LPI) in OMEGA-scale implosions and thereby recover the advantages for energy coupling that were expected for direct drive (i.e., >200 Mbar ablation pressures) [1-2].To test these predictions, LLE is building the first UV long-pulse prototype beamline to have that amount of bandwidth—the Fourth-generation Laser for Ultrabroadband eXperiments (FLUX). FLUX utilizes high-efficiency amplification of broadband spectrally incoherent pulses based on optical parametric amplifiers [3] along with a novel sum frequency generation technique that preserves the large bandwidth into the third harmonic [4]. FLUX will be coupled to the OMEGA-60 target chamber to enable focused physics studies using the LPI platform, with operation expected to begin in early 2024. Platform development in anticipation of FLUX has been underway for some time. We will discuss plans to study the impact of laser bandwidth on a wide range of processes, including CBET, hot-electron generation, filamentation, plasma characterization, laser imprint, and more. This material is based upon work supported by the DOE NNSA under Award Number DE-NA0003856, the DOE Fusion Energy Sciences program under Award Number DE-SC0021032, the University of Rochester, and the New York State Energy Research and Development Authority.

[1] R. K. Follett et al., Physics of Plasmas 28, 032103 (2021).

[2] R. K. Follett et al., Physics of Plasmas 30, 042102 (2023).

[3] C. Dorrer et al., Optics Express 28, 451-471 (2020).

[4] C. Dorrer et al., Optics Express 29, 16135-16152 (2021).   

NIF INVITED: The Road to Ignition: An Historical Overview

This talk reviews the many twists and turns in the long journey that culminated in ignition in late 2022 using the laser heated indirect-drive approach to imploding DT filled targets at the National Ignition Facility (NIF), located at the Lawrence Livermore National Laboratory (LLNL). 

We describe early origins of the Laser Program at LLNL, and key developments such as the paradigm shifting birth of High Energy Density (HED) studies with lasers, changes in choice of laser wavelength, and the development of key diagnostics and computer codes. Fulfilling the requirements of the multi-faceted Nova Technical Contract was a necessary condition for the approval of the NIF, but more importantly, the end of the Cold War and the cessation of Nuclear Testing, were key catalysts in that approval, along with the ready-and-waiting field of HED. 

The inherent flexibility of the field of laser driven ICF played a huge role in achieving success at the NIF. We describe how the original "point design" target evolved, through the lessons of experiment, in every single way: The capsule's materials and size were changed; The hohlraum's materials, size, laser entrance holes, and gas fills were also all changed, as were the laser pulse shapes that go along with all those changes. The philosophy to globally optimize performance for stability (by lowering the implosion convergence) was also key, as was progress in target fabrication, and in increasing NIF's energy output. The persistence of the research staff, and the steadfast backing of our supporters were also necessary elements in this success. 

 We gratefully acknowledge 7 decades of researchers' endeavors, and 4 decades of the dedicated efforts of many hundreds of personnel across the globe who have participated in NIF construction, operation, target fabrication, diagnostic and theoretical advances, that have culminated in ignition.   

Simulated Signatures of Ignition

Ignition on the National Ignition Facility [1] provides a novel opportunity to evaluate past data [2] to identify signatures of inertial confinement fusion capsule failure mechanisms as well as signatures of the ignition cliff, where capsule performance is highly sensitive to small perturbations.  We have used new simulations of high yield implosions as well as some from past studies [3-5] in order to identify unique signatures of different failure mechanisms: jetting due to the presence of voids or defects [3], interfacial mixing due to instabilities (using a Reynolds-Averaged Navier-Stokes model for the development of material mixing) or due to plasma transport, radiative cooling due to hot spot contaminant, long-wavelength drive asymmetry, and preheat.  Individually, these failure mechanisms involve very different physics, which exhibit unique performance trajectories as they traverse the ignition cliff that can be distinguished through the relationships between experimental observables such as neutron yield, down-scattered ratio, and burn width.  Our simulations include both plastic and high density carbon capsule implosions spanning early low-foot [6] through modern HybridE [7] designs, and the variability across designs is much smaller than the differences due to failure mechanisms.  In all cases, there is a clear distinction between three regimes that we identify as ignition failure, ignition cliff, and robust ignition.  The experimental trajectories are most consistent with preheat and voids/defects, which are indistinguishable in our analysis, and least consistent with pre-mix and interfacial mixing.  This suggests that compression improvements have played a primary role in enabling high yields.

 

References

[1]    Abu-Shawareb et al, PRL 129, 075001, 2022.

[2]    Landen et al, HEDP 36, 100755, 2020.

[3]    Haines et al, Phys. Plasmas 29, 042704, 2022.

[4]    Haines et al, Phys. Plasmas 27, 082703, 2020.

[5]    Haines et al, Phys. Plasmas 24, 052701, 2017.

[6]    Haan et al, Phys. Plasmas 18, 051001, 2011.

[7]    Kritcher et al, PRE 106, 025201, 2022.