Description
Cross beam energy transfer (CBET) is central to controlling implosion symmetry in all ignition designs at the National Ignition Facility (NIF), and can possibly be even more critical as laser driver energies increase from the current multi megajoule regime toward next generation facilities operating at approximately 10 MJ. In indirect drive inertial confinement fusion (ICF) experiments at NIF, CBET redistributes laser power among the 192 beams via ion wave mediated coupling in the hohlraum plasma, strongly influencing the spatial and temporal drive symmetry on the fusion capsule. While CBET has been essential to achieving ignition, its quantitative behavior remains poorly constrained; current radiation hydrodynamics simulations cannot reliably predict low mode shape and symmetry, and an empirical plasma wave amplitude limiter is routinely applied to CBET models. This limiter must be retuned for each design, laser pulse, and wavelength detuning configuration, creating ambiguity as to whether it compensates deficiencies in the CBET physics or other inaccuracies in hohlraum and capsule modeling. These uncertainties increase the number of required tuning shots and limit confidence in extrapolations to higher energy facilities.
We report on a new suite of NIF experiments, conducted in 2025 and 2026, designed to directly measure CBET in a hohlraum-like environment. A novel experimental platform has been developed that, for the first time, quantifies the time resolved transfer of power from outer to inner beam cones, as a function of wavelength detuning Δλ and hohlraum gas fill density. High fidelity time gated x-ray images of the inner cone footprint provide unprecedented maps of post CBET beam powers and intensities over an entire NIF hemisphere.
This work was performed under the auspices of U.S. DOE by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344