Ignition Scaling

The National Ignition Facility (NIF) is the largest laser facility in the world and the flagship facility for the national Inertial Confinement Fusion (ICF) Program. High-performance cryogenic direct-drive ICF implosions on OMEGA at 30 kJ project to near ignition conditions and several hundred kJ yields on NIF at 2-MJ laser energy, but the physics underpinning that extrapolation must be studied and verified.

Plot showing Hydrodynamic scaling.

Hydrodynamic scaling of cryogenic direct-drive implosion performance from OMEGA to NIF energies.

LPI: laser–plasma instability
PDD: polar direct drive
SDD: smoothing by spectral dispersion

Polar-Direct-Drive (PDD) Implosions

The MegaJoule Direct Drive (MJDD) Campaign consists of experiments on the NIF to study direct-drive physics at ignition scale, primarily related to laser–plasma interactions. These include laser–energy coupling, cross-beam energy transfer (CBET), hot-electron preheat, laser imprint, and hydrodynamic scaling. Low-convergence implosions are studied in pursuit of high yield on the NIF.

The NIF is configured with beams concentrated near the poles of the target chamber, optimized for illuminating the inside of an indirect-drive ICF hohlraum. For direct-drive implosions, the beams are repointed to directly irradiate the capsule in the polar direct drive (PDD) configuration.1,2

NIF target chamber.

NIF target chamber with beam ports highlighted.

Cone-swapping illustration.

Cone-swapping to produce hemispheric wavelength detuning for CBET mitigation in NIF PDD experiments.

2-D DRACO-simulated and measured x-ray radiographs.

2-D DRACO-simulated and measured x-ray radiographs of PDD implosions demonstrating wavelength detuning to partially mitigate CBET.

PDD implosions have been studied to validate modeling of CBET and mitigation using wavelength detuning.3 Currently, thin-shell, low-convergence implosions are studied to demonstrate control of laser–energy coupling and PDD implosion symmetry using shell contouring, laser pointing, and pulse shaping. Solid spheres are used as a focused experiment to study energy coupling.

Laser–energy coupling and implosion symmetry are diagnosed using x-ray radiography and self-emission imaging, as well as through nuclear measurements (yield, ion temperature, bang time) and scattered light.

The Omega Experiments Group is implementing the new scattered light time-history diagnostic (SLTD) on the NIF to diagnose scattered light in direct-drive experiments.4

SLTD illustration.

Schematic of the scattered light time-history diagnostic (SLTD).

Hot-Electron Preheat

Cartoon of hot electron preheat.

Cartoon of hot-electron preheat in a cryogenic direct-drive implosion.

Laser–plasma instabilities and hot-electron preheat are another physics issues that may behave differently at the NIF scale than at Omega scale. Planar experiments on the NIF have identified that stimulated Raman scattering (SRS) is the primary hot-electron source, in contrast to two-plasmon decay (TPD) on OMEGA.5,6 Hot-electron energy deposited in PDD implosions has been diagnosed using hard x-ray measurements and is close to levels thought to be tolerable in direct-drive ignition designs.

Preheat experiments in PDD implosions have also demonstrated the use of Si layers to mitigate preheat in implosions. Further NIF experiments are needed to extrapolate and estimate the level of preheat that is expected in ignition-scale cryogenic implosions.

Plot showing Fraction of laser energy converted to hot electrons.

Fraction of laser energy converted to hot electrons as a function of laser intensity at quarter-critical density in NIF planar experiments.

NIF PDD implosions with a Si layer.

Profile of hot-electron preheat in NIF PDD implosions with a Si layer.

Laser Imprint

Hydrodynamic instabilities seeded by short-scale nonuniformities in the laser spot intensity can degrade direct-drive implosion performance. Experiments on the NIF are used to study laser imprint and its mitigation.

Planar experiments on the NIF have used x-ray radiography to diagnose growth of single-beam imprint instabilities to benchmark imprint models and to assess imprint mitigation using multi-FM laser smoothing.

X-ray radiography data of Rayleigh-Taylor instability growth.

Planar imprint platform and x-ray radiography data of Rayleigh–Taylor instability growth seeded by laser imprint on the NIF.

Spherical (cone-in-shell) experiments have measured instability growth with many overlapping beams, as in a PDD implosion, and shown partial mitigation of imprint-related instabilities using gold overcoats.

Exploring laser and target solutions to mitigate laser imprint and enable high-performing implosions on the NIF is a critical aspect of the research program.